Competitive Bond Rupture in the Photodissociation of Bromoacetyl Chloride and 2- and 3-Bromopropionyl Chloride: Adiabatic versus Diabatic Dissociation

• Published in: Chemphyschem Vol. 14; no. 5; pp. 936 - 945
• Main Authors: Hsu, Ming-Yi, Tsai, Po-Yu, Wei, Zheng-Rong, Chao, Meng-Hsuan, Zhang, Bing, Kasai, Toshio, Lin, King-Chuen
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 02.04.2013; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: Atomic and molecular physics; bond energy; cleavage reactions; diabatic effects; Exact sciences and technology; Molecular properties and interactions with photons; Photon interactions with molecules; Physics; reaction mechanisms; velocity ion imaging; bond energy; Imaging; Reaction mechanism; reaction mechanisms; Ions; Dissociation; Photodissociation; velocity ion imaging; Velocity; diabatic effects; cleavage reactions; Organic compounds
• ISSN: 1439-4235; 1439-7641; 1439-7641
• DOI: 10.1002/cphc.201200957

Competitive bond dissociation mechanisms for bromoacetyl chloride and 2‐ and 3‐bromopropionyl chloride following the 1[n(O)→π*(CO)] transition at 234–235 nm are investigated. Branching ratios for CBr/CCl bond fission are found by using the (2+1) resonance‐enhanced multiphoton ionization (REMPI) technique coupled with velocity ion imaging. The fragment branching ratios depend mainly on the dissociation pathways and the distances between the orbitals of Br and the CO chromophore. CCl bond fission is anticipated to follow an adiabatic potential surface for a strong diabatic coupling between the n(O)π*(CO) and np(Cl)σ*(CCl) bands. In contrast, CBr bond fission is subject to much weaker coupling between n(O)π*(CO) and np(Br)σ*(CBr). Thus, a diabatic pathway is preferred for bromoacetyl chloride and 2‐bromopropionyl chloride, which leads to excited‐state products. For 3‐bromopropionyl chloride, the available energy is not high enough to reach the excited‐state products such that CBr bond fission must proceed through an adiabatic pathway with severe suppression by nonadiabatic coupling. The fragment translational energies and anisotropy parameters for the three molecules are also analyzed and appropriately interpreted. Busted open: Insight into the mechanisms causing CCl and CBr bond fission of bromoacetyl chloride and 2‐ and 3‐bromopropionyl chloride by following the 1[n(O)→π*(CO)] transition is obtained. The figure shows the center‐of‐mass translational energy distributions of ground‐state Br formation through a diabatic pathway for the dissociation of 2‐bromopropionyl chloride.