Pentazole-Based Energetic Ionic Liquids: A Computational Study

Transcription

1 Chemistry Publictions Chemistry Pentzole-Bsed Energetic Ionic Liquids: A Computtionl Study In S. O. Pimient Iow Stte University Sherrie Elzey Iow Stte University Jerry A. Botz Air Force Reserch Lbortory Mrk S. Gordon Iow Stte University, mgordon@istte.edu Follow this nd dditionl works t: Prt of the Chemistry Commons The complete bibliogrphic informtion for this item cn be found t chem_pubs/491. For informtion on how to cite this item, plese visit howtocite.html. This Article is brought to you for free nd open ccess by the Chemistry t Iow Stte University Digitl Repository. It hs been ccepted for inclusion in Chemistry Publictions by n uthorized dministrtor of Iow Stte University Digitl Repository. For more informtion, plese contct digirep@istte.edu.

2 Pentzole-Bsed Energetic Ionic Liquids: A Computtionl Study Abstrct The structures of protonted pentzole ctions (RN5H+), oxygen-contining nions such s N(NO2)2-, NO3-, nd ClO4- nd the corresponding ion pirs re investigted by b initio quntum chemistry clcultions. The stbility of the pentzole ction is explored by exmining the decomposition pthwys of severl monosubstituted ctions (RN5H+) to yield N2 nd the corresponding zidinium ction. The hets of formtion of these ctions, which re bsed on isodesmic (bond-type conserving) rections, re clculted. The proton-trnsfer rection from the ction to the nion is investigted. Disciplines Chemistry Comments This rticle is from Journl of Physicl Chemistry A 111 (2007): 691, doi: /jp Rights Works produced by employees of the U.S. Government s prt of their officil duties re not copyrighted within the U.S. The content of this document is not copyrighted. This rticle is vilble t Iow Stte University Digitl Repository:

3 J. Phys. Chem. A 2007, 111, Pentzole-Bsed Energetic Ionic Liquids: A Computtionl Study In S. O. Pimient, Sherrie Elzey, Jerry A. Botz, nd Mrk S. Gordon*, Deprtment of Chemistry, Iow Stte UniVersity, Ames, Iow nd Air Force Reserch Lbortory, Spce nd Missile Propulsion DiVision, AFRL/PRS, 10 Est Sturn BouleVrd, Edwrds Air Force Bse, Cliforni ReceiVed: September 25, 2006; In Finl Form: NoVember 5, 2006 The structures of protonted pentzole ctions (RN 5 H + ), oxygen-contining nions such s N(NO 2 ) 2-,NO 3-, nd ClO 4- nd the corresponding ion pirs re investigted by b initio quntum chemistry clcultions. The stbility of the pentzole ction is explored by exmining the decomposition pthwys of severl monosubstituted ctions (RN 5 H + ) to yield N 2 nd the corresponding zidinium ction. The hets of formtion of these ctions, which re bsed on isodesmic (bond-type conserving) rections, re clculted. The protontrnsfer rection from the ction to the nion is investigted. Introduction There is growing interest in ionic liquids (ILs) s solvents for mny pplictions including environmentlly benign synthesis, ctlysis, electrochemistry, nd ion seprtion. 1-5 The chemicl stbilities nd very low vpor pressures of ILs mke them suitble s lterntives to other more voltile orgnic solvents, hence the nme green solvents. 1-3 By crefully selecting the strting ction nd nion, the IL cn be customized bsed on preselected chrcteristics (e.g., moisture stbility, viscosity, density, miscibility), thus llowing the needed flexibility in the design of these mterils. Despite the designer solvent clssifiction of stble ILs, it is importnt to remember 6,7 tht they re not required to be stble. In fct, substntil number of ILs re chemiclly unstble, relesing lrge mounts of energy in the decomposition process. These so-clled energetic ionic liquids my find some other useful pplictions. 8 It is interesting to note tht these properties of ILs re nlogous to the highly desirble chrcteristics of high-energy density mterils (HEDMs) studied elsewhere The lrge positive hets of formtion of cyclic nitrogen-rich ctions mke them promising precursors for the synthesis of highly energetic mterils. One of the most widely studied IL ctions is butyl-methyl-imidzolium (BMIM + ). Moleculr dynmics simultions of the IL BMIM + /PF - 6 indicte t lest two different time scles for diffusion in such solvents nd provide informtion on preferred structures. 12,13 The interction of the surfce lyer of this IL with wter hs been studied experimentlly 14 nd the observed structurl chnges re consistent with the results of the computer simultions. To increse the energy content (with the side effect of decresing the stbility) of the ction, BMIM + my be replced by the trizolium nd tetrzolium ctions with three nd four nitrogens, respectively, in the ring. Recently, theoreticl studies hve been completed on ionic liquids bsed on the trizolium (N 3 C 2 H + 4 ) nd tetrzolium (N 4 CH + 3 ) ctions, combined with nions such s dinitrmide (N(NO 2 ) - 2 ), nitrte (NO - 3 ), nd perchlorte (ClO - 4 ). 15,16 Another pproch to incresing the * Corresponding uthor. E-mil: mrk@si.fi.meslb.gov. Iow Stte University. Air Force Reserch Lbortory. Figure 1. Generl structures of monosubstituted 1-H,3-R-1,3-pentzole (I), 1-H,2-R-1,2-pentzole (II), nd 1-H,1-R-1,1-pentzole (III) ctions showing the tom numbering round the ring. energy of the ctions is to introduce high-energy substituents into the ring, such s zido (-N 3 ) nd nitro (-NO 2 ). The present work is systemtic investigtion of the structures nd energetics of possible ionic liquids formed by protonted pentzole ctions, RN 5 H +, nd the dinitrmide, nitrte, nd perchlorte nions. There hve been few ttempts to synthesize HN 5 H + s even its prent precursor, pentzole (HN 5 ), hs proved to be difficult to isolte nd detect. The first pentzole compounds to be synthesized were substituted by phenyl ring Huisgen nd Ugi prepred their phenyl-substituted pentzole by recting the zide nion, N 3 -, with dizonium ions, ArN 2 +, under creful temperture control. 18,19 The ryl pentzole products redily lost N 2, but the resulting dimethylnilino pentzole ws stble enough to llow n X-ry crystl structure determintion. However, they were not successful in isolting HN 5. Since then significnt gol of heterocyclic chemistry hs been to prepre the prent pentzole. In erlier ttempts, ozonolytic degrdtion of the ryl ring with controlled mount of ozone ws mde but neither HN 5 nor its nion N 5 - ws found. 21 Recently, Butler et l., working under controlled conditions, were ble to isolte nd detect pentzole in solution s N 5 - with Zn 2+ ion. 22 In ddition to the experimentl investigtions, severl theoreticl studies hve been performed on HN 5 s well Clcultions predict tht the hlf-life of HN 5 is only bout 10 min; 21 this explins the difficulty in the direct detection of this compound. Chen 23 hd initilly suggested tht HN 5 is strong cid, even stronger thn nitric cid; hence, in solution significnt portion is esily converted to the nion, fct finlly corroborted by the experimentl results of Butler et l. 22 On the other hnd, ttempts to isolte the protonted pentzole /jp CCC: $ Americn Chemicl Society Published on Web 01/09/2007

