We have used the quasiclassical trajectory method and a diatomics-in-molecules potential energy surface to study the state-to-state dynamics of the H2+ + H2 → H3+ + H reaction, with an emphasis on the product H3+ vibration/rotation distributions. The product vibration/rotation states were determined by a recently developed Fourier transform method for calculating the H3+ good action variables. For H2+ (v1 = 0, j1 = 2) + H2 (v2 = 0, j2 = 1), we find that over 70% of the available energy goes into H3+ internal excitation. This result is only weakly dependent on reagent translational energy, but the fraction increases to over 80% when the reagent H2+ is excited to v1 = 3. Approximately 20% of the available energy goes into product rotation at low translational energies, and larger amounts at higher energies. The distribution of H3+ symmetric stretch and degenerate mode (bend plus asymmetric stretch) quantum states is found to be highly nonstatistical, with an average stretch quantum number Ns of about 0.6 and an average degenerate quantum number Nd of 2.4 for ground-state reagents. The values of Ns and Nd for the most probable final states are found to be strongly correlated, with Nd increasing nonlinearly with increasing Ns. The distribution of vibrational angular momentum l is found to show a strong propensity for l = 0, and we correlate this with the most favored orientation of the reagents along the pathway leading to reaction. The product rotational distributions were found to peak near J = 12 at low translational and vibrational energy, with K = 0 being the most likely projection state. This departure from the usual K = J propensity arises because strong vibration/rotation coupling in H3+ causes the body-fixed Z projection to be oscillatory in time. Product angular distributions were found to be mainly controlled by a stripping reaction mechanism, but product translational distributions reflect the partitioning of the reaction exoergicity into product degrees of freedom. Where comparison is possible, the product translational, angular, and vibrational distributions agree with experiment. The total reaction cross section and its division into proton-transfer and atom-transfer contributions is also considered and compared with experiment by using a surface hopping model in which the dynamics is assumed to be diabatic up to a certain intermolecular separation and then adiabatic thereafter. We find that our total reactive cross sections are in reasonable agreement with experiment as a function of both reagent translational and vibrational energy. These cross sections are found to arise from roughly equal probabilities of proton and atom transfer.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry