A thermochemical and theoretical study of the phenylpyridine isomers

The standard (p degrees = 0.1 MPa) molar enthalpies of formation for 2-, 3- , and 4-phenylpyridine in the, gas phase were derived from the standard mol ar enthalpies of combustion, in oxygen, at 298.15 K, measured by static bom b combustion calorimetry. The standard molar enthalpies of vaporization for 2-, 3-, and 4-phenylpyridine at T = 298.15 K were measured by correlation- gas chromatography. The enthalpy of sublimation of 4-phenylpyridine was obt ained as a weighted mean of the value derived from the vaporization and fus ion enthalpy values and the value measured directly by Calvet microcalorime try. The following enthalpies of formation were then derived: 2-phenylpyrid ine, g = 228.3 +/- 5.8 kJ.mol(-1); 3-phenylpyridine, g = 240.9 +/- 5.5 kJ.m ol(-1); 4-phenylpyridine, g = 240.0 +/- 33 kJ.mol(-1). The most stable geom etries of all phenylpyridine isomers were obtained using both restricted Ha rtree-Fock (RHF) and density functional theory (DFT/B3LYP) methods. The res ulting geometries were then used to obtain estimates of enthalpies of forma tion of the three isomers of phenylpyridine, which are in good agreement wi th the experimental values. A theoretical interpretation of the effect of t he phenyl ring has on the relative stabilities of the three molecules is pr esented.