Abstract
A previous model for the calculation of proton chemical shifts in substituted alkanes based upon partial atomic charges and steric interactions has been modified by the replacement of the C—C bond anisotropy term with an orientation-dependent γ effect (i.e. C·C·C·H). The new scheme (CHARGE5) predicts the proton chemical shifts of a variety of acyclic, cyclic and polycyclic hydrocarbons over 188 data points spanning 2 ppm to within <0.1 ppm, an improvement over the previous model with three fewer variable parameters. Systems considered include cyclobutanes, cyclopentanes, cyclohexanes, norbornanes, cis- and trans-decalin, perhydrophenalene, anthracene, adamantane, androstane, methylbutanes and tert-butylmethanes. The significance of these results is discussed with respect to the development of a comprehensive theory of proton chemical shifts and it is concluded that C—C bond anisotropy does not in general contribute significantly to proton chemical shifts, although a possible specific shielding effect in planar eclipsed C·C·C·C fragments is noted. © 1998 John Wiley & Sons, Ltd.
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