Variation of one-bond X-Y coupling constants 1J(X-Y) and the components of 1J(X-Y) with rotation about the X-Y bond for molecules HmX-YHn, with X, Y = 15N, 17O, 31P, 33S: the importance of nonbonding pairs of electrons

Ab initio EOM-CCSD calculations have been performed on molecules HmX-YHn, for X, Y = 15N, 17O, 31P, and 33S, to investigate the variation of one-bond X-Y spin-spin coupling constants 1J(X-Y) and the components of J with rotation about the X-Y single bond. The reduced Fermi-contact (FC) terms for all 10 molecules are negative and decrease in absolute value as the rotational angle theta changes from 0 degrees, at which point the lone pairs of electrons are on the same side of the X-Y bond, to 180 degrees where they are trans with respect to the X-Y bond. The signs of reduced paramagnetic spin-orbit (PSO) and spin-dipole (SD) terms are opposite that of the FC term and exhibit extremum values as theta approaches 90 degrees, the gauche conformation. While the FC term tends to dominate for molecules H2X-YH2 and H2X-YH, such is not the case for HX-YH, where the PSO and SD terms assume increased importance. Curves for 1K(X-Y) as a function of rotational angle are readily grouped according to formula H2X-YH2, H2X-YH, and HX-YH, which suggests that it is the lone pairs of electrons on X and Y which are primarily responsible for the trends observed.