How residual entropy and hindered rotation influence sublimation equilibria in polyphenylbenzenes


C. Lima1, M. Rocha1, A. Melo1, L. Gomes2 and L. Santos1

1CIQ, Departamento de Quimica e Bioquimica, Faculdade de Ciencias da Universidade do Porto, Porto, Portugal
2CIAGEB, Faculdade de Ciencias da Saude, Universidade Fernando Pessoa, Porto, Portugal

Keywords: residual entropy, hindered rotation, vapour pressures, sublimation, gas phase
property: vapour pressures, enthalpies, entropies
material: polyphenylbenzenes

For a series of crystalline polyphenylbenzenes the vapour pressures as a function of temperature were measured using a recently developed Knudsen/Quartz crystal apparatus [1]. From this data the standard molar enthalpies, ΔsubH0, entropies, ΔsubS0, and Gibbs energies, ΔsubG0, of sublimation were derived. A poor correlation between ΔsubH0 or ΔsubS0 and the number of phenyl substituents, n(Ph), was found. A significant improvement in the correlation was obtained for ΔsubS0 = f{n(Ph)} when considering the residual entropy in the solid phase, Sresidual(cr), and the entropy associated to the hindered rotation of the phenyl substituents in the gas phase, SHR(g). The entropic corrections are based on the molecular symmetry derived from the X-ray data and from quantum chemical calculations in gas phase. These corrections take into account that:

- higher symmetry leads to higher absolute entropy of the crystal;

- less hindered rotation leads to a more pronounced increase in ΔsubS0, since the corresponding molecular vibrations are freed to a higher extent in the gas phase.

Concerning ΔsubH0, the results can be rationalized based on the molecular surface area available to establish cohesive interactions in the supramolecular structure.

Thanks are due to Fundação para a Ciencia e Tecnologia (FCT) and Programa Operacional Ciencia e Inovaçao 2010 (POCI 2010) for the financial support to Project POCI/QUI/61873/2004, supported by the European Community Fund FEDER. C.F.R.A.C. Lima also thanks FCT and the European Social Fund (ESF) under the third Community Support Framework (CSF) for the award of Ph.D. Research Grant SRFH/BD/29394/2006.

References
  1. L.M.N.B.F. Santos, L.M.S.S. Lima, C.F.A.C. Lima, F.D. Magalhães, M.C. Torres, B. Schröder, M.A.V. Ribeiro da Silva, J. Chem. Thermodyn. (2011) doi: 10.1016/j.jct.2010.12.022

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