Proton chemical shifts in NMR spectroscopy. Part 7.1 C–C anisotropy and the methyl effect

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 to include C–C anisotropy contributions and an orientation-dependent methyl γ 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 5α-androstane.The new scheme (CHARGE4) 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.