Infrared absorption of 3-propenonyl (ṡCH ₂ CHCO) radical generated upon photolysis of acryloyl chloride [cH ₂ CHC(O)Cl] in solid para-H ₂

Irradiation at 193 nm of a p-H ₂ matrix containing acryloyl chloride CH ₂ CHC(O)Cl at 3.2 K yielded infrared absorption lines at 3143.6 (ν ₁ ), 3057.0 (ν ₂ ), 3048.0 (ν ₃ ), 2103.1 (ν ₄ ), 1461.0 (ν ₅ ), 1349.8 (ν ₆ ), 1223.7 (ν ₁₁ +ν ₁₂ or 2ν ₁₂ ), 1092.8 (ν ₈ ), 918.1 (ν ₉ ), 691.0 (ν ₁₀ ), 624.3 (ν ₁₁ ), and 597.1 (ν ₁₂ ) cm ^-1 that are assigned to the 3-propenonyl (ṡCH ₂ CHCO) radical. The assignments are based on the photolytic behavior and a comparison of observed vibrational wavenumbers and infrared intensities with those predicted with the B3PW91/aug-cc-pVDZ method. The observation is consistent with a major radical formation channel CH ₂ CHCO + Cl followed by escape of the Cl atom from the original p-H ₂ cage. The observation of 3-propenonyl (ṡCH ₂ CHCO) radical but not 3-propenalyl (s-cis- or s-trans-CH ₂ CHĊO) radical indicates that the former is the most stable isomer and that the barrier heights for conversion from s-cis- or s-trans-CH ₂ CHĊO to ṡCH ₂ CHCO are small.