Linking structure and vibrational mode coupling using high-resolution infrared spectroscopy: A comparison of gauche and trans 1-chloro-2-fluoroethane

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Abstract
The high‐resolution infrared spectrum of 1‐chloro‐2‐fluoroethane in a molecular beam was collected over the 2975–2994 cm−1 spectral region. The spectral region of 2975–2981 cm−1 contains a symmetric C–H stretching vibrational band of the gauche conformer containing the 35Cl isotope. The spectral region of 2985–2994 cm−1 contains three vibrational bands of the trans conformer. Two of the three bands are assigned as an antisymmetric C–H stretch of each of the two different chlorine isotopes. The third band is assigned as a symmetric C–H stretch of the 35Cl isotope. The gauche conformer of 1‐chloro‐2‐fluoroethane showed doublet patterns similar to those previously observed in 1,2‐difluoroethane. The model for 1,2‐difluoroethane is further refined in the present work. These refinements suggest that the coupling dark state in 1,2‐difluoroethane is composed of 1 quantum C–H bend, 1 quantum C–C stretch, and 12 quanta of torsion. For 1‐chloro‐2‐fluoroethane the dark state could not be identified due to a small data set. The trans conformer of 1‐chloro‐2‐fluoroethane showed no evidence of mode coupling in the three vibrational bands. Including 2‐fluoroethanol in this series of molecules, the extent of vibrational mode coupling did not correlate with the density of states available for coupling. Therefore, density of states alone is insufficient to explain the observed trend. A correlation was observed between the degree of intramolecular interaction and vibrational mode coupling.