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Influence of the chloro substituent position on the triplet reactivity of benzophenone derivatives: a time‐resolved resonance Raman and density functional theory study
Author(s) -
Li Wen,
Xue Jiadan,
Cheng Shun Cheung,
Du Yong,
Phillips David Lee
Publication year - 2012
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.3078
Subject(s) - chemistry , benzophenone , density functional theory , substituent , radical , reactivity (psychology) , photochemistry , hydrogen atom abstraction , raman spectroscopy , resonance (particle physics) , triplet state , computational chemistry , medicinal chemistry , molecule , organic chemistry , medicine , physics , alternative medicine , pathology , particle physics , optics
A nanosecond time‐resolved resonance Raman (ns‐TR 3 ) spectroscopic investigation of the photoreduction reactions and ability of several chloro‐substituted benzophenone (Cl‐BP) triplets is described. The TR 3 results show that the 3‐chlorobenzophenone (3‐Cl‐BP), 4‐chlorobenzophenone (4‐Cl‐BP) and 4,4′‐dichlorobenzophenone (4,4′‐dichloro‐BP) triplets exhibit similar hydrogen abstraction ability with the parent BP triplet. In 2‐propanol, the 3‐Cl‐, 4‐Cl‐ and 4,4′‐dichloro‐diphenylketyl (DPK) radicals were observed and they appear to react with dimethylketyl radicals at the para ‐position to form a light absorption transient species. These transient species were characterized with TR 3 spectra, and identified with the help of results from density functional theory calculations. In an acetontitrile/water (MeCN:H 2 O) 1:1 mixed solvent, these DPK radicals were also observed but with slower formation rates. However, the 2‐Cl‐DPK radical was observed to form with a lower yield and a significantly slower formation rate than the other chloro‐substituted benzophenones examined here in 2‐propanol under the same experimental conditions. These results reveal that the 2‐chloro substituent reduces the hydrogen abstraction ability of the substituted BP triplet, which was not as expected based on the assumption that the electron‐withdrawing group could increase its photoreduction ability. This unusual ortho effect of the chlorine substitution is briefly discussed. Copyright © 2011 John Wiley & Sons, Ltd.