The nuclear spin-spin coupling constants in methanol and methylamine: geometry and solvent effects

The effects of internal rotation and bond stretching on the spin-spin coupl ing constants in CH3OH and CH3NH2 have been calculated on MCSCF level. The reaction field theory has been used to simulate the effect of the water env ironment. The internal rotation causes considerable changes not only in (3) J(HH) but in (1)J(CH) and (2)J(OH) ((2)J(NH)) as well. The coupling constan ts in the methyl group and some of the geminal couplings in polar moieties ((2)J(OH), (2)J(NH) and (2)J(HNH)) exhibit a differential sensitivity to bo nd length variations. This phenomenon does not emerge for the single bond c ouplings involving nuclei with lone pairs. The simulation of the aqueous en vironment leads to the conclusion that solvent effects are substantial for the single bond coupling constants and for some of the geminal coupling con stants but negligible for (3)J(HH). In the case of (1)J(CH) and (2)J(HCH), solvent effects depend considerably on the molecular conformation. All effe cts under study are dominated by the changes in the Fermi contact terms, wi th the exception of the internal rotation effects on (1)J(CO) and (1)J(CN). (C) 2000 Published by Elsevier Science B.V. All rights reserved.