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

摘要

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 K and 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 Ogawa 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)] molecular crystals.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.