Semi-Empirical Study of a pH-Switchable [2] Rotaxane
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Semi-Empirical Study of a pH-Switchable [2] Rotaxane Laura Frankfort & Karl Sohlberg Department of Chemistry, Drexel University 3141 Chestnut Street, Philadelphia, PA, 19104 Abstract Semi-empirical (AM1) electronic structure calculations are reported on a pH-switchable [2] rotaxane whose synthesis was reported in [Angew. Chem. Int. Ed., 36 (1997) 1904]. The [2] rotaxane possesses two cationic binding sites, with different affinities toward dibenzo [24] crown-8 (the ring component of the [2] rotaxane) depending upon the pH of the system. The computational results capture this pH-sensitive bistability, the key device-like characteristic of the rotaxane. Introduction Rotaxanes are assemblies of two or more molecules that are mechanically linked but chemically independent [1]. A typical system consists of two components, a ring molecule threaded by a dumbbell-shaped molecule. Rotaxanes are related to, but topologically distinct from catenanes, which consist of two or more interlocked rings. In a rotaxane, the bulky ends on the dumbbell-shaped component prevent spontaneous unthreading of the ring. If the ring can be induced to move from one initially favored “station” on the dumbbell to a second “station” as a consequence of some external stimulus, then a very basic molecular shuttle has been produced. (Effectively, the external stimulus is changing the preferential binding site.) This property, that relative movement of their ring and dumbbell-shaped components may be induced with an external stimulus, endows the rotaxane with device-like character. Switchable rotaxanes therefore represent some of the simplest systems with the essential features of a molecular device. These features suggest that such systems hold promise for use as components in real molecular devices [2,3]. It is our goal to develop computational modeling techniques for switchable rotaxane systems. Modeling currently plays an important role in the design and development of mechanical devices. It is desirable in order to circumvent the need for time-consuming and expensive fabrication of numerous prototype systems. Modeling and simulation are expected to provide a shortcut to functional molecular devices as well, by enabling the screening of proposed designs and modifications based on predicted performance. Herein we report semi-empirical electronic structure calculations for a switchable rotaxane that has been demonstrated experimentally by Martinez-Diaz et al. [4] and show that these calculations capture the essential “device-like” character of the system. Theoretical Methods The AM1 semi-empirical method [5] was chosen to carry out the computations on this rotaxane system. Since the system contains only H, C, N, and O, it lies within the scope of applicability of the AM1 method [5], which is widely accepted as one of the most reliable semiempirical methods [6]. Unlike some other semi-empirical methods, AM1 has been found to be qualitatively acceptable for intermolecular hydrogen bonding [7], the dominant interaction
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