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Dependence of Isotropic Hyperfine Coupling in the Fluoromethyl Radical Series on Inversion Angle

Cramer, C. J.
J. Org. Chem. 1991, 56, 5229.

A rapid method for calculating isotropic hyperfine coupling constants (hfs) from MP2-derived spin densities is demonstrated for the particular case of the series of fluoromethyl radicals (CFnH3-n, n = 0-3). The MP2 corrections to the UHF spin densities provide significant improvement when compared to experiment. At their equilibrium geometries, agreement with experiment for CHF2 and CF3 is excellent; consideration of out-of-plane vibrational motion is required for CH3 and CH2F. Starting from planar geometries, hfs values for all atoms increase with pyramidalization. However, this effect is opposed by a tendency for increasing pyramidality to reduce either the rate of increase or even the magnitude of carbon and fluorine coupling as fluorine substitution progresses. Delocalization of positive, unpaired spin density occurs preferentially into hydrogen AO's as opposed to fluorine AO's in CH2F and CHF2.