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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
This comprehensive book presents a detailed account of research and recent developments in the field of green energetic materials, including pyrotechnics, explosives and propellants. This area is attracting increasing interest in the community as it undergoes a transition from using traditional processes, to more environmentally-friendly procedures. The book covers the entire line of research from the initial theoretical modelling and design of new materials, to the development of sustainable manufacturing processes. It also addresses materials that have already reached the production line, as well as considering future developments in this evolving field.
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Contents
Cover
Title Page
Copyright
List of Contributors
Preface
Chapter 1: Introduction to Green Energetic Materials
1.1 Introduction
1.2 Green Chemistry and Energetic Materials
1.3 Green Propellants in Civil Space Travel
1.4 Conclusions
References
Chapter 2: Theoretical Design of Green Energetic Materials: Predicting Stability, Detection, Synthesis and Performance
2.1 Introduction
2.2 Computational Methods
2.3 Green Propellant Components
2.4 Conclusions
References
Chapter 3: Some Perspectives on Sensitivity to Initiation of Detonation
3.1 Energetic Materials and Green Chemistry
3.2 Sensitivity: Some Background
3.3 Sensitivity Relationships
3.4 Sensitivity: Some Relevant Factors
3.5 Summary
Acknowledgments
References
Chapter 4: Advances Toward the Development of “Green” Pyrotechnics
4.1 Introduction
4.2 The Foundation of “Green” Pyrotechnics
4.3 Development of Perchlorate-Free Pyrotechnics
4.4 Removal of Heavy Metals from Pyrotechnic Formulations
4.5 Removal of Chlorinated Organic Compounds from Pyrotechnic Formulations
4.6 Environmentally Friendly Smoke Compositions
4.7 Conclusions
Acknowledgments
Abbreviations
References
Chapter 5: Green Primary Explosives
5.1 Introduction
5.2 Green Primary Explosive Candidates
5.3 Conclusions
Acknowledgments
References
Chapter 6: Energetic Tetrazole N-oxides
6.1 Introduction
6.2 Rationale for the Investigation of Tetrazole N-oxides
6.3 Synthetic Strategies for the Formation of Tetrazole N-oxides
6.4 Recent Examples of Energetic Tetrazole N-oxides
6.5 Conclusion
Acknowledgments
References
Chapter 7: Green Propellants Based on Dinitramide Salts: Mastering Stability and Chemical Compatibility Issues
7.1 The Promises and Problems of Dinitramide Salts
7.2 Understanding Dinitramide Decomposition
7.3 Vibrational Sum-Frequency Spectroscopy of ADN and KDN
7.4 Anomalous Solid-State Decomposition
7.5 Dinitramide Chemistry
7.6 Dinitramide Stabilization
7.7 Conclusions
References
Chapter 8: Binder Materials for Green Propellants
8.1 Binder Properties
8.2 Inert Polymers for Binders
8.3 Energetic Polymers
8.4 Energetic Plasticisers
8.5 Outlook for Design of New Green Binder Systems
References
Chapter 9: The Development of Environmentally Sustainable Manufacturing Technologies for Energetic Materials
9.1 Introduction
9.2 Explosives
9.3 Pyrotechnics
9.4 Propellants
9.5 Formulation
9.6 Conclusions
Acknowledgments
Abbreviations and Acronyms
References
Chapter 10: Electrochemical Methods for Synthesis of Energetic Materials and Remediation of Waste Water
10.1 Introduction
10.2 Practical Aspects
10.3 Electrosynthesis
10.4 Electrochemical Remediation
10.5 Current Developments and Future Directions
References
Index
This edition first published 2014
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Library of Congress Cataloging-in-Publication Data
Green energetic materials / [compiled by] Tore Brinck.
pages cm
Includes bibliographical references and index.
ISBN 978-1-119-94129-3 (cloth)
1. Fuel. 2. Explosives. 3. Green chemistry. I. Brinck, Tore, editor ofcompilation.
TP318.G74 2014
662.6–dc23
2013034800
A catalogue record for this book is available from the British Library.
HB ISBN: 9781119941293
List of Contributors
Tore Brinck, Applied Physical Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, Sweden
David E. Chavez, WX Division, Los Alamos National Laboratory, USA
Carina Eldsäter, Swedish Defence Research Agency, FOI, Sweden
Thomas M. Klapötke, Department of Chemistry and Biochemistry, Energetic Materials Research, Ludwig-Maximilian University of Munich, Germany
Eva Malmström, School of Chemical Science and Engineering, KTH Royal Institute of Technology, Sweden
Jane S. Murray, Department of Chemistry, University of New Orleans, USA and CleveTheoComp, USA
Karl D. Oyler, U.S. Army Armament Research, Development and Engineering Center (ARDEC) Picatinny Arsenal, USA
Peter Politzer, Department of Chemistry, University of New Orleans, USA and CleveTheoComp, USA
Martin Rahm, Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, USA
Jesse J. Sabatini, US Army RDECOM-ARDEC, Pyrotechnics Technology & Prototyping Division, Pyrotechnics Research, Development & Pilot Plant Branch, USA
Jörg Stierstorfer, Department of Chemistry and Biochemistry, Energetic Materials Research, Ludwig-Maximilian University of Munich, Germany
Lynne Wallace, UNSW Canberra, University of New South Wales at the Australian Defence Force Academy, Canberra, Australia
Preface
It has become increasingly apparent that the use, or production, of many common energetic materials leads to the release of substances that accumulate in nature and can be harmful to humans or to the environment. As a consequence, the use of several important compounds has already been restricted or banned, and many more are expected to face restrictions in the near future. This has resulted in considerable research efforts aimed at developing sustainable alternatives with preserved performance. This book is an attempt to review the current status of the field of green energetic materials. My objective has been to cover the entire process in the development of a new energetic material, from the initial theoretical design to the optimization of the manufacturing process. In addition, the aim has been to consider all different types of energetic materials, including propellants, explosives and pyrotechnics, and both military and civilian applications. To ensure the quality and relevance of the description, I have invited active scientists that are all highly regarded experts in their respective fields to write the individual chapters.
The book is intended as a reference and an inspiration for academic, industrial, and government researchers active in the field of energetic materials. It should also find use as a textbook for courses at the graduate level.
