Conformational analysis, rigid polycycles

Since carbocations are structurally similar to ketones, they are discussed here. Schleyer s force field incorporates carbocation parameters, and Harris is exploring their application in conformational analysis (188). The calculated angles in a series of rigid polycyclic carbocations correlated well with ketone infrared frequencies (188a). The calculated relative stabilities among various conformers of tertiary cations of methylcyclohexane, methylcycloheptane, and methyl-cyclooctane do not contradict the limited MNR observations of these species at low temperature (188b). 

The ramifications of conformational analysis of flexible and rigid ring systems are of considerable importance to the understanding of stability and reactivity in polycyclic systems. This will become increasingly evident in later discussions. 

The chemistry of centrotetraindane 8 has not been explored in detail. It may be noted, however, that the rigidity of the tribenzotriquinacene backbone renders the framework of 8 also conformationally rigid, giving rise to a single, Cs-symmetric conformer, as confirmed by X-ray structure analysis [114]. Bridgehead-functionalized derivatives of 8 should provide a wealth of further carbon-rich polycyclic compounds. 

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