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The reaction of OH radicals with HBr plays a key role in atmospheric chemistry as the reaction, OH + HBr → Br + H2O, produces Br atoms that destroy ozone. The experimental measurements of the kinetic isotope effect of k(OH + HBr)/k(OH + DBr) found that the kinetic isotope effects are temperature-independent. However, previous quasi-classical trajectory calculations on an accurate ab initio potential energy surface showed that the kinetic isotope effect is temperature-dependent. By contrast, the present full-dimensional time-dependent quantum dynamics calculations on the same potential energy surface find that the kinetic isotope effect is temperature-independent, agreeing well with the experimental studies both qualitatively and quantitatively. Furthermore, the rate constants from both quantum dynamics and quasi-classical trajectory calculations have a peak at around 15 K whereas the experimental data are not available in this low temperature range. The good agreement of the temperature-dependence of kinetic isotope effects between the present quantum dynamics calculations and the experimental measurements indicates that the kinetic isotope effect of k(OH + HBr)/k(OH + DBr) should be temperature-independent and the peak of the rate constants from the theoretical calculations call for experimental measurements at a very low temperature range. [ABSTRACT FROM AUTHOR]

Relative state-to-state cross sections of OH molecules in the 2Π3/2, v=0, J=3/2, MJ=3/2, f state have been determined for transitions up to 2Π3/2, v=0, J=11/2, f and 2Π1/2, v=0, J=7/2, e states by collisions with HBr molecules (1Σ, v=0, J<4) at 750 cm-1 collision energy. In order to investigate features of the anisotropy of the OH–HBr potential energy surface, the steric asymmetries, which account for the effect of the OH orientation with respect to the collision partner, have been measured. A comparison with other systems previously studied shows strong similarities with the OH–HCl system. [ABSTRACT FROM AUTHOR]

A step-scan Fourier-transform spectrometer coupled with a 6.4 m multipass absorption cell was employed to detect time-resolved infrared absorption spectra of the reaction intermediate CH3SO2 radical, produced upon irradiation of a flowing gaseous mixture of CH3I and SO2 in CO2 at 248 nm. Two transient bands with origins at 1280 and 1076 cm-1 were observed and are assigned to the SO2-antisymmetric and SO2-symmetric stretching modes of CH3SO2, respectively. Calculations with density-functional theory (B3LYP/aug-cc-pVTZ and B3P86/aug-cc-pVTZ) predicted the geometry, vibrational, and rotational parameters of CH3SO2 and CH3OSO. Based on predicted rotational parameters, the simulated absorption band of the SO2-antisymmetric stretching mode that is dominated by the b-type rotational structure agrees satisfactorily with experimental results. In addition, a band near 1159 cm-1 observed at a later period is tentatively attributed to CH3SO2I. The reaction kinetics of CH3+SO2→CH3SO2 and CH3SO2+I→CH3SO2I based on the rise and decay of absorption bands of CH3SO2 and CH3SO2I agree satisfactorily with previous reports. [ABSTRACT FROM AUTHOR]

The reaction of OH with ClO has been investigated by ab initio molecular orbital and variational transition state theory calculations. Both singlet and triplet potential energy surfaces predicted by the G2M method are presented. The reaction was shown to take place primarily over the singlet surface by two main channels producing HO[sub 2]+Cl and HCl+O[sub 2]([sup 1]Δ), with the former being dominant. The predicted total rate constant, k[sub t]=5.27×10[sup -9] T[sup 1.03] exp (-40/T) cm[sup 3] molecule[sup -1] s[sup -1], and product branching ratios in the temperature range 200–500 K at P<200 atm agree satisfactorily with experimental values. The computed branching ratios, k[sub 2]/(k[sub 1]+k[sub 2])=0.073 for HCl+[sup 1]O[sub 2] and 0.045–0.048 for DCl+[sup 1]O[sub 2] in the temperature range 200–500 K based on the recent experimental heat of formation for HO[sub 2] (4.0±0.8 kcal/mol) compare closely with the experimental values, 0.07±0.03 and 0.05±0.02, respectively. At higher temperatures (1000–2500 K), the branching ratios increase slightly to 0.084–0.137 and 0.061–0.111 for the OH and OD reactions, respectively. The rate constant for HO[sub 2]+Cl and HCl+O[sub 2] production from OH+ClO in the temperature range, 500-2500 K, can be given by k[sub 1]=3.4×10[sup -13] T[sup 0.3] exp (725/T) and k[sub 2]=5.85×10[sup -19] T[sup 1.67] exp (1926/T) cm[sup 3] molecule[sup -1] s[sup -1], respectively. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]