The Electronic Spectrum of o-Benzyne Experiment and Theory, a Critical Evaluation

Semiempirical valence electron calculations using the CNDO/S SECI, HAM/3 CI, LNDO/S PERTCI, MNDOC PERTCI, and MNDO/S PERTCI methods have been applied to the interpretation of the recently reported UV/vis absorption spectrum of transient o-benzyne. Detailed calculations were performed for benzene, o-benzyne, cyclopentadienylidenecarbene, cyclopentadienylideneketene, and cyclopentadienylideneketone (6-fulvenone). From these calculations and experimental data for benzene, obenzyne, and cyclopentadienylideneketone, an interpretation of the electronic absorption spectrum of o-benzyne is proposed. The interpretation includes the proposal that the first band and a part of the third band originally assigned to o-benzyne may be due to cyclopentadienylideneketene, a common by-product (or intermediate) of the photolyses of benzocyclobutenedione and 3-diazobenzofuranone.