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Chemical physics of solids and their surfaces Volume 7; A by M W Roberts; J M Thomas; R I Bickley

By M W Roberts; J M Thomas; R I Bickley

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C 40 K), In the case of p-terphenyl both constrained and unconstrained lattice energy minimizations have been carried out 5 2 with respect to external and internal variables and the results are in excellent agreement with the X-ray studies;44 on an average the molecules take up non-planar conformations with two different twist angles for the extreme rings and the lattice energies were lower compared with that of the room temperature phase (with planar molecules). Calculations 53 on biphenyl, however, do not give rise to a lower energy structure with non-planar molecules; both constrained and unconstrained optimizations yield only the planar conformations for the molecules in the crystal.

1978,2§, 237. The Use of Atom-Atom Potentials 43 Figure 3 The paired molecules in the unit cell of Pyrene: (a) in the room temperature phase: (b) in low temperature phase of these calculations one may well ask whether the stability of different phases of polymorphic materials can be predicted by atom-atom potentials ? Though one may be tempted to say affirmatively (according to one's experience), the approximations in the derivations of the potentials in the present functional form, preclude prediction of such small energy changes involved during a phase transition.

42 The resulting molecular conformations indicated an increased overlap between the excimeric molecules in confirmity with the shift to longer wavelengths observed in the low temperature phase (see Figure 3). These results support further experimental evidence from optical transmittance mea~urements:~ which indicated a first order transition occurring through domain by domain formed temporarily in the crystal, In the light 36 R. Zallen, C. H. Griffiths, M. L. Slade, M. Hayek, and 0. Brajman, Chem.

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