In the first chapter I introduce the concept of green energetic materials and define it in the context of the principles of green chemistry. The particular issues that separate the production and use of energetic materials from other chemical products are highlighted and discussed. Furthermore, the development of green propellants for civil space travel is used as a case study to analyze the problems of taking an energetic material from the initial design phase to its final application.
In the subsequent chapter, Martin Rahm and I discuss the use of quantum chemical methods to design green energetic materials with targeted properties. We show that, while such procedures are very effective for identifying compounds with extreme performance levels, the real challenge often lies in devising routes for their synthesis and large-scale production. The focus of Chapter 3 is on the sensitivity for detonation of energetic materials. Peter Politzer and Jane Murray describe the current understanding of sensitivity at the molecular level, and identify molecular and crystalline features that can be used to design compounds with decreased risks of unintended detonations.
The advances in the development of green pyrotechnics are impressive, considering the diversity of the applications and the large number of toxic chemicals that traditionally have been used to achieve the special effects. Jesse Sabatini reviews the efforts that have been made to remove perchlorates, toxic heavy metals, and harmful organic compounds from pyrotechnic applications. In the subsequent chapter, Karl Oyler writes about similar developments in the field of primary explosives. Here, the main challenge has been to find replacements for lead azide and lead styphnate. Thomas Klapötke and Jörg Stierstorfer describe current efforts to develop greener secondary explosives. Their chapter focuses on the synthesis and properties of tetrazole N-oxides; a class of compounds that shows great promise to hold a green alternative for the most common military explosive, RDX.
The following two chapters turn attention to the development of green propellants. In Chapter 7, Martin Rahm and I discuss recent advances in the understanding of ammonium dinitramide (ADN). This oxidizer, first synthesized in the 1970s in the former Soviet Union, has long been considered a potential green replacement for ammonium perchlorate in solid propellants. However, its implementation in solid formulations has been plagued with stability and compatibility issues. Due to new knowledge, gathered from theoretical and spectroscopic analyses, the chemical rationale behind these issues has been resolved and the potential for designing stable ADN-propellants has improved. In the following chapter, Carina Eldsäter and Eva Malmström discuss binder materials for green solid composite and homogeneous rocket propellants. They consider propellants based on ADN as well as other green oxidizers. This chapter highlights the many factors that need to be optimized in order to obtain a functional propellant formulation.
David Chavez reviews the development of sustainable manufacturing technologies for energetic materials in Chapter 9. According to the principles of green chemistry, it is not sufficient that the end product is benign, the entire manufacturing process should be environmentally friendly and make efficient use of natural resources. Challenges that are discussed include minimization of waste, reduced use of organic solvents, and the introduction of energy efficient synthesis procedures. Electrochemical synthesis is one such procedure that is discussed in more detail in Chapter 10. In this chapter, Lynne Wallace also describes the use of electrochemical methods for remediation of wastewater arising from the use and manufacture of energetic materials.
I am very grateful to all the authors that have contributed to this book. My hope is that you will appreciate their chapters as much as I have, and that you will find this book useful for your future endeavors in the exciting field of green energetic materials.
Tore Brinck
Sweden
1
Introduction to Green Energetic Materials
Tore Brinck
Applied Physical Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, Sweden
1.1 Introduction
The first energetic materials were developed in ancient China. Around 200 BC, Chinese alchemists were already starting to experiment with heating mixtures of saltpeter and sulfur. In the seventh century, saltpeter and sulfur were combined with charcoal to create an explosive material resembling what we today refer to as gunpowder. It was originally used for fireworks but soon became increasingly important for a range of military applications. The use of gunpowder in mining and civil engineering did not begin until the seventeenth century. Gunpowder remained the base for all energetic materials in practical use until the isolation of mercury(II) fulminate in 1799 by Edward Charles Howard. However, the first revolution in the development of energetic materials since the discovery of gunpowder started with the inventions of nitrocellulose (NC) in 1846 and nitroglycerine (NG) in 1847. NC was used as a propellant, whereas NG was mainly an explosive. Both these compounds had greatly enhanced performance compared to gunpowder. In 1866, Alfred Nobel introduced the original dynamite, a mixture of 75% NG with 25% kiselguhr, with a minor addition of sodium carbonate. It had much reduced sensitivity compared to pure NG and was, in contrast to NG, relatively safe to handle and transport. Nobel later developed gelatinous dynamite by combining NG with NC in a jelly. This material performed considerably better than the original dynamite and additionally improved safety. These examples illustrate the two main objectives, to improve performance and safety, that have traditionally driven research on energetic materials. It is important to remember that, in the wider definition of energetic materials – which include propellants, explosives, and the large area of pyrotechnics – the definition of performance depends largely on the purpose of the intended device. For example, in the area of pyrotechnics, performance can relate to light intensity, gas generation or smoke production.
Since the end of the twentieth century it has been increasingly realized that the use, or production, of many energetic materials leads to the release of substances that are harmful to human health or to the environment. In some instances the use of certain compounds has been restricted, or even banned, as a consequence of legislative actions. The result is that new objectives have been enforced on the development of energetic materials. Today, almost all research in the area is focused on designing new materials that can be considered “green.” This book intends to summarize the most recent developments in the area of green energetic materials, and to introduce the reader to some tools that are used in the research. However, before we embark on this journey, it may be valuable to try to define “green”, and how this extra requirement relates to the objectives of maximizing performance and safety of handling.
1.2 Green Chemistry and Energetic Materials
The concept of “green chemistry” was first introduced in 1990s by the US Environmental Agency (EPA), and is briefly defined on their web site as [1]:
To promote innovative chemical technologies that reduce or eliminate the use or generation of hazardous substances in the design, manufacture, and use of chemical products.
