The temperature dependence of gas chromatographic retention times of hexaheptacontane to dononacontane is reported. These data are used with a summary of earlier work to evaluate and compare the vaporization enthalpies and vapor pressures of the n-alkanes from T = (298.15 to 540) K for heneicosane to dononacontane. The vapor pressure and vaporization enthalpy results obtained are compared with estimated data from Morgan’s “PERT2” extended corresponding states principle (CSP) which uses n-alkane input parameters based on the works of Kudchadker and Zwolinski and of Tsu. The results are also compared with a model previously developed from empirical data which predicts that vaporization enthalpies measured at the boiling temperatures should approach a maximum value and then asymptotically approach zero as the chain length approaches infinity. Some curvature in the enthalpy of transfer from the gas chromatographic column to the gas phase, expected to show the same dependence on size, is indeed observed as the number of carbon atoms exceeds 60. The vapor pressure equations generated from the gas chromatographic results are used to predict boiling temperatures. A comparison of these temperatures with those obtained by extrapolation of an empirical fit of experimental boiling temperatures to a hyperbolic function is remarkably good.

The effects of temperature and flow rate on enthalpies of transfer and the resulting vaporization enthalpies measured by correlation-gas chromatography are examined at the mean temperatures of measurement for a series of alkanes. The study is an effort to understand the parameters affecting the magnitude of the enthalpy of transfer of the analyte from the stationary phase of the column to the gas phase. An endothermic enthalpy of solution or association of the analyte with the stationary phase of the column, ∆ sln H m (T m ), is observed which appears quite sensitive to temperature. A considerably less endothermic ∆ sln H m (T m ) value is observed with (D)-and (L)-menthol, and this becomes weakly exothermic with (D)-and (L)-limonene on chiral columns containing cyclodextrin and its derivatives. The endothermicity and sensitivity to temperature observed on achiral columns offers an explanation of how it is possible to measure enthalpies of vaporization of large hydrocarbons that exceed the strength of the weakest bond in the molecule.

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The vaporization enthalpies and liquid vapor pressures of quinoxaline, quinazoline, phthalazine, benzo[c]cinnoline, 1,10-phenanthroline, phenazine, and triphenylamine are measured by correlation−gas chromatography and compared to values calculated from available literature vaporization, sublimation, and fusion enthalpy data. Good agreement with the literature is observed for all compounds except the 1,2-diazines and the sublimation enthalpy of triphenylamine. The results suggest that liquid 1,2-diazines exhibit an additional intermolecular stabilizing interaction of approximately 6 kJ·mol−1 not present in other diazine isomers.

The vaporization enthalpies of a series of heterocyclic compounds some suspected of showing evidence of a higher degree of self-association in comparison to many other nitrogen heterocycles have been measured both by correlation gas chromatography (CGC) and for comparison by transpiration. The compounds share some structural similarities. They include 2- and 4-(N,N-dimethylamino)pyridine, 1,5-diazabicyclo[4.3.0]non-5-ene, 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,2,4-triazolo[1,5-a]pyrimidine and imidazo[1,2-a]pyridine. The vaporization enthalpies of the first three of the compounds are well reproduced by a standard series of reference compounds, mainly pyridine derivatives. The latter two compounds and 4-(N,N-dimethylamino)pyridine exhibit vaporization enthalpies approximately 7 kJ·mol(-1) larger. Their values are reproduced by using standards previously shown to exhibit stronger but similar self-association properties. The crystal structure of 1,2,4-triazolo[1,5-a]pyrimidine has been determined in an effort to understand the nature of the self-association. The molecule crystallizes in planar stacks slightly offset with a stacking distance of 3.24 Å.