4 692 J. Phys. Chem. A, Vol. 111, No. 4, 2007 Pimient et l. Figure 2. Optimized structures of dinitrmide, nitrte, nd perchlorte nions (lower structures) nd their protonted counterprts (nitrogen ) blue, oxygen ) red, chlorine ) green, crbon ) blck, hydrogen ) white). Figure 4. Monosubstituted 1-H,2-R-pentzole ction minimum energy structures. Atom colors re defined in Figure 3. Figure 3. Monosubstituted 1-H,3-R-pentzole minimum energy structures (nitrogen ) blue, oxygen ) red, fluorine ) green, crbon ) blck, hydrogen ) white). ction hve so fr been unsuccessful nd theoreticl clcultions re sprse. 28,29 The lck of experimentl dt on protonted pentzole mkes ccurte quntum chemicl clcultions essentil in predicting the properties of these compounds. The cidity of the zoles hs been known for quite sometime nd increses in the order pyrrole < pyrzole < imidzole < 1,2,3-trizole < 1,2,4-trizole < tetrzole < pentzole. 30 This mens tht the protonted pentzole ction, which is the most cidic mong the protonted zoles, is the most difficult to isolte experimentlly. Mo et l. investigted 29 the bsis set effects on the theoreticl (Hrtree-Fock) gs-phse bsicities of severl zoles including pentzole nd its protonted form with the STO-3G, 3-21G, nd 6-31G bsis sets, using the conjugte grdient optimiztion procedure 31 to obtin the strting structures. The bsicity of these compounds is predicted to depend on the contributions of different tutomers of either the nonprotonted or protonted forms. The monosubstituted pentzole ction hs three possible isomers (i.e., 1-H,3-R-1,3-pentzole (I), 1-H,2-R-1,2-pentzole (II), nd 1-H,1-R-1,1-pentzole (III) ions) (see Figure 1). The only difference mong the three isomers is in the loction of the substituent R. To find new high-energy IL precursors nd to obtin more comprehensive picture of the stbility of these compounds, severl topics will be ddressed below. First, the stbility nd structure of severl monosubstituted pentzole ctions re investigted by determining the ctivtion brrier nd the energy of decomposition to N 2 nd the corresponding zidinium ction. It is expected tht substituents my ply mjor role in the thermodynmic stbility of the protonted pentzole ring. Second, π-bonding nd popultion nlysis re performed to determine the mount of chrge delocliztion round the ring in these compounds. One of the chrcteristics of low melting ILs is the presence of chrge-deloclized ions. Finlly, the structures of dimers composed of the protonted pentzole nd severl energetic nions including dinitrmide, nitrte, nd perchlorte re presented. All possible structures, including the neutrl pirs, trnsition sttes, nd ionic pirs, re described. The smll brriers for proton trnsfer from the ction to the nion indicte tht deprotontion my be first step in the decomposition of these pentzole-bsed ILs. Computtionl Detils The geometries of the monosubstituted pentzole ctions RN 5 H + (R ) H, F, CH 3, CN, NH 2, OH, OCH 3,CH 2 NO 2,C 2 H 3, N 3,NF 2 ), trnsition stte structures nd decomposition products s well s the nions dinitrmide, nitrte, nd perchlorte were

5 Pentzole-Bsed Energetic Ionic Liquids J. Phys. Chem. A, Vol. 111, No. 4, TABLE 1: Monosubstituted 1-H,3-R-1,3-Pentzole (I) Optimized Geometries (Bond Lengths A-B in Å nd Angles A-B-C in Degrees) H F CH 3 CN NH 2 OH CH 2NO 2 N 3 NF 2 C 2H 3 OCH 3 N3-R N1-N N2-N N3-N N4-N N1-N N1-N2-N N2-N3-N N3-N4-N N4-N5-N N2-N1-N N2-N3-R N4-N3-R TABLE 2: Monosubstituted 1-H,2-R-1,2-Pentzole (II) Optimized Geometries (Bond Lengths A-B in Å nd Angles A-B-C in Degrees) H F CH 3 CN NH 2 OH CH 2NO 2 N 3 NF 2 C 2H 3 OCH 3 N2-R N1-N N2-N N3-N N4-N N1-N N1-N2-N N2-N3-N N3-N4-N N4-N5-N N2-N1-N N1-N2-R N3-N2-R fully optimized with second-order Møller-Plesset perturbtion theory (MP2), 32,33 using the restricted Hrtree-Fock (RHF) 34 determinnt s the reference. The completely renormlized coupled-cluster with singles, doubles, nd nonitertive triples (CR-CCSD(T)) method ws used to clculte single-point energies for the ctions t the MP2 geometries. The bsis set used ws G(d,p) 38,39 for ll clcultions. Intrinsic rection coordinte (IRC) clcultions were performed to connect the trnsition sttes with the rectnts nd products. 40 A multiconfigurtion self-consistent field (MCSCF) 41 clcultion using Edmiston-Ruedenberg moleculr orbitls (LMO) 42 ws performed to determine the mount of chrge delocliztion in the ring. 15,16 Gussin-2 (G2) 43,44 clcultions were employed to extrct dt for the thermochemicl nlysis. Ion pirs of severl neutrl, trnsition stte, nd ionic dimers were optimized t the RHF level, nd MP2 single-point energies t the RHF geometries were clculted. All results were obtined from the GAMESS 45,46 electronic structure pckge, except for the G2 results, which were obtined from the Gussin94 code. 47 All molecules were visulized with McMolPlt. 48 Results nd Discussion Minimum Energy Structures. The MP2/ G(d,p) optimized structures for the nions nd their protonted counterprts tht re used to construct the dimer structures re shown in Figure 2. There is very little chnge in the geometries between the protonted nd deprotonted structures of the nitrte nd perchlorte species. However, the dinitrmide nion hs protontion sites t both the centrl nitrogen nd ny of the four oxygens; only the structure with the preferred site of protontion (i.e., the centrl nitrogen) is shown. Figure 5. Isomer III minimum energy structures showing no stble ring species from the 1-H,1-R-pentzole ction. See lst figure in Figure 1 for the strting structures. Atom colors re defined in Figure 3.

