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Competitive Bond Rupture in the Photodissociation of Bromoacetyl Chloride and 2‐ and 3‐Bromopropionyl Chloride: Adiabatic versus Diabatic Dissociation
Author(s) -
Hsu MingYi,
Tsai PoYu,
Wei ZhengRong,
Chao MengHsuan,
Zhang Bing,
Kasai Toshio,
Lin KingChuen
Publication year - 2013
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201200957
Subject(s) - chemistry , diabatic , excited state , dissociation (chemistry) , photochemistry , bond cleavage , photodissociation , chloride , vibronic coupling , adiabatic process , molecule , atomic physics , catalysis , organic chemistry , physics , thermodynamics
Competitive bond dissociation mechanisms for bromoacetyl chloride and 2‐ and 3‐bromopropionyl chloride following the 1 [n(O)→π*(CO)] transition at 234–235 nm are investigated. Branching ratios for CBr/CCl 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 CO chromophore. CCl bond fission is anticipated to follow an adiabatic potential surface for a strong diabatic coupling between the n(O)π*(CO) and n p (Cl)σ*(CCl) bands. In contrast, CBr bond fission is subject to much weaker coupling between n(O)π*(CO) and n p (Br)σ*(CBr). 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 CBr 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.