The author flashes back over 35 years of ESR spectroscopy of organic radical ions. A few examples from five favourite classes of compounds are presented.
I. Nonalternant p-systems: acenaphth[1,2-a]acenaphthylene.
II. Bridged annulenes: 1,6:8,13-ethanediylidene[14]annulene and N,N'-dimethyl-syn-1,6:8,13-diimino[14]annulene. III. Cyclophanes: [2.2]paracyclophane.
IV. Multiple p-electron acceptors: 4', 4", 5', 5"-tetraphenyl-1,2:9,10-dibenzo[2.2]paracyclophane-1,9-diene and 11,11,12,12-tetracyanopyreno-2,7-quinodimethane.
V. Small hydrocarbons: s-trans-buta-1,3-diene, cyclobutene, bicyclo[1.1.0]butane, benzvalene, bicyclopropylidene and tetramethyleneethane.
The ENDOR technique was amply used to analyze complex or incompletely resolved ESR spectra. The interest focused on structural features of the radicalions, such as lack pairing properties of frontier orbitals in the class I, effects of the bridging on such orbitals of the p-perimeters in II, lowering of symmetry by association with the counterion in III, consequences of passing from mono- to polyions in IV, and geometry, shape of the singly occupied MO, and rearrangements in V.