Structural Investigations by Means of Nuclear Magnetism. I. Rigid Crystal Lattices

Abstract

Experimental absorption line shapes at nuclear magnetic resonance are given for several simple assemblies of nuclei, with spin ½, in the solid state at temperatures between 90° and 95°K. Analysis of these data is facilitated by the theory of Van Vleck which relates the second moment of the absorption line in a rigid lattice to nuclear spins, gyromagnetic ratios, and internuclear distances, and which provides, therefore, an objective and general method for determining structural parameters from the experimental line shapes. 1,2-dichloroethane exhibits a proton resonance with line structure characteristic of nuclear magnetic moments interacting in pairs to produce the broadening of the absorption line. Consideration of both fine structure and second moment leads to an inferred H–H distance of 1.71±0.02A in the —CH2Cl group. In conjunction with the expected C–H bond distance of from 1.09A to 1.10A, this implies that the H–C–H bond angle is between 4° 30′ and 9° less than tetrahedral. Absorption line shapes for the proton resonance in 1,1,1-trichloroethane and ammonium chloride are given as characteristic respectively of equilateral three-spin and tetrahedral four-spin systems. Less conclusive studies of perfluoroethane and diborane are presented. A detailed analysis of data on ammonium fluoride, chloride, and bromide yields a value of 1.025±0.005A for the N–H distance in the ammonium ion. Some general aspects of the use of nuclear magnetism in structural investigations are discussed briefly.