ELECTRON SPIN RESONANCE STUDIES OF HYDROXYSEMIQUINONE RADICALS - HYDROGEN-DEUTERIUM ISOTOPE EFFECTS IN INTRAMOLECULAR HYDROGEN BONDS

Electron spin resonance spectra have been obtained of semiquinone radicals containing intramolecular hydrogen bonds produced by reduction of l,4-dihydroxy-5,8-naphthoquinone, 1,4-dihydroxy-, 1,5-dihydroxy-, and 1,8-dihydroxy-9,10-anthraquinone, and 6,11-dihydroxy-5,12-tetracenequinone. The same radicals were also investigated with the hydrogen in the hydroxyl position replaced by deuterium. Some of the species were studied in several different solvent systems, and some over a wide temperature range. The proton-deuteron splitting-constant ratios for the protons and deuterons in the hydroxyl positions were found to have values ranging from (αhoh/ αdod) =7 2 to 7.8 instead of the value of (αH/αD) = 6.514 predicted solely on the basis of the magnetic properties of the two isotopes. Substitution of deuterium for hydrogen in the hydroxyl positions was also found to cause significant changes in the ring-proton splitting constants. These two isotope effects have been correlated with changes in the π-electron spin-density distribution determined by the Hückel molecular orbital method. In these calculations, the value of the Coulomb integral for the oxygen atom in the deuterated radicals was chosen to be more negative than the value for the undeuterated species. The deuterium isotope effects in hydrocarbon radicals cannot be accounted for in this way, and the different interpretations are attributed to differences between the structure of carbon-hydrogen bonds and intramolecular hydrogen bonds. An attempt to correlate the hydroxyl-proton splittings with calculated spin densities on the oxygen atoms was not on the whole successful, and the detailed analysis suggests that the σ-π parameters for an intramolecular hydrogen bond vary from radical to radical, and with the solvent employed to study a particular radical, because the structure of these bonds is extremely sensitive to the surroundings. A tentative assignment of the proton splitting constants previously reported for the 1,4-dihydroxyanthrasemiquinone ion has been shown to be incorrect. © 1969, American Chemical Society. All rights reserved.