The vacuum UV photoabsorption spectrum of methyl chloride (CH3Cl) and its perdeuterated isotopomer CD3Cl: II. A vibrational analysis Original Research Article

The fine structure of the vacuum UV photoabsorption spectrum of CH3Cl and CD3Cl has been analyzed in the 7.5–10.5 eV photon energy range. A large number of lines have been observed, classified and assigned to the vibrational excitation accompanying a series of Rydberg transitions. The vibronic transitions involve both Jahn–Teller distortion and spin–orbit splitting. The former effect has been evaluated by ab initio calculations showing that the 2E state (in the C3v symmetry group) splits into 2A′ and 2A′′ states in the CS symmetry group. The 2A′ state is energetically the lowest component whereas the 2A′′ is found to be a transition state. The Jahn–Teller stabilization energy and the wave numbers associated with all vibrational modes have been calculated. Experimentally, the entire fine structure could be described by using three vibrational modes, i.e. hcω5=104±7 meV, hcω6=77±7 meV and hcω=162±3 meV respectively, as resulting from an average over all the analyzed Rydberg states. In CD3Cl the corresponding energies are hcω5=81±4 meV, hcω6=66±5 meV and hcω=124±4 meV. These values are in good agreement with those predicted by the present ab initio calculations for the the ion ground state. However, the agreement is not so good for the wave number ω which could be assigned to the ν3 or to the ν4 vibrational modes. Though some arguments favour ν4 (the CH3 umbrella mode), within the error limits on the present measurements it is formally not possible to ascribe this wave number to one of these two vibrations.