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

The 4,4'-bis(diphenylhydroxymethyl)biphenyl/acetone (DHMB-Ac) system was studied by NPT molecular dynamics (MD) calculations; the study was prompted by the availability of X-ray single-crystal diffraction and calorimetric data (L. Johnson, L. Nassimbeni, E. Weber and K. Skobridis, J. Chem. Soc. Perkin Trans. 2, 1992, 2131). Potential energies were calculated by using the UNI-FF or the OPLS all-atom force fields. The kinetics of formation of hydrogen-bonded aggregates between the DHMB molecule and acetone was sampled, and the persistence and fluxionality of the O-H...O=C hydrogen bond are clearly revealed. Extensive MD runs at variable temperature on the inclusion compound in its crystalline state allow a description of the thermal motion and eventual reorientation within its cavity of the acetone molecule; the host crystalline matrix does not survive when guests are removed. The dynamic evolution of a slab of 60 DHMB and 120 acetone molecules. which has the experimental crystal structure, was also simulated at the temperature experimentally observed for guest desorption, and the loss of solvent molecules was 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 back 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 of the computational sample.