Since its first appearance, green chemistry has gradually evolved due to organized efforts in both Europe and the USA, and has been widely adopted within the chemical industry as a method to promote sustainability in the design and manufacturing of new chemicals. The basic ideas behind green chemistry were concretized in 1998 by Anastas and Warner by their definition of the Twelve Principles of Green Chemistry [2]:
The principles of green chemistry are largely geared towards guiding the design of the manufacturing process, since for many chemicals the manufacturing has the largest impact on human health and the environment. Energetic materials are to some extent different, in that their use prohibits recycling and proper waste disposal; they disintegrate, and the decomposition or combustion products are directly released into the environment. It is, therefore, particularly important to consider the health and environmental effects of the final product and its usage. With this aspect in mind, the principles of green chemistry can be applied to the design of energetic materials and their manufacturing. It may seem very difficult to adhere to some of the principles, such as 2, 5, 8, and 9. After all, most energetic materials are complex structures that are very high in energy. Energy and complexity are often afforded by employing reactive reagents, specialized solvents, extreme reaction conditions, and through the use of protecting groups or other derivatization. However, most drugs are of equal or larger complexity, and therefore it is encouraging that green chemistry has been very successfully implemented in the pharmaceutical industry [3,4]. It is also obvious from reading Chapter 9 that great progress is currently being made in adopting green chemistry principles to the design of manufacturing processes for energetic materials. In some cases the achievements are clearly at the forefront of sustainable manufacturing, such as the use of biocatalysts or the implementation of continuous processes. The implementation of electrochemical methods is also likely to become of increased importance. Electrochemical processes often constitute energy efficient approaches for synthesis and for remediation of chemical waste. The use of water as the prevalent solvent in many such processes is an added advantage.
The principles of green chemistry provide no direct indication of how to determine the sustainability of a chemical or manufacturing process. One attempt to remedy this deficiency is the E-factor, which has been introduced as a method for quantifying the greenness of processes and products [5–7]. It is defined as the quotient of mass of waste over the mass of product, that is, mwaste/mproduct. The waste is often considered to include all compounds formed during the process, including gases and water. It is generally a better measure for comparing different processes for making the same product than for comparing products, since it does not explicitly consider the constitution of the waste or its toxicity. Even if the E-factor is a relatively blunt tool, it provides a rapid and often very revealing method for assessing different process alternatives from an environmental perspective. A lifecycle assessment (LSA) is a much preferred approach as it attempts to give an assessment of the overall environmental impact of a product. It considers the entire process from the extraction and acquisition of the raw material, via the manufacturing and use of the product, to the end-of-life management [5,8]. There are also methods that attempt to combine LSA with lifecycle cost analysis to get a total assessment of the costs for a product. Such a technique was recently used to analyse the life-cost of a toxic monopropellant (hydrazine) propellant versus a green one [9]. The analysis demonstrated that replacing the toxic propellant would give large cost reductions, even though the actual manufacturing cost for the green replacement is higher. It is a common observation that it pays off, from a direct cost perspective, to replace old products with greener alternatives. This also holds for the manufacturing of chemical products; converting to processes that adhere to the green principles of chemistry is often cost effective.
Reduction of costs can actually be considered one of main forces that drive the implementation of green products and manufacturing technologies; the others are societal pressure due to public awareness and government legislation. In Europe, chemicals and their use are regulated through the European Community regulation REACH (Registration, Evaluation, Authorization and Restriction of Chemical substances) [10]. REACH was introduced in 2007 and will be gradually phased in over 11 years. “The aim of REACH is to improve the protection of human health and the environment through the better and earlier identification of the intrinsic properties of chemical substances” [10]. REACH significantly increases the responsibilities of manufacturers and importers of chemicals. They are required to gather information on the properties of their chemical substances and provide safety information that ensures their safe handling. The Regulation further calls for the progressive substitution of the most dangerous chemicals when suitable alternatives have been identified. This also has implications for the energetic materials industry. It is becoming increasingly important to identify early on energetic materials that are in danger of being phased out, and to begin the development of green replacements.
So far we have not touched on the subject of whether the considerations according to the principles of green chemistry should be prioritized over performance and safety of handling when designing green energetic materials. Safety of handling is partly considered in principle 12. However, the potential consequence of an accident in the case of an energetic material is often of such magnitude that safety of handling must take precedence over other priorities. The prioritization of performance is a slightly more complicated matter, and is somewhat dependent on the application. It is obvious that the performance of a product will affect its atom-economy, for example, if a new material has half the performance of an old material, we need to use twice the amount of the new material to accomplish the same task. However, for some applications the consequence of lowered performance can be detrimental. In the case of rockets for space exploration, the mass of the propellant can easily be up to 90% of the total weight, whereas the payload typically constitutes only a few percent; even a minor reduction in performance (specific impulse) will significantly reduce the size of the payload. Thus, high performance is essential for efficient energy utilization and is often needed for mission completion. There are many other applications of energetic materials where high performance is also of great importance.
With this discussion in mind we can attempt to define the concept of a green energetic material:
A green energetic material is a material designed and manufactured in accordance with the principles of green chemistry, with the minimum requirement to preserve the performance level, and safety of handling, of the energetic material it is intended to replace.
It is important to remember that it is not always possible to design a material that fully satisfies this definition and, if the need to replace an old composition is sufficiently urgent, it may be necessary to settle for something that is not entirely green, just greener. In many cases, such an approach can lead to a significant improvement over the existing situation.
1.3 Green Propellants in Civil Space Travel
We are facing environmental challenges related to the use of energetic materials in many areas of society. In the following section, we will use the civil space sector as an example, and discuss some past and present attempts to develop and implement green propulsion technology. This is not intended to be a comprehensive survey, but rather to indicate some of the challenges associated with taking a green material from the initial development state to its final application.
1.3.1 Green Oxidizers to Replace Ammonium Perchlorate
Ammonium perchlorate (AP, NH4ClO4, Scheme 1.1) is the most common oxidizer in solid propellants. Its use extends from civil pyrotechnics, via a range of military applications, to the large booster engines in space shuttles and heavy-lift launchers. One of the environmental concerns relating to AP is its high chlorine content, which is converted to hydrochloric acid (HCl) upon combustion. As an example, it is easy to calculate that each launch of the European heavy-lift launcher, Ariane 5, produces HCl corresponding to 270 metric tons of concentrated hydrochloric acid at full conversion [11,12]. The large amount of HCl released into the atmosphere as consequence of the extensive use of AP may contribute to acidic rain and ozone depletion. The direct release of perchlorates into the environment, either due to incomplete combustion, or leakage from undisposed material, is considered to be an even bigger problem. It is estimated that drinking water in at least 35 of the states in the USA is contaminated by perchlorates [13]. This is a matter of concern since it has been suggested that high intakes of perchlorates may lead to an increase in thyroid disorders [14]. The EPA published in 2009 an interim health advisory with a maximum limit of 15 ppb for drinking water, and announced in 2011 that they are moving forward with regulations [15,16].
