Infrared and Raman spectra, conformational stability, ab initio calculations of structure and vibrational assignment of butyronitrile

Abstract

The infrared spectra (3500–50 cm⁻¹) of the gas and solid and the Raman spectra (3500–50 cm⁻¹) of the liquid and solid were recorded for butyronitrile (propyl cyanide), CH₃CH₂CH₂CN. Both the anti (methyl group trans to the cyanide group) and gauche conformers were identified in the fluid phases. Variable temperature studies of the infrared spectrum (3500–400 cm⁻¹) of the sample dissolved in liquid xenon (−60 to − 100°C) were recorded. Utilizing nine sets of conformer pairs, the enthalpy difference was determined as 40 ± 3 cm⁻¹ (0.48 ± 0.04 kJ mol⁻¹) with the gauche conformer the more stable form. At ambient temperature there is approximately 30% of the anti conformer present in the vapor state. Either the anti or gauche conformers could be obtained in the crystalline state depending on whether the solid was obtained from the liquid or from the gas sprayed on a cold substrate. A complete vibrational assignment is proposed for both conformers based on infrared contours, relative intensities, depolarization values and group frequencies which are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6–31G(d) calculations. Complete equilibrium geometries were determined for both rotamers by ab initio calculations employing the 6–31G(d), 6–311 + G(d,p) and 6–311 + G(2df,2pd) basis sets at the level of Møller–Plesset with full electron correlation by the perturbation method to the second order (MP2). From all of these calculations the gauche conformer is predicted to be the more stable form. The r₀ adjusted structural parameters were obtained from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.