Ab initio study of the Wolff rearrangement of C6H4O intermediate in the gas phase

Ab initio calculations of the geometry and reactivity of 1,2-ketocarbene C₆H₄O as an intermediate in organic reactions were performed using the second-order Møller-Plesset (MP2) perturbation theory in the 6-311G^* basis set. Only the singlet state of the intermediate was considered. An oxirene-like structure (6) with a six-membered ring and a ketene-like structure (5) with a five-membered ring were localized on the potential energy surface. Attempts to locate a quinone type structure characteristic of aliphatic ketocarbenes failed. The energy of structure5 is ?70 kcal mol^?1 lower than that of structure6. Harmonic frequencies and intensities of normal vibrations in the IR spectra of6 and5 were calculated. The activation energy of the Wolff rearrangement6→5 was estimated at 12.5 kcal mol^?1. The geometry of the transition state of this reaction resembles the quinone-like structure.