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

The reorientational dynamics of benzene-d(6) molecules hosted into the cavity of a cavitand-based, self-assembled capsule was investigated by Molecular Dynamics (MD) simulations and temperature-dependent solid-state H-2 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 (similar to 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 degrees (B2 motion, 373 K) with an activation barrier (similar to 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 H-2 NMR scale has been predicted between 293 and 173 K. H-2 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 degrees at 373 K and 35 degrees 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 B1. A molecular mechanism for the encapsulated guest accounting for the B1 and B2 motions was also provided