Electric field effects on the carbon-13 nuclear magnetic shielding in several organic molecules

Abstract

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 of 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 wavefunctions 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.