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The Structure of O3 -CH4 and the Implications for the O + CH4 Precursor-Initiated Reaction



Angela R. Hight Walker, Gerald T. Fraser, R D. Suenram, Francis J. Lovas


The rotational spectrum of the O3--CH4 complex has been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. An a-type pure-rotation and a c-type rotation-inversion electric-dipole spectrum is observed, complicated by the nearly free internal rotation of the CH4 top and the inversion tunneling of the O3. The nuclear-spin statistics of the equivalent oxygen nuclei leads to only one tunneling component existing for each rotation-internal-rotation state, indicating that the transition state has a heavy-atom, C2v-symmetry geometry. The tunneling splitting is determined to be 30 MHz to 40 MHz, dependent on the CH4 internal-rotor state. Only two of the three methane internal-rotor states have been assigned. These two states of A and F symmetry have asymmetric-rotor energy-level structures, weakly perturbed by the ozone-inversion tunneling. Transitions have been observed for the E internal-rotor state, verified by their linear frequency shifts with electric field, however no definitive rotational assignment of these lines could be made. The E internal-rotor-state energy-level structure is complicated by the unquenched internal-rotation angular momentum of the methane top, which leads to a strong Coriolis interaction between the rotation and internal-rotation angular momenta. The zero-point structure of the complex has a heavy-atom plane of symmetry with the two terminal O atoms equidistant above and below this plane. The angle between the line joining the center of masses of the two subunits and the O3, C2 axis is 118.2(5) , with the planar O directed away form the CH4. The shortest O--C separation is 3.57 . The geometry of the complex suggests two outcomes for the reaction of the O atom produced by 267 nm photolysis of O3 in the complex, either non reaction or reaction by stripping of a hydrogen atom at high impact parameters, leading to fast, highly rotationally excited, OH product.
Journal of Chemical Physics
No. 6


intermolecular forces, methane, microwave spectroscopy, ozone, ozone-methane, rotational spectroscopy, van der Waals complex


Hight, A. , Fraser, G. , Suenram, R. and Lovas, F. (2000), The Structure of O<sub>3</sub> -CH<sub>4</sub> and the Implications for the O + CH<sub>4</sub> Precursor-Initiated Reaction, Journal of Chemical Physics (Accessed September 30, 2023)
Created August 1, 2000, Updated February 17, 2017