Quantum rate coefficients and kinetic isotope effect for the reaction Cl + CH4 → HCl + CH3 from ring polymer molecular dynamics

Thermal rate coefficients and kinetic isotope effect have been calculated for prototypical heavy-light-heavy polyatomic bimolecular reactions Cl + CH4/CD4 → HCl/DCl + CH3/CD3, using a recently proposed quantum dynamics approach: ring polymer molecular dynamics (RPMD). Agreement with experimental rate coefficients, which are quite scattered, is satisfactory. However, differences up to 50% have been found between the RPMD results and those obtained from the harmonic variational transition-state theory on one of the two full-dimensional potential energy surfaces used in the calculations. Possible reasons for such discrepancy are discussed. The present work is an important step in a series of benchmark studies aimed at assessing accuracy for RPMD for chemical reaction rates, which demonstrates that this novel method is a quite reliable alternative to previously developed techniques based on transition-state theory.

Relative Tropospheric Photolysis Rates of HCHO, H13CHO, HCH18O, and DCDO Measured at the European Photoreactor Facility

The relative photolysis rates of HCHO, H13CHO, HCH18O, and DCDO were studied in pseudo-natural tropospheric conditions in July 2003 at the European Photoreactor Facility (EUPHORE) in Valencia, Spain. The photolytic decay of HCHO, H13CHO, and HCH18O is measured relative to DCDO by long path FT-IR detection during the course of about 3 h of sunlight. The relative photolysis rates obtained are as follows: JH13CHO/JHCHO = 0.894 +/- 0.006, JHCH18O/JHCHO = 0.911 +/- 0.011, and JDCDO/JHCHO = 0.597 +/- 0.001. The errors represent 1sigma and do not include possible systematic errors. The atmospheric implications of the large isotope effects in the photolysis of formaldehyde are discussed.

Temperature dependence of carbon-13 kinetic isotope effects of importance to global climate change

We report here a theoretical study of the 13C kinetic isotope effect (KIE) and its temperature dependence for the reaction OH + CH4 --> H2O + CH3, the major sink of atmospheric methane in the troposphere. The KIE values at various atmospherically significant temperatures were determined by direct dynamics using variational transition state theory with multidimensional tunneling contributions (VTST/MT). The potential energy surfaces (PESs) were generated by hybrid density functional theory as well as by recently developed doubly hybrid density functional theory methods. Comparisons of our calculated KIEs with experimental data and theoretical values in the literature reveal the critical contributions due to multidimensional tunneling and torsion anharmonicity as well as the critical issue of the choice of internal rotational axis.