Carbon-13 Chemical Shift Tensors in Pentaerythritol

Abstract

The carbon-13 chemical shift tensors of pentaerythritol (C(CH2OH)4, tetrakis(hydroxymethyl)methane) are measured with use of the two-dimensional chemical shift correlation technique. The results are interpreted with quantum-chemical ab initio gauge invariant atomic orbital computations of the nuclear shielding tensors based on X-ray and neutron diffraction structures, and on corresponding structures modified by optimizing the CH2 proton positions with the Gaussian 94 program. The X-ray based computations correlate with the experimental tensors with a root-mean-square deviation of 4.45 ppm, while the neutron structure gives a root-mean-square deviation of 0.86 ppm. After optimizing the CH2 proton positions for the X-ray and neutron structures, root-mean-square deviations of 1.19 and 0.84 ppm, respectively, are calculated. Tensor computations for the central carbon were done with hypothetical structures obtained by altering the C−C−C angle. The difference between the two computed principal values of this axially symmetric tensor varies at a rate of 2.7 ppm/deg as the angle changes. The experimentally measured chemical shift difference of 14.6 ppm corresponds to a C−C−C bond angle of 106.6°, in good agreement with the X-ray diffraction value of 107.3° and the neutron diffraction value of 107.1°.