Proton chemical shifts in NMR spectroscopy .7. C-C anisotropy and the methyl effect

A previous model for the calculation of proton chemical shifts in substituted alkanes based upon partial atomic charges and steric interactions has been modified to include C-C anisotropy contributions and an orientation-dependent methyl gamma effect (i.e. Me-C-C-H), The ring inversion in 1,1-dimethylcyclohexane and cis-decalin has been slowed at low temperature and the individual proton chemical shifts assigned, along with those for 5 alpha-androstane. The new scheme (CHARGEA) predicts the proton chemical shifts of a variety of acyclic, cyclic and polycyclic hydrocarbons over 188 data points spanning 2 ppm to within 0.11 ppm, a 40% improvement over the previous model, Systems considered include substituted cyclohexanes and norbornanes, cis- and trans-decalin, perhydrophenalene and anthracene, adamantane and androstane, as well as methylbutanes and tert-butylmethanes.