Rotational Dynamics in a Crystalline Molecular Gyroscope by Variable-Temperature 13C NMR, 2H NMR, X-Ray Diffraction, and Force Field Calculations

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• 作者：Tinh-Alfredo V. Khuong ; Hung Dang ; Peter D. Jarowski ; Emily F. Maverick ; Miguel A. Garcia-Garibay
• 刊名：Journal of the American Chemical Society
• 出版年：2007
• 出版时间：January 31, 2007
• 年：2007
• 卷：129
• 期：4
• 页码：839 - 845
• 全文大小：295K
• 年卷期：v.129,no.4(January 31, 2007)
• ISSN：1520-5126

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.