6 694 J. Phys. Chem. A, Vol. 111, No. 4, 2007 Pimient et l. Figure 6. Contributing resonnce structures for monosubstituted 1,3- pentzole (IA nd IB) nd 1,2-pentzole (IIA nd IIB). The optimized geometries for the monosubstituted pentzole ctions I (Figure 3) nd II (Figure 4) re summrized in Tbles 1 nd 2, respectively. MP2/ G(d,p) geometry optimiztions of the 1,1-pentzole ctions III led to the instntneous decomposition to N 2 nd the corresponding zidinium ctions, s illustrted in Figure 5 for R ) H-,F-,CH 3 -,CN-,NF 2 -, nd -CH 2 NO 2. For R ) NH 2 -, OH-, nd -OCH 3, the zidinium ctions brek down further, ejecting nother N 2. For R ) C 2 H 3, the 1,1-pentzole ction decomposes into two N 2 molecules, with cycliztion of the remining NHC 2 H 3 + species. It is interesting to see in Figure 5 tht the N 3 -substituted ction spontneously decomposes into pieces, forming N 2 molecules nd H +. Like their neutrl prent HN 5, 27 the ctionic pentzole rings in ll monosubstituted species for isomers I nd II re plnr with lmost equl N-N bond lengths of 1.32 Å (Tbles 1 nd 2). The mximum difference in N-N bond distnces is only Å in isomer I for R ) CN with the N3-N4 nd N1-N5 bonds lwys slightly longer thn the other N-N bonds in the ring (see Figure 1 for the tom numbering). This difference is slightly smller in isomer II (bout Å for R ) F) where, in generl, the N3-N4 bond is little longer thn the remining N-N bonds. These results indicte tht the ctionic pentzole ring is hrdly ffected by the ttched substituents regrdless of how smll (-H) or bulky (-NF 2 ) the groups re, nd the strength of their electron-donting (-OH nd -NH 2 ) or electron-withdrwing (-CN) chrcter. The very smll vrition in the N-N distnces lso suggests significnt electron delocliztion. MP2 geometry optimiztion of the nitro group (-NO 2 ) results in rther long N-N distnce of Å, suggesting very wek binding to the ring nitrogen. This is not surprising, since it ws shown previously tht -NO 2 does not bind well to nitrogen of the tetrzolium ring. 13 In the present work, methylene spcer, -CH 2 NO 2, is inserted to fcilitte binding. Reltive Energies of Monosubstituted Pentzole Ctions. The two stble ring isomers of the monosubstituted pentzole ctions (lbeled I nd II in Figure 1) hve two possible resonnce structures, shown in Figure 6, tht differ in the positions of the double bonds nd the loction of the ctionic chrge. The moleculr nd electronic structure of ech isomer is some composite of these contributing resonnce structures (i.e., IA nd IB for isomer I nd IIA nd IIB for isomer II). For ll substituted species, MP2/ G(d,p) predicts tht isomer I is lower in energy thn isomer II by kcl/mol (see Tble 3). For comprison, the CR-CCSD(T) single-point Figure 7. The MP2/ G(d,p) reltive energies in kcl/mol for proton rerrngement from the 1-H,2-H-pentzole to the 1-H,3-Hpentzole. energies t the MP2 geometries predict tht isomer I is lower by kcl/mol. The trnsition stte for proton rerrngement in the hydrogensubstituted ctions (HN 5 H + ) from the 1-H,2-H-N 5 + species to its more stble 1-H,3-H-N 5 + counterprt lies 50 kcl/mol bove the less stble isomer (see Figure 7). This mens tht it is rther unlikely for proton migrtion to occur, lthough chemicl intuition suggests tht steric effects due to bulky substituents ttched to N2 in the pentzole ring, such s -CH 2 NO 2 nd -NF 2, my lower this brrier. To ssess the reltive importnce of the contributing resonnce structures with regrd to the mount of chrge delocliztion round the pentzole ring in the prent isomers, n MCSCF π-lmo popultion nd π-bond order nlysis 15,16 hs been performed. Recll tht the possibility of obtining low melting point IL is enhnced by substntil chrge delocliztion round the protonted pentzole ring. The digonl elements of the density mtrix cn be regrded s the number of electrons occupying the LMO, while the off-digonl elements give the π-bond orders. A positive bond order reflects bonding interction nd negtive bond order indictes n ntibonding interction. The strength of these interctions is mesured by the mgnitude of their respective bond orders. The results of the MCSCF bonding nlysis for the prent pentzole ctions re presented in Tble 4. For the resonnce TABLE 3: MP2 nd CR-CCSD(T)/ G(d,p) Electronic Energy Differences, in kcl/mol H F CH 3 CN NH 2 OH CH 2NO 2 N 3 NF 2 C 2H 3 OCH 3 1,3-Isomer MP CR-CCSD(T) ,2-Isomer MP CR-CCSD(T)

