View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Digital Repository @ Iowa State University Chemistry Publications Chemistry 1-2007 Pentazole-Based Energetic Ionic Liquids: A Computational Study Ian S. O. Pimienta Iowa State University Sherrie Elzey Iowa State University Jerry A. Boatz Air Force Research Laboratory Mark S. Gordon Iowa State University, [email protected] Follow this and additional works at: http://lib.dr.iastate.edu/chem_pubs Part of the Chemistry Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ chem_pubs/491. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Chemistry at Iowa State University Digital Repository. It has been accepted for inclusion in Chemistry Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Pentazole-Based Energetic Ionic Liquids: A Computational Study Abstract The trs uctures of protonated pentazole cations (RN5H+), oxygen-containing anions such as N(NO2)2-, NO3-, and ClO4- and the corresponding ion pairs are investigated by ab initio quantum chemistry calculations. The ts ability of the pentazole cation is explored by examining the decomposition pathways of several monosubstituted cations (RN5H+) to yield N2 and the corresponding azidinium cation. The heats of formation of these cations, which are based on isodesmic (bond-type conserving) reactions, are calculated. The proton-transfer reaction from the cation to the anion is investigated. Disciplines Chemistry Comments This article is from Journal of Physical Chemistry A 111 (2007): 691, doi:10.1021/jp0663006. Rights Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The onc tent of this document is not copyrighted. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/chem_pubs/491 J. Phys. Chem. A 2007, 111, 691-703 691 Pentazole-Based Energetic Ionic Liquids: A Computational Study Ian S. O. Pimienta,† Sherrie Elzey,† Jerry A. Boatz,‡ and Mark S. Gordon*,† Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011 and Air Force Research Laboratory, Space and Missile Propulsion DiVision, AFRL/PRS, 10 East Saturn BouleVard, Edwards Air Force Base, California 93524 ReceiVed: September 25, 2006; In Final Form: NoVember 5, 2006 + - - The structures of protonated pentazole cations (RN5H ), oxygen-containing anions such as N(NO2)2 ,NO3 , - and ClO4 and the corresponding ion pairs are investigated by ab initio quantum chemistry calculations. The stability of the pentazole cation is explored by examining the decomposition pathways of several + monosubstituted cations (RN5H ) to yield N2 and the corresponding azidinium cation. The heats of formation of these cations, which are based on isodesmic (bond-type conserving) reactions, are calculated. The proton- transfer reaction from the cation to the anion is investigated. Introduction There is growing interest in ionic liquids (ILs) as solvents for many applications including environmentally benign syn- thesis, catalysis, electrochemistry, and ion separation.1-5 The chemical stabilities and very low vapor pressures of ILs make them suitable as alternatives to other more volatile organic solvents, hence the name “green” solvents.1-3 By carefully Figure 1. General structures of monosubstituted 1-H,3-R-1,3-pentazole selecting the starting cation and anion, the IL can be customized (I), 1-H,2-R-1,2-pentazole (II), and 1-H,1-R-1,1-pentazole (III) cations based on preselected characteristics (e.g., moisture stability, showing the atom numbering around the ring. viscosity, density, miscibility), thus allowing the needed flex- ibility in the design of these materials. energy of the cations is to introduce high-energy substituents into the ring, such as azido (-N ) and nitro (-NO ). Despite the “designer solvent” classification of stable ILs, it 3 2 is important to remember6,7 that they are not required to be The present work is a systematic investigation of the structures and energetics of possible ionic liquids formed by stable. In fact, a substantial number of ILs are chemically + unstable, releasing large amounts of energy in the decomposition protonated pentazole cations, RN5H , and the dinitramide, nitrate, and perchlorate anions. process. These so-called “energetic” ionic liquids may find some + other useful applications.8 It is interesting to note that these There have been few attempts to synthesize HN5H as even properties of ILs are analogous to the highly desirable charac- its parent precursor, pentazole (HN5), has proved to be difficult teristics of high-energy density materials (HEDMs) studied to isolate and detect. The first pentazole compounds to be 17-20 elsewhere.9-11 The large positive heats of formation of cyclic synthesized were substituted by a phenyl ring. Huisgen and Ugi prepared their phenyl-substituted pentazole by reacting the nitrogen-rich cations make them promising precursors for the - + synthesis of highly energetic materials. One of the most widely azide anion, N3 , with