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P. F. Scholander, H. T. Hammel, and F. H. Fisher for stimulating discussions. This work was supported by National Science Foundation Grant No. GB-18749 and
COMMUNICATIONS TO THE EDITOR Public Health Service Research Career Development Award No. GN-47352 from the National Institute of General Medical Sciences.
C O M M U N I C A T I O N S T O THE E D I T O R
Reversible Line Broadening in the Electron Spin Resonance Spectra of tert-Butyl Radicals in y-Irradiated Crystalline tert-Butyl Isothiocyanate' Publication costs assisted by the U.S . Atomic Energy Commission
Sir: We wish to report an unusual example of reversible esr line broadening which is understandable in terms of static and dynamic states of tert-butyl radicalthiocyanate anion pairs in y-irradiated crystalline tertbutyl isothiocyanate. I n addition, the esr results distinguish clearly between these two states of the radicalanion pair and the radical anion. As shown in the upper spectrum of Figure 1, the esr spectrum of y-irradiated tert-butyl isothiocyanate in the dark at 77°K consists of an even multiplet superimposed on a conspicuous central feature which henceforth will be referred to as a singlet. I n the lower spectrum recorded while the sample ~ 7 a sexposed to red light (A >640 nm), the singlet is virtually unchanged whereas the multiplet spectrum is much sharper and eight lines can be identified as the inner components of the 10-line spectrum of the tert-butyl radical produced during y irradiation. After the light was turned off, the sharp spectrum of the tert-butyl radicals reverted to the original broad spectrum. There was no significant decrease in signal intensity, even after several cycles, showing that this effect of line sharpening is completely reversible. On illuminating the y-irradiated sample with unfiltered tungsten light, both the singlet esr spectrum and the color center responsible for the light brown appearance of the sample mere bleached irreversibly. This change was accompanied by a large increase in the integrated intensity of the tert-butyl radical spectrum, as illustrated by a comparison of the lower spectrum in Figure 1 with the upper spectrum in Figure 2 recorded at a slightly lower gain. This photobleaching process is similar to that observed in y-irradiated acetonitrile where the reaction involves the dissociation of either a monomer or dimer radical anion depending upon the crystalline phase.2 Accordingly, the photobleachable The Journal of Physical Chemistry, Vol. 7 6 , No. 1g3 1972
esr singlet spectrum is assigned to the radical anion of tert-butyl isothiocyanate. After turning off the light, the lines of the tert-butyl radical spectrum broadened to yield the lower spectrum in Figure 2. Subsequent illumination of the sample with red light induced the reversible line-sharpening effect as observed for the tert-butyl radical spectrum before photobleaching. The relative increase in signal height was similar before and after photobleaching despite the greater intensity of the tert-butyl spectrum in the latter case. Therefore the phenomenon applies to thc tert-butyl radicals produced by photodissociation of the radical anion as well as to the tert-butyl radicals formed during y irradiation. This photodynamic effect was observed with samples which had been stored at 77°K for up to 40 days after y irradiation and photobleaching. Experiments with various combinations of color filters demonstrated that light in the near-infrared region (A >1250 nm) *wasrequired to bring about the effect. This suggested the possibility that absorption of near-infrared radiation might lead t o bulk heating of the matrix. However, this is unlikely to be the case since it was found that the sharp spectrum obtained at 77°K by illumination could only be simulated thermally in the dark by warming the sample to 124"I
