Structure and conformation of β-thiodan in the solid state and in solution. Application of the infrared–X-ray method

Abstract

We have determined the structure of the β isomer (m.p. 208–210 °C) of the commercial insecticide thiodan by single-crystal X-ray techniques and determined its solution conformation by the i.r.–X-ray method. β-Thiodan, 6,7,8,9,10,10-hexachloro-1,2,2a,4,4a,5-hexahydro-6,9-methano-2,4,3-benzodioxathiepin-3-oxide, crystallizes in space group P2₁/c with a= 8.244(2), b= 10.104 (3), c= 17.32(1)Å, β= 95.48 (3)°, and Z= 4. The structure was solved by direct methods and refined to R 0.074. The structure is consistent with the assignment based on n.m.r. and stability considerations and inconsistent with that assigned based on dipole-moment measurements.

I.r. spectra were used to compare the solid-state and solution conformation. The solid-state and solution i.r. spectra showed almost identical absorption maxima, but the i.r. difference spectra had substantially different intensities of the absorptions in these two states. Thus β-thiodan was probably in the same conformation in the solid state and in solution and care must be taken in interpreting i.r. difference spectra.

Dihedral angles for thiodan indicated a Lambert R value of 2.17 while the observed R value was ca. 1.0 based on the reported ratio of trans and cis vicinal coupling constants, indicating that this method does not apply to such systems.

The results of the i.r., dipole-moment, and R value studies indicate the necessity of using several methods when comparing solid-state and solution conformations.