PROTON CHEMICAL-SHIFT TENSORS OF METHYL-GROUPS - A MULTIPLE-PULSE NMRAND LORG IGLO AB-INITIO STUDY/

High-resolution proton spectra of single crystals of dimethyl terephth alate (DMT) and rotation patterns of line shifts are reported. The sup pression of the proton-proton dipole-dipole interactions by multiple-p ulse cycles works sufficiently well to achieve linewidths of less than 0.5 ppm and to follow the angular dependence of the chemical shifts o f all 12 inequivalent protons in DMT in the rotation patterns. In this paper, the data from the methyl groups are analyzed. The anisotropy o f the methyl proton chemical-shift (CS) tensor is +(2.9 +/- 0.1) ppm, The asymmetry 77 is 0.25. LORG and IGLO ab initio CS calculations for the methyl ester fragment of DMT, supplemented by a hydrogen, i.e., fo r methyl formate, are also presented. Calculated and measured methyl p roton CS tensors agree on the +/-0.5 ppm accuracy level. The calculate d CS tensor of the C-H proton in methyl formate is compared with the i ntramolecular part of the previously measured CS tensors in calcium an d lead formate. Again, remarkable agreement is found between calculate d and measured proton CS tensors. (C) 1995 Academic Press, Inc.