Scheme 1.1
Hydrazinium nitroformate (HNF) has been considered as a potential green oxidizer in solid booster propellants. HNF was discovered already in 1951 [17]. However, the real interest in HNF as a replacement for AP started in the late 1980s, when the European Space Agency (ESA) began to realize the potential environmental problems associated with acid release from AP combustion. The ESA initiated a large research program with the aim of developing a green solid propellant for use in space applications. This included the establishment of a production plant for producing HNF at a scale of 300 kg per year [17]. HNF is not considered to be entirely green, since one of the reactants used to manufacture HNF is hydrazine, which is a highly toxic and carcinogenic compound. Still, HNF is chlorine free, and the burning is clean and without acid release. It was soon realized that HNF has severe compatibility issues with common binder systems. However, HNF was later shown to be compatible with the energetic binder glycidyl azide polymer (GAP), and a propellant based on HNF, aluminum (Al), and GAP has been developed [18]. The performance, in terms of specific impulse, is reported to be 2–7% higher than for a standard formulation of AP-Al with hydroxyl-terminated polybutadiene (HTPB) as the binder [19]. This increase would, according to some estimates, translate into a reduction in the cost/kg payload of a launch system of 5–50% [19]. In spite of these very encouraging results, and the large investments made, ESA seems to have largely lost interest in the development of an HNF-based propellant; the research has almost halted and there are no indications of industrialization in the near future. We can only speculate about the reasons. However, it may be connected to problems with the thermal stability and sensitivity of HNF [17]. In addition, the manufacturing costs of both HNF and GAP are high and it is expected that a HNF propellant would be too expensive to use on a larger scale. This should be added to the large costs associated with the development of a new engine system and the related infrastructure.
Ammonium dinitramide (ADN, Scheme 1.2) is another oxidizer that has received considerable interest as a potential green replacement for AP. ADN was first prepared in 1971 in Moscow, and is believed to have been used for propulsion of Soviet intercontinental missiles during the Cold War [11]. Reportedly, the main advantage of the propellant was the smokeless signature (a consequence of the clean burning of ADN), which prevented radar detection of the missiles. ADN was rediscovered in the late 1980s by US scientists. Since then, it has been the focus of intense research efforts worldwide. Like HNF it is chlorine free, and combustion of pure ADN without a fuel leads to formation of N2, H2O, and O2 as the thermodynamic products. ADN is today produced on a large scale in a relatively green process at the SNPE Eurenco plant in Karlskoga, Sweden.
Scheme 1.2
ADN is very hygroscopic in nature. Although it is relatively stable in pure form, it has, like HNF, been found to be incompatible with a number of binder and curing systems, most notably the common isocyanate curing system [17,20] (see Chapter 8 for details). The reactivity of ADN has been very difficult to understand and anomalous solid state decomposition behavior has been reported [17,20]. As described in Chapter 7, many of the issues with ADN are now finally beginning to be resolved. New compatible binder systems have also recently been presented [11,21] (see also Chapter 8). Although they need considerable optimization before a working solid ADN propellant can be realized, the preliminary results are promising. Theoretical calculations indicate that an optimized ADN-propellant would have a high performance and increase the specific impulse over a standard AP -Al propellant by up to 5–7% (see Chapter 2).
Even though ADN-based solid propulsion is on the verge of becoming a mature propulsion technology, we do not see any initiative that in the near future is likely to lead to the implementation of ADN in a larger propulsion system. In 2008, NASA signed an agreement with the Swedish Defence Research Agency (FOI) with the title Initial evaluation of ADN as oxidizer in solid propellants for large spacelauncher boosters [22], and it was rumored that ADN was considered for the next-generation ARES booster. However, NASA, in an unoffical statement, denounced this rumor and explained that ADN needed further evaluation and development before eventual implementation [23]. The reason for the current relatively low interest in an ADN-based launch system from private investors and space agencies is not obvious. However, the answer is probably connected to the size of the investment and the large risk associated with such a project. It would not only involve the development of the propellant, but also of the engine system, and the accompanying infrastructure, and would take many years from start until an eventual first launch. In addition, considering the large quantities of propellant used at each launch, it must be possible to produce the propellant at a relatively low cost.
1.3.2 Green Liquid Propellants to Replace Hydrazine
For obvious reasons, it is easier to attract funding for the development of propellants for smaller engine systems, where the costs and risks are lower. One such area has been finding a replacement for the hydrazine monopropellant, which is mainly used for satellite station-keeping motors. Hydrazine is highly toxic and carcinogenic and cannot be handled without stringent safety precautions. Researchers at the FOI had already realized in the late 1990s the potential of ADN to be used in monopropellant formulations because of its high solubility in polar solvents [11]. This led the Swedish National Space Board to fund the first attempt to develop a liquid ADN monopropellant; the project was a collaboration between the FOI and the Swedish Space Corporation (SSC). This investigation found compositions of ADN, methanol and water (AMW) to be particularly promising for replacing hydrazine [11]. ECAPS, a subsidiary of the SSC, subsequently continued the development of an AMW propellant and an associated rocket engine. In the final design the 1N thruster system was demonstrated to provide a specific impulse that was 5% higher than a corresponding hydrazine thruster [24]. The improvement in density impulse was even larger due to the 24% higher density of the developed propellant, LMP-103S. The first in-flight demonstration came in 2010, when the system was tested for rendezvous and formation-flying satellite maneuvers on the PRISMA technology demonstration mission [25].
ECAPS today claims to be the market leader in green propulsion technology, and LMP-103S is marketed worldwide. Their ADN-based technology is one of the first examples of a green propellant that has made the full journey from the initial development stage to finally reaching the commercial market. In this context it is interesting to note the time span of nearly 15 years from the initial research project in 1997 to the market release in 2012. This is similar to the development time in drug design, where it typically takes up to 15 years from the initial discovery phase to market release. The pharmaceutical industry is normally considered, of all technology industries, to hold the record for long development times. The significance of the development time for LMP-103S is difficult to estimate, considering we only have this example. However, one can only hope that ECAPS has broken the ice and that new and improved propellants will soon be ready for commercialization. There are some candidates that have already shown great promise. The FOI has their own ADN-based liquid propellant, which is reported to provide a significantly higher density impulse than the ECAPS propellant [11]. A green propellant with even better performance has been developed by the US Air Force Division [26]. It is an ionic liquid formulation of hydroxyl ammonium nitrate (HAN, Scheme 1.3). This propellant is set to be tested on a satellite on the 2015 GPIM mission [27].
