INVESTIGATION OF CHARGE-TRANSFER COMPLEXES BY COMPUTER-SIMULATION .2. IODINE IN PYRIDINE SOLUTION

In this article we report molecular-dynamics (MD) simulation results for the thermodynamic, structural, and dynamic properties of a dilute solution of iodine in liquid pyridine (Py). The molecules of the simulated solution (10I2 + 246Py) interact through site-site intermolecular potentials for which the pyridine molecules are modeled by 12 sites (11 atomic sites plus a lone-pair site on the nitrogen atom) and the iodine molecules by four sites (two sites coinciding with the iodine atoms plus two extra sites). The site-site potentials contain isotropic terms to describe the electrostatic, dispersion and repulsion interactions between molecules. The potential parameters are determined from a Monte Carlo (MC) simulation of the iodine-pyridine complex in the gaseous phase where it appears that the most stable conformation is realized when iodine and pyridine are in a linear arrangement (the iodine molecule interacting with the nitrogen atom of the pyridine molecule). The MC study also shows that the complex becomes unstable near room temperature. On the contrary, in liquid pyridine our molecular-dynamics simulation predicts that iodine and pyridine molecules associate to form long-lasting (1:2) complexes, the latter ones (Py-I2-Py) adopting a nearly linear conformation. Concerning the intermolecular motions the simulation indicates that the stretching mode of the I-I...N bonds produces the predominant contribution to the far-infrared absorption spectrum of the iodine solution and is responsible of the sharp band observed on the experimental spectra reported in the literature.