Importance of Correlated Motions on the Low Barrier Rotational Potentials of Crystalline Molecular Gyroscopes
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- 作者：Peter D. Jarowski ; K. N. Houk ; Miguel A. Garcia-Garibay
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：March 21, 2007
- 年：2007
- 卷：129
- 期：11
- 页码：3110 - 3117
- 全文大小：418K
- 年卷期：v.129,no.11(March 21, 2007)
- ISSN：1520-5126
文摘
The energetic and structural changes taking place upon rotation of the central phenylene of1,4-bis(3,3,3-triphenylpropynyl)benzene in the solid state were computed using molecular mechanicscalculations. Pseudopolymorphic crystals of a benzene clathrate (1A) and a desolvated form (1B) wereanalyzed with models that account for varying degrees of freedom within the corresponding lattices. Thecalculated rotational barriers in a rigid lattice approximation, 78 kcal/mol for 1A and 72 kcal/mol for 1B, areabout 5 times greater than those previously measured by variable-temperature 13C CPMAS NMR andquadrupolar echo 2H NMR line-shape analysis: 12.8 kcal/mol for 1A and 14.6 kcal/mol for 1B. The potentialenergy barriers calculated with a model that restricts whole body rotation and translational motions butallows for internal rotations give results that are near the experimental free-energy barriers. The calculatedbarriers for 1A and 1B are 15.5 and 16.2 kcal/mol, respectively. The differences between the rigid andpartially relaxed models are attributed to the effect of correlated motions of the lattice and the rotatinggroup, which are evident from the structural analysis of the atomic position data as a function of the dihedralangle of the rotator. The displacements of neighboring molecules near the rotary transition states for 1Aand 1B can be as large as 2.7 and 1.1 Å, respectively. The displacement and oscillation (C2) ofinterpenetrating phenyl rings from neighboring rotors proximal to the event are significant for both 1A and1B. In addition, 6-fold (C6) benzene rotations in clathrate 1A were found to be directly correlated to therotation of the phenylene rotator.