ELECTRON PARAMAGNETIC RESONANCE STUDIES ON THE PRODUCTION OF FREE RADICALS IN HYDROGEN PEROXIDE AT LIQUID NITROGEN TEMPERATURE¹

Abstract

Sir:Since the first e.p.r. observation by Ingram? and his colleagues on the photolytic formation of free radicals in hydrogen peroxide a t liquid nitrogen temperature, a considerable amount of research has been done on the identification of paramagnetic species produced under these or similar conditions. Preparation of the samples for e.p.r. examination has been performed either by condensation a t liquid nitrogen temperature of the products of electric discharge in various gaseous mixtures containing H 2 0 2 or by direct photolysis of aqueous H20s solutions frozen in liquid nitrogen.2,5,6 I n all the cases reported, the e.p.r. spectrum seems to represent the same species, believed by most authors to be the H 0 2 radical. However, Kaitmazov and Prokhorov5 indicate !hat: (1) ultraviolet irradiation (with X >2fi00 A,) of Hz02 may result only in the breakage of the HO-OH bond, and (2) no secondary reactions occur during photolq sis, since the change of H202 concentration in the range from 3-987, does not affect the spectrum. Thus, according to Kaitmazov and Prokhorov, the e.p.r. spectrum obtained by H202 photolysis a t liquid nitrogen temperature must be derived from OH radicals. The present report summarizes the results of e.p.r. experiments carried out with H202 solutions irradiated by TP-particles and ultraviolet light a t liquid nitrogen temperature Irradiation by Tp-Particles.-6 -11 H 2 0 2 solution was tritiated with T20 to an activity of 0.3 c./'ml. and frozen in glass sample tubes in liquid nitrogen. Tritium has been chosen as an internal source of radiation since, as reported previously,' i t allows one to avoid the background of irradiated glass in the e.p.r. spectrum. The resulting e.p.r. spectrum (Varian V4500, X-band e.p.r. spectrometer) shown in Fig. 1 9 , is very similar to the spectra reportedj for H?OZ irradiated by ultraviolet light and consists essentially of a fairly broad band, poorly resolved into two peaks, with separation of about 12 gauss. This spectrum is, however, distinctly different from that usually attributed to OH radicals and obtained by irradiation with CoiiO ?-rayss or Ta-particles' of pure HzO ice.The e.p.r. spectrum produced by T,!j'-particle: in H20 ice at liquid nitrogen temperature is shown in Fig. 1B. Lipart from their spectral appearan :e, radicals , ?, (produced in H20s) and B (in H 2 0 ) di; 'er markedly in thermal stabilities. , I s shown in I ig.(1) Supported b y a grant from the Sational Research Counc of C m a d s .

Electron spin resonance detection of radicals and electrons condensed from water vapor after irradiation with 1 MeV helium ions

“…Spectroscopic evidence that our radical is not OH is derived from the g values in the papers by Dibdin (21) and by Brivati et al (22). From these papers one can infer that HO, was also formed in the earlier work (3,(12)(13)(14)(15)(16).…”

“…Both groups find that the yield of hydrogen atoms and electrons is about 5 and 3 molecules per 100 eV respectively, giving a ratio of yields It must be pointed out that such a close correspondence with the conventional gas phase ; 0.5 0 results may be coincidental, since reactions [14], [15], and [16] may occur to an unknown extent.…”

“…This spectrum has been observed in the condensate from a microwave discharge in H 2 0 and H202 vapors (12), from an electric discharge in H 2 0 and D 2 0 vapors (13), in frozen aqueous solutions of 5 to 98% H202 photolyzed by ultraviolet light (14,15) For personal use only. of 6 M H 2 0 2 irradiated by tritium beta particles (16), and in the condensate from a jet of H,O or D,O vapor irradiated by 100 eV electrons (3). The radical must be oxygenated since a similar spectrum is obtained from H 2 0 or D20.…”

“…data (3,(12)(13)(14)(15)(16), it would be hazardous to identify this radical from its e.s.r. spectrum alone.…”

“…In these cases neither spectrum looks like the one obtained by condensing water vapor after electron irradiation in the vapor phase (3). In addition, previously published examples of this spectrum (12)(13)(14)(15)(16), in a water or water -hydrogen peroxide matrix cannot on their own merit be definitely ascribed to the HO, radical because of the uncertainty in the process of radical formation.…”