Phase transitions in molecular crystal 4,4′-bis(6-hydroxy-1-hexyloxy)biphenyl studied by molecular dynamics simulations and IR spectroscopy

Molecular dynamics (MD) simulations in an NPT ensemble combined with temperature dependent IR spectroscopic measurements were used to study the phase transitions in a molecular crystal of a mesogenic diol -4,4'-bis(6-hydroxy-1-hexyloxy)biphenyl (D). A molecule of D consists of two distinct parts: a stiff biphenyl group and two flexible ICH:),OH chains. The potential energy was calculated using the peff_300 force field in Cerius(2) modelling environment. The results of MD calculations revealed the changes of molecular acid crystal structure at two phase transitions temperatures, T-1 similar to 365 K (crystal-crystal) and T-2 similar to 445 K (crystal-isotropic state); these temperature estimated by differential scanning calorimetry measurements correspond to those observed by IR spectroscopy. MD calculations showed that the structural changes occurring during the first and second phase transitions could be attributed to changes in the conformation of the flexible part of D (connected with T-1 transition) and the stiff part of D (connected with T-2 transition. The distortions in the flexible parts of D molecules above T-1-transition result in the break up of a regular network of hydrogen bonds between diols and consequently a disturbance in the layer ordering of molecules The distortions in the stiff biphenyl group above T-2-transition indicate the premelting stage with a large departure from the three-dimensional molecular ordering. (C) 2001 Elsevier Science B.V. All rights reserved.