Electronic spectroscopy of the ÃA″1↔X̃A′1 system of CDBr

Abstract

We report fluorescence excitation and single vibronic level emission spectra of jet-cooled CDBr in the 450–750nm region. A total of 32 cold bands involving the pure bending levels 20n with n=3–10 and combination bands 20n301 (n=2–10), 20n302 (n=2–9), 10120n (n=7–10), and 10120n301 (n=6,8–9) in the ÃA″1←X̃A′1 system of this carbene were observed; most of these are reported and/or rotationally analyzed here for the first time. Rotational analysis yielded band origins and effective (B¯) rotational constants for both bromine isotopomers (CD79Br and CD81Br). The derived ÃA″1 vibrational intervals are combined with results of Yu et al. [J. Chem. Phys. 115, 5433 (2001)] to derive barriers to linearity for the 2n, 2n31, and 2n32 progressions. The ÃA″1 state C–D stretching frequency (2350cm−1) is determined for the first time, in excellent agreement with theory, as are the Br79–Br81 isotope splittings in the excited state. Our emission spectra probe the vibrational structure of the X̃A′1 and ãA″3 states up to ∼9000cm−1 above the vibrationless level of the X̃A′1 state; the total number of levels observed is around twice that previously reported. Unlike CHBr, where even the lowest bending levels are perturbed by spin-orbit interaction with the triplet origin, the term energy of every level save one below 3000cm−1 in CDBr is reproduced by a Dunham expansion to within a standard deviation of 1cm−1, and a spin-orbit coupling matrix element of ∼330cm−1 is derived from a deperturbation analysis of the triplet origin. The multireference configuration interaction (MRCI) calculations of Yu et al. [J. Chem. Phys. 115, 5433 (2001)] well reproduce triplet perturbations in the pure bending manifold, and globally, the vibrational frequencies of X̃A′1, ãA″3, and ÃA″1 are in excellent agreement with theoretical predictions.