Vibrational effects on structural parameters obtained by solid-state NMR are studied by theoretical calculations and molecular-dynamics simulations. The structural parameters treated contain internuclear distances between directly bonded or remote nuclei including nonproton pairs in a molecule and bond and dihedral angles. In addition to the intramolecular normal mode vibrations, the libration of the whole molecule is considered in the theory. It is shown that the molecular libration as well as the intramolecular vibrations reduce dipolar interactions, and consequently lengthen the internuclear distances obtained from the dipolar interactions (RNMR). In contrast, the internuclear distances obtained by single crystal x-ray or neutron diffraction (Rcor) are proved to be shortened by the molecular libration. Molecular-dynamics simulations for glycine molecules in the crystal at room temperature reveal that RNMR are 1%–4% longer than Rcor, confirming the theoretical results. It is also demonstrated that the effect of the molecular libration on distances between nonproton nuclei is dominant over that of the intramolecular vibrations. Especially for long distances, the molecular libration is shown to be an almost unique vibrational effect and to give differences of 1% to 2% between RNMR and Rcor. On the other hand, the theoretical calculations on the vibrational effects on bond and dihedral angles determined by correlating two dipolar tensors show very little angular deviations, and it is confirmed by molecular-dynamics simulations for glycine molecules.

No abstract

We have studied the proton spin-lattice relaxation times (Td of a series of benzoic acid (BAC) derivatives and decanoic acid (DAC) over a wide range of temperature and analyzed the results in terms of the double proton switching along the hydrogen bonds. The proton T, in the high temperature region are analyzed using the classical jump model and the barrier heights for the proton transfer are determined. The thermodynamic parameters for the equilibria between the two configurations in the solid state are also determined by the Ff -IR measurements. It is shown that the energetics and dynamics of the proton transfer in DAC and the para-and meta-substituted BAC are all similar, but they are very different in the ortho-substituted ones. It is suggested that the low temperature behavior of the proton T, of the dimers of carboxylic acid is due to the tunneling and the asymmetry of the potential brings in a small activation energy.