A study of crystal structure and molecular dynamics by XRD and ¹H NMR in bis-thiourea N-methylpyridinium iodide

Abstract

Crystal and molecular structures of two orthorhombic phases of bis-thiourea N-methylpyridinium iodide [(NH₂)₂CS]₂C₅H₅NI⁻ were determined by a single-crystal X-ray diffraction method. Both phases were discovered earlier by DSC analysis. At 297 K the complex crystallizes in the space group Cmcm with a = 15.742(3) Å, b = 11.349(2) Å, c = 8.360(2) Å, and at 100 K in the space group Pbcn with the cell parameter c tripled. The first-order reversible phase transition occurs at 155 K on cooling and at 160 K on heating. At room temperature all N-methylpyridinium cations are dynamically disordered, while at 100 K the ratio of ordered to statically disordered cations is 2:1. Thiourea-iodide ribbons are well ordered in both crystal phases. ¹H NMR analysis performed for the title complex and its deuterated counterpart has revealed a reorientation of methyl group in the low-temperature phase; and in the high temperature phase, a reorientation of N-methylpyridinium cation over inequivalent barriers followed by a probable flip-reorientation of the thiourea molecules. For steric reasons the latter motion is possible only if preceded by a flip-motion of the NH₂ group. Activation energy values for the proposed cation and thiourea motions are given.

Acknowledgement

We wish to thank Dr S. Mielcarek for the DSC study.