R. H. Schuler, G. P. Laroff, and R . W. Fessenden
456
lectron Spin Resonance Spectra of the Radicals Produced in the Radiolysis of queous Solutions of Furan and Its Derivatives1 Robert H. SchuYer,* Gary P. Laroff, and Richard W. Fessenden Radiation Research Laboratories, Center for Special Studies and Department of Chemistry, Mellon lnstitute of Science, Carnegie-Mellon University, Pittsburgh, Pennsylvania 15273 (Received October 2, 1972) Publication costs assisted by the U . S. Atomic Energy Commission and Carnegie-Mellon University
The esr spectra of radicals produced by addition of H atoms and OH radicals to the position adjacent to the heterocyclic oxygen atom of furan and a number of its derivatives have been observed during the continuous radiolysis of their acidic solutions. The radicals are allylic in character and exhibit hyperfine constants for the protons-at the 2, 3, and 4 positions which are typical of an allylic system. The H atom adducts are characterized by hyperfine constants of the protons a t the 5 position of 30-35 G. Such values are considerably larger than that of 23.0 G for the p protons in the structurally related hydrocarbon radical cyclopentenyl. This observed difference is attributable to the presence of a small but significant spin density on the ring oxygen atom which causes a large increase in the value of the electronic wave function at the positions of these protons. The radicals produced by addition of OH have hyperfine constants for the protons at the allylic positions which are similar to those of the H atom adducts. The CH proton of the -CHOH- group in these radicals, however, has an hyperfine constant of only 19-21 G and reflects an effect of OH substitution similar to that found in the comparison of cyclohexadienyl and hydroxycyclohexadienyl radicals. In basic solutions of furan, ring opening follows OH addition and the radical anion of butenedial is formed. Related highly conjugated radicals are produced from furfuryl alcohol, furfural, furylacetonitrile, 2-furoic acid, 3,4-furandicarboxylic acid, 2,5-f&andicarboxylic acid, 2acetylfuran, 2-furonitrile, and furylacrylic acid. Because of conjugation, these radicals can exist in a number of isomeric forms. Several isomeric radicals were observed to be produced siniultaneously in the ease of carboxylic acids. In very basic solutions of 2,5-dimethylfuran abstraction of one of the methyl hydrogen atoms occurs and results in the formation of a very interesting cyclic conjugated radical. The spin distribution in this radical shows that the oxygen atom induces a significant negative contribution on the atonis to which it is attached.
In a study of the esr spectra of an irradiated single crystal of furoic acid Cook, Fbwlands, and Whiffen2 have shown that hydrogen atoms add to the 5 position of this compound to produce a conjugated radical that exhibits an unusually large proton hyperfine constant (31.4 G). From the 9.0-G hyperfine constant of the proton on the carbon atom a t the 4 position one can estimate the spin density there to be 0.37. If the only contribution to the wave function ai, the position of the CS protons results from spin density on this adjacent atom, one expects the observed /3 hyperfine constant to be only 12.9 G.3 This situation is very much like that in cyclohexadienyl radical where the splitting by the CH2 protons is twice as large as otherwise expected from simple considerations of the spin densities on the adjacent carbon atoms. These large splittings are now untlerstood to arise because of the nonlinear way in which the effects of the spin density on these adjacent atonis ndd.43" Cyclic radicals will, in general, carry the possibility of unusual hyperfine constants at such positions. The high v d u e in the case of furoic acid appears to be the consequence of a contribution which results from the preseiace of appreciable spin density on the ring oxygen atom in addition to that on the Cq carbon. It has been previously estimated that a spin density of only 0.06 on this oxygen atom would be sufficient to produce the observed effect.2.3 T o this point esr information on the radicals produced by H atom and OH radical addition to the furan ring system i3 of considerable interest. We have examined the isotropic spectra of a number of these radicals The Journal of Physicai Chemistry, Vol. 77, No. 4, 1973
in aqueous solution by the in situ radiolysis-esr approach and wish to report the results here. In acidic solutions the principal radicals observed are those which result from the addition of H and OH to the position adjacent to oxygen of the furan ring. In alkaline solutions, however, OH addition to this position results in sing opening to form conjugated radical anions having terminal aldehyde groups. Such ring opening has previously been observed by Lilie6 in pulse radiolysis studies on furan where the radical anion of butenedial was shown to be an important intermediate. The esr results reported here substantiate Lilie's conclusions and show that, because of the conjugation and consequent hindered internal rotation, these radical anions can exist in a number of isomeric forms. A number of other interesting radicals are also observable during the radiolysis of solutions of derivatives of furan and the results on these are included here.
