Paramagnetic Resonance Study of Liquids during Photolysis: Hydrogen Peroxide and Alcohols

Abstract

Paramagnetic resonance spectra of free radicals in liquids have been studied during the course of intense irradiation with ultraviolet light. Equipment is described in which the liquid sample flows through the microwave cavity of the spectrometer and is photolyzed as it passes the sensitive region of the cavity. In this way the steady-state concentration of short-lived radicals have been observed. Temperatures from about −70° to 60°C have been used. A number of alcohols containing up to 1% H2O2 have been studied. In the absence of dissolved oxygen the main products are the radicals formed by abstraction of a hydrogen from the position alpha to the hydroxyl group. The radicals RĊHOH where R is H, CH3, C2H5, C3H7, (CH3)2CH, and C6H5 have values of aα near room temperature ranging from 14.7 to 17.4 G and g values ranging from 2.00298 to 2.00334. Those radicals with beta hydrogens have values of aβ ranging from 20.0 to 22.2 G. The ring protons of C6H5ĊHOH are inequivalent. When the alpha hydrogen is abstracted from allyl alcohol two radicals are formed that are geometric isomers with the relative abundances depending on temperature. Coupling values for all hydrogens in each isomer are given. All couplings are also given for the radicals formed from ethylene glycol and isopropyl alcohol. In many cases couplings have been measured at several temperatures. Values for hydrogens in hydroxyl groups are strongly temperature dependent and in some cases become zero. Alpha and beta coupling constants are less temperature dependent. The various results are discussed. In the presence of dissolved oxygen, peroxy radicals are formed and their spectra are discussed. Their linewidths are strongly dependent on temperature and on concentration of dissolved oxygen. Radicals have been seen in photolyzed hydrogen peroxide solutions for concentrations ranging from 10% to 45%. In the presence of acid a single absorption line is observed. Without the acid the spectra are complex. These results are also discussed.