Detection and characterisation of oxocyclohexan[b]indoles by combined application of vibrational spectroscopy and mass spectrometry

The vibrational and mass spectra of a representative set of nine oxocyclohexan[b]indoles (tricyclic indoles with a 6,5,6 ring pattern) with a carbonyl group in each of the four positions of the third non-aromatic ring and up to two methyl substituent(s) attached to the carbon atoms in the other positions of this ring are reported and discussed. Particular attention is focused on the use of infrared spectroscopy to determine whether these compounds may best be described as ketoindoles or hydroxyindolenines. Most of these spectra indicate that the tricyclic heterocycles exist preferentially at ketoindoles, but the solid state infrared spectra of oxocyclohexan[b]indoles with the oxygen function in the 4-position are highly unusual, giving the initial impression that these compounds are hydroxyindolenines. Computational modelling supports the interpretation that the 4-ketoindole tautomer has a lower energy than either of its most stable isomeric indolenines conformers. Mass spectrometry is shown to provide valuable confirmatory evidence to differentiate the isomeric ionised oxocyclohexan[b]indoles, which dissociate via distinctive and informative fragmentation routes, in which the stability of intermediate distonic ions appears to be profoundly influenced by participation of the lone pair of electrons on the nitrogen atom. Many of these fragmentations are better described by stepwise mechanisms, rather than as concerted cycloreversions.