Study of Spectral Characteristics, Kinetics, and Equilibria of Radicals Derived from Hydroxy Benzophenones

Abstract

Studies on radical equilibria and the spectral evaluation have been carried out for three monohydroxy-substituted benzophenones (HOBP's) in aqueous solution. The transient ketyl or anion radicals were generated via hydrated electron (eaq -) reaction or dimethyl ketyl radical reaction in a pulse radiolysis experiment. The reactivity of both the undissociated (HOBP) as well as dissociated (-OBP) forms toward these reducing agents were studied. eaq - was found to react with the HOBP's and -OBP's with diffusion-controlled rates which are of the order 3 × 1010 dm3 mol-1 s-1. In the case of the para derivative, two acid−base equilibria were identified (pK a 9 and 11) in the pH range 6−13, one corresponding to the protonation−deprotonation at the OH site of the ketyl radical and the other for the protonation−deprotonation at the carbonyl site. For the ortho derivative, the intramolecularly hydrogen bonded structure of the radical anion has been found to be stable even in strongly alkaline solution (pH 13). Three different radical forms for the meta derivative have been identified at different pH conditions. The spectral evaluation of these radical species at different pH conditions was carried out. Reactions of dimethyl ketyl radicals with HOBP's and -OBP's were found to be different at different pH conditions. At acidic pH, it forms an adduct with the HOBP's, whereas at alkaline pH, the reaction with the ortho and meta derivatives occurs by H atom transfer. However, no reaction is seen with the para derivative in alkaline solution. Detailed spectral and kinetic data on the formation and decay of the various transient intermediates have been obtained.

Time‐resolved resonance Raman and density functional theory study of the deprotonation reaction of the triplet state of para‐hydroxybenzophenone in mixed acetonitrile/water solutions

“…Singlet states of benzophenone itself and the p-hBP derivative have been found to be very short-lived and undergo ISC to the triplet manifold within a few picoseconds. 7,8 This fast process and the generally high yield for ISC as reported the deactivation channel and the major relaxation pathway of photoexcited benzophenone and p-hBP derivative molecules to the ground state occurs via their T 1 states in the solution phase. The para-HA molecule has a structure similar to the p-hBP molecule where a methyl group in HA is replaced by the second phenyl ring to make p-hBP.…”

Thermal Reduction of 7H-Benz[d,e]anthracen-7-one and Related Ketones under Hydrogen-Transfer Conditions

“…Ketyl radicals in solution are known to be more strongly acidic species than the corresponding alcohols. Further, a substantial difference in p K a (in water) values has been found for alkyl and aryl ketyls; for example, for m -hydroxy benzophenone ketyl and dimethyl ketyl, Me 2 C( • )OH, p K a s of 6.5 and 12, respectively, have been reported . The variation of almost 6 p K a units can be associated with the better delocalization of the oxygen's lone pair due to the aromatic system.…”

Time-Resolved Resonance Raman and Density Functional Theory Study of Hydrogen-Bonding Effects on the Triplet State of p-Methoxyacetophenone

“…12,[20][21][22][23][24][25][26][27][28][29][30][31] For instance, it was found that, depending on the kinds of compounds, introducing the solute-solvent H-bonding can enhance or inhibit the rate of ISC (to triplet) and/or IC (to ground state) to deactivate the initial photopopulated singlet exited state. 20,29,30 There have been many studies that have reported different triplet chemical reactivity in water containing solvent systems as compared to organic solvent systems, and these differences have been mostly associated with the triplet H-bond interaction with solvent water molecule(s), although no explicit solid evidence has been presented. 12,21,27,28,31 The profound H-bonding effects have been mostly attributed to the shifts (sometimes inverse) of electronic levels due to the destabilization of the nπ* states and stabilization of the ππ* states.…”