Electronic mechanisms of intra and intermolecular J couplings in systems with C-H...O interactions

Correlation effects on the change of (1)J(CH) couplings in model systems I:NCH...H2O and II:CH4...H2O as a function of the H...O distance are discussed. RPA and SOPPA results follow a similar trend in system II. In system I RPA values decrease monotonously as the H...O distance decreases, while SOPPA ones exhibit flat maximum near equilibrium. Such different behavior is ascribed to the pi-transmitted component. Intermolecular couplings at the equilibrium geometry of I are analyzed by means of the CLOPPA approach. The larger absolute value of (2h)J(CO) compared to (1h)J(HO) is found to arise from contributions involving a vacant LMO localized in the C-H...O moiety.