THE DIPOLE-MOMENT OF 18-CROWN-6 - MOLECULAR-DYNAMICS STUDY OF THE STRUCTURE AND DYNAMICS OF THE MACROCYCLE IN VACUO AND IN CYCLOHEXANE

We have performed very long simulations of the 18-crown-6 molecule in the gas phase and in cyclohexane. For the isolated molecule we have used two different sets of charges. For all simulations the average dipole moment was in moderate agreement with experiment. Therefore we have examined in some detail the theoretical models used for the interpretation. of the experiments. We propose a new formula, based on the Kirkwood equation, to calculate the molecular dipole moment from the experimental dielectric constants. With previously published experimental data, we have calculated a dipole moment that is somewhat larger than the originally reported value. We conclude that the charges that have been used in all potential models up to now may, at best, be treated as effective charges and that polarization is expected to be important. We made an extensive investigation of the structure of 18-crown-6 during the simulations. It was observed that conformational statistics was almost the same in vacuo and in the apolar cyclohexane. The structure of the crown ether is found to fluctuate around the centrosymmetric Ci conformation. A comparison is made with previously published statistical mechanical studies. We also examined the average shape of 18-crown-6 by looking at the mass distribution within the molecule. Again it was found that the crown ether, on average, displays an elliptical shape, consistent with the other results. Further, it was found that many of the samples show the same structural features, although they do not exhibit the same conformation. Finally, the dynamics of the different systems was investigated. As expected, it has been found that the solvent slows down the dynamics of the crown ether molecule. (C) 1995 American Institute of Physics.