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• Vibrational spectroscopy shows oxocyclohexan[ b ]indoles are ketoindole tautomers. • Computational data confirms oxocyclohexan[ b ]indoles are most stable as ketoindoles. • Vibrational spectroscopy differentiates isomeric oxocyclohexan[ b ]indoles. • Mass spectrometry gives complementary information on oxocyclohexan[ b ]indoles. • Fragmentation of ionised oxocyclohexan[ b ]indoles involves stepwise mechanisms. 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. [Display omitted] [ABSTRACT FROM AUTHOR]

RATIONALE When subjected to positive ion electrospray ionisation (ESI+) mass spectrometry (MS), indoles with a 3-alkyl substituent show a propensity to form novel [2M-H]+ 'covalently bound dimers'. This process, which appears to be initiated in the nebuliser of the instrument, is mechanistically interesting, analytically useful and potentially significant in organic synthesis. METHODS A selection of 2- and 3-substituted indoles have been synthesised and analysed by ESI-MS. The formation of the 'homo' and 'hetero' dimers of these compounds has been investigated using ESI+ mode. The mechanism of formation of the observed 'dimeric' species has been probed by synthesising authentic samples of the dimeric compounds. RESULTS 'Dimeric' species corresponding to [2M-H]+ have been observed for all 3-substituted indoles studied, but not for indoles substituted in just the 2-position. By infusing equimolar mixtures of labelled and unlabelled indoles through the instrument, the expected approximately statistical mixture of homo- and heterodimeric species has been observed. Further experiments have established that this novel dimerisation occurs in the droplets formed in the nebuliser of the instrument. CONCLUSIONS It has been shown that 3-substituted indoles form [2M-H]+ dimers in high abundance in the spray obtained from the nebiliser of an ESI+ instrument. The mechanism for the dimerisation does not involve the known 2M dimeric species that is readily formed in the solution-phase chemistry of indoles. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]