High level ab initio, DFT, and RRKM calculations for the unimolecular decomposition reaction of ethylamine

Abstract

Mechanisms for the decomposition reaction of ethylamine, CH3CH2NH2, were investigated using ab initio, DFT, and RRKM calculations. Optimized geometries of reactants, transition states, intermediates, and products were determined at HF, MP2, and B3LYP levels of theory using the 6-31G(d) and 6-31+G(d) basis sets. Single point energies were also determined at G3MP2B3 and G3B3 levels of theory. Thermodynamic properties, activation energies, enthalpies and Gibbs energies of activation were calculated for each reaction pathway investigated. Intrinsic reaction coordinate (IRC) analysis was performed to characterize the transition states on the potential energy surface. The conformational change and planarity of the ethylamine moiety along with the twist angle of the amino group about the CN axis are examined. Four pathways for the decomposition reaction of ethylamine were studied. All pathways involve a 1,2-elimination reaction and 1,3-proton shift to produce ethene, ethanimine, ethenamine, and methanimine. All pathways are single-step mechanisms. Elimination of the NH3 dominates the decomposition behavior up to 1200K whereas after this temperature, secession of the C–N gradually holds more importance. While pathways signifying departures of NH3 and NH2 exhibit pressure-dependent behavior, branching ratios for these two channels are generally not influenced by variation in pressure higher than the atmospheric pressure.