A b i n i t i o study of electronic energy transfer in the quenching of N2(A 3Σ+u) and N2(B 3Πg) by H2

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Abstract
The interactions of N2(A 3Σ+u) and N2(B 3Πg) with H2(X 1Σ+g) have been characterized through potential energy calculations, which were performed using a combination of MCSCF and multiple-reference SDCI methods. In C2v geometry with the N2 and H2 bond axes parallel, at the highest level of theory used, energy transfer from N2(A 3Σ+u) to H2(b 3Σ+u) and dissociation of the latter into H atoms is found to proceed through an adiabatic reaction path with a barrier of 0.513 eV. In C2v geometry with perpendicular orientation of N2 and H2, the two lowest 3B2 surfaces are shown to exhibit a strongly avoided crossing; the lower surface shows a favorable pathway for energy transfer from N2(B 3Πg) to H2(b 3Σ+u). In each case, energy transfer occurs via a two-electron exchange mechanism as a result of mixing between orbitals with the proper energy and symmetry. Consistent with the isoconfigurational electronic structure of N2(B) and CO(a 3Π), the results for N2(B) are similar to those which we found previously for quenching of CO(a) by H2. The overall results are shown to be consistent with available experimental kinetics data, which show quenching of N2(A) by H2 to be inefficient. Application of these results to electronic quenching and vibrational relaxation of N2(A) by other small molecules is discussed.