ESR studies of some nonbenzoid radical ions
Two classes of nonbenzoid compounds have been considered in this lecture: (1) nonalternant hydrocarbons, and (2) bridged [10]and [14]-annulenes. (1) The radical anion and cation of a nonalternant hydrocarbon should exhibit dissimilar e.s.r. spectra, in contrast to the two corresponding radical ions of an alternant system. This prediction of a simple MO model has been verified for four nonalternant compounds: acepleiadylene (I), acenaphth[ 1 ,2-a] acenaphthylene (II), acepleiadiene (III), and 3,5,8, 1 0-tetramethylcyclopenteno[ef]heptalene (IV). In all cases there is an excellent agreement between ex:periment and theory. (2) The degeneracies of both the lowest antibonding and the highest bonding perimeter orbitals are removed in a bridged [10]or [14]-annulene. The resulting energy sequence can be inferred from an MO perimeter model, if an adequate substituent effect is assumed for the bridging group. Since the e.s.r. spectra of the radical ions of bridged annulenes are determined by the energy sequence in question, they offer an experimental test of such an assumption. The substituent effect has been discussed for the bridging groups in 1,6methano[ lO]annulene (V), its iminoand oxido-analogues (VI and VII), syn-1,6: 8, 13-bisoxido[14] annulene (VIII), trans-15,16-dimethyl-15, 16-dihydropyrene (IX), and cyc1 [3,2,2] azine (X). INTRODUCTION Electron spin resonance (e.s.r.) spectroscppy is a powerful tool for testing the reliability of current n-electron models. With the exception of a few cases, only minor changes in the n-electron structure result, when an electron is added to or removed from an extended aromatic system. The radical ions can therefore be described by the same kind of n-electron models as the parent neutral compounds, and the information extracted from the e.s.r. spectra of the radical ions will in general also be characteristic of the neutral compounds1·2. Since the benzoid compounds are readily available, and their radical ions usually enjoy a remarkable stability, numerous studies of such radicals have been reported. In contrast, the number of radical ions derived from nonbenzoid aromatics is relatively modest1. A few examples will be presented in this lecture which attempts to give some insight into the potential of e.s.r. spectroscopy in the sturlies of interesting 7t-electron systems. * The topics dealt with in this lecture have been recently reviewed in ref. 1. 131