The Influence of Aromaticity in Gas Chromatography Retention: The Case of Polycyclic Aromatic Sulfur Heterocycles

Aromaticity has been used as a criterion to explain the gas chromatographic (GC) retention of cata-condensed polycyclic aromatic sulfur heterocycles (PASHs) C12H8S, C16H10S, C20H12S; and peri-condensed PASHs C18H10S, in a GC column with 50% phenyl/50% dimethyl silphenylene polymer. To establish the aromaticity, nucleus-independent chemical shifts at the level of the molecular plane, NICS(0), and at 1 A above the surface of the molecular plane, NICS(1), have been used. It has been found that the GC retention of cata-condensed PASHs C12H8S, C16H10S, and C20H12S is satisfactorily defined by the aromaticity of the entire molecule, and the GC retention of peri-condensed PASHs C18H10S is satisfactorily defined by the local aromaticity in the sulfur pentagonal ring. In addition, the positive slope between GC retention and NICS(0) of the entire molecule for cata-condensed PASHs, C12H8S and C16H10S, and by NICS(1) in the pentagonal ring for peri-condensed PASHs, C18H10S, is explained by the interaction between the electrons of the heterocycle molecule and the positive pole of the silicon atom in the GC column, as suggested with PAHs. In contrast, the negative slope between GC retention and aromaticity for cata-condensed C20H12S is explained by the presence of bay, cove, or fjord regions in the vicinity of the sulfur atom that generates either higher GC retention and lower aromaticity or lower GC retention and higher aromaticity.