Determination of differential quantum yields in solution by electron paramagnetic resonance spectroscopy

Quantitative kinetic studies on the photochemistry of paramagnetic species in solution may be carried out by electron paramagnetic resonance (EPR) spectroscopy. A cylindrical cell can be used as photochemical reactor, but the internal diameter should be less than 1.7 mm in order to achieve the resonance of an aqueous sample in an X-band (9–10 GHz) spectrometer. In this paper we present a detailed analysis of the fractions of incident light that are reflected, transmitted and absorbed by a liquid solution in a quartz cylindrical cell placed in the optical cavity of an X-band EPR spectrometer. Since the photolysis cell is irradiated perpendicularly to its axis, variable angles of incidence have been considered to calculate the transmission and reflection coefficients from Fresnel equations. Polarization of light has been also taken into account in the evaluation of the coefficients. The procedure proposed here is adequate for the evaluation of the absorbed light in the determination of quantum yields. The continuous photolysis at 366 nm of symmetric chlorine dioxide (OCIO) in aqueous solution was considered as an example. The initial differential quantum yield obtained for OCIO photodecomposition in aqueous solution was Θ₃₆₆ = 0.55 ± 0.04.