In their crystalilne phases, the conformation of non-rigid polyphenyl molecules results from a delicate balance between competing intramolecular and intermolecular forces. At low temperature, the polyphenyl crystals undergo structural phase transitions associated with stabilization of a nonplanar conformation, with respect to a torsional angle between the phenyl rings. An interesting feature is the different natures of these phase transitions: displacive in biphenyl and order-disorder in p-terphenyl. In order to illustrate this point we consider the following features: the thermal motion in the high-temperature phases, the influence of hydrostatic pressure on the phase transition in p-terphenyl, the transitional heat capacities, the observation of the critical slowing down of fluctuations in p-terphenyl and the existence of incommensurate phases in biphenyl.

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Articles you may be interested inStructural optimization of molecular clusters with density functional theory combined with basin hopping Conformational distribution of a ferroelectric liquid crystal revealed using fingerprint vibrational spectroscopy and the density functional theory Structure and conformation properties of 1-alkyl-3-methylimidazolium halide ionic liquids: A density-functional theory study Investigation of structure of liquid 2,2,2 trifluoroethanol: Neutron diffraction, molecular dynamics, and ab initio quantum chemical study First the conformations of various ortho and di-ortho substituted toluenes calculated by quantum chemistry methods are discussed as well as the hindering potentials deduced from the latter results and those established experimentally by microwaves and fluorescence techniques. It appears that methyl ͑Me͒ groups are much less hindered in di-ortho than in ortho substituted compounds. Then the study of the 1,3,5-triiodo-2,4,6-trimethylbenzene ͑triiodomesitylene or TIM͒ is reported. Density functional theory ͑DFT͒ calculations indicate that two conformations of the TIM molecule have the same formation energies. One has C 3h symmetry, the other with the C s symmetry is obtained from the C 3h by a rotation of 60°for one Me. Experimentally, the TIM structure has been determined at 15 K using single-crystal neutron diffraction data. TIM crystallizes in the triclinic space group P Ϫ1. Molecules are stacked in an antiferroelectric manner along the oblique a axis. For two Me groups the experimental conformation is close to the C s one, but the third Me group has a C-H bond nearly orthogonal to the ring plane. Such conformation is unstable in the gas state, but it is stabilized in the crystal by intermolecular interactions, nevertheless DFT predicts accurate bond lengths and angles when the Me conformations are constrained to the experimental ones. The three Me groups, having different environments, experience different hindering potentials, this explains why they are tunnelling at different energies as found by inelastic neutron scattering ͑INS͒. Using INS results, we deduced the potentials hindering the Me groups rotation in the crystal. The proton probability densities ͑PPD͒ calculated from these potentials are in concordance with the crystallographic results. Hence the quantum origin of the broad spreading of PPD observed at 15 K is established. Moreover, it is demonstrated that the crystal field is responsible for the larger part of the potentials hindering the Me groups in the solid state.