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The sublimation enthalpies of 17 hydrocarbons are obtained by combining the technique of correlation gas chromatography (CGC), to evaluate vaporization enthalpies at 298.15 K, and differential scanning calorimetry (DSC) to measure fusion enthalpies. Vaporization enthalpies at 298.15 K obtained by CGC are compared to values measured directly from vapor pressure measurements at temperatures above the melting point by adjusting the experimental vaporization enthalpy for the effects of temperature. Vaporization enthalpies obtained by these two methods agree within AE3877 J mol À1 . Fusion enthalpies are similarly adjusted for temperature. Sublimation enthalpies, obtained by combining temperature adjusted fusion, and vaporization enthalpies agree within AE2580 J mol À1. The sublimation enthalpies of azulene and 1,8-cyclotetradecadiyne are also measured by head-space analysis resulting in values of 76880 and 94348 J mol À1 at 298.15 K, respectively. # 1998 Elsevier Science B.V.

The vaporization enthalpies and vapor pressures of 1-, 6-, 7-, and 9-heptadecanol, 1-octadecanol, 1-eicosanol, 1-docosanol, 1-hexacosanol and cholesterol at T = 298.15 K have been measured by correlation gas chromatography using as standards, the even carbon n-alkanols from 1-decanol to 1-octadecanol and 1-pentadecanol. Fusion enthalpies for the all of these compounds were either measured by DSC or obtained from the literature. Adjusted to T = 298.15 K, the fusion and vaporization enthalpies were combined to provide sublimation enthalpies. The sublimation enthalpies were compared to existing literature values. Agreement between the two sets of values when available was generally very good.

The standard (p°) 0.1 MPa) molar enthalpies of formation for 2-, 3-, and 4-phenylpyridine in the gas phase were derived from the standard molar enthalpies of combustion, in oxygen, at 298.15 K, measured by static bomb 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 obtained as a weighted mean of the value derived from the vaporization and fusion enthalpy values and the value measured directly by Calvet microcalorimetry. The following enthalpies of formation were then derived: 2-phenylpyridine, g) 228.3 (5.8 kJ‚mol-1 ; 3-phenylpyridine, g) 240.9 (5.5 kJ‚mol-1 ; 4-phenylpyridine, g) 240.0 (3.3 kJ‚mol-1. The most stable geometries of all phenylpyridine isomers were obtained using both restricted Hartree-Fock (RHF) and density functional theory (DFT/B3LYP) methods. The resulting geometries were then used to obtain estimates of enthalpies of formation of the three isomers of phenylpyridine, which are in good agreement with the experimental values. A theoretical interpretation of the effect of the phenyl ring has on the relative stabilities of the three molecules is presented.

The melting behavior of a homologous series is described in terms of the melting of the parent molecule and of the polymer the series eventually forms. For those series characterized by a parent melting below the melting temperature of the related polymer, the melting behavior can be described quantitatively by the hyperbolic functionwhere T f (n) refers to the melting temperature of a compound with n repeat units, T f (∞) is the melting temperature of the polymer, and m and b are two variables used in fitting the data. A plot of [1/(1 -T f (n)/T f (∞))] against n results in a straight line with slope m and intercept b. This linear relationship provided the analytical form of the equation described above. For series with parents exhibiting melting temperatures higher than those of the related polymer, a linear correlation is observed when]. These equations appear applicable for the quantitative evaluation of the melting behavior of any homologous series, provided care is taken to consider compounds characterized by the same symmetry number. Molecules containing odd and even numbers of repeat groups are generally treated separately. The hyperbolic behavior exhibited by the melting temperature in most series appears characteristic of molecules that seem to pack similarly in the solid state. Series with members exhibiting liquid-crystal behavior are successfully modeled by these equations, provided the transition correlated is the temperature at which the liquid becomes isotropic. The usefulness of these equations was tested by selecting three data points from each series to provide values for m and b. The melting temperatures of most compounds in the series were estimated using these parameters. This resulted in a standard deviation of (6.6 K between experimental and calculated values based on a total of 995 compounds.

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