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 and six observed isolated CH stretching frequencies nu(is)CH. Eleven more nu(is)CH values are obtained from the spectra through the determination of scale factors. Estimation of the remaining three undetermined nu(is)CH values is discussed. Problems of assignment remain both in the nu CH 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. nu CH 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, unsealed valence force constants are given, and their relation to nu(is)CH, bond lengths, and bond length "offset" values is discussed. Evidence for variations in the latter is reviewed. An unsealed nu(is)CH value reflects very closely the corresponding CH stretching force constant in any of the groups methyl, methylene, and methine.