MOLECULAR-DYNAMICS OF GUEST RADICALS IN UREA INCLUSION-COMPOUNDS

Molecular dynamics simulations of several radical guests included in u rea clathrates have been performed for a period of 1 ns. The starting conformations of the radicals have been obtained by AM1 quantum mechan ical computations. The host matrix has been modelled as a single chann el formed by 47 or 72 urea molecules, and different constraints impose d on the urea molecule positions have been tested to optimize the effe ct of the host confinement on the guest radicals. Two kinds of motion have been considered, the internal dynamics of the methylene group bet a to the radical centre and the rotation of the guest inside the host channel. The simulation results have been compared with experimental d ata obtained by electron paramagnetic resonance spectroscopy. Good agr eement is found for the internal motion when a finite harmonic constra int is imposed on the urea carbonyl. The correlation time for the mole cular rotation is estimated to be longer than the simulation period. T he slow reorientation process should be controlled by the concurrent r earrangement of the host matrix.