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?

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-TR³) 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.