DIFFUSIVE MOTIONS IN LIQUID 18-CROWN-6 - A MOLECULAR-DYNAMICS STUDY

Transport properties of 18-crown-6 in the liquid phase-act investigate d by means of molecular dynamics simulations. Three different force fi elds are used. It is attempted to separate molecular motions into inde pendent contributions from translations, rotations, and deformations. Translational diffusion coefficients are calculated and they are found to depend very much on the molecular flexibility, i.e., on the potent ial model. With two potential models, diffusion: coefficients are obta ined which are in good agreement with experimental data. With one of t hese force fields the possibility is investigated to define molecule-f ixed frames which allow a separation of relations and deformations. Tw o different definitions are suggested for this purpose. Combining cont ributions to the hydrogen displacements from translational, rotational , and intramolecular motions, and comparing them to the actual displac ements, it is found that one of the definitions fails, and the other p erforms reasonable well, It is found that the hydrogen displacements m ay very well be modeled by assuming independent translational and rota tional motions. Attempts to obtain rotational diffusion coefficients f rom fitting the data using a symmetric diffusor model were unsuccessfu l. This was imputed to the large difference between the timescales for the different orientational motions and illustrates that experimental results should be met with reservation. (C) 1997 American Institute o f Physics.