Probing OH stretching overtones of CH3OOH through action spectroscopy: Influence of dipole moment dependence on HOOC torsion.

The OH stretching overtones and OH stretch-HOOC torsion combination bands of methyl hydroperoxide are investigated using action spectroscopy initiated through vibrational state selected photodissociation. Our results for the room temperature spectra covering the 2nu(OH)-5nu(OH) regions suggest that the coarse vibrational structures appearing in the spectra can be understood using a simple two-dimensional vibration-torsion model involving the OH stretch and COOH torsion consistent with what has been previously reported. However, investigation of the jet-cooled spectrum for the 2nu(OH) band along with the results of ab initio calculations using coupled cluster methods reveals that the dependence of the transition dipole moment on the HOOC torsion angle cannot be neglected when simulating intensities of OH stretching overtone bands, as has been suggested by earlier room temperature studies. The present results demonstrate that transitions between torsional levels of different symmetries, which arise from the dependence of the dipole moment mu(r,tau) on the torsional angle, contribute significantly to the intensities of the vibrational overtone bands and are important in interpreting the temperature dependence of the spectral band profiles. Contributions from these transitions are largest for the 2nu(OH) and 3nu(OH) levels and fall off gradually for the higher overtones. In addition, results are presented investigating the orbital interactions in CH(3)OOH that influence changes in the HOOC adiabatic torsion potential with increased OH stretching excitation.

Hydroxyl-radical-initiated oxidation mechanism of bromopropane.

Bromopropane has been considered as a replacement for chlorofluorocarbons used as the active component of industrial cleaning solvents, more specifically for HCFC-141b. The proposed mechanism for the atmospheric oxidation of bromopropane is studied via ab initio methodology. Ab initio molecular orbital methods at the CCSD(T)/6-311++G(2df,2p)//MP2/6-31G(d) level of theory have been used to determine the structure and energetics of the 58 species and transition states involved in the atmospheric oxidation of bromopropane. The calculations show that the major oxidation species is bromoacetone. Other brominated species that result from the oxidation are BrCH 2CH 2C(O)H, BrC(O)CH 2CH 3, and BrC(O)H, potential new bromine reservoir species that result from bromopropane in the atmosphere.

Atmospheric oxidation mechanism of bromoethane.

A mechanism for the atmospheric oxidation of bromoethane is proposed from an ab initio study. Using CCSD(T)/6-311++G(2df,2p)//MP2/6-31G(d) level of theory, the structure and energetics of the 35 species and transition states involved in the atmospheric oxidation of bromoethane are examined. From these calculations, reaction enthalpies and activation energies to characterize the potential energy surface of the proposed mechanism for the complete atmospheric degradation of bromoethane are determined. The studies revealed that the hydrogen abstraction from the alpha carbon has the lowest activation energy barrier of all the possible abstractions, making this pathway the most energetically favored pathway for the atmospheric oxidation process. The brominated species that result from the oxidation at the alpha carbon are BrC(O)CH(3) and BrC(O)H. Other species resulting from oxidation initiated at the beta carbon are also identified.