Acetylketene: Conformational Isomerism and Photochemistry. Matrix Isolation Infrared and Ab Initio Studies

The s-Z and s-E conformers of acetylketene (5) were generated by flash vacuum pyrolysis (FVP) and matrix photolysis of precursors 1-4. Both conformers were characterized by low-temperature IR spectroscopy in argon, xenon, and nitrogen matrices. The observed frequencies and intensities are in excellent agrement with ab initio calculations at the MP2/6-31G* level. UV irradiation of s-Z/s-E-5 in nitrogen or xenon matrices allows the clean interconversion of conformers. Prolonged UV irradiation resulted in decarbonylation and formation of methylketene (7). However, the photolysis of argon matrix isolated 5 produced a new set of bands in the IR spectrum very close to the s-Z/s-E absorptions. Annealing of the matrix to 35 K removed these bands. This unusual phenomenon was shown to be due to a photochemically induced matrix site effect. Consistent with experimental observations, high-level calculations (QCISD(T)/6-311+G(2d,p)+ZPVE) predict that the s-Z and s-E conformers have virtually identical energies.