METHYL NITRATE, HYDRAZOIC ACID, AND THEIR CONJUGATE ACIDS - A CONFIGURATION-INTERACTION STUDY OF THE GAS-PHASE PROTON-TRANSFER EQUILIBRIUM AND OF ACID-CATALYZED FRAGMENTATION REACTIONS

Methyl nitrate, hydrazoic acid, their conjugate acids, and selected reactions were studied with ab initio methods. Potential energy surfaces were explored at the RHF/6-31G* level, and electron correlation effects were estimated with third- or full fourth-order perturbation theory and with configuration interaction methods using the 6-31G*, 6-311G**, and 6-311G (dfp) basis sets. CH3ONO2 and its conjugate acids were also studied with semiempirical methods (MNDO, AM1, PM3). Relative isomer stabilities and conformational preferences of protonated CH3ONO2 are reported. Ester-O protonation is preferred over O(term) protonation, and intermolecular proton transfer is more likely for their gas-phase isomerization. Aminodiazonium ion is greatly preferred over iminodiazenium ion, by 27.1 kcal/mol at CISD/6-311G**//MP2/6-31G*+DELTA-VZPE. In good agreement with experiment, the reaction energy of DELTA-E3 = 2.9 kcal/mol was found for the proton-transfer reaction CH3ONO2 + H2N3+ double-line arrow pointing left and right (CH3ONO2)H+ + HN3 at CISD/6-311G**//RHF/6-31G*+DELTA-VZPE. At this level, the gas-phase proton affinities of HN3 and CH3ONO2 are DELTA-E1 = 187.6 and DELTA-E2 = 184.7 kcal/mol, respectively. The PA(HN3) at this level is within 1 kcal/mol of our best value obtained at CISD/6-311G(dfp)//MP2/6-31G*. These theoretical results suggest that the reported experimental proton affinity of HN3 (176.6 kcal/mol) might be too low as a consequence of evaluating the proton transfer equilibrium with an underestimated proton affinity of CH3ONO2. Ester-O protonated CH3ONO2 is best described as methanol-solvated NO2+, and its dissociation requires only 17.5 kcal/mol at CISD/6-311G**//RHF/6-31G*+DELTA-VZPE. In sharp contrast to prior semiempirical results, dediazotization of aminodiazonium ion is endothermic by 73.6 kcal/mol at CISD/6-311G(dfp)//RHF/6-31G*+DELTA-VZPE.