Scheme 1.3
At this point, it may be relevant to reflect upon how green an ADN-based propellant, like LMP-103S, really is. It should first be realized that ADN is considerably less toxic than hydrazine, as indicated by the LD50 values for oral intake in rats, which are 823 mg/kg [11] and 59 mg/kg, respectively. In reality the difference in health hazard is much larger due to the carcinogenicity of hydrazine and its higher vapor pressure. It is actually argued that the methanol is the reason for the hazard symbol labeling of LMP-103S as being toxic [24]. In any case, the safety precautions for handling the propellant are similar to most organic solvents. This should be compared to the handling of hydrazine, which requires the use of an advanced protective suit. All the subcomponents of LMP-103S are biodegradable and it is considered environmentally benign [24]. The propellant is insensitive to impact and friction, and it has even been approved for transportation on commercial aircrafts [24]. Thus, it seems like LMP-103S fulfills many of the criteria for being considered green.
In this context, it is also relevant to evaluate the manufacturing of ADN from a green perspective. ADN is currently produced at the SNPE Eurenco plant in Karlskoga. Beginning in the mid 2000s, substantial efforts were made to optimize the process for capacity, reduction of impurities, recirculation of chemicals, and reduction of chemical waste. Some of the results have been published and are discussed in Chapter 9 of this book [28,29]. The optimized process is described in a company newsletter from 2004 [30]:
Dinitramide is synthesized with a mixed acid nitration of a sulphamic acid salt. The dinitramide is then precipitated as GUDN (guanylurea dinitramide) from a water solution by adding guanylurea. The spent nitration acids are regenerated by using standard procedures. In a following step GUDN is transformed to ADN in two separate ion exchange reactions. First, GUDN is changed to potassium dinitramide (KDN) using potassium hydroxide. Finally, KDN is transformed to ADN in an ion exchange reaction with ammonium sulphate. ADN is crystallized from the solution and potassium sulphate is generated as a waste. The solvents used in all steps are recycled.
The process for manufacturing ADN seems to be a very good example of the advances in adopting green chemistry and engineering to energetic materials production.
At this stage, we can safely conclude that replacing hydrazine with a monopropellant based on ADN, such as LMP-103S, is also a significant improvement in sustainability if the propellants' entire lifecycle is considered. This has also been verified by a lifecycle cost analysis, which in addition showed a 66% reduction in the operational mission cost for LMP-103S compared with hydrazine [9]. The largest improvements where found in environmental costs, mainly associated with facility operations and maintenance, and costs associated with health and human safety protection. Thus, this example clearly illustrates that the implementation of a green energetic material can also be advantageous from a direct cost perspective.
1.3.3 Electric Propulsion
Another major step towards greener propulsion is related to the recent advances in the development and implementation of electric propulsion. The basic idea behind electric propulsion is to use electrical rather than chemical energy to accelerate the propellant. Thrusters for electric propulsion can be divided into three categories: electrothermal, electrostatic, and electromagnetic. The first is most similar to a conventional thruster, in that the electric energy is used to heat the propellant and the thermal energy is converted to kinetic energy. In the two latter approaches the propellant is ionized and the resulting ions are accelerated in an electromagnetic field; the most common propellant is xenon, as it combines a relatively low ionization potential with high atomic mass. Electric thrusters are characterized by a much higher specific impulse, and therefore much lower propellant consumption, compared to conventional thrusters. Their disadvantage is the low thrust, which means that the time taken to accomplish a particular maneuver will be much longer. Typical applications of electric propulsion include station keeping, and the transfer of satellites from the geostationary transfer orbit (GTO) to their geostationary earth orbit (GEO); maneuvers that are traditionally performed using chemical thrusters driven by a hydrazine monopropellant or toxic bipropellants based on methylhydrazine and nitrogen tetroxide.
Electric propulsion is particularly advantageous when combined with solar energy. For example, for transferring a satellite to GEO, it may be possible to use electric power from solar panels that are otherwise not in use during that phase. The potential of solar-driven electric propulsion was demonstrated by the ESA SMART-1 mission to the Moon in September 2003 [31]. The 370 kg satellite built by SSC used a solar-powered Hall effect thruster for the entire transfer from the GTO to the lunar orbit. It was kept in orbit using the same propulsion system until September 2006, when it was finally crashed into the Moon's surface. At the end of the mission, the thruster had been operating 5000 h and had used only 82 kg of xenon propellant in total. This is a very impressive example of the advantages of solar-driven electric propulsion. A system based on traditional chemical propulsion would have required a much heavier propulsion system, and the total cost of the mission would have been substantially higher. From a green perspective the advantages are considerable.
Xenon cannot be considered an energetic material in its traditional form. However, in a wider definition, it could be designated as a green energetic material, or at least a green propellant. Xenon is commercially produced as a byproduct of the separation of air into oxygen and nitrogen. It is relatively chemically inert, and nontoxic. Its disadvantage is a relatively high price due to its low natural abundance; a price that is rising because of the increased usage of xenon, in particular in lightning devices. Recent research shows that more traditional energetic materials, like those based on ADN, could potentially be used for electric propulsion too [32]. In addition to lower propellant cost, this could also have other benefits, in particular in terms of saving weight, since systems for electrical and chemical propulsion could share the same fuel system.
1.4 Conclusions
We can safely conclude that the time when energetic materials could be developed with the sole aims of improving performance and safety of handling is now gone. It is becoming increasingly important to also consider the potential health and environmental effects of a new material during its entire lifetime. In this respect, the principles of green chemistry can be a great aid to the design of new products and their manufacturing processes. This type of approach serves not only to improve the sustainability of the product but can also often lead to reduced costs. Lifecycle assessment is another tool that can be very effective in the process. We have further suggested a definition of a green energetic material from the principles of green chemistry.
We have analyzed some of the past and present attempts to develop green propulsion technologies for civil space travel. Our limited analysis shows that the entire development process, from the initial discovery of a new propellant component to the implementation of the final propellant in an engine system, is a lengthy journey full of potential risks of failure. The costs, and the chances of succeeding, seem to largely depend on the size and type of the engine system. The development of green solid propellants to replace ammonium perchlorate formulations for heavy-launch systems has not yet reached the stage where new propellants are ready to be implemented. On the other hand, there have been significant advances in green technologies aimed at replacing smaller engines that use liquid propellants based on hydrazine or hydrazine derivatives. The recent development and implementation of a green liquid propellant based on ADN is one obvious example. This propellant has recently been tested for formation flying of satellites, and has been released on the commercial market. Electric propulsion is another green alternative that is advancing. The great potential of this technology was demonstrated in 2003 by the SMART-1 mission to the Moon.
