Reaction dynamics of Mg(3s3p 1P1) with CH4: Elucidation of reaction pathways for the MgH product by the measurement of temperature dependence and the calculation of ab initio potential energy surfaces

Abstract

Using a pump–probe method, we have obtained the nascent bimodal rotational distribution of MgH (v″=0 and 1) products formed in the reaction of Mg(3s3p 1P1) with CH4. The low-N component of the distribution in the v″=0 state is much larger than that in the v″=1 state, whereas the high-N component in the v″=0 state is roughly equivalent to that in the v″=1 state. The MgH (v″=0) rotational distributions at three temperatures, 770, 830, and 880 K, were measured. The bimodal distribution does not change with temperature within a small experimental error. The findings suggest that the bimodal nature results from the same process, supporting a mechanism of Mg insertion into the C–H bond, irrespective of the geometry of the entrance approach. The result is consistent with that of Kleiber et al. using the far-wing scattering technique, and is supported by Chaquin et al.’s theoretical calculations. We also calculated two-dimensional potential energy surfaces for the excited and ground states of the reaction system. The calculation suggests that two possible trajectories are responsible for the production of MgH following a nonadiabatic transition. One trajectory, weakly dependent on the bending angle of H–Mg–CH3, is related to formation of the low-N component. The other trajectory evolves through a linear geometry of the intermediate complex prior to dissociation, causing a strong anisotropy in the PES. This second trajectory corresponds to the population of rotationally and vibrationally hot states. An alternative explanation of the low-N distribution is also discussed.