ELECTRIC-FIELD EFFECTS ON THE SHIELDING OF PROTONS IN C-H BONDS

The coefficients which determine the electric field dependence at first and second order of the shielding of protons in C-H bonds were calculated by a finite-field SCF method for the molecules, CH4, C2H6, CH3F, CH3Cl, CH3CN, C2H6, H2CO, HCN and C2H2. Values are given for individual tenser components and for the rotationally averaged coefficients. Diamagnetic and paramagnetic parts of the coefficients are generally of the same order of magnitude. This differs from carbon, nitrogen, oxygen and fluorine shielding studied previously where the paramagnetic parts are often very much larger than their diamagnetic counterparts. The linear electric field coefficients A(x) (electric field along the C-H bond from C to H) for the above molecules (except H2CO and HCN) are very similar in value, with a mean of 70.0 ppm a.u. This is very close to an empirical value of 72.2 ppm a.u. obtained by Zurcher in a classic study of steroids. It further appears that the value may be transferable among all C(sp(3))-H bonds. A representative value for the mean quadratic coefficient B is 68.0 ppm a.u. Empirical values of this vary widely.