Dynamics of Benzene Guest Inside a Self-Assembled Cylindrical Capsule:A Combined Solid-State ~2H NMR and Molecular Dynamics Simulation Study

摘要

The reorientational dynamics of benzene-fife molecules hosted into the cavity of a cavitand-based,self-assembled capsule was investigated by Molecular Dynamics(MD)simulations and temperature-dependent solid-state ~2H NMR spectroscopy.MD simulations were preliminarily performed to assess the motional models of the guest molecules inside the capsules.An in-plane fast reorientation of the benzene guest around the C_6 symmetry axis(B1 motion),characterized by correlation times of the order of picoseconds,was predicted with an activation barrier(approx 8 kJ/mol)very similar to that found for neat benzene in the liquid state.An out-of-plane reorientation corresponding to a nutation of the C_6 symmetry axis in a cone angle of 39 deg(B2 motion,373 K)with an activation barrier(approx 39 kJ/mol)definitely larger than that of liquid benzene was also anticipated.In the temperature range 293-373 K correlation times of the order of a nanosecond have been calculated and a transition from fast to slow regime in the ~2H NMR scale has been predicted between 293 and 173 K.~2H NMR spectroscopic analysis,carried out in the temperature range 173-373 K on the solid capsules containing the perdeuterated guest(two benzene molecules/capsule),confirmed the occurrence of the B1 and B2 motions found in slow exchange in the ~2H NMR time scale.Line shape simulation of the ~2H NMR spectral lines permitted defining a cone angle value of 39 deg at 373 K and 35 deg at 173 K for the nutation axis.The T_1 values measured for the ~2H nuclei of the encapsulated aromatic guest gave correlation times and energetic barrier for the in-plane motion B1 in fine agreement with theoretical calculation.The experimental correlation time for B2 as well as the corresponding energetic barrier are in the same range found for Bl.A molecular mechanism for the encapsulated guest accounting for the B1 and B2 motions was also provided.