A nuclear quadrupole resonance and X-ray study of the crystal structure of 1,2,4,5-tetrachlorobenzene

Abstract

The chlorine nuclear quadrupole resonance has been used to determine the C-C1 bond directions in a single crystal of 1,2,4,5-tetrachlorobenzene. With these data, the phases for the (0kl) structure amplitudes were determined directly and used in a Fourier projection. The results for the n.q.r. measurements and the two-dimensional X-ray analysis agree within 1-2 ° in the bond orientations. The angle between adjacent C-C1 bonds in the molecule is 63± l°. The interpretation in terms of strained models is discussed in relation to the n.q.r, experiments. al information. In some circumstances, this can be rather precise data relating to the relative orientation of bond directions other- wise accessible only to the more laborious techniques of a complete three-dimensional X-ray crystal- structure analysis. Alternately it can be used to give the X-ray crystallographer enough preliminary struc- tural data to enable him to proceed directly to the solution of the phase problem. In this paper, the Zeeman sphtting of the C135 n.q.r, spectrum of 1,2,4,5-tetrachl orobenzene is de- scribed. The results were interpreted on the basis of the simple model in which the electric field gradient at a chlorine nucleus is considered to arise solely from the ~ and ~ character of the C-CI bonds. This gave the relative C-CI bond and benzene ring orientations. The validity of this generally accepted interpretation was supported by the close correlation with the results from a two-dimensional X-ray analysis. Furthermore, although no great difficulty would be anticipated in solving the phase problem for this particular crystal structure by the standard methods of trial and error or Patterson synthesis, the use of the n.q.r, orientation results did in fact permit a direct solution.