Influence of solvents on the conformation of benzoin

Photochemical α-Cleavage Reaction of 3',5'-Dimethoxybenzoin: A Combined Time-Resolved Spectroscopy and Computational Chemistry Study

Benzoin is one of the most commonly used photoinitiators to induce free radical polymerization. Here, improved benzoin properties could be accomplished by the introduction of two methoxy substituents, leading to the formation of 3',5'-dimethoxybenzoin (DMB) which has a higher photo-cleavage quantum yield (0.54) than benzoin (0.35). To elucidate the underlying reaction mechanisms of DMB and obtain direct information of the transient species involved, femtosecond transient absorption (fs-TA) and nanosecond transient absorption (ns-TA) spectroscopic experiments in conjunction with density functional theory/time-dependent density functional theory (DFT/TD-DFT) calculations were performed. It was found that the photo-induced α-cleavage (Norrish Type I reaction) of DMB occurred from the nπ* triplet state after a rapid intersystem crossing (ISC) process (7.6 ps), leading to the generation of phenyl radicals on the picosecond time scale. Compared with Benzoin, DMB possesses two methoxy groups which are able to stabilize the alcohol radical and thus result in a stronger driving force for cleavage and a higher quantum yield of photodissociation. Two stable conformations (cis-DMB and trans-DMB) at ground state were found via DFT calculations. The influence of the intramolecular hydrogen bond on the α-cleavage of DMB was elaborated.

Spectroscopic and computational study of the major oxidation products formed during the reaction of two quercetin conformers with a free radical

The goal of this research was to determine whether there are differences between the major oxidation products formed during the reaction of quercetin unhydrate (QUH) or quercetin dihydrate (QDH) with the 2,2-diphenyl-1-picrylhydrazyl free radical (DPPH), as well as to identify some properties of these products. The study was carried out employing spectroscopic and computational methods, in order to know the effect of different conformations of quercetin on the mechanism of free radical scavenging. The results demonstrated that although the same oxidation products may be formed from QUH and QDH, their properties and the predominant product were different in each. The o-quinone was the predominant oxidation product of QUH, whereas in QDH it was established an equilibrium between o-quinone and extended p-quinone.