Phenylnaphthalenes: Sublimation Equilibrium, Conjugation, and Aromatic Interactions


In this work, the interplay between structure and energetics in some representative phenylnaphthalenes is discussed from an experimental and theoretical perspective. For the compounds studied, the standard molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, were determined by the measurement of the vapor pressures as a function of T, using a Knudsen/quartz crystal effusion apparatus. The standard molar enthalpies of formation in the crystalline state were determined by static bomb combustion calorimetry. From these results, the standard molar enthalpies of formation in the gaseous phase were derived and, altogether with computational chemistry at the B3LYP/6-311++G(d,p) and MP2/cc-pVDZ levels of theory, used to deduce the relative molecular stabilities in various phenylnaphthalenes. X-ray crystallographic structures were obtained for some selected compounds in order to provide structural insights, and relate them to energetics. The thermodynamic quantities for sublimation suggest that molecular symmetry and torsional freedom are major factors affecting entropic differentiation in these molecules, and that cohesive forces are significantly influenced by molecular surface area. The global results obtained support the lack of significant conjugation between aromatic moieties in the alpha position of naphthalene but indicate the existence of significant electron delocalization when the aromatic groups are in the beta position. Evidence for the existence of a quasi T-shaped intramolecular aromatic interaction between the two outer phenyl rings in 1,8-di([1,1-biphenyl]4-yl)naphthalene was found, and the enthalpy of this interaction quantified on pure experimental grounds as -(11.9 +/- 4.8) kJ.mol(-1), in excellent agreement with the literature CCSD(T) theoretical results for the benzene dimer.



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Lima, CFRAC; Rocha, MAA; Schroder, B; Gomes, LR; Low, JN; Santos, LMNBF

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Thanks are due to Fundacao para a Ciencia e Tecnologia (FCT) and Programa Operacional Ciencia e Inovacao 2010 (POCI 2010) supported by the European Community Fund FEDER for the financial support to Project POCI/QUI/61873/2004. C.F.R.A.C.L., M.A.A.R., and B.S. also thank FCT and the European Social Fund (ESF) under the third Community Support Framework (CSF) for the award of the research grants: SFRH/BD/29394/2006, SFRH/BD/60513/2009, and SFRH/BPD/38637/2007, respectively.

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