Far infrared and low frequency gas phase Raman spectra and conformational stability of the 1-halopropanes

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Abstract
The far infrared (375–50 cm−1) and low frequency Raman (400–70 cm−1) spectra of the gaseous 1-halopropanes CH3CH2CH2F, CH3CH2CH2Cl, and CH3CH2CH2Br have been recorded and both the methyl and asymmetric torsional modes have been observed and assigned for both the gauche and trans conformers for all of these molecules. The asymmetric torsions for each molecule have several observed excited states which fall on the low frequency side of the fundamental. The asymmetric torsional potential functions have been calculated and, from these potential functions, the enthalpy differences between the high energy trans and low energy gauche conformers have been determined to be 122±10 cm−1 for the fluoride, 127±10 cm−1 for the chloride, and 37±10 cm−1 for the bromide. The trans and gauche methyl torsions have also been observed and assigned for the three 1-halopropanes. The resulting barriers in cm−1 are: 936±4 (trans), 986±9 (gauche) for 1-fluoropropane; 929±2 (trans), 1080±3 (gauche) for 1-chloropropane; and 841 (trans), 1016±8 (gauche) for 1-bromopropane. A complete vibrational assignment has also been made for the 1-fluoropropane molecule and, from the spectral data for the solid, it appears that there are two or more molecules per primitive cell. Attempts to obtain experimental values for the enthalpy differences in the gas phase were made and these results, as well as the determined potential functions, are discussed in relation to previous studies.