The structure of O_3-CH_4 and the implications for O+CH_4 precursor-initiated reaction

摘要

The rotational spectrum of the 03.-CH4 complex has been measured in a molecular beam using a i-szzle Fourier-transform microwave spectrometer. An a-type pure-rotation and a c-type ~mi-inversion electric-dipole spectrum is observed, complicated by the nearly free internal m of the CH4 top and the inversion tunneling of the 03. The nuclear-spin statistics of the ~~alent oxygen nuclei leads to only one tunneling component existing. for each rotation—liii ~rnal-rotation state, indicating that the transition state has a heavy-atom, C2~-symmetry geometry. The tunneling splitting is determined to be 30 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. The zero-point structure of the complex has a heavy-atom plane of symmetry with the two terminal 0 atoms equidistant abovc and below this plane. The angle between the line joining the center of masses of the two subunits and the 03. C2 axis is 118.2(5)0. with the central 0 directed away from the CH4. The shortest 0—C separation is 3.57 A. The geometry of the complex suggests two outcomes for the reaction of an 0 atom produced by 267 nm photolysis of 03 in the complex (assuming that· the initial 03 photodissociation dynamics are not perturbed by complexation), either nonreaction or reaction by stripping of a hydrogen atom at high impact parameters, leading to fast, highly rotationally excited. OH product.