The relationship between partition enthalpy and connectivity characteristics of chlorinated biphenyl isomers under isothermal GC conditions

Abstract

The capillary gas chromatograph column retention time under isothermal conditions was observed for 46 pentachlorobiphenyls (PeCBs). Relative changes of the partition enthalpy (Δ(ΔH)) and entropy (Δ(ΔS)) were calculated on the basis of van' Hoff relation. A marked linear relationship between them, so called classical enthalpy & entropy compensation, was observed. A clear decrease of Δ(ΔH) was found with the decrease in the number of chlorine atoms substituted at 2, 2', 6, and 6' positions. The decrease indicated the enhancement of the interaction between the liquid phase in the column and the PeCB molecule, and seemed to be related to the planarity of the molecule. Furthermore, the difference of Δ(ΔH) from the linear relationship (ΔH-LFER) was investigated in terms of the number of a pair of proximitive hydrogen atoms (Npx-H). Negative difference was observed when Npx-H was 0 or 1, and positive difference was observed when Npx-H was 3 or 4. In addition, almost linear dependence of ΔH-LFER on Randic''s 2χ, known as a typical connectivity descriptor, was also revealed. To characterize the ΔH-LFER, which is discrepancy from the linear free energy relationship, quantitative evaluation of the effect induced by the atoms substituted at proximitive positions will be an important part.