7 Pentzole-Bsed Energetic Ionic Liquids J. Phys. Chem. A, Vol. 111, No. 4, TABLE 4: MCSCF/ G(d,p) π Orbitl Popultions nd Bond Orders π orbitl popultions N1 N2 N3 N4 N5 Totl bonding 1,3 ion ,2 ion djcent π bond orders N1-N2 N2-N3 N3-N4 N4-N5 N5-N1 1,3 ion ,2 ion next neighbor π bond orders N1-N3 N2-N4 N3-N5 N4-N1 N5-N2 1,3 ion ,2 ion TABLE 5: Hets of Formtion t 298 K for Gs-Phse Monosubstituted 1,3- nd 1,2-Pentzole Ctions R H f (IA) 1,3-isomer H f (IB) verge H f (IIA) 1,2-isomer H f (IIB) verge H F CH CN NH OH CH 2NO N NF C 2H OCH IA, IB, IIA, nd IIB refer to prticulr resonnce hybrids with their verge vlues being the predicted hets (the verge is given in both kcl/mol nd kcl/g). structures of isomer I, if IA dominted over IB, N1 would hve two π-electrons nd ll other toms in the ring would possess one π-electron (see Figure 6). Tble 4 shows tht N1 nd N3 hve π-popultions of 1.37 nd 1.39 electrons, respectively, indicting nerly equl contributions from the two resonnce structures. So, these two nitrogens eqully shre the forml chrge of +1 in isomers IA nd IB. For isomer II, both N1 nd N2 hve the sme popultion of 1.42 π-electrons, which mens tht the ring is hybrid between IIA nd IIB nd tht the resonnce structures re of equl importnce. These observtions re further vlidted by the clculted π-bond orders of between djcent toms. These bond orders between 0 nd 1 lso suggest tht the ring is deloclized. For both isomers, ll next nerest neighbor interctions re ntibonding. The clculted totl π-bonding of 1.89, obtined by summing ll π-bonding nd ll π-ntibonding interctions, is very close to the totl bonding of 2 π-bonds in digrms IA, IB, IIA, nd IIB shown in Figure 6. Hets of Formtion. The mount of energy contined in the monosubstituted pentzole ctions my be quntified vi their hets of formtion, clculted here by combining their MP2/ G(d,p) electronic energies with the dt obtined from the G2 clcultions. It is expected tht the pentzole ction in which ll the toms in the ring re nitrogens will hve lrger het of formtion thn tetrzolium or trizolium. The clculted gs-phse hets of formtion for both monosubstituted pentzole ctions re presented in Tble 5. Figure 8. Monosubstituted 1-H,3-R-pentzole trnsition sttes. Atom colors re defined in Figure 3. To obtin ccurte hets of formtion for these ctions, severl isodesmic (bond-type conserving) rections re utilized. Becuse there re two contributing resonnce structures ech for both isomers, four isodesmic rections re considered (two for isomer I, i.e., IA nd IB, nd two for isomer II, i.e., IIA nd IIB): IA + 7NH 3 f 2H 2 N-NH 2 + 2HNdNH + H 2 N-R + H 4 N + IB + 7NH 3 f 2H 2 N-NH 2 + 2HNdNH + H 3 N + H 3 N-R + IIA + 7NH 3 f 2H 2 N-NH 2 + 2HNdNH + H 2 N-R + H 4 N + IIB + 7NH 3 f 2H 2 N-NH 2 + 2HNdNH + H 3 N + H 3 N-R + Becuse isodesmic rections conserve the number of forml bond types on both sides of rection, the correltion energy 49 error is minimized, nd hence the computed enthlpies of formtion re more ccurte becuse of error cncelltions. Tble 5 shows tht the clculted hets of formtion for IA nd IB (or IIA nd IIB) re lmost lwys equl with the lrgest devition of only 3.3 kcl/mol obtined from the NF 2 -substituted ion. Thus, the verge vlues of IA nd IB nd of IIA nd IIB cn be regrded s the hets of formtion for the monosubstituted pentzole ctions I nd II, respectively. As expected, the verge hets of formtion for the monosubstituted pentzole ctions re lrge, bout 4.1 kcl/g for the prent H-substituted pentzole ctions. In comprison, the predicted hets of formtion for the prent trizolium nd tetrzolium ctions re nd kcl/g, respectively. This is further evidence tht these pentzole ions re reltively unstble nd, therefore, will likely be difficult to synthesize, isolte, nd detect much like their prent neutrl pentzole. The hets of formtion of the monosubstituted pentzole ctions I re lwys smller by bout kcl/mol reltive to

