High-Resolution Pulse NMR in Solids

Abstract

In liquids the scalar chemical shifts and the indirect spin-spin coupling constants are the two important NMR parameters that hold clues as to molecular structure and conformation. In solids, however, because of the relatively high rigidity of the environment and the frozen state of the molecules, these two interactions become minor and the homo and heteronuclear dipolar Hamiltonians become the most dominant interactions. One can easily calculate the dipolar interaction from a knowledge of the positional parameters of the dipoles in a rigid lattice and can also calculate what would be the energy levels in a randomly oriented powder assuming statistical distribution of the magnetic dipoles. The dipolar interaction in solids is on the order of several kilohertz and the resulting broad structureless resonance absorption will mask completely the chemical shift information (~kHz) and wipe out spin-spin coupling (~0.01–0.2 kHz). One cannot get much information from such broad lines except in favorable cases, where dipolar splitting between neighbor interactions far exceeds the lattice contribution so that distances and direction cosines of dipolar-coupled pairs can be derived. In fact, this method has been used successfully to locate proton positions in hydrated crystals, as a supplement to x-ray crystallography. One may also analyze the moments of spectral distribution in solids and the effect of molecular motions on these spectral moments. Temperature dependence of lineshapes has been successfully analyzed in terms of specific models of molecular libration, rotation, etc.