Experimental Section The steady-state in situ radiolysis -esr approach as applied to studies of aqueous systems has been described Supported in part by the U. S . Atomic Energy Commission, R. J . Cook, J. R . Rowlands, and 0 . H. Whiffen, Mal. Phys., 7, 57 (1963). R. W. Fessenden and R. H. Schuler, Advan. Radiat. Chem., 2, 103 (1970). D. H. Whiffen, Mol. Phys,, 6, 223 (1963). R. W. Fessenden and R. H. Schuler, J. Chem. Phys., 39, 2147 (1963). J. Lilie, 2. Naturforsch., 26, 197 (1971),
Radiolysis of ~
q ~ SN3lutions ~ e of o Furan ~ ~ ~
457
p r e v i ~ u s l y .Apprcpriate ~ solutions of furan and its derivatives (furan arid f ~ ~ r f u r yalcohol, l Eastman Kodak Co.; furfural, Fisher Scien tific Co.; %furonitrile, 2-furanacetonitrile, 2-acetylfuraa, 2-furylacrylonitrile, 2-furylacrylic acid, and 3,4-furandicarboxylic acid, Aldrich Chemical Co.; ~ ~ 5 - d i m e ~ ~ y ~Columbia f ~ ~ . r a nOrganic , Chemicals Co.; 2-furoic acid, Analabs; and 2,5-furandicarboxylic a.cid, rg Inc.) were prepared and deoxygenated er with nitrogen or nitrous oxide depending on whether or n:)t it was desired to convert eaq- to .OH.Perchloric acid and potassium hydroxide were used to adjust the pH ,to the desired value. Electron irradiations were c o n ~ i ~ i twith i ~ ) radical ~ ~ ~ production rates being up to 40-2 M sec-:'. Residence times of the solution within the esr cavity were -50 msec. Double modulation at 200 Ks and IOQ kWz was used and the spectra recorded as the second d e r ~ v a t i ~ .of e s the esr absorptions. In most cases the line widths observed approached the limits of the experiment (-0.07 G). The magnetic field and microwave frequency were continuously monitored during the recording of the spectra as described in ref 5 and 7 . Goupiing constants are cnnsidered t u be accurate to -0.02 6 and absolute values cf the g factors to -0.00003. Comparison of two radicals within a single spectrum allows relative g factors to be determined to -0.00001.
H Atom Addition. Radicals formed by addition of hydrogen atoms produced in the radiolysis of water to the 2 position of furan arid 3,4-furandicarboxylic acid and to the 5 position of fiurfuryl alcohol, 2-furoic acid, and 2-furonitrile have been obstxved in the irradiation of 10-3-10-2 M solutions a t pfl 1. In each of these cases the spectrum is complicated by the presence of other radicals. However, the 1.arge hyperfine constant of the two N atoms on the carbon atom adjacent to the ring oxygen makes the spectrum of the E1 atom adduct sufficiently wide (80-100 G) that the outerrnost. 1i:nes arc. well separated from the lines of the other radica!s. The sums of the coupling constants and centers of the spectra are directly determinable from the positions of these outermost lines so that analysis of the remaining stru8r:tureis readily tractable. H atom addition was also obswved to occur a t the 2 position of 2,s~ u r a ~ d ~ c a ~ b oacid. x y l In i~~ this case the'total spread of the spectrum i s only 43 13, NaO saturated) where 0 - is the reactant. Even here the central region of the spectrum is cluttered by the presence of several radicals. The lines of a radical containing three equivalent and four nonequivalent protons were apparent in the region above the quartz signal. which is relatively free of interference. The total available intensity for such a radical is distributed over a pattern of 64 lines SO that the individual lines are still relatively weak (signal/noise of th lines is -3-9) but nevertheless observable. chemically induced spin polarization, the law-field lines are so weak as not to be observable. The hyperfine constants determined were 8.34 ( 3 ) , 12.84, 13.19, 9.44. and The Journal of Physical Chemistry, Voi. 7i7, No 4 , 1973
R. 5. Wesley and C.W.5eKock