8 696 J. Phys. Chem. A, Vol. 111, No. 4, 2007 Pimient et l. TABLE 6: Monosubstituted 1-H,3-R-1,3-Pentzole Trnsition Stte Geometries (Bond Lengths A-B in Å nd Angles A-B-C in Degrees) H F CH 3 CN NH 2 OH CH 2NO 2 N 3 NF 2 C 2H 3 OCH 3 N3-R N1-N N2-N N3-N N4-N N1-N N1-N2-N N2-N3-N N3-N4-N N4-N5-N N2-N1-N N2-N3-R N4-N3-R ctions II. These re ll lrger thn the inferred hets of formtion of the oxdiziridine-bsed HEDMs. 11 Of course, the nture of the substituent ttched to the pentzole ring ffects the computed enthlpy of formtion. By replcing the -H substituent in the prent pentzole ction by electron-donting groups (EDGs), the clculted hets of formtion decrese, while electron-withdrwing groups (EWGs) tend to increse the enthlpies. For the monosubstituted isomer I, the hets of formtion (in kcl/mol) re decresed by the presence of EDGs by s little s 2.8 (-OH) to s much s 17.6 (-CH 3 ) nd incresed by EWGs by s little s 22.0 (-F) to s much s (-N 3 ) reltive to the enthlpy for the prent H-substituted 1,3-pentzole ion (285.6 kcl/mol). These differences in enthlpies re 3.7, 19.4, 24.2, nd kcl/mol for the -OH, -CH 3, -F, nd -N 3 substituents, respectively, reltive to the prent 1,2-pentzole ion (303.9 kcl/mol). However, the ctions with nitrogen-contining EDGs such s -NH 2 nd -NF 2 hve higher hets of formtion in opposition to the min trend. In these instnces, the resulting increse in the clculted enthlpy is due to the dditionl nitrogen djcent to the pentzole ring. All listed hets of formtion in Tble 5 re lrge nd positive, but the species with the lrgest enthlpies on per weight bsis include the prent pentzole ctions with hets of formtion of bout 4.1 kcl/g nd the CN-substituted ions with energies of bout 3.8 kcl/g. Trnsition Sttes nd Decomposition Products. The trnsition stte structures for the decomposition of the monosubstituted ctions I, RN 5 H + f RN 3 H + + N 2, (Figure 8) show lengthening of the N1-N5 nd N3-N4 bonds by bout Å nd shortening of the N4-N5 distnce by bout 0.13 Å (see Figure 1 nd Tble 6), in the process of eliminting n N 2 molecule. In generl, the N3-N4 bond length is longer thn the N1-N5 distnce, implying tht the trnsition stte is obtined vi n symmetric ring opening. The exception is the prent 1,3-pentzole ion (i.e., the H-substituted) formed through symmetric ring opening mechnism with equl N3-N4 nd N1-N5 bond lengths. On the other hnd, the decomposition of the monosubstituted ctions II involves two steps. First, the N2- N3 distnce increses by Å, while the N3-N4 bond decreses by 0.17 Å forming ring-opened trnsition sttes (see Figure 9 nd Tble 7), leding to the locl minimum structures shown in Figure 10 nd quntified in Tble 8. There is slight increse in the N4-N5 bond length of bout 0.05 Å, which indictes tht succeeding step to form N 2 is imminent. Indeed, the second step involves the formtion of trnsition sttes (Figure 11) in which the N4-N5 distnce is incresed further by bout 0.6 Å (Tble 9). It is interesting to note from the structures in Figure 11 tht the hydrogen in the zidinium ction locted in the N1 position is oriented in such wy tht it my be involved in rerrngement rection into the more stble form in which the hydrogen is frthest wy from the substituents. The one exception to this is the CH 2 NO 2 -substituted ction in which the hydrogen in the N1 position is trnsferred to one of the oxygens in the CH 2 NO 2 moiety. The structures of the decomposition products of the vrious monosubstituted pentzole ctions for both isomers re shown in Figures 12 nd 13. The optimized geometries for the products re listed in Tbles 10 nd 11. The decomposition of the monosubstituted pentzole ctions involves the formtion of N 2. The resulting bond distnce of 1.12 Å in the product N-N bonds for ll isomer I nd II decomposition products is in good greement with the experimentl vlue of Å. Figure 13 confirms tht the hydrogen in the zidinium ction, originlly locted in the N1 position, migrtes to form the more stble isomer, similr to tht of the isomer I decomposition products. As noted bove, the exception is the CH 2 NO 2 -substituted zidinium ion in which the hydrogen is trnsferred to the oxygen Figure 9. Monosubstituted 1-H,2-R-pentzole ring-opened trnsition sttes. Atom colors re defined in Figure 3.

9 Pentzole-Bsed Energetic Ionic Liquids J. Phys. Chem. A, Vol. 111, No. 4, TABLE 7: Monosubstituted 1-H,2-R-1,2-Pentzole Ring-Opened Trnsition Stte Geometries (Bond Lengths A-B in Å nd Angles A-B-C in Degrees) H F CH 3 CN NH 2 OH CH 2NO 2 N 3 NF 2 C 2H 3 OCH 3 N2-R N1-N N2-N N3-N N4-N N1-N N1-N2-N N2-N3-N N3-N4-N N4-N5-N N2-N1-N N1-N2-R N3-N2-R in the nitro group insted, producing three decomposition products: N 2, HONO, nd H 2 CN 3 +. The ctivtion energies for the decomposition rections re listed in Tble 12. Isomer I decomposes in single step vi n symmetriclly ring-opened trnsition stte (see Figure 8 nd Tble 6), while isomer II decomposes in two steps vi ring opening followed by subsequent proton migrtion (Figures 9 nd 11 nd Tbles 7 nd 9). All of the monosubstituted isomer I species hve CR-CCSD(T) ctivtion energies in the kcl/mol rnge. The CR-CCSD(T) E 1 ctivtion energies of isomer II re lrger by 2-7 kcl/mol, except for R ) F, CN, nd NF 2. Previous studies on severl neutrl zoles, including trizole, tetrzole, nd pentzole, show tht the ctivtion energies for this fmily of zoles depend on the number of ring nitrogen toms nd increse in the order pentzole < tetrzole < trizole. 27 It is expected tht their monosubstituted ctionic counterprts will follow the sme trend. Figure 10. Locl minim structures connecting the trnsition sttes of the monosubstituted 1-H,2-R-pentzole ctions. Atom colors re defined in Figure 3. Tble 12 shows tht EDGs in isomer I such s -CH 3, -CH 2 - NO 2, nd -C 2 H 3 tend to give the lrgest ctivtion energies (24.0, nd 21.8 kcl/mol, respectively) while EWGs such s -N 3 (16.0 kcl/mol) nd -CN (19.0 kcl/mol) give the smllest brriers. The E 1 ctivtion energies re slightly lrger in isomer II, but the observed trend is the sme. Isomer II with EDGs ttched (e.g., -CH 3 (37.0 kcl/mol) nd -C 2 H 3 (32.0 kcl/mol)) possess lrger ctivtion energies thn their EWGsubstituted counterprts such s -N 3 (23.4 kcl/mol) nd -CN (19.0 kcl/mol). In generl, the CR-CCSD(T) ctivtion brriers re t most 5 kcl/mol lrger thn the MP2 ctivtion energies. The inductive effects (e.g., in -CH 3 nd -CH 2 NO 2 ) nd resonnce donting effects (e.g., in -C 2 H 3 ) brought bout by the EDGs help to stbilize the ring by incresing the electron density in the ring, thereby resulting in higher ctivtion energies. Although isomer I is lower in energy, isomer II is more stble to decomposition. An importnt fctor in the design of high-energy mterils is the energy relesed on decomposition. The performnce of highly energetic system is gretly enhnced with n increse in the relesed energy. The decomposition rections for ll monosubstituted species re exothermic (see Tble 13). In generl, the decomposition of isomer II is more exothermic by t lest 14 kcl/mol reltive to isomer I. As for the ctivtion energy, the decomposition energy is gretly ffected by the substituent ttched in the ctionic pentzole rings of the two isomers. The decomposition becomes more exothermic with n increse in the electron-withdrwing chrcter of the substituent. The highest decomposition energies re found in the NH 2 - nd N 3 -substituted ctions for isomer I (-24.5 nd kcl/mol, respectively) nd the N 3 - nd CH 2 NO 2 - substituted ctions for isomer II (-58.9 nd kcl/mol, respectively). The observed trend in which more exothermic rection is ccompnied by lower ctivtion brrier seems to follow Hmmond s postulte, 50 ccording to which lower ctivtion energy corresponds to more exothermic rection. By combining the energy profiles for the decomposition of the monosubstituted pentzole ctions including the minimum energies, the ctivtion energies, nd the energies for decomposition, generl potentil energy pthwy for decomposition cn be deduced for isomers I nd II (Figure 14). Finlly, note tht there re severl cses in both Tbles 12 nd 13 for which there is significnt (>10 kcl/mol) disgreement between the MP2 nd CR-CCSD(T) energy differences. For exmple, the two levels of theory differ by 14 kcl/mol in their prediction of the net rection energy for the prent species (Tble 13), with R ) H. Such disgreement is often indictive of some nontrivil configurtionl mixin