Finally, we note that the relatively slow progress of introducing green propulsion technologies to civil space travel is not representative of the impact of green energetic materials in other areas. Despite many challenges, particularly large advances have been made in the area of green pyrotechnics. Here, the development has largely been driven by military research. However, the new materials are being increasingly implemented in civilian pyrotechnics. Similar progress is also being made in other areas of energetic materials, but at different rates. Overall, the future of energetic materials seems bright and green.
References
1. US Environmental Protection Agency, (last updated September 13, 2013), http://www.epa.gov/p2/pubs/partnerships.htm (last accessed in September, 2013).
2. Anastas, P.T. and Warner, J.C. (1998) Green Chemistry: Theory and Practice, Oxford University Press, Oxford.
3. Dunn, P.J. (2012) The importance of green chemistry in process research and development. Chemical Society Reviews, 41 (4), 1452–1461.
4. Bandichhor, R., Bhattacharya, A., Diorazio, L.et al. (2013) Green chemistry articles of interest to the pharmaceutical industry. Organic Process Research & Development, 17 (4), 615–626.
5. Rothenberg, G. (2008) Catalysis: Concept and Green Applications, Wiley-VCH Verlag GmbH & Co., Weinheim, Germany.
6. Sheldon, R.A. (1994) Consider the environmental quotient. Chemtech;(US), 24 (3) 38–47
7. Sheldon, R.A. (1997) Catalysis: The key to waste minimization. Journal of Chemical Technology and Biotechnology (Oxford, Oxfordshire: 1986), 24 (3), 381–388.
8. Lankey, R.L. and Anastas, P.T. (2002) Life-cycle approaches for assessing green chemistry technologies. Industrial & Engineering Chemistry Research, 41 (18), 4498–4502.
9. Johnson, C.C. and Duffey, M.R. (2013) Environmental life cycle criteria for propellant selection decision-making. International Journal of Space Technology Management and Innovation (IJSTMI), 2 (1), 16–29.
10. European Commission, REACH - Environment - European Commission, (last updated September 12, 2013) http://ec.europa.eu/environment/chemicals/reach/reach_en.htm, (last accessed in September, 2013).
11. Larsson, A. and Wingborg, N. (2011) Green propellants based on ammonium dinitramide (ADN), in Advances in Spacecraft Technologies (ed. J. Hall), InTech, Available from http://cdn.intechweb.org/pdfs/13473.pdf.
12. Arianespace (2013) http://www.arianespace.com/launch-services-ariane5/ariane-5-intro.asp (last accessed in July 2013).
13. Schor, E. (2010) As EPA Moves to Regulate Perchlorate, Groups Await Pentagon Response, The New York Timeshttp://www.nytimes.com/gwire/2010/10/04/04greenwire-as-epa-moves-to-regulate-perchlorate-groups-aw-43754.html (last accessed July 6, 2013).
14. Sellers, K., Weeks, K., Alsop, W., and Clough and, H.M. (2007) Perchlorate Environmental Problems and Solutions, CRC Press – Taylor & Francis Group, Boca Raton, FL, US.
15. US Environmental Protection Agency (January, 2009) Revised Assessment Guidance for Perchlorates: http://www.epa.gov/fedfac/documents/perchlorate_memo_01-08-09.pdf (last accessed in July 2013).
16. US Environmental Protection Agency (February 2011) Fact Sheet: Final Regulatory Determination for Perchlorate - US Environmental Protection Agency http://water.epa.gov/drink/contaminants/unregulated/upload/FactSheet_PerchlorateDetermination.pdf (last accessed in July 8, 2013).
17. Agrawal, J.P. (2010) High Energy Materials - Propellants, Explosives, Pyrotechnics, Wiley-VCH Verlag GmbH & Co., Weinheim, Germany.
18. Gadiot, G.M.H.J.L., Mul, J.M., Meulenbrugge, J.J.et al. (1993) New solid propellants based on energetic binders and HNF. Acta Astronautica, 29 (10), 771–779.
19. APP Aerospace Propulsion Products B.V., HNF (2006–2012) http://www.appbv.nl/index.asp?grp_id=3&sub_id=23 (last accessed in July 6, 2013).
20. Rahm, M. (2010) Green Propellants, PhD thesis, Physical Chemistry, KTH The Royal Institute of Technology, Stockholm, Sweden.
21. Rahm, M., Malmstrom, E., and Eldsater, C. (2011) Design of an ammonium dinitramide compatible polymer matrix. Journal of Applied Polymer Science, 122 (1), 1–11.
22. A–Initial evaluation of ADN as oxidizer in solid propellants for large spacelauncher boosters, NASA Presolicitation December 2008, http://www.fbo.gov/?s=opportunity&mode=form&id=6e876911f91a4be2cc6bfc8314b39242&tab=core&_cview=1 (last accessed in 2013).
23. Flightglobal/Blogs, December 2008, NASA denies imminent Ares I first-stage oxidiser change, http://www.flightglobal.com/blogs/hyperbola/2008/12/nasa-denies-ares-i-crew-launch.html (last accessed in 2013).
24. Anflo, K. and Möllerberg, R. (2009) Flight demonstration of new thruster and green propellant technology on the PRISMA satellite. Acta Astronautica, 65 (9–10), 1238–1249.
25. D'Amico, S., Ardaens, J.S., and Larsson, R. (2012) Spaceborne autonomous formation-flying experiment on the PRISMA mission. Journal of Guidance Control and Dynamics, 35 (3), 834–850.
26. Masse, R., Overly, J., Allen, M., and Spore, R. (2012) A New State-of-The-Art in AF-M315E Thruster Technologies 48th AIAA/ASME/SAE/ASEE, Joint Propulsion Conference and Exhibits, American Institute of Aeronautics and Astronautics.
27. Green Propellant Infusion Mission (GPIM), (last updated November 6, 2012) http://www.nasa.gov/mission_pages/tdm/green/gpim_overview.html (last accessed in 2013).
