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Probing the Ignored Elimination Channel of Br 2 in the 248 nm Photodissociation of 1,1‐Dibromoethylene by Cavity Ring‐Down Absorption Spectroscopy
Author(s) -
Lee PingChen,
Tsai PoYu,
Hsiao MingKai,
Lin KingChuen,
Huang C. H.,
Chang A. H. H.
Publication year - 2009
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.200800665
Subject(s) - photodissociation , chemistry , dissociation (chemistry) , ab initio quantum chemistry methods , ab initio , spectroscopy , population , isomerization , absorption spectroscopy , ground state , singlet state , atomic physics , photochemistry , excited state , molecule , biochemistry , physics , demography , organic chemistry , quantum mechanics , sociology , catalysis
A process of elimination : Optical spectra of the Br 2 product eliminated from the photodissociation of 1,1‐dibromoethylene at 248 nm are obtained (see picture), and the subsequent dissociation pathways are proposed with the aid of ab initio potential energy calculations. The behavior of the compound is compared with that of 1,2‐dibromoethylene.In the photodissociation of 1,1‐C 2 H 2 Br 2 at 248 nm, the Br 2 elimination channel is probed by using cavity ring‐down absorption spectroscopy (CRDS). In terms of spectral simulation, the vibrational population ratio of Br 2 ( v =1)/Br 2 ( v =0) is found to be 0.55±0.05, which indicates that the Br 2 fragment is vibrationally hot. The rotational population is thermally equilibrated with a Boltzmann temperature of 349±38 K. According to ab initio potential energy calculations, the obtained fragments are anticipated to result primarily from photodissociation of the ground electronic state that undergoes 1) H migration followed by three‐center elimination, and 2) isomerization forming either trans ‐ or cis ‐1,2‐C 2 H 2 Br 2 from which Br 2 is eliminated. RRKM calculations predict that the Br 2 dissociation rates through the ground singlet state prevail over those through the triplet state. Measurements of temperature and Ar pressure dependence are examined to support the proposed pathway via internal conversion. The quantum yield for the Br 2 elimination reaction is determined to be 0.07±0.04. This result is smaller than that obtained in 1,2‐C 2 H 2 Br 2 , probably because the dissociation rates are slowed in the isomerization stage.