Z. Olender et al., C-13 CHEMICAL-SHIFT TENSORS AND RELAXATION PHENOMENA IN SOLID TRIMETHYLSULFOXONIUM IODIDE - A SINGLE-CRYSTAL NMR-STUDY, Journal of magnetic resonance. Series A, 114(2), 1995, pp. 179-187
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.