Direct dynamics simulation of the thermal O(3P) + dimethylamine reaction in the triplet surface. I. Rate constant and product branching

In order to provide atomistic details for the mechanism of the collisional dynamics of O(P-3) and dimethylamine (DMA) in the triplet electronic surface, direct dynamics simulations are reported herein. The simulations are performed at the U-HSE06/aug-cc-pVDZ level of theory. The results are reported for the relative collision energy of 7.8 kcal/mol. For the vibrational and rotational excitations, following temperature regimes have been considered: 200 and 10 K, respectively. Simulations reveal that the reaction can lead to two product channels in the considered energy regime: (1) (OH)-O-2 + (CH3NHCH2)-C-2 and (2) (OH)-O-2 + (CH3NCH3)-C-2. The computed reaction cross section for pathways 1 and 2 are as follows: 17.89 +/- 0.20 angstrom(2) and 3.28 +/- 0.03 angstrom(2), whereas the computed microcanonical reaction rate constants for pathways 1 and 2 are as follows: (4.21 +/- 0.05)*10(-10) and (7.72 +/- 0.07)*10(-11) cm(3)/(molecule sec). Both pathways follow direct and indirect H abstraction processes. Among the direct pathways, stripping and rebound mechanisms have been observed, whereas the indirect pathway involves formation of a post-reaction complex having life-time similar to 0.4-0.5 ps. The velocity scattering angle distribution for the reaction is dominated by scattering in the sideways (60-120 degrees) and backward (120-180 degrees) directions with some contribution from the scattering in the forward direction (0-60 degrees).