Spin - spin coupling tensors in fluoromethanes

All spin-spin coupling tensors J of the fluoromethanes CH3F, CH2F2 and CHF3 are obtained theoretically by multiconfiguration self-consistent field lin ear response (MCSCF LR) ab initio calculations. Furthermore the principal v alues and the orientation of the principal axis systems of each theoretical J tensor are specified. Experimental liquid crystal NMR (LC NMR) data on t he tensorial properties of the CF spin-spin coupling in CH3F and CH2F2, and the FF spin-spin coupling in CHF3 are also reported. In the analysis of th e experiments, the contributions from molecular vibrations, as well as that of the correlation of vibrational and rotational motion to the experimenta l anisotropic couplings, D-exp, are taken into account. The information of the anisotropic indirect coupling, 1/2J(aniso), is detected as the differen ce between D-exp and the calculated dipolar coupling. D-calc. The extracted indirect contributions, 1/2J(aniso), are in fair agreement with the ab ini tio results. All relative (experimental and theoretical) CF and FF indirect contributions, 1/2J(aniso)/ D-exp, are negative and under 1.7% in magnitud e, when the observed molecular orientations are used. Therefore, in the one bond CF couplings and in the two bond FF couplings, the indirect contribut ion can normally be ignored without introducing serious error to the determ ination of molecular orientation and/or structure. However, a more accurate method is to partially correct fur the indirect contribution by utilising the transferability of the spin -spin coupling tensors in related molecules . This is due to the fact that even small contributions may be significant, if the order parameter of the internuclear direction is negligibly small, leading to dominating indirect contributions. The very good agreement of th e experimental values with the calculated coupling constants and the reason able agreement in the anisotropic properties, which are experimentally much more difficult to define, indicates that the MCSCF LR method is capable of producing reliable J tensors fur these systems, contrary to the case of de nsity-functional theory.