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 ν is CH. Eleven more ν is CH values are obtained from the spectra through the determination of scale factors. Estimation of the remaining three undetermined ν is CH 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 ν is CH, bond lengths, and bond length "offset" values is discussed. Evidence for variations in the latter is reviewed. An unscaled ν is CH value reflects very closely the corresponding CH stretching force constant in any of the groups methyl, methylene, and methine.

The vibrational spectrum of 1,4-dioxane in aqueous solution – theory and experiment

“…This is due to the fact that those bonds differ in length by 0.008 Å (Table 2), which results in a difference in force constant values high enough to prevent local vibrations mixing. There is a well-known correlation [25][26][27] between isolated CH vibrational frequencies and bond lengths, giving a rule of thumb that a 0.001 Å contraction in the CH bond length increases the isolated CH frequency by 10 cm À1 . The value we obtain is slightly larger, 13.5 cm À1 /(À0.001 Å).…”

High resolution FTIR spectroscopy of pentafluoroethane: perturbations in the Coriolis doublet of ν4 and ν13

“…Baker and Pulay [10] questioned this assignment, because their scaled quantum mechanical force field based on B3-PW91 calculations predicted a value of 1433 cm À1 . McKean [11] also had difficulties fitting both m 2 (1392 cm À1 ) and m 3 (1310 cm À1 ) using a uniform set of force field coordinates. A number of combination bands appear in this region making confident assignment of the m 2 band centre difficult.…”

“…The recent microwave study by Blanco et al [8] included a revised set of rotational and centrifugal distortion constants for the ground state, together with rotational analyses of the first five excited states of the CF 3 torsion vibration (m 18 of the CF 2 rocking vibration (m 17 ). Several ab initio calculations for R125 have been performed [8][9][10][11][12], with vibrational frequencies, rotational constants and structural parameters reported.…”

Infrared Spectra and Quantum-Chemical Studies of Disilylmethanes

“…The failure of a simple approach to scaling may be seen in different ways. The use of a single factor for the stretching constant of a particular type of CH or SiH bond rarely succeeds in reproducing accurately the small but precise differences in ν is CH or ν is SiH values found for these bonds, either in the same molecule or in related ones. − Differing scale factors are then needed. More dramatic instances are the markedly different scale factors needed for the symmetric and antisymmetric SiN stretching force constants in N(SiH 3 ) 3 19 and MeN(SiH 3 ) 2 …”