28. Skifs, H., Stenmark, H., and Thormahlen, P. (2012) Development and scale-up of a new process for production of high purity ADN, International Conference of ICT, 43rd (Energetic Materials) 6/1-6/4, Karlsruhe, Germany.
29. Stenmark, H., Skifs, H., and Voerde, C. (2010) Environmental improvements in the dinitramide production process, International Conference of ICT, 41st (Energetic Materials: for High Performance, Insensitive Munitions and Zero Pollution) 1/1-1/5, Karlsruhe, Germany.
30. EURENCO Group SNPE (October, 2004) Dinitramide News, http://www.eurenco.com/en/high_explosives/newsletters/Dinitramide_oct_2004.pdf_ (last accessed in 2013).
31. Estublier, D., Soccoccia, G., and del Amo, J.G. (2007) Electronic propulsion on SMART-1 - A technology milestone. ESA BULL-EUR Space, 129, 40–46.
32. Kleimark, J., Delanoë, R., Demairé, A., and Brinck, T. (2013) Ionization of ammonium dinitramide: decomposition pathways and ionization products, Theoretical Chemistry Accounts, 132 (12), 1412.
2
Theoretical Design of Green Energetic Materials: Predicting Stability, Detection, Synthesis and Performance
Tore Brinck1 and Martin Rahm2
1Applied Physical Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology Sweden
2Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California USA
2.1 Introduction
This book is a testament to the need for development of greener energetic materials. However, to be considered truly green, the material properties must be refined at many levels. For example, a potential replacement for today's solid propellant compositions should not only produce substantially less toxic waste during combustion, but also be perform better or on par with today's solutions. Reduced performance would lower payloads and increase propellant consumption, and thereby have a negative impact on the environment. There are also numerous other issues to consider. The complexity of the design problem, together with the inherit safety issues associated with the synthesis and handling of energetic materials, makes the road to progress much slower to travel without invoking rational design based on modeling and theoretical considerations. The energetic materials community has also been ground breaking in the employment of computational chemistry methods in general, and quantum chemical techniques in particular, for the design of new materials with optimized properties. However, it must be emphasized that chemical knowledge and intuition are necessary inputs to the design process, and without the contribution of chemists experienced in synthesis new compounds will never materialize.
In this chapter our main focus is on the use of computational techniques for the development of new propellant components. However, not only the techniques, but also the basic considerations, are to a great extent similar to those employed when designing other energetic materials, such as explosives and pyrotechnics.
Most propellant compositions combine an oxidizer with one or more fuel components. For example, the typical solid propellant consists of ammonium perchlorate and solid aluminum in a hydroxyl-terminated polybutadiene binder. Most common oxidizers have a rather low energy content by themselves. It is instead through the combustion of fuels, and particularly metal fuels, that a rocket motor can release energy. The combustion temperature rapidly becomes a concern with increased metal loadings, as state of the art materials used in throat and nozzles of rocket engines fail at approximately 2500 K [1]. Some modern highly aluminized propellants exhibit combustion temperatures up towards 3600 K and can only be employed together with engine designs that employ techniques such as, regenerative cooling, film cooling, or ablative protection [1,2]. Further performance increases cannot rely much further on improved engine designs and larger metal loadings, as combustion temperatures above 5000 K are considered unrealistic for practical applications. Maintaining high performance, while reducing combustion temperatures through reduced metal loadings, will require new oxidizers with higher internal energy, for example compounds with highly energetic bonds. Compounds that are able to combust themselves entirely, without the addition of a fuel, are for many purposes desirable, as they can be used with lighter and less expensive engines. We will refer to such compounds as having a neutral oxygen balance.
Good candidate compounds for green high performance propulsion are generally high in nitrogen, have a positive or neutral oxygen balance, and are free of halogens. Nitrogen-rich compounds are desirable as they release large amounts of energy upon decomposition when forming the exceedingly stable N=N triple bond of molecular nitrogen from the much weaker single and double N=N bonds of the parent molecules. Average bond energies for the triple, double, and single bonds of nitrogen are 226, 98, and 39 kcal/mol, respectively. The extraordinary relative stability of the N2 triple bond is easily realized when comparing it to the same energies for C=C bonds, which have the values 230, 146, and 83 kcal/mol.
Whereas compounds featuring a large number of N=N single bonds are desirable for their high internal energy, they generally have problems with low kinetic stability. Fundamental theory, such as the Bell–Evans–Polanyi principle [3], clearly shows that there is a general correlation between thermodynamic and kinetic stability. From a more simplistic perspective, it can be realized that the activation barrier towards dissociation is not likely to exceed the energy of the weakest bond of the molecule. A common design objective is therefore to strengthen weak N=N and N=O single bonds by invoking resonance delocalization to attain a partial double bond character. Common strategies include resonance stabilization between sp2 hybridized nitrogen and oxygen, and aromatic stabilization in ring systems with 4n +2 π-electrons [4]. Resonance stabilization is generally a prerequisite of planar molecules. This is also of interest from a stability perspective, as planar molecules and ions are more likely to crystalize in sheet-like structures, a structure type that is believed to reduce the impact sensitivity.
Another alternative design strategy for combining high energy content (low thermodynamic stability) with sufficient kinetic stability is to utilize caged structures. Besides invoking single bonds that are high in energy, for example N=N bonds, such structures often benefit thermodynamically from strain energy. Despite the fact that a caged structure often is made up of seemingly weak bonds, the kinetic stability can be relatively high. The reason for the high decomposition barrier is usually that it is not possible to form a thermodynamically favored intermediate by breaking only one bond. Instead, the main dissociation pathway either involves several bonds breaking in one step, or passes via a high lying intermediate; both types of transformations are often associated with a high barrier. This should be taken as a general consideration for identifying energetic compounds with high kinetic stability, that is, molecules that cannot decompose to a stable intermediate via a single simple chemical transformation are likely to have a larger kinetic stability than can be anticipated from the strengths of their individual chemical bonds. In the following discussion we will provide several examples where this is the case.
