AN EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE OLEFINIC CARBON CHEMICAL-SHIFT TENSORS IN TRANS-STILBENE AND PT(ETA(2)-TRANS-STILBENE)(PPH3)(2)

The olefinic carbon chemical shift tensors of trans-stilbene-alpha,bet a-C-13(2) (1) and (trans-stilbene-alpha,beta-C-13(2))[bis- (triphenylp hosphine)]platinum(0) (2) have been characterized by solid-state C-13 NMR spectroscopy. Analyses of the C-13 NMR spectra obtained for statio nary powder samples of 1 and 2 at 4.7 and 9.4 T yield the principal co mponents of the carbon chemical shift tensors. The presence of a homon uclear spin pair in these compounds provides information about the ori entation of the chemical shift tensors in the dipolar frame of referen ce. The span, Omega, and skew, kappa, of the olefinic carbon chemical shift tensor of 1 are 166 ppm and -0.145, respectively, comparable to those of other known olefinic carbons. The carbon chemical shift is la rgest when the applied magnetic field is in the plane of the vinyl gro up, perpendicular to the olefinic C,C bond (delta(11) = 215 ppm) and s mallest when it is perpendicular to the vinyl group (delta(33) = 49 pp m) The intermediate principal component of the chemical shift tensor, delta(22) = 120 ppm, is oriented along the olefinic C,C bond. Upon coo rdination to platinum(0), the isotropic carbon chemical shift changes from 128 to 68 ppm, and the span of the chemical shift tensor decrease s dramatically, Omega = 51 ppm. The experimental results are compared with. those from ab initio shielding calculations performed with a com bination of density functional theory (DFT) and the gauge-including at omic orbitals (GIAO) method. Overall, agreement with experiment is goo d. The combined experimental and theoretical approach allows one to su ggest the most likely orientation of the carbon chemical shift tensor for 2 in the molecular axis system.