THE ELECTRON SPIN RESONANCE SPECTRA OF THE POSITIVE AND NEGATIVE IONS OF DIPHENYLENE1

Transcription

1 THE ELECTRON SPN RESONANCE SPECTRA OF THE POSTVE AND NEGATVE ONS OF DPHENYLENE1 C. A. MCDOVELL AND J. R. RO\VL~\NDS ABSTRACT The electron spin resonance spectra of the positive and negative ions of diphen).lene have been measured. t has been found that these spectra consist of five lines showing that the obser\.ccl hyperfine interaction is caused by four equivaleni protons. The over-all extent of the positive ion spectrum is 18 gauss coinpared with that of 12.9 gauss for ihe negative ion. The hyperfine splittings observed are 4.0 gauss ancl 2.75 gauss respectively. NTRODUC''ON 'The electron spin resonance spectra (e.s.r.) of both positive and negative ions of a number of alternant hydrocarbons have been nleasured in recent years (). t has been shown that the qualitative features of the experimental spectra can be interpreted by use of unpaired electron spin densities calculated from the simple -ucltel niolecular orbital theory. The hyperfine interactions found in the e.s.r. spectra of these aronlatic hydrocarbon ions are attributed to interactions between the odd electron spin and the nuclear spins of the protons attached to the carbon atoms. t has been shown that the hyperfine splitting produced by a proton attached to a given carbon atom is linearly proportional to the unpaired electron density on that carbon atom (2), i.e., \vhere A is the hyperfine splitting observed, and p, is the odd electron spin density on carbon atom p. From the properties of even alternant hydrocarbons it is evident that the odd electron spin densities on carbon atom p, of an electron moving in a molecular orbital &, are given by the equation t has been sho\vn (3) that the atomic orbital coefficients c,, in paired orbitals are equal in magnitude, i.e, if orbitals and 4, are paired in energ)., then [31 G, = =tc,,. We can, therefore, expect that if A+ is the hyperfine splitting lound in the nlonopositive ion, and A- that of the lllononegative ion of a particular alternant hydrocarbon, and if Q is the same for both species, then t has been found experinlentally for alternant hydrocarbons tlzat although both species give the same number of components of hyperfine interaction, the over-all width of the monopositive ion spectra is in general about 4 gauss greater than the corresponding mononegative ions (). The complexity of the spectra has made the determination of hyperfine splitting constants difficult. Where an analysis of the experimental spectra is possible, we have a choice of (i) assuming that the unpaired spin densities in both positive and negative ions are equal and are predicted accurately by the -ucltel theory, and determining suitable values of the proportionality constant Q for the two species, or (ii) l~l~anuscri#t received Decentber 22, Contribz~tion fronz the Departnzent of Clze~nistry, Can. J. Chem. Vol. 38 (1960) 8, B.C.

2 504 CANADAN JOURNAL OF CEMSTRY. VOL. 38, 1960 assuming that this proportionality constant Q is the same for the two species, and is in fact constant for all even alterilant hydrocarbons, and coinparing the experi~nentally determined spin densities with those predicted from the Hiicliel inolecular orbital theory. n this paper on the ions of diphenylene the latter procedure has been followed. EXPERMENTAL The illononegative ion of diphenylene was prepared by distilling dry tetrahydrofuran under vacuum into a bulb containing a freshly prepared sodium film. Attached to the bulb via a side arm was a narrow tube which held the diphenylene. The bulb was sealed off from the solvent source by freezing both sides with liquid nitrogen, puinping off ally unfrozen tetrahydrofuran, and then sealing off at a constriction just above the necli of the bulb. When the solvent in the bulb was at room temperature the diphenylene was dissolved in it by shaking. The diphenylene then inzmediately reacted with the sodium to give a solution of the mononegative ion. Measurements of the e.s.r. spectra were then made by tipping the solution into the narrow tube, which was then placed in the cavity of the spectrometer operating in a TE012 mode. All the spectra were recorded on a highresolution electron spin resonance spectroineter operating at 9000 Mc/sec. The magnetic field of approximately 3300 gauss was supplied by a Varian &in. magnet with shimmed pole faces. A modulating frequency of 10 kc/sec was used and the derivation of the spectra recorded 011 a Leeds & Northrup recorder as phase-sensitive detection was used. The monopositive ion of diphenylene was prepared by dissolving the hydrocarbon in concentrated sulphuric acid. Calibratioils of all runs were made, by determining with the e.s.r. spectrum of Fremi's salt under identical sweep conditions. RESULTS The spectra of the negative and positive ions of diphenylene are shown in Figs. 1 and 2, together with the integrated absorption curves for each case. Each ion gives a spectrum containing five completely resolved lines with approximate intensity ratios of 1 :4:6:4:1. No further hyperfine splitting could be detected for either ion, although the spectra were FG. 1. 'The electron spill resonance absorption spectruln of the diphenylene positive ion in concentrated sulphuric acid.

