Cyclopentadienylideneketene: theoretical consideration of an infrared spectrum frequently mistaken for that of benzyne

Ab initio molecular quantum mechanics has been applied to the cyclopentadienylideneketene molecule, a C 7 H 4 O isomer. Until very recently, it was thought that this molecule had not been observed spectroscopically. However, recent experiments appear to show that an infrared feature near 2085 cm -1 (and often attributed to benzyne) is due to cyclopentadienylideneketene. Here the structure of the latter molecule has been optimized using three theoretical methods, including second-order perturbation theory with a triple-ζ plus double-polarization (TZ2P) basis set