Substituent chemical shifts in NMR spectroscopy. Part 6. A model for the calculation of proton chemical shifts in substituted alkanes

Abstract

A development of a previous calculation of partial atomic charges (CHARGE3) is given which allows the prediction of the proton chemical shifts in a variety of substituted alkanes.

This is accomplished by identifying the effects of substituents at the α, β, γ and the more distant protons. The hydrogen electronegativity is changed to a value close to the Pauling value, the γ(H.C.C.X) SCS (substituent chemical shift) is shown to be a function of the polarisability of X rather than the electronegativity and the problem of multi-substitution of electronegative substituents is overcome by an explicit correction for oxygen and fluorine substituents. These amendments allow the proton chemical shifts of CH_{4 –n}X_n and CH₃CH_{3 –n}X_n(n= 1–3, X = H, NH₂, OH, F, Cl, Br, I, SH) to be predicted generally to 0.1 ppm, apart from some of the Br and I compounds.

The method has also been tested on a variety of cyclic alkanes, including substituted cyclohexanes and norbornanes, cis- and trans-decalin, bicyclo[2.2.2]octane, perhydrophenalene and anthracene and some tert-butylmethanes, providing a wide variety of steric interactions and strain energies, and also on fluoro and chloro substituted cyclohexanes and norbornanes.

For these compounds the orientation dependence of the γ methyl SCS is considered both explicitly and as a result of steric effects. In contrast the effects of fluorine and chlorine SCSs at the γ(i.e. vicinal) proton are non-orientational.

The long range effects of proton–proton interactions are shielding at the protons but the long range effects of C, F and Cl deshield the affected protons. For H, C and Cl an r^–6 distance dependence was found but fluorine steric effects were better reproduced with an r^–3 distance dependence. The calculations reproduced the observed proton chemical shifts of the compounds studied to 0.17 ppm. It was not necessary to invoke in these calculations either the magnetic anisotropy or the electric field effects of the fluorine and chlorine substituents, and the implication of these results on present theories of proton chemical shifts is discussed.