10 698 J. Phys. Chem. A, Vol. 111, No. 4, 2007 Pimient et l. TABLE 8: Monosubstituted 1-H,2-R-1,2-Pentzole Geometries for the Intermedites Connecting the Two Trnsition Sttes (Bond Lengths A-B in Å nd Angles A-B-C in Degrees) H F CH 3 CN NH 2 OH CH 2NO 2 N 3 NF 2 C 2H 3 OCH 3 N2-R N1-N N2-N N3-N N4-N N1-N N1-N2-N N2-N3-N N3-N4-N N4-N5-N N2-N1-N N1-N2-R N3-N2-R g in which cse the coupled cluster method is generlly more relible. Ion Pir Interctions. The RHF/ G(d,p) optimized structures of the neutrl nd ionic pirs, s well s the trnsition sttes tht connect them, for ll dimers formed by combining either of the H-substituted ctionic pentzole isomers with the three selected nions re shown in Figures All MP2/ G(d,p) optimiztions nd structure determintions were unsuccessful in locting ionic dimers; only hydrogen-bonded neutrl pirs seem to exist. However, MP2 single-point energies t the RHF geometries, referred to s MP2//RHF, re clculted for comprison. The results of the RHF/ G(d,p) geometry optimiztions on severl different orienttions of either of the two minimum energy protonted pentzole isomers nd the minimum energy dinitrmide nion indicte tht the dimer structure in Figure 15, brought bout by the trnsfer of the proton from the 1,2-isomer to the dinitrmide, is the most stble neutrl Figure 11. Monosubstituted 1-H,2-R-pentzole structures for the second trnsition stte. Atom colors re defined in Figure 3. dimer. Henceforth, this structure is tken s the zero energy structure ginst which ll other energies of the remining protonted pentzole-dinitrmide dimer structures re compred. However, the neutrl dimer structure formed by the protonted 1,3-pentzole nd dinitrmide is higher in energy by only 0.4 nd 1.0 kcl/mol t the RHF nd MP2 levels of theory, respectively. The MP2//RHF energy is tken to be the MP2 zero of energy. In the serch for the minimum energy protonted pentzoledinitrmide dimer structures, severl RHF optimiztions were performed. The strting structures were vried with n dequte ction-nion seprtion to remove ny bises towrd the finl optimized structures. Figures 16, 18, 19, nd 21 show tht only few neutrl nd ionic dimer pirs out of bout 20 strting structures were found, depending on the loction of the protontion in dinitrmide. In prticulr, the number of unique protonted pentzole-dinitrmide ionic dimer structures is only four for isomer I (Figure 16) nd three for isomer II (Figure 19). The smll number of dimer structures found is due to the highly symmetric chrcter of the strting protonted pentzole nd tht the ctionic pentzole ring is composed of ll nitrogens, which mens tht some of the initil dimer orienttions converge to the sme finl structure. The corresponding neutrl dimer structures resulting from proton-trnsfer rections from the ction to the nion re shown in Figures 18 nd 21. The trnsition sttes linking the ionic pirs to the neutrl dimers re shown in Figures 17 nd 20. As noted previously (Figure 16), the common zero of energy for ll structures is the most stble pentzole-dinitrmide neutrl dimer, shown here s the leftmost structure in Figure 21. The clculted MP2 ctivtion energies show shllow brriers for proton trnsfer with the highest vlue only 5.6 kcl/mol. This is indictive of the very short lifetimes of these ionic pirs if they were synthesized. In fct, only two of the ionic dimers hve brrier; in lmost ll cses, the proton is spontneously trnsferred to the dinitrmide nion to form the neutrl pirs. The clculted energies for the neutrl dimer structures in Figures 18 nd 21 show tht the preferred protontion site in the dinitrmide nion is indeed the centrl nitrogen. For ll seven neutrl structures of the pentzole-dinitrmide pirs, the energy seprtion is smll with mximum difference t the RHF nd MP2 levels of only 11.9 nd 14.0 kcl/mol, respectively. The structures in Figures 18 nd 21 show tht the predominnt form of interction is intermoleculr hydrogen bonding, either the N-HsN or O-HsN type, but few dimer structures possess intrmoleculr hydrogen bonding within the dinitrmic cid moiety such s the O-HsO bonding interction found in Figure 18(d).

