ELECTRIC-FIELD EFFECTS ON THE C-13 NUCLEAR MAGNETIC SHIELDING IN SEVERAL ORGANIC-MOLECULES

In a uniform electric field changes take place in the tensor components of nuclear shielding. For fields of small magnitude, a Taylor series expansion of each component in terms bf the field can be defined. In the present work the coefficients of such expansions-sometimes called shielding polarizabilities and hyperpolarizabilities-have been calculated for the carbon nuclei in the molecules CH4, C2H6, C2H4, C2H2, HCN, CH3F, CH3CN and H2CO. The calculations have been carried out at the SCF level with large basis set wave-functions in which derivative orbitals are added to basis sets suggested by Sadlej. The results show that the paramagnetic part of the carbon shielding is usually far more sensitive to an electric field than the diamagnetic part. The coefficients depend very strongly on the chemical bonding of the carbon atom containing the nucleus of interest.