Add time:08/15/2019         Source:sciencedirect.com

Some time ago empirical observation of a heretofore unpublished set of electronic spectra of substituted tropylium ions led to tentative classification of them into three types based on degree of electronic interaction between substituent and ring: Type I, perturbation of symmetry from D7h to C2v with no interaction; Type II, intermediate type resulting from increased polarizabilty of substituent; and Type III, substituent to ring charge transfer. The spectra have now been prepared for publication, and the nature of the substituent–ring electronic interactions examined with ab initio (3-21(G∗)) molecular orbital methods. There are three types, although the assignment of specific compounds to type varies some from the original. In Type I the substituent π-electron pair resides on substituent in the lowest π-orbital, and the HOMO to LUMO (HL) transition is essentially that of an unaffected tropylium ion. In Type II, the substituent π-electron pair is delocalized over the ring in an extended conjugated system; the HL transition has a small bias towards increased ring electron density in the LUMO. In Type III the substituent π-electron pair resides on substituent in the highest π-orbital, and the HL transition is a substituent to ring charge-transfer absorption.

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