11 Pentzole-Bsed Energetic Ionic Liquids J. Phys. Chem. A, Vol. 111, No. 4, TABLE 9: Monosubstituted 1-H,2-R-1,2-Pentzole Geometries for the Second Trnsition Stte (Bond Lengths A-B in Å nd Angles A-B-C in Degrees) H F CH 3 CN NH 2 OH CH 2NO 2 N 3 NF 2 C 2H 3 OCH 3 N2-R N1-N N2-N N3-N N4-N N1-N N1-N2-N N2-N3-N N3-N4-N N4-N5-N N2-N1-N N1-N2-R N3-N2-R The interctions between ech of the protonted pentzole isomers with the nitrte nd the perchlorte nions lso were exmined. The MP2 optimiztions result in n instntneous proton trnsfer from the protonted pentzole to one of the oxygens of the nion, wheres RHF optimiztions predict tht both neutrl nd ionic dimers re locl minim. So, electron correltion is clerly importnt in determining which structures exist. Figures 22 nd 23 show the neutrl nd ionic dimer structures nd the trnsition sttes tht connect them. The pir comprising the H-substituted 1,2-pentzole nd either the nitrte or perchlorte nion is predicted to be lower in energy thn the interction between the H-substituted 1,3-pentzole nd these nions. The zero energy structure for the neutrl pentzolenitrte pir, resulting from the trnsfer of proton from isomer II to the nitrte, is shown in the bottom right corner of Figure 22. The lowest energy protonted 1,3-pentzole-nitrte pir obtined in the MP2 (RHF) clcultion is only 1.7 kcl/mol (1.3 kcl/mol) higher in energy reltive to this structure. In the protonted pentzole-perchlorte pir, the MP2 energy seprtion between the two neutrl dimer structures is 2.1 kcl/mol. Finlly, the MP2 hets of rection for the formtion of either the neutrl or ionic dimers from the individul ions re included s the third number in Figures 16, 18, 19, 21, 22, nd 23. The mixing of the protonted pentzole with ny of the three nions produces either neutrl or ionic pirs, lthough MP2 optimiztions result in proton trnsfer. These rections re ll highly exothermic. As expected, the hets of rection for the formtion of the neutrl pirs re lwys lrger (more exothermic) thn those for the ionic pirs, by t lest 16 kcl/mol in the protonted pentzole-dinitrmide, 31 kcl/mol in the protonted pentzole-nitrte, nd 20 kcl/mol in the protonted pentzoleperchlorte systems. The neutrl dimer contining 1,3-pentzole is lower in energy thn the corresponding 1,2-isomer by only 1.2, 1.7, 2.2 kcl/mol for the dinitrmide, nitrte, nd perchlorte Figure 12. Monosubstituted 1-H,3-R-pentzole decomposition products. Atom colors re defined in Figure 3. Figure 13. Monosubstituted 1-H,2-R-pentzole decomposition products. Atom colors re defined in Figure 3.

12 700 J. Phys. Chem. A, Vol. 111, No. 4, 2007 Pimient et l. TABLE 10: Monosubstituted 1-H,3-R-1,3-Pentzole Decomposition Product Geometries (Bond Lengths A-B in Å nd Angles A-B-C in Degrees) H F CH 3 CN NH 2 OH CH 2NO 2 N 3 NF 2 C 2H 3 OCH 3 N3-R N1-N N2-N N3-N N4-N N1-N N1-N2-N N2-N3-N N3-N4-N N4-N5-N N2-N1-N N2-N3-R N4-N3-R TABLE 11: Monosubstituted 1-H,2-R-1,2-Pentzole Decomposition Product Geometries (Bond Lengths A-B in Å nd Angles A-B-C in Degrees) H F CH 3 CN NH 2 OH CH 2NO 2 N 3 NF 2 C 2H 3 OCH 3 N2-R N1-N N2-N N3-N N4-N N1-N N1-N2-N N2-N3-N N3-N4-N N4-N5-N N2-N1-N N1-N2-R N3-N2-R TABLE 12: Activtion Energies (E ) for the Decomposition of Protonted Pentzole To Form the Corresponding Azidinium Ctions nd N 2 (Including Zero-Point Energy Correction, in kcl/mol) 1,3-Isomer 1,2-Isomer E E 1 E 2 MP2 CR-CCSD(T) MP2 CR-CCSD(T) MP2 CR-CCSD(T) H F CH CN NH OH CH 2NO N NF C 2H OCH Subscripts 1 nd 2 lbel the first nd second ctivtion energies, respectively, for the 1,2-isomer. E 2 is clculted reltive to the globl minimum energy structures shown in Figure 4. nions, respectively. The differences in the hets of rection between the ionic dimers of the two isomers re t lest 2.0, 3.5, nd 6.5 kcl/mol, respectively, for the dinitrmide, nitrte, nd perchlorte nions. Although the prent isomer I is more stble thn isomer II, the nions seem to prefer to bind to the ltter. Conclusions Ionic liquid dimers composed of the five-membered monosubstituted pentzole ction nd the dinitrmide, nitrte, nd perchlorte nions hve been investigted. Of the two monosubstituted pentzole ction isomers, it is predicted tht the 1,3- isomer is lower in energy thn the 1,2-isomer. The reltive ctivtion energies nd decomposition energies for the decomposition of these ctions to N 2 plus the corresponding zidinium ction cn be relted to the electron-donting chrcter of the TABLE 13: Rection Energies Including Zero-Point Energy Correction for the Decomposition of Protonted Pentzole To Form the Corresponding Azidinium Ctions nd N 2 (kcl/mol) 1,3-Isomer 1,2-Isomer MP2 CR-CCSD(T) MP2 CR-CCSD(T) H F CH CN NH OH CH 2NO N NF C 2H OCH

