Raman and infrared spectra, conformational analysis, ab initio calculations and vibrational assignment of 3-bromopropionitrile

The Raman (3200—10 cm−1) and infrared (3100—400 cm−1) spectra were recorded for liquid and solid 3‐bromopropionitrile, BrCH2CH2CN. These data are interpreted on the basis that the molecule exists as a mixture of trans (Br atom oriented trans to the CN group) and gauche conformers in the liquid phase with the gauche form the more stable rotamer and the only conformer in the solid. From variable‐temperature Raman studies of the liquid, the enthalpy difference between conformers was determined to be 434 ± 19 cm−1 [(1.24 ± 0.05 kcal mol−1) (1 kcal = 4.184 kJ)]. A complete vibrational assignment is proposed for both conformers. Ab initio calculations were performed employing the RHF/LANL1DZ basis set to obtain the structural parameters, barriers to internal rotation, the fundamental wavenumbers for both conformers and the infrared and Raman intensities. The calculated values are compared with the corresponding experimental quantities. These results are discussed and compared with similar quantities obtained for some related molecules.