An Experimental and Theoretical Investigation of the Olefinic Carbon Chemical Shift Tensors in trans-Stilbene and Pt(η-trans-stilbene)(PPh3)2

The olefinic carbon chemical shift tensors of trans-stilbene-R,â-C2 (1) and (trans-stilbene-R,â-C2)[bis(triphenylphosphine)]platinum(0) (2) have been characterized by solid-state 13C NMR spectroscopy. Analyses of the 13C NMR spectra obtained for stationary powder samples of 1 and 2 at 4.7 and 9.4 T yield the principal components of the carbon chemical shift tensors. The presence of a homonuclear spin pair in these compounds provides information about the orientation of the chemical shift tensors in the dipolar frame of reference. The span, Ω, and skew, κ, 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 largest when the applied magnetic field is in the plane of the vinyl group, perpendicular to the olefinic C,C bond (δ11 ) 215 ppm) and smallest when it is perpendicular to the vinyl group (δ33 ) 49 ppm). The intermediate principal component of the chemical shift tensor, δ22 ) 120 ppm, is oriented along the olefinic C,C bond. Upon coordination to platinum(0), the isotropic carbon chemical shift changes from 128 to 68 ppm, and the span of the chemical shift tensor decreases dramatically, Ω ) 51 ppm. The experimental results are compared with those from ab initio shielding calculations performed with a combination of density functional theory (DFT) and the gauge-including atomic orbitals (GIAO) method. Overall, agreement with experiment is good. The combined experimental and theoretical approach allows one to suggest the most likely orientation of the carbon chemical shift tensor for 2 in the molecular axis system.