Cation dynamics in pyridinium nitrate and bis-thiourea pyridinium nitrate inclusion compound studied by H-2 NMR spectroscopy

摘要

H-2 NMR ( nuclear magnetic resonance) line-shape measurements were performed over a wide temperature range for pyridinium nitrate (d(5)PyH) NO3 and bis-thiourea pyridinium nitrate inclusion complex T-2(d(5)PyH) NO3 in order to compare the dynamics of d(5)-pyridinium cation in both compounds. It was revealed that in both systems the pyridinium cation undergoes reorientations about the axis perpendicular to its plane, among inequivalent potential energy barriers. However, in (d(5)PyH) NO3 (no phase transition) the population of the deepest minima decreases monotonically up to the melting point, whereas in T-2(d(5)PyH) NO3 (two phase transitions) this population decreases rapidly in a transition to the high-temperature phase and its value does not depend on temperature. In bis-thiourea pyridinium nitrate inclusion compound the out-of-plane motion of the cation is also found in the intermediate- and high-temperature phases, and the amplitudes of the motion are equal to 20 degrees and 35 degrees, respectively. On the basis of the assumptions of these models, it is possible to reproduce the experimental H-2 NMR line shapes in an excellent way.