The Radical Cation ofsyn-Tricyclooctadiene and Its Rearrangement Products

Abstract

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.

Photochemistry of Reactive Intermediates

“…In this way we have studied several photoinduced ring-opening reactions of radical cations containing three- [17][18][19][20][21] or four-membered rings [22][23][24][25], phototautomerizations of ionized ketones and enols [26][27][28], and rotamer inter-conversions in polyene radical cations [29][30][31]. I will use two examples to illustrate the photochemistry of radical cations:…”

Matrix Isolation

“…Nevertheless, if applied with due circumspection, CASPT2 is a very reliable method and has been applied succesfully to the modeling of electronic spectra of a wide variety of compounds, including many matrix-isolated reactive intermediates in the author's laboratory. 8,[134][135][136][137][138][139][140][141][142] An entirely different approach to ab initio calculations of excited states is to look at the response of the ground-state electron distribution to oscillating electric fields of different frequencies. If the frequency of such an oscillation is in resonance with an electronic excitation, then the polarizability will increase discontinuously, that is, the frequency-dependend polarizability will show a pole whose intensity is proportional to the electronic transition moment.…”

Unusual Rearrangements of Radical Cations: The Role of Vibronic Coupling

“…Three examples of surprising radical cation rearrangements. [1] For the purpose of this paper I have rerun all the calculations with a modern functional (M06-2X), and the relative energies are based on single-point CCSD(T)/cc-pVTZ calculations, which was not possible at the time when we did the original work. [1a] syn-TOD •+ has a totally symmetric ground state ( 2 A 1 ) which, according to the photoelectron spectrum of TOD, [4] is separated from the first excited state ( 2 B 1 ), by ca.…”