Theoretical Study of C–H Nuclear Spin—Spin Coupling Using a Constrained SCF Model

Abstract

A constrained SCF model is used in an ab initio calculation of C–H nuclear spin—spin coupling constants. The C–H bond of some molecule of the general form, R≡C–H, is treated as a diatomic molecule in the molecular orbital approximation. The subsequent self-consistent-field calculation is performed subject to the constraint of orthogonality between the resulting molecular orbitals and the valence hybrid atomic orbitals centered on the carbon atom, representing the residue of the original R—C bond(s). Calculations are carried out for the sp3, sp2, and sp hybrid situations using both an optimized minimal basis set and an extended basis set. The further dependence of the coupling constant JCH on the C–H bond distance is examined. The extended-basis-set treatment is found to be preferable to the use of a minimal basis set from the standpoints of accuracy and of the efficiency of parameter variation. The computed coupling constants resulting from such extended-basis-set calculations differ from experimental JCH values for representative compounds of sp3, sp2, and sp hybridization by 11.2%, 13.7%, and 23.0%, respectively.