Rotational Dynamics in a Crystalline Molecular Gyroscope by Variable-Temperature 13C NMR, 2H NMR, X-Ray Diffraction, and Force Field Calculations
文摘
A combination of solid-state 13C CPMAS NMR, 2H NMR, X-ray-determined anisotropic displacement parameters (ADPs), and molecular mechanics calculations were used to analyze the rotationaldynamics of 1,4-bis[3,3,3-tris(m-methoxyphenyl)propynyl]benzene (3A), a structure that emulates agyroscope with a p-phenylene group acting as a rotator and two m-methoxy-substituted trityl groups actingas a stator. The line shape analysis of VT 13C CPMAS and broad-band 2H NMR data were in remarkableagreement with each other, with rotational barriers of 11.3 and 11.5 kcal/mol, respectively. The barriersobtained by analysis of ADPs obtained by single-crystal X-ray diffraction at 100 and 200 K, assuming asinusoidal potential, were 10.3 and 10.1 kcal, respectively. A similar analysis of an X-ray structure solvedfrom data acquired at 300 K suggested a barrier of only 8.0 kcal/mol. Finally, a rotational potential calculatedwith a finite cluster model using molecular mechanics revealed a symmetric but nonsinusoidal potentialthat accounts relatively well for the X-ray-derived values and the NMR experimental results. It is speculatedthat the discrepancy between the barriers derived from low and high-temperature X-ray data may be dueto an increase in anharmonicity, or to disorder, at the higher temperature values.