X-ray-induced transformation of o-vinylbenzaldehyde and 2-methylbenzocyclobutenone to an o-quinoid ketene and its radical cation

Upon ionization in argon matrices by X-irradiation, o-vinylbenzaldehyde (VBA) partially undergoes transfer of the formyl hydrogen atom to the vinylic CH2 group, thereby forming a quinoketene radical cation of the type previously described (Bally, T.; Michalak, J. J. Photochem. Photobiol. A: Chem. 1992, 69, 185). This process does not manifest itself clearly in the optical spectra because the transitions of the quinoketene cation coincide with the much more intense ones of VBA(.+) and are mostly obscured by those. However, the IR spectra show unambiguously that the rearrangement occurs rather efficiently. Also they show that in the course of the X-irradiation, the tautomerized cations are reneutralized to from the quinoketene tautomer of VBA in good yield. The quinoketene and its radical cation were independently obtained from 2-methylbenzocyclobutenone which undergoes spontaneous ring-opening on photolysis and/or ionization. The optical spectra of the radical cations are assigned on the basis of CASPT2 calculations and the different species involved in this study are characterized at the B3LYP/6-31G* level.