Proton-transfer reactions in nitromethane at 297 K

Abstract

Rate constants for proton-transfer reactions of the type XH + + CH 3NO 2 → CH 3NO 2H + + X (1) where X = H 2, D 2, N 2, CO 2, CH 4, N 2O, CO, H 2O, HCN, CH 3CHCH 2, and CH 3OH, and of the type CH 3NO 2 + X − → XH + CH 2NO 2 − (2) where X = NH 2, H, D, OH, CH 3O, C 2H 5O, C 2H, CH 3COCH 2, and CH 3S have been measured at 297 ± 2 K using the flowing afterglow technique with values ranging from 0.1 to 1.3 × 10 −8 cm 3 molecule −1 s −1. Rate constant and equilibrium ion concentration measurements for proton transfer from C 3H 7 + to CH 3NO 2, from CH 3NO 2H + to CH 3OH, and from CH 3NO 2 to CH 3S − provided values for equilibrium constants of 3 ± 1, 1.3 ± 0.4 and 1.9 ± 0.3, respectively, from which values for PA 298(CH 3NO 2) = PA 298(CH 3OH) = 181 ± 3 kcal mol −1 and PA 298(CH 2NO 2 −) = 357.6 ± 2.8 kcal mol −1 were deduced. Several of the more exothermic members of reactions of type (1) appeared to proceed, at least in part, by dissociative proton transfer to produce NO + and CH 3NO +. In the case of reactions of type (2), proton transfer predominated over exothermic NO 2 − displacement. The measured rate constants are compared with the predictions of the Langevin, average-dipole-orientation and locked-dipole theories of ion-molecule collisions. The average-dipole-orientation theory provides the most realistic collision rates for these reactions although these appear to be underestimated on average by ca. 10% and ca. 5% for reactions of type (1) and (2), respectively. The observed variations of reaction efficiency with exothermicity are discussed in terms of current models of the mechanism for proton transfer.