A broad band at λmax ≈ 620 mm confirms the singlet character of the carbene 1, since no other conceivable compounds in the system should absorb in this region. Moreover, calculations on dialkylcarbenes yield similar λ values. Compound 1 rearranges photochemically to 2, and on warming in a CO‐doped argon matrix it affords 3.

The photoelectron (PE) spectra of cyclopentadiene (CP) and its perdeuterated isotopomer (cP-d6) and the electronic absorption (EA) and infrared (IR) spectra of the corresponding radical cations are presented. The electronic structure of C P + is discussed on the basis of CASSCF/CASPT2 calculations which reproduce the excited-state energies as derived from the PE and EA spectra very well. Density functional force fields are scaled to fit the observed bands in the IR spectra and vibrational progressions in the first PE bands. The changes in valence force constants upon ionization of CP are discussed on the basis of qualitative expectations and compared to those found earlier for butadiene.

The syn dimer of cyclobutadiene (tricyclo[4.2.0.0(2.5)]octa-3,7-diene, TOD) is subjected to ionization under different conditions and the resulting species are probed by optical and ESR spectroscopy. By means of quantum chemical modelling of the potential energy surfaces and the optical spectra, it is possible to assign the different products that arise spontaneously after ionization or after subsequent warming or illumination of the samples. Based on these findings, we propose a mechanistic scheme which involves a partitioning of the incipient radical cation of TOD between two electronic states. These two states engage in (near) activation-less decay to the more stable valence isomers, cyclooctatetraene (COT*+) and a bis-cyclobutenylium radical cation BCB*+. The latter product undergoes further rearrangement, first to tetracyclo[4.2.0.0(2,4).0(3,5]oct-7-ene (TCO*+) and eventually to bicyclo[4.2.0]octa-2,4,7-triene (BOT*+) which can also be generated photochemically from BCB*+ or TCO*+. The surprising departure of syn-TOD*+ from the least-motion reaction path leading to BOT*+ can be traced to strong vibronic interactions (second-order Jahn-Teller effects) which prevail in both possible ground states of syn-TOD*+. Such effects seem to be more important in determining the intramolecular reactivity of radical cations than orbital or state symmetry rules.

The recently discovered access paths to the radical cation of bicyclo[3.3.0]octa-2,6-diene-4,8-diyl (BOD'+) are explored by electronic absorption (EA) spectroscopy whereby previous ESR results are confirmed. The electronic and molecular structure of BOD'+ and of its photoprecursor, the radical cation of cyclooctatetraene (COT+), are discussed on the basis of their EA spectra and ab initio calculations. The ground and excited state potential surfaces common to the title cations are explored, and it is shown that the COT+ -BOD'+ photorearrangement proceeds mainly by virtue of a pronounced Jahn-Teller distortion of the second excited state of COT+ (2E). This distortion competes effectively with internal conversion to the first excited state, leads to an inversion of the ground state symmetry, and covers a substantial part of the reaction path leading to the bisallylic cation.

The anti dimer of cyclobutadiene (anti-tricyclo[4.2.0.0(2.5)]octa-3,7-diene, TOD) is subjected to ionization by gamma-irradiation in Freon matrices, pulse radiolysis in hydrocarbon matrices, and photoinduced electron transfer in solution. The resulting species are probed by optical and ESR spectroscopy (solid phase) as well as by CIDNP spectroscopy (solution). Thereby it is found that ionization of anti-TOD invariably leads to spontaneous decay to two products, that is bicyclo[4.2.0]octa-2,4,7-triene (BOT) and 1,4-dihydropentalene (1,4-DHP), whose relative yield strongly depends on the conditions of the experiment. Exploration of the C8H8*+ potential energy surface by the B3LYP/6-31G* density functional method leads to a mechanistic hypothesis for the observed rearrangements which involves a bifurcation between a pathway leading to the simple valence isomer, BOT*+, and another one leading to an unprecedented other valence isomer, the anti form of the bicyclo[3.3.0]octa-2,6-diene-4,8-diyl radical cation (anti-BOD*+). The latter product undergoes a very facile H-shift to yield the radical cation of 1,3a-dihydropentalene (1,3a-DHP*+) which ultimately rearrranges by a further H-shift to the observed product, 1,4-DHP*+.