Carbonyl Carbon Chemical Shift Tensors for a Typical Aryl Aldehyde and Formaldehyde . NMR Studies of the Isolated 13 C - 2 H Spin Pair

The magnitude and orientation of the principal components of the carbonyl carbon chemical shift tensor of a typical aryl aldehyde, 3,4-diben~yloxybenzaldehyde-~~C,~H,, 1, have been determined from the analysis of I3C NMR spectra of static powder samples obtained at 4.7 and 9.4 T. The principal components of the carbon shielding tensor of formaldehyde, 2, have been calculated from the experimental I3C nuclear spinrotation tensor, which is available in the literature. The experimental results are compared with those of ab initio chemical shielding calculations using the GIAO (Gauge-Zncluding Atomic Orbitals) method. For 1, the most shielded component of the carbonyl carbon chemical shift tensor, 833, is perpendicular to the 2H--’3C=0 plane, whereas the least shielded component, 81 I , is within this plane approximately perpendicular to the C-0 bond. The orientation of the carbon shielding tensor of formaldehyde is analogous to that of 1; however, the precise orientation of the three principal components is dictated by the higher molecular symmetry (e.g., 8 2 2 is coincident with the C-0 bond). The deshielding associated with 811 is attributed to a large local magnetic dipole transition moment of the a n* type. Analysis of I3C NMR spectra of solid 3,4-diben~yloxybenzaldehyde-’~C~~H, obtained with magic-angle spinning yields a value for the indirect 13C’H spin-spin coupling constant, 1J(13C,2H)iSo = 26.4 f 0.5 Hz. From 2H NMR spectra of a static powder sample of 1, the 2H nuclear quadrupolar coupling constant was estimated to be 151.5 f 1 kHz.