Quasi-classical trajectory calculations for the reaction of Mg(3s3p 1 P 1 ) with H 2 are performed on two potential energy surfaces (PES), the excited state 1 A′ (or 1 B 2 in the C 2V symmetry) in the entrance channel and the ground state 1 A′ (or 1 A 1 ) in the exit channel. A many-body expansion procedure is adopted for the construction of the analytical fit functions from the ab initio results. The title reaction involves a nonadiabatic transition between the two potential surfaces. For simplicity, the transition probability is assumed to be unity when the trajectory goes through the region of surface crossing and changes to the lower surface. The calculated total collisional deactivation and reaction cross sections decrease with the increase of translational collision energy. The calculated rotational product distributions are characterized by a bimodal feature both for the MgH V ) 0 and 1 states. The trend of bimodality is consistent with the observation reported in experimental studies. Our inspection of individual trajectories reveals that the low-rotational and high-rotational populations are caused by two distinct reaction pathways. This observation supports our previous expectation for the microscopic branching via the PES anisotropy. The angular product distribution indicates that the reaction proceeds predominantly via a linear collision complex. An increase of the collision energy from 2.026 to 8.104 kcal/ mol has resulted in a shift of the distribution toward forward direction. The vibrational product distribution tends to decrease with the quantum numbers. The ratio of MgH(V ) 1) to MgH(V ) 0) yields a value of ∼0.3, which is nevertheless underestimated as compared with the observation of 0.7 ( 0.2. The reasons for the discrepancy are also discussed.
Two ab initio methods have been employed to calculate the dynamical potential energy surfaces (PES’s) for the excited (B21 or A'1) and the ground (A11 or A'1) states in the Mg(3s3p1P1)–H2 reaction. The obtained PES’s information reveals that the production of MgH in the Σ+2 state, as Mg(1P1) approaches H2 in a bent configuration, involves a nonadiabatic transition. The MgH2 intermediate around the surface crossing then elicits two distinct reaction pathways. In the first one, the bent intermediate, affected by a strong anisotropy of the interaction potential, decomposes via a linear HMgH geometry. The resulting MgH is anticipated to populate in the quantum states of rotational and vibrational excitation. In contrast, the second pathway produces MgH in the low rotational and vibrational states, as a result of the intermediate decomposition along the stretching coordinate of the Mg–H elongation. These two tracks may account for the previous experimental findings for the MgH distribution, which the impulsive model has failed to comprehend. By far, different interpretations have been proposed especially for the low-N MgH product. The supply of a detailed PES’s information in this work helps to clarify the ambiguity. It is also conducive to an interpretation of the isotope and temperature effects on the product rotational distribution.
Articles you may be interested inAn ab initio analytical potential energy surface for the O(3 P)+CS(X 1Σ+)→CO(X 1Σ+)+S(3 P) reaction useful for kinetic and dynamical studies Ab initio transition state theory calculations of the reaction rate for OH+CH4→H2O+CH3Using a pump-probe method, we have obtained the nascent bimodal rotational distribution of MgH ͑vЉϭ0 and 1͒ products formed in the reaction of Mg(3s3 p 1 P 1 ) with CH 4 . The low-N component of the distribution in the vЉϭ0 state is much larger than that in the vЉϭ1 state, whereas the high-N component in the vЉϭ0 state is roughly equivalent to that in the vЉϭ1 state. The MgH ͑vЉϭ0͒ 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-CH 3 , 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.
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