Abstract
Gas chromatographic measurements of the relative retention times of alkyl-substituted arenes and phenols on capillary columns at temperatures of 125 – 140°C have shown that logarithms of retention times increase bilinearly with the number of carbon atoms in the molecule. It was found that in a high density stationary phase, the longer alkyl side chains of compounds in question are subject to cyclization as a result of the resistance force of this phase affecting molecules during their thermal and diffusion motion. Consequently, common conventional aromatic-aliphatic molecules become new molecules with quasi-alicyclic rings. In comparison with the conventionally conceived molecules, the resulting aromatic–quasi-alicyclic molecules are characterized by rather different, possibly even completely different non-covalent interactions between the molecules, which then affect the retention characteristics. Moreover, cyclization facilitates the mixing of n-alkyl arenes and n-alkyl phenols with high-molecular stationary phases, because the thermodynamic condition for mixing is better fulfilled.
Highlights
The following considerations presented were based on gas chromatographic measurements on capillary columns (50 m, an internal diameter of 0.25 mm) with stationary phases Apiezon K and tri (2,4-xylenyl) phosphate-phosphoric acid (95:5) [1,2,3,4,5,6]
The side-chain cyclization was based on the concept that in a high-density stationary phase, the longer side alkyl chains of n-alkyl phenols and n-alkyl benzenes are subject to deformation as a result of the resistance of this phase affecting the molecules of these compounds in motion
The calculation results for the energies of the covalent bonds and non-covalent interactions are demonstrated for meta n-alkyl phenols and n-alkyl benzenes both common and cyclized into the meta position with respect to the substituent, are summarized in Tables 2 and 3
Summary
The following considerations presented were based on gas chromatographic measurements on capillary columns (50 m, an internal diameter of 0.25 mm) with stationary phases Apiezon K (nonpolar) and tri (2,4-xylenyl) phosphate-phosphoric acid (95:5) (polar) [1,2,3,4,5,6]. With the increasing number of carbons in the molecule of n-alkyl phenols and nalkyl benzenes, the intermolecular interactions between the compounds in question and the stationary phase might change non-linearly, but these interactions must manifest themselves differently in polar and nonpolar phases. The aim of the presented work is to explain the bi- linearity in the logarithmic dependence of the relative retention time on the number of carbons in the molecule in the case of n-alkyl phenols and n-alkyl benzenes by molecular mechanics modeling and to clarify the behavior of these compounds in the environment of a stationary phase of a capillary column. The potential energies of the bonds and interactions have been considered for the cal- culations
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