ELECTRON SPIN RESONANCE STUDIES OF RADICALS PRODUCED BY ELECTROLYSIS (1967)

The ESR spectra of the radical anions of pyridine, pyrimidine, and pyrazine in liquid ammonia were measured.The coupling constants obtained N N H from these spectra allowed values of the quantities ~ , Q CN ' and Q CH to be determined without relying on calculated spin densities.The values found are: ~N = + 27.3 gauss, Q CN N = gauss.Theoretical values for the parameters H -1.7 gauss, and Q CH = -24.5 ~N (pyridine, pyridine N-oxide, nitrosobenzene, and nitrobenzene), QCN N , and Q ON N were calculated using configuration interaction in an MO framework.The ESR spectra of several substituted pyridine radical anions in liquid ammonia are reported including: 4-picoline, 3,5-lutidine, pyridine N-oxide, 4-picoline N-oxide, and 2,6-lutidine N-oxide o A series of McLachlan type spin density calculations were carried out in order to determine values of the variable parameters appropriate for the various N-heterocyclic radical anions.Excellent agreement between experimental and calculated spin densities was obtained for the simple heterocyclics and for the methyl substituted pyridines~ The parameters for the N-oxides were not as well determined.USCF calculations including an approximate doublet state projection operator were carried out for the unsubstituted N-heterocyclics and for pyridine N-oxide with the resulting spin densities slightly better than those calculated using .' -viiithe' Mclachlan approximation.The ESR spectra of the radical anions of nitrosobenzene in DMSO and THF and of ortho-and para-nitrosotoluene and 2-nitroso-m-xylene in liquid ammonia were measured.The values of the coupling constants determined for the methyl nitrosobenzenes corresponded to an assignment of the larger ortho proton coupling constant observed in the nitroso-,benzene radical anion to the proton nearest the oxygen atom.This is consistent with the assignment pr'edicted by including non-neighbor resonance.integrals in the MO calculations of spin density distribution.An additional mechanism whereby the ISN electrons are polarized by spin density in the oxygen pi orbital was proposed and values of the parameters Q OHC H and. QCHO~ were estimated using simple three center MO's.McLachlan calculations for the nitrosobenzene radical anions were carried out using several models to account for the effects of restricted rotation.In each case it was found that several sets of parameters gave equally good predictions of the ring position spin densities, with wide variation of .thepredicted distribution in the nitroso group.USCF calcUlations were carried out for the nitrosobenzene radical anion and similar ambiguity in the choice of parameters "TaS found.In the absence of non-neighbor integrals, the USCF wave functions predict a larger spin density in the ortho carbon atom which is further from the oxygen atom.~.f .reduction of various single and multiring" poly-nitrogen heterocyclic 12 "'compounds and by Hauser reporting electrolytic reduction of phenazine and dipr6to phenazine.Much work in this area followed and the recent' article by Henning 13 includes a review'.' . .' ''-'"' ~. ' "' . .,-~~~-'--<-...