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Abstract:
The carbon-13 chemical-shift tensors of the methyl groups in solid trimethylsulfoxonium 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 performed at −40°C. The compound crystallizes in the space group Pnma with the TMSI molecules lying in crystallographic reflection planes so that one methyl group (A) is in the plane and two others (B and B′) are situated symmetrically on both sides of the plane. The principal tenser components of both types of methyl groups are similar but not identical. From the rotation patterns, the following principal values were derived for the A methyls, δ11 = 62.0 ppm, δ22 = 44.0 ppm, and δ33 = 7.5 ppm (relative to TMS),while for methyls B and B′, δ11 = 66.5 ppm, δ22 = 45.5 ppm, and δ33 = 8.0 ppm. The overall chemical-shift range of the low-temperature powder spectrum of TMSI is considerably smaller (by similar to 8 ppm) than that expected from the single-crystal results. The discrepancy is ascribed to shape-related susceptibility effects on the spectra of the single crystals. The results show that the most-shielded direction, corresponding to δ33, is close to the SC bond direction (the angle between them being ϵ ∼ 5°) while the least-shielded direction (δ11) is perpendicular, or nearly perpendicular, to the corresponding OSC plane. The results are compared with those obtained for other sulfur-bound methyl groups and with quantum-mechanical calculations on related compounds. The observed 13C NMR linewidth, even below -20°C, where the effect of the threefold molecular jumps is negligible, is much larger (∼ 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 the I-ions which is partially self decoupled by the 127I quadrupolar relaxation.
