Diffusive motions in liquid 18-crown-6: A molecular dynamics study

Transport properties of 18-crown-6 in the liquid phase-act investigated by means of molecular dynamics simulations. Three different force fields are used. It is attempted to separate molecular motions into independent 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 potential model. With two potential models, diffusion: coefficients are obtained which are in good agreement with experimental data. With one of these force fields the possibility is investigated to define molecule-fixed frames which allow a separation of relations and deformations. Two different definitions are suggested for this purpose. Combining contributions to the hydrogen displacements from translational, rotational, and intramolecular motions, and comparing them to the actual displacements, it is found that one of the definitions fails, and the other performs reasonable well, It is found that the hydrogen displacements may very well be modeled by assuming independent translational and rotational motions. Attempts to obtain rotational diffusion coefficients from fitting the data using a symmetric diffusor model were unsuccessful. 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 of Physics.