Controlling the frequency of macrocyclic ring rotation in benzylic amide [2]catenanes
Controlling the frequency of macrocyclic ring rotation in benzylic amide [2]catenanes
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DOI：
10.1021/ja974065m
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发表时间：
1998-07-08
影响因子：
15
通讯作者：
Zerbetto, F
中科院分区：
文献类型：
--
作者：
Leigh, DA;Murphy, A;Zerbetto, F
变温H-1 NMR光谱和分子力学计算的组合已被用来探测的因素，确定在苄酰胺[2]索烃的大环旋转速率。结果表明，互锁的大环动力学是由一个微妙的组合的空间位阻效应，错综复杂的大环间的氢键阵列，π-π堆积，和T人字形的相互作用。氢键断裂和形成的级联是回旋过程中的主要事件（一个大环围绕另一个大环完全旋转），但伴随着一系列有助于稳定分子能量的协同构象和共构象重排。实验图片是一致的，当活化能从NMR信号的聚结测量时，速率常数直接测量自旋极化转移通过选择性反转恢复（SPT-SIR）方法。旋转过程的性质意味着二酰基芳族单元的精确结构对大环旋转的频率有巨大的影响：在室温下，2，5-噻吩基索烃的旋转速度比类似的2，6-吡啶基系统快320万倍！环境的极性也在决定环间动力学中起着至关重要的作用：通过使用氢键破坏溶剂（甲醇，DMSO）降低基态氢键网络的强度，通过降低旋转的活化能（通常在11-20 kcal mol（-1）的范围内）增加旋转速率，最高可达3.2 kcal mol（-1）。这允许精细控制互锁分子系统的各个组分的平移行为的动力学，这是它们作为纳米级穿梭机、开关和信息存储系统发展的关键要求。
A combination of variable-temperature H-1 NMR spectroscopy and molecular mechanics calculations have been used to probe the factors that determine the rate of macrocyclic ring rotation in benzylic amide [2]catenanes. The results show that the interlocked macrocycle dynamics are governed by a delicate combination of steric effects, intricate inter-macrocyclic arrays of hydrogen bonds, pi-pi stacking, and T herringbone-type interactions. A cascade of hydrogen-bond ruptures and formations is the principal event during circumvolution (complete rotation of one macrocyclic ring about the other) but is accompanied by a series of cooperative conformational and co-conformational rearrangements that help to stabilize the energy of the molecule. The experimental picture is consistent both when activation energies are measured from the coalescence of NMR signals and when rate constants are directly measured by spin polarization transfer by selective inversion recovery (SPT-SIR) methods. The nature of the circumrotational process means that the precise structure of the diacylaromatic units has a tremendous effect on the frequency of macrocyclic ring rotation: a 2,5-thiophene-based catenane rotates 3.2 million-fold faster than the analogous 2,6-pyridine-based system at room temperature! The polarity of the environment also plays a crucial role in determining the inter-ring dynamics: reducing the strength of the ground-state hydrogen-bonding network by employing hydrogen bond-disrupting solvents (methanol, DMSO) increases the rate of rotation by lowering the activation energy for circumvolution (normally in the region of 11-20 kcal mol(-1)) by up to 3.2 kcal mol(-1). This allows exquisite control over the kinetics of the translational behavior of the individual components of an interlocked molecular system, a key requirement for their development as nanoscale shuttles, switches, and information storage systems.