Theoretical investigation of the ground state dissociation pathways of CH2NO2

Abstract

We report theoretical calculations of the nitromethyl radical (CH2NO2) ground state dissociation pathways. Calculations were performed using a combination of density functional theory (DFT) and coupled-cluster singles doubles and triples (CCSD(T)) with the cc-pVTZ basis set. The ground state unimolecular dissociation of the nitromethyl radical (CH2NO2) occurs via three dissociation pathways and we have characterized the transition state associated with the formation of NO + CH2O (channel I) which has a barrier height of 167.5 kJ/mol. We have also calculated the variational transition states associated with the CH2 + NO2 (channel II) and CH2NO + O (channel III) product channels. Thermal rate constants were calculated to model dissociations at temperatures relevant to shock tube experiments on nitromethane combustion. We find that the branching fractions vary over the range of temperatures relevant to nitromethane combustion, with channel II rising from <1% at 800 K to 14% at 2450 K, and channel I decreasing from >99% to 86% over the same range at one bar. RRKM theory was used to calculate unimolecular microcanonical rate constants at total energies corresponding to photoexcitation over a range of UV wavelengths from 279 to 228 nm to assess the relative branching fractions and compare to previous experimental data on the photodissociation of CH2NO2. The comparison reveals a disparity in the branching fraction between the calculation and experimental results, suggesting the possibility of an excited state dissociation pathway.