Sternheimer shielding in molecules

The effect of perturbing electric fields and electric-field gradients on the nuclear quadrupole coupling in molecules is investigated. The theory for the first-order effects is developed and it is shown that not only a perturbing field gradient but also a perturbing field, and higher terms, induce a finite field gradient at a nucleus in a molecule. The so-called Sternheimer (anti)shielding is thus considerably more complex in molecular than in atomic systems. The different shielding components have been calculated for HCl, HCN and NH₄ ⁺ using a finite perturbation ab initio molecular orbital method. The calculated values are then used to rationalize experimentally determined ¹⁴N quadrupole coupling constants in condensed media for HCN and NH₄ ⁺.