Quantum-Chemical Studies of Fluoroethanes: Vibrational Assignments, Isolated CH Stretching Frequencies, Valence Force Constants, and Bond Length Relationships

Abstract

Harmonic force fields for the series of fluoroethanes, derived from HF and B3LYP calculations with the 6-311G** basis set, are scaled individually for each molecule and for each type of CH bond. Reassignments are made in the νCH region with the aid of earlier correlations between CH bond length and six observed isolated CH stretching frequencies νisCH. Eleven more νisCH values are obtained from the spectra through the determination of scale factors. Estimation of the remaining three undetermined νisCH values is discussed. Problems of assignment remain both in the νCH region and also between 1200 and 1000 cm-1, which call for further study. Improved predictions are made of unobserved frequencies in the less abundant conformers of the 1,2-, 1,1,2-, and 1,1,2,2- fluoroethanes. νCH scale factors vary significantly according to the number of fluorine atoms attached to the same carbon and also to a lesser degree among those of the same type. Scale factors for other types of motion also vary both between molecules and also within a given molecule. Changes in the factor for torsional motion are particularly large. For the CH, CF, and CC bonds, unscaled valence force constants are given, and their relation to νisCH, bond lengths, and bond length “offset” values is discussed. Evidence for variations in the latter is reviewed. An unscaled νisCH value reflects very closely the corresponding CH stretching force constant in any of the groups methyl, methylene, and methine.