Quantum-chemical studies of fluoroethanes: Vibrational assignments, isolated CH stretching frequencies, valence force constants, and bond length relationships

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 nu CH region with the aid of earlier correlations between CH bond length an d six observed isolated CH stretching frequencies nu(is)CH. Eleven more nu( is)CH values are obtained from the spectra through the determination of sca le factors. Estimation of the remaining three undetermined nu(is)CH values is discussed. Problems of assignment remain both in the nu CH region and al so between 1200 and 1000 cm(-1), which call for further study. Improved pre dictions are made of unobserved frequencies in the less abundant conformers of the 1,2-, 1,1,2-, and 1,1,2,2- fluoroethanes. nu CH scale factors vary significantly according to the number of fluorine atoms attached to the sam e carbon and also to a lesser degree among those of the same type. Scale fa ctors for other types of motion also vary both between molecules and also w ithin a given molecule. Changes in the factor for torsional motion are part icularly large. For the CH, CF, and CC bonds, unsealed valence force consta nts are given, and their relation to nu(is)CH, bond lengths, and bond lengt h "offset" values is discussed. Evidence for variations in the latter is re viewed. An unsealed nu(is)CH value reflects very closely the corresponding CH stretching force constant in any of the groups methyl, methylene, and me thine.