A molecular dynamics study of the structure and evolution of the 4,4 '-bis(diphenylhydroxymethyl)biphenyl/acetone host-guest system

The 4,4'-bis(diphenylhydroxy-methyl)biphenyl/acetone (DHMB-Ac) system was s tudied by NPT molecular dynamics (MD) calculations; the study was prompted by the availability of X-ray single-crystal diffraction and calorimetric da ta (L, Johnson, L. Nassimbeni, E. Weber and K. Skobridis, J. Chem. Soc Perk in Trans. 2, 1992, 2131). Potential energies were calculated by using the U NI-FF or the OPLS all-atom Force fields. The kinetics of formation of hydro gen-bonded aggregates between the DHMB molecule and acetone was sampled, an d the persistence and fluxionality of the O-H ... O=C hydrogen bond are cle arly revealed, Extensive MD runs at variable temperature on the inclusion c ompound in its crystalline state allow a description of the thermal motion and eventual reorientation within its cavity of the acetone molecule; the h ost crystalline matrix does not survive when guests are removed. The dynami c evolution of a slab of 60 DHMB and 120 acetone molecules, which has the e xperimental crystal structure, was also simulated at the temperature experi mentally observed for guest desorption. and the loss of solvent molecules w as monitored. Crystalline order is very quickly lost; then, guest molecules can easily diffuse away from the surface without a noticeable swelling of the material. The molecular reorganization of the remaining host liquid bac k to the crystal structure of one of the two polymorphs of pure DHMB could not be observed, owing to the short simulation times and to the smallness o f the computational sample.