High-resolution infrared spectroscopy of supersonically cooled singlet carbenes: Bromomethylene (HCBr) in the CH stretch region

First high-resolution spectra of cold (& SIM;35 K) singlet bromomethylene HCBr in the CH stretching (v(1)) region from 2770 to 2850 cm(-1) are reported using near quantum shot-noise limited laser absorption methods in a slit jet supersonic discharge expansion source. Three rovibrational bands are identified at high S/N (20:1-40:1) and rotationally assigned to (i) the CH stretch fundamental (v(1)) band X ~ 1,0,0 & LARR;X ~ 0,0,0 and (ii) vibrational hot bands [X ~ (1,1,0)& LARR;X ~ (0,1,0) and X ~ (1,0,1)& LARR;X ~ (0,0,1)] arising from vibrationally excited HCBr populated in the discharge with single quanta in either the H-C-Br bend (v(2)) or C-Br stretch (v(3)) modes. Precision rotational constants are reported for a total of six states, with an experimentally determined CH stretch vibrational frequency (2799.38 cm(-1)) in good agreement with previous low-resolution fluorescence studies [M. Deselnicu et al., J. Chem. Phys. 124(13), 134302 (2006)]. Detailed analysis of the fundamental v(1) band highlights the presence of perturbations in the X ~ 1,0,0 level, which we tentatively attribute to arise from the nearby triplet state a ~ (0,0,1) through spin-orbit interaction or the multiple quanta X ~ 0,2,1 singlet state via c-type Coriolis coupling. Reduced-Doppler resolution (60 MHz) in the slit-jet IR spectrometer permits for clear observation of a nuclear spin hyperfine structure, with experimental line shapes well reproduced by nuclear quadrupole/spin-rotation coupling constants from microwave studies [C. Duan et al., J. Mol. Spectrosc. 220(1), 113-121 (2003)]. Finally, the a-type to b-type transition intensity ratio for the fundamental CH stretch band is notably larger than that predicted by using a bond-dipole model, which from high level ab initio quantum calculations [CCSD(T)/PVQZ] can be attributed to vibrationally induced charge-sloshing of electron density along the polar C-Br bond.

Automated summary

The first high-resolution spectra of cold singlet bromomethylene HCBr in the CH stretching region were obtained using laser absorption methods. Several bands were identified and rotational constants were determined. The study also revealed the presence of perturbations in the fundamental band and observed nuclear spin hyperfine structures. Additionally, the intensity ratio of the a-type to b-type transitions in the CH stretch band was found to be larger than predicted by theory.