Photoionization of Phenolates and Scavenging of Hydrated Electrons by No3-: A Study of the Reaction Mechanism by FT-EPR

The mechanism of photoionization of phenol, p -cresol, and tyrosine in alkaline aqueous solution was investigated with Fourier Transform EPR. The spectra given by the phenoxyl radicals and hydrated electron ( ) display a low-frequency (high-field) absorption/high-frequency (low-field) emission (A/E) polarization pattern generated by radical pair mechanism chemically induced dynamic electron polarization (RPM CIDEP). Upon addition of nitrate the NO 3 2- radical is formed by capture of . With [NO 3 - ] < 10 -2 M the three-line spectrum due to NO 3 2- shows a 1:1:1 absorption pattern reflecting spin polarization carried over from the hydrated electron. At high nitrate concentration, [NO 3 - ] > 5 × 10 -2 M, both the phenoxyl and NO 3 2- signals display an E/A polarization pattern at short delay times (<1 μs) between pulsed-laser excitation and microwave pulse. This is attributed to RPM CIDEP generated by the [PhO • ···NO 3 2- ] radical pair formed in the reaction [PhO • ··· ] + NO 3 - → [PhO • ···NO 3 2- ]. At longer delay times (>1 μs), the spin polarization pattern in the spectrum of NO 3 2- changes to A/E as a result of F-pair CIDEP produced by the back reaction PhO • + NO 3 2- → PhO - + NO 3 - . The experimental data show that the sign of the exchange interaction J between the unpaired electron in [PhO • ··· ] is opposite to that in [PhO • ···NO 3 2- ]. From the CIDEP effects, it can be concluded further that the photoionization must occur from a singlet excited state of the phenolates so that in the radical pair [PhO • ··· ] J must have the positive sign.