Doppler-limited high-resolution spectrum and VPT2 assisted assignment of the C-H stretch of CHBr.

The Doppler limited non-saturated rotationally resolved infrared spectra of the symmetric and asymmetric CH-stretch bands of CHBr have been measured. A continuous wave cavity ringdown setup with a widely tunable Mid-IR-OPO laser light source yielded a single-shot minimum absorption of 4.9×10cm. In contrast to the heavily congested ν band, the ν band showed partially resolved rotational features that may serve as suitable absorption targets in future environmental detection schemes for CHBr. A straightforward, VPT2 (second-order vibrational perturbation theory) assisted quantum-chemical approach for assigning the rotational structure has been tested using different model chemistries. The molecular structures, anharmonic frequencies and the structural changes upon vibrational excitation of CHBr have been investigated. The predicted changes of the anharmonic rotational constants have been used together with available spectroscopic ground state constants to simulate the rovibrational structures of the ν and ν bands of CHBr. A refined analysis of the ν band is presented yielding accurate values for the band origin and the rotational constants. A fit of the line positions of 312 prominent transitions of the three isotopologues revealed a low standard error of 0.00056cm, hence within the absolute 0.0009cm wavelength accuracy of the used spectrometer setup. A combined analysis of the predicted line strengths and positions of the strong Q sub-branches of the ν band has been performed to test the ability of the different density functionals for VPT2 prediction of anharmonic molecular constants. The M06/6-311++G(d,p) model chemistry turned out to yield reliable state-dependent rotational constants that are accurate enough to reproduce the overall rotational structure even without fitting.