The effects of librations on the 13C13C chemical shift and 2H2H electric field gradient tensors in β-calcium formate

Abstract

The magnitudes and orientations of the principal elements of the 13C13C chemical shift anisotropy (CSA) tensor in the molecular frame of the formate ion in β-calcium formate is determined using one-dimensional dipolar-shift spectroscopy. The magnitudes of the principal elements of the 13C CSA13CCSA tensor are σ11C = 104 ppm,σ11C=104ppm, σ22C = 179 ppm,σ22C=179ppm, and σ33C = 233 ppm.σ33C=233ppm. The least shielding element of the 13C CSA13CCSA tensor, σ33C,σ33C, is found to be collinear with the C–H bond. The temperature dependence of the 13C CSA13CCSA and the 2H2H quadrupole coupling tensors in β-calcium formate are analyzed for a wide range of temperature (173–373 K). It was found that the span of the 13C CSA13CCSA and the magnitude of the 2H2H quadrupole coupling interactions are averaged with the increasing temperature. The experimental results also show that the 2H2H quadrupole coupling tensor becomes more asymmetric with increasing temperature. A librational motion about the σ22Cσ22C axis of the 13C CSA13CCSA tensor is used to model the temperature dependence of the 13C CSA13CCSA tensor. The temperature dependence of the mean-square amplitude of the librational motion is found to be 〈α2〉 = 2.6×10−4(T) rad2〈α2〉=2.6×10−4(T)rad2 K−1.K−1. The same librational motion also accounts for the temperature-dependence of the 2H2H quadrupole coupling tensor after the relative orientation of the 13C CSA13CCSA and 2H2H electric field gradient tensors are taken into account. Reconsideration of the results of a previous study found that the librational motion, not the vibrational motion, accounts for an asymmetry in the 1H–13C1H–13C dipolar coupling tensor of α-calcium formate at room temperature. © 2000 American Institute of Physics.