title: meta versus para substitution: how does C-H activation in a methyl group occur in 3-methylbenzophenone but does not take place in 4-methylbenzophenone?
abstract: The photophysical and photochemical reactions of 3-methylbenzophenone (3-MeBP) and 4-methylbenzophenone (4-MeBP) were investigated using femtosecond transient absorption (fs-TA) and nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy and density functional theory (DFT) calculations. 3-MeBP and 4-MeBP were observed to behave similarly to their parent compound benzophenone (BP) in acetonitrile and isopropyl alcohol solvents. However, in acidic aqueous solutions, an unusual acid-catalyzed proton exchange reaction (denoted the m-methyl activation) of 3-MeBP (with a maximum efficiency at pH 0) is detected to compete with a photohydration reaction. In contrast, only the photohydration reaction was observed for 4-MeBP under the acidic pH conditions investigated. How the m-methyl activation takes place after photolysis of 3-MeBP in acid aqueous solutions is briefly discussed and compared to related photochemistry of other meta-substituted aromatic carbonyl compounds.
authors.affiliation: Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, People's Republic of China.
authors.forename: Jiani
authors.initials: J
authors.lastname: Ma
authors.forename: Tao
authors.initials: T
authors.lastname: Su
authors.forename: Ming-De
authors.initials: MD
authors.lastname: Li
authors.forename: Xiting
authors.initials: X
authors.lastname: Zhang
authors.forename: Jinqing
authors.initials: J
authors.lastname: Huang
authors.forename: David Lee
authors.initials: DL
authors.lastname: Phillips
mesh_terms: D001577:Benzophenones / Q000737:chemistry*; D015394:Molecular Structure; D011789:Quantum Theory
chemical_list: D001577:Benzophenones