Collect. Czech. Chem. Commun. 1998, 63, 1094-1106

Bent Cyclopenta-2,4-dienylideneketene: Spectroscopic and ab initio Study of Reactive Intermediate

Juliusz G. Radziszewskia, Piotr Kaszynskib, Anders Friderichsena and Jens Abildgaardc

a National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 and Department of Chemical Engineering and Petroleum Refining, Colorado School of Mines, Golden, CO 80401, U. S. A.
b Department of Chemistry, Vanderbit University, Nashville, TN 37235, U. S. A.
c Department of Life Sciences and Chemistry, Roskilde University, DK-4000 Roskilde, Denmark


Results of an experimental and theoretical study of cyclopenta-2,4-dienylideneketene (3), a highly unstable reactive intermediate, are reported. The ketene was prepared, under matrix isolation conditions at 4.2 or 10 K, by laser photocarbonylation of 1,2-didehydrobenzene (1) photogenerated earlier from phthalic anhydride (2). FTIR polarization measurements performed on partially photooriented samples of 3 immobilized in solid neon or argon provide infrared transition moment directions for most of the observed vibrations. Experimental results confirm that the ketene is bent, as predicted by ab initio calculations. Utilizing two isotopically modified 3, 3b and 3c, on the basis of the infrared absorption spectrum alone, we have analyzed and assigned its vibrations in a way, which leaves no doubt about the bent ketene structure. This work was motivated by a long standing confusion surrounding the assignments of the vibrations in 1,2-didehydrobenzene (1), especially of its "triple" bond stretch.

Keywords: Matrix isolation; Photolysis of phthalic anhydride; IR spectrum of cyclopenta-2,4-dienylideneketene; Ketenes; Benzyne; Ab initio calculations.