It is obvious that predicting performance and stability is key to the design of green energetic materials. Traditionally, it is also in these areas that computational chemistry has played its most important role. Enthalpies of formation for gaseous molecules are readily obtained by quantum chemical methods. However, as most energetic materials are used in solid or liquid phases, methods have been developed to also calculate phase transition energetics and condensed phase densities. The kinetic stability is first approximated by evaluating the potential decomposition pathways and their barriers in the gas phase by quantum chemistry. Invoking the effects of interactions with neighboring molecules in condensed phases increases the complexity considerably, but significant progress has been made in this area too. However, the true bottleneck in obtaining new functional energetic materials has been devising synthetic routes. Here, we believe that computational chemistry has the potential to play a more important role. Potential synthetic routes can be evaluated by a similar analysis to that used for evaluating stability and decomposition pathways. Finally, we note that detection and characterization often are key issues in the discovery of highly energetic molecules of high symmetry and low molecular weight. Theoretically predicted spectra for different spectroscopies can be great aids to the process.
2.2 Computational Methods
Kohn–Sham density functional theory (KS-DFT) is today the standard quantum chemical method for the optimization of molecular geometries and calculation of vibrational frequencies. It is generally used to probe potential energy surfaces and characterize transition states. The widely used B3LYP exchange-correlation functional has also been quite successful for energetic molecules and their reactions. In general, B3LYP geometries are more accurate than energies. However, there are examples of nitro-group rich systems where B3LYP produces both erroneous geometries and energies, and for some of these the more recent M06-2X functional [5] of Truhlar and coworkers performs considerably better. A general observation is that when the two functionals give geometries in consensus, these can be trusted for energy calculations at higher level. Double zeta basis sets augmented by diffuse and polarization functions, for example 6–31+G(d,p), are recommended for geometry optimizations of energetic molecules.
In order to obtain energies that are close to chemical accuracy, post-Hartree Fock methods are generally needed. In particular, methods such as CBS-QB3 [6,7] and G3MP2 [8], that combine coupled cluster [CCSD(T)] calculations with smaller basis sets and basis set extrapolation at the MP2 level, are recommended for small and medium sized energetic molecules. Heats of formation () of new systems should not be computed via atomization energies, but rather from hypothetical reactions involving molecules with well-defined experimental gas phase heats of formations. For larger molecules, it may be necessary to resort to DFT-methods. We note that the B3LYP-based atom-equivalent parameterizations of Rice and coworkers often provide accurate for energetic molecules at a very modest computational cost [9,10]. However, for novel compounds whose structures deviate from the parameterization set, or for computing activation energies, we recommend direct calculations using some modern, more accurate, functionals, such as M06-2X and the double hybrid functional B2-PLYP of Grimme et al. [11].
For probing decomposition reactions and analyzing synthetic routes, it is often necessary to account for the effects of neighboring molecules in the liquid environment. Solvation models based on a continuum description of the liquid state, such as the polarizable continuum model (PCM) [12] and the conductor like screening model (COSMO) [13], are surprisingly accurate for correcting geometries and energies. However, for computing barriers of reactions where a neighboring molecule takes active part, such as in solvent-assisted catalysis, incorporation of explicit molecules from the environment is necessary.
Most energetic compounds are either liquids or solids, and computed gas phase heats of formation need to be corrected by computational estimates of heats of vaporization (ΔHvap) or sublimation (ΔHsub). Parameterized methods that utilize descriptors obtained from quantum chemical calculations produce reliable estimates for reasonable computational effort. These methods are generally based on a statistical analysis of the surface electrostatic potential together with descriptors that describe molecular size, such as surface area or molecular volume. The methodology stems from original work by Brinck, Murray, and Politzer [14–16]. Recent developments for the prediction of molecular (non-ionic) compounds are due to Politzer et al. [17,18] and Rice et al. [9,10]. It is important to remember that these methods are only reliable for compounds similar to those used in the parameterization. In this respect, the recently developed parameters of Politzer et al. [18] are more generally applicable than those of Rice et al. [9,10]. There are also methods for predicting lattice enthalpies of molecular salts based on electrostatic considerations and estimates of molecular size. Rice and coworkers have recently analyzed the accuracy of several such methods for the prediction of solid phase heats of formation of energetic salts [19].
In addition to heats of formation, computational estimates of liquid and solid densities are necessary to compute the performance of propellants and explosives. These can be estimated from molecular volumes determined by quantum chemical calculations [20–22]. Whereas such estimates often are sufficiently accurate for rocket performance calculations, these methods provide no structural information. Crystal structure prediction is extremely difficult, and one of the current frontiers in development of computational chemistry. The challenge lies in that there are usually millions of possible structures, and hundreds to thousands of these may lie within a few kcal/mol of the global minimum. In energetic materials research, Herman Ammon has been the most prominent pioneer [23]. His approach, which is also being used in pharmaceutical research, involves initial large-scale packing simulations, followed by structure optimization with specially tuned force fields. In recent years, it has been shown that more accurate predictions can be obtained by invoking optimizations of the lowest energy structures with periodic DFT calculations. This methodology was pioneered by Neumann and Perrin [24], and has been further refined and introduced into the area of energetic materials by Rice and coworkers [25–27]. The obtained structures are generally more than adequate for evaluating sensitivity issues, and provide starting points for computational analysis of solid state decomposition pathways. Presently, the accuracy of the free energy predictions is not sufficient to analyze potential issues due to polymorphism or predict transition temperatures between different crystal phases.
Prediction of spectral properties to guide the experimental identification of new compounds is attainable by quantum chemical methods. Vibrational spectra, including both IR and Raman intensities, are often well predicted by standard DFT methods. Since the computations generally are conducted within the harmonic approximation, and do not account for anharmonicity effects, the computed frequencies have to be scaled before comparison with experiment. Solvation normally has very small effects on the frequencies, and gas phase calculations are usually sufficient to estimate liquid spectra. UV-Vis spectra can readily be computed using time-dependent DFT, which also has been implemented in the major quantum chemistry codes. However, the transition energies are sensitive to the choice of exchange-correlation functional and, especially for analyzing charge-transfer bands, long-range corrected functionals, such as CAM-B3LYP [28] or ω B97X [29], are highly recommended. Even better accuracy can be obtained using equation of motion (EOM) or linear response (LR) coupled-cluster calculations [30]. The most exact methods, EOM-CCSD and LR-CC3, can only be used for smaller systems. In contrast, LR-CC2 is tractable even for larger systems and is in most cases sufficiently accurate. Solvent effects sometimes induce large shifts in spectral bands. The molecular interactions behind these shifts are often complex, and although continuum solvation effects can be included in TD-DFT calculations, the predicted shifts are not seldom in the wrong direction.
