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

Abstract

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.

Vibrational Energy Transfer

“…331 Spectral resolution substantially better than 150 MHz is needed simply to resolve individual quantum states. When the symmetry-selected state densities become so high, even small coupling matrix elements induce strong mixing between nearby lying states, 269,270,[279][280][281]291,[386][387][388][389][390][391][392][393][394][395][396][397][398] meaning that the quantum number assignment becomes problematic at best, even if the spectrum can be resolved.…”

Rotational spectroscopy of the first excited state of the acetylenic C–H stretch of 3-fluoropropyne performed by infrared-Fourier transform microwave–microwave triple-resonance spectroscopy

“…We believe that these deviations are caused by weak perturbations to the acetylenic C-H spectrum. These are the same perturbations that eventually lead to extensive fragmentation of the vibrational spectrum in larger molecules where the density of possible perturbers is significantly higher [38][39][40][41][42][43][44][45].…”