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

摘要

Crystal and molecular structures of two orthorhombic phases of bis-thiourea N-methylpyridinium iodide [(NH2)(2)CS](2)C5H5NCH3+I- were determined by a single-crystal X-ray diffraction method. Both phases were discovered earlier by DSC analysis. At 297K the complex crystallizes in the space group Cmcm with a 15.742( 3) angstrom, b 11.349(2) angstrom, c = 8.360(2) angstrom, and at 100K in the space group Pbcn with the cell parameter c tripled. The first-order reversible phase transition occurs at 155K on cooling and at 160K on heating. At room temperature all N-methylpyridinium cations are dynamically disordered, while at 100K the ratio of ordered to statically disordered cations is 2: 1. Thiourea-iodide ribbons are well ordered in both crystal phases. H-1 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.