1.03 G. The g factor i s 2.00261 and is typical of aliphatic radicals. A t pH values below -12 lines of this radical are not present. The above spectrum is clearly assignable to radical VII. This radical is extremely interesting in that it possesses conjugation of au allylic type which extends over the entire ring. Because of the conjugation the two protons of the CZI2 group are not equivalent. They are, however, not expected to have very different hyperfine constants and accordingly tae constants of 12.8 and 13.2 G assigned to this position. The 8.3 G of the quartet pattern is directly assignable to Ihe methyl group and, by analogy with allyl, the 9.4- and 2.0-6 hyperfine constants are assigned to the protons at the starred and unstarred positions of the ring. The 8.34 splitting of the methyl protons indicates a spin density of 0.S1 OM the adjacent carbon. The other hyperfine constants indicate spin densities of 0.52 at the CI32 carbon atom, and 0.37 at the starred and -0.05 at the unstanxed positions of the ring to which protons are at. tached. The total of the above is 1.15 so that appreciable negative spin density must exist at the unprobed carbon position arid ox1 the oxygen atom. Since methyl groups are known to have little effect on spin distribution it must be concluded that it is the oxygen linkage that induces conBiderable asymmetry into the distribution in this particular radical.
VII
Summary Furan and its derivatives provide a sodrce of a large variety of interesting radicals, most of which exhibit a high degree of conjugation and thus are of interest in both experimental and theoretical determinations of spin distribution. It is clear that the H and OH reactions exemplified here are general and that with appropriate effort it should be possible to obtain similar information on other derivatives of furan. The esr approach can be put to extremely good use here since a large number of different radicals are producible from each single solute. It is noted, for example, that the esr parameters of nine distinguishable radicals from 2,5-furandicarboxylic are reported in this paper. Acknowledgment. The authors wish to thank Dr. P. Neta for many valuable discussions and suggestions during the course of this work.
ectra and Geometries of Matrix Isolated Yttrium Tri- and D i f l u ~ r i d e ~ Wesley and C. W . DeKock" Department of Chemistry. Oregon State University. Cowallis. Oregon 9733 I (Received April 5. 1972J
Irhr infrared spectra of YF3 and YF2 isolated in argon and nitrogen matrices have been measured in the region 40-800 cm-l. The fundamental frequency assignments in argon assuming Csu symmetry for YF3 are , I ~ ( E663 ) cm-1, vz(A1) 119 cm-1, v4(E) 140 cm-l, with vi unobserved. The frequency assignments for 5'Fz in nitrogen assuming CzU symmetry are VQ(B1) 538 cm-l, ul(A1)545 cm-I, and vz(A1) 134 cm-1. The results show that YF3 is easily reduced to YF2 in a tantalum Knudsen cell.
Lntroductilon As part of a continuing study of the infrared spectra of the rare-earth halides1 and dihalides2 we have measured the infrared spectra of matrix isolated YFa and YF2. These results are of particular interest since they show that J7F3(g) i s easily reduced to YF2(g) at high temperatures. YF3 has recently been studied by Margrave, et al.,3 but their reported spectrum of YF3 we attribute to YF2, Recent work on YFs includes mass spectrometric results showing it to be monomeric in the gas phase4 and more significantly very recent molecular beam electric deflecThe Journal of Phj~ssico4Cllsrnjstry, Vol. 77, No. 4 , 1973
tion studies5 showing that YFs has a dipole moment and therefore is presumably a pyramidal molecule. (However, in the later study the YF3 was vaporized from a tantalum Knudsen cell which as seen from the present study would give primarily YFz vide infra) (1) R D WesleyandC W DeKock,J Chem Phys 55,3866 (1971) (2) C W DeKock, R D Wesley, and D D Radtke, H ~ 9 hTemp S c i , 4,41 (1972). (3) R . H. Hauge, J. W. Hastie, and J. L. Margrave, J. Less Common Metals, 23,359 (1971). (4) K. F. Zmbov and J. L. Margrave, Advan. Chem. Ser.. No. 72, 267
1196a1. (5) E. W.'Kaiser. W. E. Falconer, and W, Klemperer, J. Chem. Phys.. 56,5392 (1972).
