Structure, energetics, and dynamics of pedal-like motion in stilbene from molecular simulation and ab initio calculations

Abstract

Molecular simulations of solid stilbene in the isothermal-isobaric ensemble with variable-shape simulation are reported. The structure has been characterized by means of lattice parameters and radial distribution functions. Simulations show the existence of pedal-like motion at higher temperatures in agreement with the recent X-ray diffraction measurements by Ogawa and co-workers and several others previously. The difference in energy between the major and minor conformers and the barrier to conformational change at both the crystallographic sites have been calculated. The temperature dependence of the equilibrium constant between the two conformers as well as the rate of conversion between the conformers at the two sites have also been calculated. These are in agreement with the recent analysis by Harada and Ogawa of nonequilibrium states obtained by rapid cooling of stilbene. (Harada, J.; Ogawa, K. J. Am. Chem. Soc. 2004, 126, 3539.) An estimate of the activation energies for interconversion between the two conformers at the two sites is reported. The volume and the total intermolecular energy suggests the existence of two transitions in agreement with previous Raman phonon spectroscopic and calorimetric studies. They seem to be associated with change from order to disorder at the two sites. Ab initio calculations coupled with simulations suggest that the disorder accounts for only a small part of the observed shortening in ethylene bond length.