Vase-Kite Equilibrium of Resorcin[4]arene Cavitands Investigated Using Molecular Dynamics Simulations with Ball-and-Stick Local Elevation Umbrella Sampling

A key feature of resorcin[4]arene cavitands is their ability to switch between a closed/contracted (Vase) and an open/expanded (Kite) conformation. The mechanism and dynamics of this interconversion remains, however, elusive. In the present study, the Vase-Kite transitions of a quinoxaline-based and of a dinitrobenzene-based resorcin[4]arene are investigated using molecular dynamics (MD) simulations in three environments (vacuum, chloroform, and toluene) and at three temperatures (198.15, 248.15, and 298.15K). The challenge of sampling the Vase-Kite transition, which occurs experimentally on the millisecond time scale, is overcome by calculating relative free energies using ball-and stick local elevation umbrella sampling (B&S-LEUS) to enhance the statistics on the relevant states and to promote interconversion transitions. Associated unbiased MD simulations also evidence for the first time a complete Vase-to-Kite transition, as well as transitions between degenerate Kite1 and Kite2 forms and solvent-exchange events. The calculated Vase-to-Kite free-energy changes G are in qualitative agreement with the experimental magnitudes and trends. The level of quantitative agreement is, however, limited by the force-field accuracy and, in particular, by the approximate treatment of intramolecular interactions at the classical level. The results are in line with a less stable Vase state for the dinitrobenzene compared to the quinoxaline compound, and a negative entropy change S for the Vase-to-Kite transition of the latter compound. Relative free energies calculated for intermediates also suggest that the Vase-Kite transition does not follow a concerted mechanism, but an asynchronous one with sequential opening of the flaps. In particular, the conformation involving two adjacent flaps open in a parallel direction (cis-p) represents a likely intermediate, which has not been observed experimentally to date.