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

摘要

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.