diazonium ions, ArN2 , under careful 18,19 studied IL cations is butyl-methyl-imidazolium (BMIM+). temperature control. The aryl pentazole products readily lost + - N , but the resulting dimethylanilino pentazole was stable Molecular dynamics simulations of the IL BMIM /PF6 indicate 2 at least two different time scales for diffusion in such solvents enough to allow an X-ray crystal structure determination. and provide information on preferred structures.12,13 The interac- However, they were not successful in isolating HN5. Since then tion of the surface layer of this IL with water has been studied a significant goal of heterocyclic chemistry has been to prepare experimentally14 and the observed structural changes are the parent pentazole. In earlier attempts, ozonolytic degradation consistent with the results of the computer simulations. of the aryl ring with a controlled amount of ozone was made - 21 To increase the energy content (with the side effect of but neither HN5 nor its anion N5 was found. Recently, Butler + et al., working under controlled conditions, were able to isolate decreasing the stability) of the cation, BMIM may be replaced - 2+ 22 by the triazolium and tetrazolium cations with three and four and detect pentazole in solution as N5 with Zn ion. nitrogens, respectively, in the ring. Recently, theoretical studies In addition to the experimental investigations, several theo- 23-27 have been completed on ionic liquids based on the triazolium retical studies have been performed on HN5 as well. + + Calculations predict that the half-life of HN is only about 10 (N3C2H4 ) and tetrazolium (N4CH3 ) cations, combined with 5 - - min;21 this explains the difficulty in the direct detection of this anions such as dinitramide (N(NO2)2 ), nitrate (NO3 ), and - 15,16 compound. Chen23 had initially suggested that HN is a strong perchlorate (ClO4 ). Another approach to increasing the 5 acid, even stronger than nitric acid; hence, in solution a significant portion is easily converted to the anion, a fact finally * Corresponding author. E-mail: [email protected]. 22 † Iowa State University. corroborated by the experimental results of Butler et al. On ‡ Air Force Research Laboratory. the other hand, attempts to isolate the protonated pentazole 10.1021/jp0663006 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/09/2007 692 J. Phys. Chem. A, Vol. 111, No. 4, 2007 Pimienta et al. Figure 2. Optimized structures of dinitramide, nitrate, and perchlorate anions (lower structures) and their protonated counterparts (nitrogen ) blue, oxygen ) red, chlorine ) green, carbon ) black, hydrogen ) white). Figure 4. Monosubstituted 1-H,2-R-pentazole cation minimum energy structures. Atom colors are defined in Figure 3. of several azoles including pentazole and its protonated form with the STO-3G, 3-21G, and 6-31G basis sets, using the conjugate gradient optimization procedure31 to obtain the starting structures. The basicity of these compounds is predicted to depend on the contributions of different tautomers of either the nonprotonated or protonated forms. The monosubstituted pentazole cation has three possible isomers (i.e., 1-H,3-R-1,3-pentazole (I), 1-H,2-R-1,2-pentazole (II), and 1-H,1-R-1,1-pentazole (III) ions) (see Figure 1). The only difference among the three isomers is in the location of the substituent R. To find new high-energy IL precursors and to obtain a more comprehensive picture of the stability of these compounds, several topics will be addressed below. First, the stability and structure of several monosubstituted pentazole cations are investigated by determining the activation barrier and the energy of decomposition to N2 and the corresponding azidinium cation. It is expected that substituents may play a major role in the thermodynamic stability of the protonated pentazole ring. Second, π-bonding and population analysis are performed to determine the amount of charge delocalization Figure 3. Monosubstituted 1-H,3-R-pentazole minimum energy struc- around the ring in these compounds. One of the characteristics tures (nitrogen ) blue, oxygen ) red, fluorine ) green, carbon ) black, of low melting ILs is the presence of charge-delocalized ions. ) hydrogen white). Finally, the structures of dimers composed of the protonated pentazole and several energetic anions including dinitramide, nitrate, and perchlorate are presented. All possible structures, cation have so far been unsuccessful and theoretical calculations including the neutral pairs, transition states, and ionic pairs, are are sparse.28,29 The lack of experimental data on protonated described. The small barriers for proton transfer from the cation pentazole makes accurate quantum chemical calculations es- to the anion indicate that deprotonation may be a first step in sential in predicting the properties of these compounds. the decomposition of these pentazole-based ILs.