The effects of librations on the C-13 chemical shift and H-2 electric field gradient tensors in beta-calcium formate

The magnitudes and orientations of the principal elements of the C-13 chemi cal shift anisotropy (CSA) tensor in the molecular frame of the formate ion in beta -calcium formate is determined using one-dimensional dipolar-shift spectroscopy. The magnitudes of the principal elements of the C-13 CSA ten sor are sigma (11C)=104 ppm, sigma (22C)=179 ppm, and sigma (33C)=233 ppm. The least shielding element of the C-13 CSA tensor, sigma (33C), is found t o be collinear with the C-H bond. The temperature dependence of the C-13 CS A and the H-2 quadrupole coupling tensors in beta -calcium formate are anal yzed for a wide range of temperature (173-373 K). It was found that the spa n of the C-13 CSA and the magnitude of the H-2 quadrupole coupling interact ions are averaged with the increasing temperature. The experimental results also show that the H-2 quadrupole coupling tensor becomes more asymmetric with increasing temperature. A librational motion about the sigma (22C) axi s of the C-13 CSA tensor is used to model the temperature dependence of the C-13 CSA tensor. The temperature dependence of the mean-square amplitude o f the librational motion is found to be < alpha (2)> =2.6x10(-4)(T) rad(2) K-1. The same librational motion also accounts for the temperature-dependen ce of the H-2 quadrupole coupling tensor after the relative orientation of the C-13 CSA and H-2 electric field gradient tensors are taken into account . Reconsideration of the results of a previous study found that the librati onal motion, not the vibrational motion, accounts for an asymmetry in the H -1-C-13 dipolar coupling tensor of alpha -calcium formate at room temperatu re. (C) 2000 American Institute of Physics. [S0021-9606(00)01048-5].