The effects of H-bonding, change of state and molecular distortion on 2H and 17O nuclear quadrupole coupling constants: general theoretical considerations and specific application to formic acid

Abstract

The major emphasis here is in analysing the effect of H-bonding on the nuclear quadrupole coupling behaviour of 2H and 17O nuclei. Using ab initio quantum mechanical methods on the test molecule formic acid, the coupling amplitudes (nuclear quadrupole coupling constants e 2 qQ/h) and, also, coupling anisotropies (asymmetry parameter η) and orientations of the electric field gradient principal axis system with respect to molecular axes have been calculated. As formic acid contains 2H and 17O nuclei in a variety of functional groups — alkyl ( 2H), hydroxyl ( 2H and 17O) and carboxyl ( 17O) — it is a particularly efficient molecule for theoretical study. In addition the structures of its H-bonded complexes vary as a function of phase from the cyclic H-bonded gaseous dimer through a probably chain-like association in the liquid to a regular chain H-bonded solid phase. Thus in this work the consequences of differing H-bond geometries on nuclear quadrupole coupling of the given nuclei are investigated, and also the relative contributions to the coupling changes of geometric and electronic disortions resulting from H-bonding and lattice field effects in the solid, are evaluated. The theoretical results are compared with the most up-to-date experimental data available. General problems in calculating molecular electric field gradients are outlined and, in particular, errors resulting from uncertainties in experimental input geometries are critically assessed. Differences in computational emphasis for 2H as compared with 17O and other “heavy” nuclei are pointed out.