Journal
JOURNAL OF ORGANIC CHEMISTRY
Volume 85, Issue 18, Pages 11635-11640Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.joc.0c01115
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Funding
- National Natural Science Foundation of China [21973075]
- Research Grants Council of Hong Kong [HKU17301815]
- Areas of Excellence Scheme [AoE/P-03/08]
- UGC Special Equipment Grant [SEG-HKU-7]
- University of Hong Kong Development Fund 2013-2014 project New Ultrafast Spectroscopy Experiments for Shared Facilities
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In this work, 2,4'-dichlorobiphenyl (1) yielded 4'-chloro-2-hydroxybiphenyl (2) after photolysis in neutral acetonitrile aqueous (ACN-H2O) solutions. Ultrafast spectroscopic measurements and density functional theory (DFT) computations were performed for 2 in ACN and ACN-H2O (v/v, 1:1). These results were compared with previously published results for 2-hydroxybiphenyl (3). The counterparts 2 and 3 went through a singlet excited state intramolecular proton transfer (ESIPT) in ACN but behaved differently in ACN-H2O with a dehydrochlorination process occurring for 2 and an ESIPT taking place for 3. Computational results indicate that the phenol O-H bond elongates after photoexcitation to induce a concerted asynchronous process with the C-Cl bond increasing first followed by HCl elimination. A biradical intermediate (IM1) is then formed with some spin located at the phenyl 4'-C radical that appears to favor a hydrogen atom transfer (HAT) process and some spin located on phenoxyl that appears to prefer a subsequent center dot CH2CN radical rebound. The hydrogen bond promotes HCl elimination, while this is disfavored for ESIPT, making 4'-Cl extrusion the predominant process in ACN-H2O solutions. The mechanistic investigations have fundamental and significant implications for the understanding of polychlorinated biphenyl photolysis in an aqueous environment and hence the photodegradation of these kinds of pollutants in the natural environment.
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