Temperature-induced dynamical conformational disorder in 4-vinyl benzoic acid molecular crystals: A molecular simulation study

Extensive molecular simulations are carried out as a function of temperature to understand and quantify the conformational disorder in molecular crystals of 4-vinyl benzoic acid. The conformational disorder is found to be dynamic and associated with a flip-flop motion of vinyl groups. The population of minor conformer is less than 3% up to 300 Kand is 13.2% at 350 K and these results are consistent with the experimental observations. At still higher temperatures, the population of minor conformer increases up to 25%. The evolution of structure at both molecular and unit-cell level of the molecular crystal as a function of temperature has been characterized by various quantities such as radial distribution functions, average cell parameters, volume,and interaction energies. The van't Hoff plot shows a nonlinear behavior at lower temperatures as it has been reported recently by O gawa and co-workers in the case of stilbene, azobenzene [J. Am. Chem.Soc. 126, 3539 (2004)], and N-(4-methylbenzylidene)-4-methylaniline[Acta Crystallogr, Sect. B. Struct. Sci. B60, 589 (2004)] molecularcrystals. A set of rigid body simulations were also carried out to quantify the effect of conformational disorder on structural quantities such as unit-cell volume and interaction energy. The anomalous shrinkage of vinyl C=C bond length as a function of temperature has been explained by combining the results of simulations and a set of constrained optimizations using ab initio electronic structure calculations for various molecular structures differing in torsional angle