Ethylcarbene versus Direct Propene Formation in the Near-UV Photodissociation of Ethylketene

The competing pathways in the photodissociation of gaseous ethylketene at excitation wavelengths of 320.0, 340.0, and 355.1 nm were studied using photofragment translational energy spectroscopy. The primary dissociation channel was C═C bond fission producing ethylcarbene (CH₃CH₂CH; also known as propylidene) and CO. Product translational energy distributions are consistent with theoretical predictions that ground state ethylcarbene lies ∼34 kJ/mol higher in energy than its isomer dimethylcarbene (CH₃CCH₃). A second dissociation channel involved direct formation of propene prior to or concurrent with CO elimination. The measured product branching ratios indicate that the effective potential energy barrier for the direct propene channel lies below the energetic threshold for ethylcarbene formation. A minor C-C bond fission channel was also observed, leading to CH₃ + CH₂CHCO products. Comparisons are made to the results of our recent studies of methylketene and dimethylketene photodissociation.