Carbonium ion salts. Part 15. Correlation of electronic spectra of substituted tropylium (cycloheptatrienylium) ions with the nature of electronic interaction between substituent and ring

Some time ago empirical observation of a heretofore unpublished set of elec tronic spectra of substituted tropylium ions led to tentative classificatio n of them into three types based on degree of electronic interaction betwee n substituent and ring: Type I, perturbation of symmetry from D-7h to C-2v with no interaction; Type II, intermediate type resulting from increased po larizabilty of substituent; and Type III, substituent to ring charge transf er. The spectra have now been prepared for publication, and the nature of t he substituent-ring electronic interactions examined with ab initio (3-21(G *)) molecular orbital methods. There are three types, although the assignme nt of specific compounds to type varies some from the original. In Type I t he substituent pi-electron pair resides on substituent in the lowest pi-orb ital, and the HOMO to LUMO (HL) transition is essentially that of an unaffe cted tropylium ion. In Type II, the substituent pi-electron pair is delocal ized 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 I II the substituent pi-electron pair resides on substituent in the highest p i-orbital, and the HL transition is a substituent to ring charge-transfer a bsorption. (C) 2000 Elsevier Science B.V. All rights reserved.