Experimental and theoretical studies of the vibrational spectra of CHD2Br

The dideuterated form of methyl bromide, CHD₂Br, has been synthesized and the gas-phase infrared spectra investigated in the range 400–10,000?cm^–1 using a medium-resolution FTIR spectrometer. The nine fundamental bands have been characterized in detail. Six of them, i.e. ν ₁, ν ₄, ν ₅, ν ₇, ν ₈ and ν ₉, have been rotationally analysed through the assignment of the partially resolved structure of the ^PQ_K and ^RQ_K cluster of lines and the spectroscopic parameters have been derived in the symmetric top limit approximation. Among the fundamental levels, anharmonic resonance occurs between ν ₇/ν ₄?+?ν ₈ and ν ₈/ν ₆?+?ν ₉. An isotopic ^79/81Br shift was found for ν ₆ and in the more complex region of the ν ₈ fundamental. High-quality ab initio calculations – carried out at coupled cluster level [CCSD(T)] employing the correlation-consistent basis set of Dunning (cc-pVTZ) – were performed to determine quadratic, cubic and quartic (semidiagonal) force constants. Using these constants and applying second-order vibrational perturbation theory (VPT2), with allowance for resonances (when necessary), permitted us to identify and assign, in addition to the fundamentals, about 70 overtones and combination bands up to three quanta.