13 Pentzole-Bsed Energetic Ionic Liquids J. Phys. Chem. A, Vol. 111, No. 4, Figure 14. Decomposition pthwys for the monosubstituted 1,3- isomer (A) nd 1,2-isomer (B). Figure 16. Ionic pirs in the prent 1,3-pentzole nd dinitrmide system locted t the RHF/ G(d,p) level. The first nd second energy vlues re from RHF/ G(d,p) nd MP2/ G- (d,p) t RHF geometries in kcl/mol. The third energy vlue is the MP2 het of rection for HN 5H + + Y - f [HN 5][HY] (or [HN 5H + ][Y - ]), where Y ) N(NO 2) 2,NO 3,orClO 4, in kcl/mol. The zero of energy for Figures is neutrl complex presented in Figure 21. Atom colors re defined in Figure 3. Figure 15. Minimum energy neutrl dimer pirs formed by the protonted 1,2-pentzole (left) nd 1,3-pentzole (right) with the dinitrmide nion. Reltive energies (beneth ech structure) re from RHF/ G(d,p). The MP2/ G(d,p) reltive energies t RHF geometries re in prenthesis. Atom colors re defined in Figure 3. TABLE 14: Hets of Proton-Trnsfer Rection, in kcl/mol, from the Protonted Pentzole to the Anion ) N(NO 2 ) 2-, NO 3-, nd ClO 4 - N(NO 2) 2 - NO 3 - ClO 4-1,3-Isomer MP CR-CCSD(T) ,2-Isomer MP CR-CCSD(T) The term pentzolium is not used becuse it refers to the deprotonted pentzole (N 5+ ). substituent. An electron-donting substituent tends to increse the ctivtion energy nd decrese the decomposition energy. The MCSCF π orbitl nlysis suggests tht the electrons in the ctionic pentzole ring re deloclized. The clculted enthlpies of formtion show tht electron-withdrwing groups, such s -CN nd -N 3, help to increse the hets of rection. Severl dimer structures composed of the H-substituted pentzole nd either of the nions mentioned bove re proposed nd found to be very unstble. The low brrier connecting the Figure 17. Trnsition sttes in the protonted 1,3-pentzole nd dinitrmide system, connecting ionic pirs to neutrl complexes. The energy vlues nd tom colors hve the sme mening s defined in Figure 16. Activtion energies (E ) from RHF nd MP2//RHF re clculted s the difference between the trnsition stte nd their corresponding ionic dimer energies in kcl/mol. ionic nd neutrl dimers nd the lrge corresponding exothermicity suggests tht proton is spontneously trnsferred to the nion. So, fundmentl step for the decomposition of protonted pentzole is likely to be deprotontion. However, the introduction of dditionl ion pirs re likely to stbilize the chrge seprtion in the ionic species. It is predicted tht the 1-H,3-H-pentzolium-dinitrmide pir with brrier for proton trnsfer of 4.7 kcl/mol is the most stble dimer mong ll of the ion-pirs studied. The ion-pir comprised of the pentzolium ction nd perchlorte nion possesses the lrgest energy content nd would hve been desirble high-energy ionic liquid but the 2.7 kcl/mol brrier for proton trnsfer is prohibitively smll for the synthesis nd detection of this ionic

14 702 J. Phys. Chem. A, Vol. 111, No. 4, 2007 Pimient et l. Figure 21. Neutrl dimers in the prent 1,2-pentzole nd dinitrmide system, resulting from proton trnsfers. The energy vlues nd tom colors hve the sme mening s defined in Figure 16. The lowest energy dimer found is the left structure nd is tken s the energy zero for Figures Figure 18. Neutrl dimers in the protonted 1,3-pentzole nd dinitrmide system, resulting from proton trnsfers. The energy vlues nd tom colors hve the sme mening s defined in Figure 16. Figure 19. Ionic pirs in the prent 1,2-pentzole nd dinitrmide system. The energy vlues nd tom colors hve the sme mening s defined in Figure 16. Figure 22. Structures for the prent 1,3-pentzole (upper) nd 1,2- pentzole (lower) with nitrte: ion pir, trnsition stte, nd neutrl pir. The energy vlues nd tom colors hve the sme mening s defined in Figure 16. E vlues re defined in Figure 17. Figure 20. Trnsition sttes in the protonted 1,2-pentzole nd dinitrmide system connecting ionic pirs to neutrl complexes. The energy vlues nd tom colors hve the sme mening s defined in Figure 16. E vlues re defined in Figure 17. liquid. The 1-H,3-H-pentzolium-dinitrmide pir is more idel with reltively lrger brrier nd energy content. Among ll the ion-pirs studied, it is clculted tht the pentzolium-nitrte pirs provide the lrgest energy relesed with het of rection of bout 130 kcl/mol. The CR-CCSD(T) ctivtion energies for decomposition of the 1,2- nd 1,3- ctions re in the rnge of nd kcl/mol, respectively. Becuse higher brrier corresponds to more stble species, the most desirble substituents from this perspective re H, CH 3,CH 2 NO 2, nd C 2 H 3 for the both isomers nd OCH 3 s well for the 1,2-isomer. Becuse ll of the substituted species re highly exothermic, the stbility to decomposition should be the overriding fctor. Acknowledgment. This work ws supported in prt by grnts from the Air Force Office of Scientific Reserch nd the Ntionl Science Foundtion. The uthors thnk Dr. Michel Figure 23. Structures for the prent 1,3-pentzole (upper) nd 1,2- pentzole (lower) with perchlorte: ion pir, trnsition stte, nd neutrl pir. The energy vlues hve the sme mening s defined in Figure 16. Atom colors re defined in Figure 2. E vlues re defined in Figure 17. Bermn for suggesting this study nd Dr. Michel W. Schmidt nd Dclt. Deborh D. Zorn for their helpful discussions. The computtions were performed on locl computers, including n