C-13 CHEMICAL-SHIFT TENSORS IN PENTAERYTHRITOL

The carbon-13 chemical shift tensors of pentaerythritol (C(CH2OH)(4), tetrakis(hydroxymethyl)methane) are measured with use of the two-dimen sional chemical shift correlation technique. The results are interpret ed with quantum-chemical ab initio gauge invariant atomic orbital comp utations of the nuclear shielding tensors based on X-ray and neutron d iffraction structures, and an corresponding structures modified by opt imizing the CH2 proton positions with the Gaussian 94 program. The X-r ay based computations correlate with the experimental tensors with a r oot-mean-square deviation of 4.45 ppm, while the neutron structure giv es a root-mean-square deviation of 0.86 ppm. After optimizing the CH2 proton positions for the X-ray and neutron structures, root-mean-squar e deviations of 1.19 and 0.84 ppm, respectively, are calculated. Tenso r computations for the central carbon were done with hypothetical stru ctures obtained by altering the C-C-C angle. The difference between th e two computed principal values of this axially symmetric tensor varie s at a rate of 2.7 ppm/deg as the angle changes. The experimentally me asured chemical shift difference of 14.6 ppm corresponds to a C-C-C bo nd angle of 106.6 degrees, in good agreement with the X-ray diffractio n value of 107.3 degrees and the neutron diffraction value of 107.1 de grees.