EXPERIMENTAL AND THEORETICAL AB-INITIO STUDY OF THE C-13-C-13 SPIN-SPIN COUPLING AND H-1 AND C-13 SHIELDING TENSORS IN ETHANE, ETHENE, AND ETHYNE

Experimentally and theoretically (ab initio) determined CC spin-spin c oupling tensors and H-1 and C-13 nuclear shielding tensors are reporte d for ethane ((C2H6)-C-13), ethene ((C2H4)-C-13), and ethyne ((C2H2)-C -13). The experimental anisotropies of the CC coupling tensors, Delta J(CC), for all these molecules, and also the combination J(CC,xx)-J(CC ,yy) for ethene, were derived from sets of anisotropic couplings (D-ex p) analyzed from the H-1 and C-13 NMR spectra of molecules partially o riented in liquid-crystalline environments. Both harmonic vibrations a nd structural deformations arising from the correlation of vibrational and reorientational motions were taken into account in the D coupling s. The ab initio calculations of all the J tensors were performed usin g MCSCF linear response theory. The best calculated and experimental D elta J(CC) values (along with J(CC,xx)-J(CC,yy) for ethene) an found t o be in good mutual agreement. Together with earlier work on the (n)J( CC) tensors in benzene, this study shows that the indirect contributio n, 1/2J(CC)(aniso), to experimental couplings between differently hybr idized carbons is small and can generally be omitted. This means that the use of experimental D-CC couplings in the determination of molecul ar order tensors and/or conformation does not introduce serious errors . The experimental determination of the H-1 and C-13 Shielding tensors was based on the liquid crystal director rotation by 90 degrees in mi xtures of thermotropic nematogens with opposite anisotropy of diamagne tic susceptibility. Ab initio SCF and MCSCF calculations utilizing gau ge-including atomic orbitals produce results in good agreement with ex periments.