An Experimental Study of the Dynamics of the Reaction H + CO₂ → OH(v‘, j‘, f) + CO:  Product State-Resolved Differential Cross Sections and Translational Energy Release Distributions

Abstract

Angular distributions and pair-correlated translational energy distributions have been measured for OH(v‘ = 0, N‘ = 1), produced through the reactive scattering of hot H atoms by CO2 at 300 K and mean collision energies of 1.8 eV and 2.5 eV. The new measurements, together with those reported previously for the OH(v‘ = 0, N‘ = 5, A‘/A‘‘) products of the reaction at 2.5 eV, reveal that the differential cross sections depend strongly on collision energy and product quantum state. For the OH(v‘ = 0, N‘ = 1) channel, the angular distribution changes from broad forward scattering to pronounced backward scattering as the collision energy is raised from 1.8 to 2.5 eV. The angular distributions at the higher collision energy shift from being backward to nearly isotropic, with forward−backward peaks, as the OH angular momentum is increased from N‘ = 1 to N‘ = 5. Although somewhat less dramatic changes are evident in the pair-correlated kinetic energy release data, the measurements confirm that the CO internal energy distributions are considerably colder than those predicted by statistical phase space theory. The results are discussed in light of earlier theoretical and experimental studies, particularly the results of femtochemical kinetic measurements.

On the theory of the reaction rate of vibrationally excited CO molecules with OH radicals

“…We recall previous studies where the reaction CO+ OH → CO 2 + H showed a nonstatistical behavior [35][36][37][38][39][40] in both bulk gas-phase and molecular-beam studies of the reverse reaction, H + CO 2 → OH+ CO. It was observed that the vibrational excitation of the CO product of that reaction was below that expected from statistical theory for the HOCO * intermediate.…”

On the theory of the CO+OH reaction, including H and C kinetic isotope effects

“…Because of this large frequency difference between OH stretching and the other modes, the internal energy transfer to OH-stretching motion via internal resonances with the other modes may be less rapid than needed for internal equilibration. ͑A higher-order resonance is possible.͒ Prompted by the experimental results of Brouard et al [63][64][65] and other results, we have explored, with details in Appendix C, the effect of a reduced intramolecular energy transfer between the OH-stretching-mode motion and the other coordinates.…”

“…[4][5][6][7][8][9][10][11][12][13]17 Nevertheless, carbon and oxygen isotope effects have been observed [29][30][31][32][33] and the anomalous effects for these heavy-atom isotopes have not yet been treated in the literature. Again, Simons and co-workers [63][64][65] observed in their molecular-beam study of the reverse reaction, H + CO 2 → OH+ CO, that the vibrational excitation of the CO product was far below that expected from statistical theory for the HOCO * intermediate. While the energies in their experiments are higher than those normally occurring in the OH+ CO reaction, it is interesting to look for other anomalies in that reaction which may be better explained by a nonstatistical modification of the Rice-Ramsperger-KasselMarcus ͑RRKM͒ theory.…”

Communication: A chemically accurate global potential energy surface for the HO + CO → H + CO2 reaction

“…2 Kinetic studies have revealed that its rate constant is essentially flat at low temperatures but increases quickly with T above 500 K. [3][4][5] This behavior has been attributed to a complexforming mechanism, 6 although the lifetime of the HOCO intermediate is not particularly long. 7,8 Molecular beam experiments on the title reaction 9 and its reverse 10 suggested complex reaction dynamics with a significant non-statistical component. In addition, significant tunneling towards H + CO 2 has been observed in photodetachment of the HOCO − anion.…”