High resolution spectroscopy of 1,2difluoroethane in a molecular beam: A case study of vibrational modecoupling J. Chem. Phys. 97, 2971 (1992; 10.1063/1.463038 Microwave, infrared, and Raman spectra, structures, quadrupole coupling constants, barrier to internal rotation, and vibrational assignment of 1chloro2methylpropene Spectra and structure of chiral molecules. III.a) Vibrational assignment and normal coordinate analysis of 1bromo 1chloro1fluoroethane and 1bromo1chloro1fluoroethane?2d 1The high-resolution infrared spectrum of 1-chloro-2-fluoroethane in a molecular beam was collected over the 2975-2994 cm Ϫ1 spectral region. The spectral region of 2975-2981 cm Ϫ1 contains a symmetric C-H stretching vibrational band of the gauche conformer containing the 35 Cl isotope. The spectral region of 2985-2994 cm Ϫ1 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 35 Cl 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.