Variation of one-bond X-Y coupling constants1J(X-Y) and the components of1J(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

摘要

Ab initio EOM-CCSD calculations have been performed on molecules H mX-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 θ changes from 0°, at which point the lone pairs of electrons are on the same side of the X-Y bond, to 180° 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 θ approaches 90°, the gauche conformation. While the FC term tends to dominate for molecules H2X-YH2 and H 2X-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-YH 2, 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. © 2007 American Chemical Society.