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IR Matrix Spectroscopy of Pentachlorocyclopentadienyl Cation C5Cl5+. Effect of Chlorine as a Substituent

The first direct observation of pentachlorocyclopentadienyl cation (C5Cl5+) by IR spectroscopy was made in cryogenic SbF5 matrix. The structure of the ion was analyzed by ab initio DFT calculations. It was found that the characteristic stabilization effect of the chlorine n-electrons which was observed in other alpha-chlorocarboications (CCl3+ or C3Cl3+) is not operative in this molecule. This phenomenon was explained by possible destabilizing SOMO - n(chlorine) interaction. Similarity between the antiaromatic D5h structure of C5Cl5+ (global potential energy minimum) and the aromatic D6h structure of hexachlorobenzene is an argument which contributes to the theory that Dnh symmetry of cyclic conjugated pi-electron molecules is not caused by aromaticity.

pentachlorocyclopentadienyl cation; matrix isolation; antimony pentafluoride

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