C-13 CHEMICAL-SHIFT TENSORS AND RELAXATION PHENOMENA IN SOLID TRIMETHYLSULFOXONIUM IODIDE - A SINGLE-CRYSTAL NMR-STUDY

The carbon-13 chemical-shift tensors of the methyl groups in solid tri methylsulfoxonium iodide (TMSI) were determined by recording chemical- shift rotation patterns of three single crystals. To avoid broadening effects due to the threefold molecular jumps, the measurements were pe rformed at -40 degrees C. The compound crystallizes in the space group P-nma with the TMSI molecules lying in crystallographic reflection pl anes so that one methyl group (A) is in the plane and two others (B an d B') are situated symmetrically on both sides of the plane. The princ ipal tenser components of both types of methyl groups are similar but not identical. From the rotation patterns, the following principal val ues were derived for the A methyls, delta(11) = 62.0 ppm, delta(22) = 44.0 ppm, and delta(33) = 7.5 ppm (relative to TMS),while for methyls B and B', delta(11) = 66.5 ppm, delta(22) = 45.5 ppm, and delta(33) = 8.0 ppm. The overall chemical-shift range of the low-temperature powde r spectrum of TMSI is considerably smaller (by similar to 8 ppm) than that expected from the single-crystal results. The discrepancy is ascr ibed to shape-related susceptibility effects on the spectra of the sin gle crystals. The results show that the most-shielded direction, corre sponding to delta(33), is close to the S-C bond direction (the angle b etween them being epsilon similar to 5 degrees) while the least-shield ed direction (delta(11)) is perpendicular, or nearly perpendicular, to the corresponding O-S-C plane. The results are compared with those ob tained for other sulfur-bound methyl groups and with quantum-mechanica l calculations on related compounds. The observed C-13 NMR linewidth, even below -20 degrees C, where the effect of the threefold molecular jumps is negligible, is much larger (similar to 500 Hz at half maximum intensity) than that commonly found in molecular crystals. This large width is ascribed to dipolar interaction with the iodine nuclei of th e I- ions which is partially self decoupled by the I-127 quadrupolar r elaxation. (C) 1995 Academic Press, Inc.