Thermodynamic insights on the structure and energetics of s-triphenyltriazine

For s-triphenyltriazine, at T = 298.15 K, were measured the standard (p(0) = 10(5) Pa) molar enthalpy of combustion, by static bomb combustion calorimetry, and the standard molar enthalpy, entropy, and Gibbs energy of sublimation by Knudsen/Quartz crystal effusion. A comparison between the entropies of sublimation of s-triphenyltriazine and the isosteric 1,3,5-triphenylbenzene gave a good indication that the higher symmetry of the former contributes significantly to the decrease of its volatility. A computational study at the MP2/cc-pVDZ and B3LYP/6-311++g(d,p) levels of theory was carried out in order to obtain the gas phase geometry, enthalpy, and barriers to internal rotation about the phenyl-triazine bonds. Making use of homodesmotic reaction schemes, a marked stabilization was observed in the molecule of s-triphenyltriazine relative to analogous systems. This result is supported both experimentally and computationally and, combined with a detailed analysis of the literature data concerning the energetics and structure of related compounds, pointed to a significant enthalpic stabilization associated with the exchange of an intramolecular Ar-H···H-Ar close contact by an Ar-H···N(Ar) one. An inspection of the ring-ring torsional profiles in azabenzenes and biphenyls, obtained computationally at the SCS-MP2/cc-pVDZ level, showed that the ring-ring torsions are the dimensions of the potential energy surface (PES) that chiefly determine the energetic differentiation in this class of compounds.