Proton chemical shifts in NMR. Part 9.1 Steric and electric field effects in chlorine substituent chemical shifts (SCS)

Abstract

A model for the prediction of the proton chemical shifts of chloroalkanes is given and applied to a variety of chloro compounds. These include chloro- and 1,1-dichloro-cyclohexane for which the proton spectra have been obtained and the individual proton chemical shifts assigned at -80 °C where the ring inversion is slowed sufficiently to give the spectra of the individual conformers. The proton spectra of 1-chloro- and 2-chloro-adamantane are also reported and completely assigned.The chlorine SCS are shown to be multifunctional. The short range effects (three bonds or less) are calculated from the partial atomic charges obtained from the CHARGE scheme. The long range (>three bonds) effects are shown to be due to the electric field of the C–Cl bond plus the steric effect of the chlorine atom.This model (CHARGE4) predicts the proton chemical shifts of a variety of chloroalkanes over 70 data points spanning ca. 6.2 ppm with an rms error of 0.15 ppm. These compounds include chloroethanes, propanes, cyclohexanes, bornanes, norbornanes, adamantanes and steroids.Analysis of the chlorine steric term shows that the quadratic term in the Buckingham equation is much too small to account for the observed steric effects which are considered to arise from van der Waal’s interactions.