MOLECULAR MODELING STUDIES ON THE THIOUREA FERROCENE CLATHRATE/

Computer modelling studies by means of molecular dynamics, molecular m echanics and Monte Carlo simulations have been carried out on the thio urea:ferrocene clathrate. The known crystal structure of this host:gue st system was used as a model system containing four ferrocene guest m olecules in a thiourea host channel of 16 Angstrom in length. Molecula r dynamics and molecular mechanics calculations indicated that the clo se packing of ferrocene molecules in the channel occurred despite stro ng steric repulsive interactions associated with neighbouring ferrocen e molecules in the channel when they are positioned along the crystall ographic sites. The neighbouring ferrocene molecules rotate and tilt w ith respect to each other in order to reduce the intermolecular repuls ions. A second model containing a single ferrocene guest molecule in t he thiourea host channel has also been studied. The guest adopts eithe r a parallel alignment where the molecular five-fold axis coincides wi th the channel trigonal axis, or a perpendicular orientation or an int ermediate orientation. Results from the molecular dynamics and molecul ar mechanics calculations show that the parallel positions are favoure d. Monte Carlo simulations indicate a population distribution of 60%/4 0% for ferrocene in the parallel/perpendicular orientations. The ferro cene molecule alternates between the two orientations in the thiourea channel in order to avoid the repulsion inherent from the ideal parall el or perpendicular arrangement. Energy calculations show that only sm all movements off the channel trigonal axis are possible.