title: Doppler-limited high-resolution spectrum and VPT2 assisted assignment of the C-H stretch of CH2Br2.
abstract: The Doppler limited non-saturated rotationally resolved infrared spectra of the symmetric and asymmetric CH-stretch bands of CH2Br2have 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 4.9×10-8cm-1. In contrast to the heavily congested ν1band, the ν6band showed partially resolved rotational features that may serve as suitable absorption targets in future environmental detection schemes for CH2Br2. 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 CH2Br2have 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 ν1and ν6bands of CH2Br2. A refined analysis of the ν6band 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-1, hence within the absolute 0.0009cm-1wavelength accuracy of the used spectrometer setup. A combined analysis of the predicted line strengths and positions of the strong Q sub-branches of the ν6band 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.
authors.affiliation: Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str. 1, Kiel D-24118, Germany.
authors.forename: Ibrahim
authors.initials: I
authors.lastname: Sadiek
authors.affiliation: Institut für Physikalische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str. 1, Kiel D-24118, Germany; KMS Kiel Marine Science - Centre for Interdisciplinary Marine Sciences, Olshausenstr. 40, Kiel D-24098, Germany. Electronic address: friedrichs@phc.uni-kiel.de.
authors.forename: Gernot
authors.initials: G
authors.lastname: Friedrichs
keywords: Dibromomethane; High resolution infrared spectrum; Spectral assignment; VPT2