31P Chemical Shielding Anisotropy in Solids. Single Crystal and Powder Studies at 99.4 MHz

Abstract

Free induction decays of a single crystal of P₄S₃ and powders of Zn₃P₂, Mg₃P₂, and P₄S₁₀ are studied at 99.4 MHz. The P₄S₃ molecule is found to have two ³¹P chemical shielding tensors, one for the single apical phosphorus and one for the three phosphorus atoms of the basal triangle in the molecule. The tensors are more or less symmetric and the eigenvectors for the unique elements are found to be colinear within experimental error, thus suggesting that there is rapid motion about the C₃ axis of the molecule. A comparison with theoretical spectra bears this out. The general character of the spectra agrees with the crystal structure of the compound found by x‐ray analysis and the isotropic shielding values agree with a previous spinning‐sample study. ³¹P powder patterns for Zn₃P₂ and Mg₃P₂ yield chemical shielding parameters and the spectra appear to disagree with previous pulsed and spinning‐sample experiments on these compounds. The spectrum of P₄S₁₀ indicates rapid molecular motion at room temperature. The possibility of low‐temperature and liquid experiments is discussed.