Separation of aromatic isomers on cyclophane-bonded stationary phases

A cyclophane (CP66)-bonded silica gel stationary phase (CP66-SP) was prepared and the retention of water-insoluble hydrophobic compounds on it was investigated in comparison with that on the CP44-bonded stationary phase (CP44-SP) reported previously. Like CP44-SP, it retained aromatic compounds more strongly than the corresponding alicyclic compounds, as was expected by the cavity size of the cyclophane. The CP66-SP also showed isomer-selectivity for monosubstituted and disubstituted naphthalenes, but its selectivity was perfectly reversed to that of the CP44-SP. On the CP66-SP, isomers having methyl and ethyl groups at beta-position were eluted prior to those having groups at alpha-position, whereas on the CP44-SP beta-substituted naphthalenes were retained more strongly than alpha-substituted ones. Isomers of three- and four-ring aromatic compounds were also separated on these cyclophane-bonded stationary phases. The retention order on the CP66-SP was almost opposite to that on the CP44-SP; on the CP66-SP, the retention order was phenanthrene > anthracene, and chrysene > 1,2-benzanthracene > 2,3-benzanthracene, whereas on the CP44-SP, anthracene > phenanthrene, and 2,3-benzanthracene > chrysene > 1,2-benzanthracene. The retention mechanism of aromatic compounds is discussed on the basis of the structure of the cyclophane-involved complex.