3 McDOWELL AND ROWLANDS: E.S.R. SPECTRA OF DPHENYLENE ONS 505 FG. 2. The electron spill resonance absorption spectrum of the diphenylene negative ion in tetrahydrofuran. FG. 3. Theoretical electron spin resonance absorption spectrum of diphenylene positive (or negative) ion on the Hiickel molecular orbital approximation. measured over a range of concentrations. Figure 3 is the theoretical spectrum built up using the calculated -uclcel spin densities, assuming that all Couloillb integrals for the carbon atoms are the same and that all resonance integrals for the C-C bonds are also equal. The theoretical spectra of both positive and negative ions are of course identical on this picture. DSCUSSON t can be seen from Figs. 1 and 2 that the observed spectra coilsist of five lines. These lines are well resolved and there is no trace of any other fine structure on the spectra of either the positive or negative ion even when spectra have been talcell with extrenle care and very high resolution. The theoretical spectrum calculated froill the -uclcel electron densities indicates that there are 25 lines to be expected (see Fig. 3). Our observed spectra must, therefore, be interpreted on the assuinption that there are only four equivaleilt protons contributing to the hyperfne splitting. These can be talcen to be the protons attached to carbon atolns 2,3, 6, and 7, which of the two sets of carbon atoms with protons

4 506 C,KADAN JOUl1N.4L OF CHEMSTRY. VOL. 38, logo attached are those having the highest electroil density on the -uckel theory. Carbon atoins 9, 10, 11, ancl 12 have no protoils attached and for this reason the e.s.r. spectra of the ions of diphenylene extend oilly over about half the range observed for the ions of other alternant hydrocarbons. Thus the spectrum of the positive ion of diphenyleile extends over only 18 gauss conlpared with about 33 gauss for the anthracene positive ion. spin densities on carbon atonls 9, 10, 11, and 12 total about 54y0 of the total spin densities at all the carboil atoms in diphenylene; hence we would expect the e.s.r. spectrunl of this substance to extend only over about half the range of spectrum of say the anthracene positive ion. This is in agreenlent with our observations. Although the intensity distributioil of the experi~nental spectra of the two ions is the same, there is a marked difference in the magnitude of the hyperfine interaction. The overall width of the positive ion spectrum is 18 gauss, with a splitting co~lstant of 4.0 gauss. The corresponding figures for the negative ion are 12.9 gauss and 2.75 gauss respectively. f we take the value of the proportionality constant Q to be the same for the two species, then there is a difference in the odd electron distribution in the two ions. n Table ne have listed the experimental spin densities for the two ions, obtained by using a value of Q derived from the benzene negative ion spectrum, which is the only alterilant hydrocarbon Electron spin densities for the mononegative and monopositive ions of cliphenylcne Diphenylene- ion Calc., Calc., assuming Espt. Hiickel theory ac, = ac+l/5b P p? ps for which accurate spin densities are known. Column 2 ol Table lists the spin densities calculated from the -uclcel theory, with the assumption of equal Coulonlb integrals for the carbon atoms. n column 3 of the table we have included spin densities obtained by assiglliilg the four central carbon atoms with a Coulo~llb integral equal to the normal carbon Coulomb integral plz~s one-jiftlz the carbon-carbon resonance integral. t may be seen fro111 the table that this makes the calculated spin densities of the positive ion different from those of the negative ion. Since in dipheilyleile we are studying a derivative of cyclobutadiene, it is quite plausible that we should treat it as containing two different types of carbon atom. t is quite obvious fro111 the laclc of agreement between experiment and theory that the sinlple -uclcel lllolecular orbitals obtained by the assulnption of equal Coulonlb and resonance integrals are inadequate. t must, however, be recalled that other workers have previously noted that exact agreement between the electron densities calculated from the simple Hiiclcel molecular orbital theory and the experinlental values

5 McDOWELL AND ROWLANDS: E.S.R. SPECTRA OF DPESYLENE ONS 507 is lacking. Further it has also been observed that in other alternant hydrocarbon systems there are significant differences between the positive and negative ion spectra which canilot be explained on the basis of the simple -iiclrel molecular orbital theory. We wish to thank the Xational Research Council and the Defence Research Board of Cailada for grants-in-aid of this work. Our thanks are due to Drs. V. Rogers and J. F. McOmie for the gift of diphenylene. REFERENCES 1. T.?. TUTTLE, Jr., R. L. \\ARD, and S.. \VESSMAN.. Chem. Phys. 25, 180 (1956). 2. S.. WElSShltzN and E. Dri BOER. J. Am. Che~n. Soc. 80, 4549 (1958). 3. S.. WESSBAN. J. Chem. Phys. 25, 890 (1956) W. ~/LCCONNELL. J. Chem. Phys. 24, 76-1 (1956). 5. H. S. JRRETT. J. Chem. Phys. 25, 1289 (1956). 6. C. A. COCLSON and G. S. RUSHBROOKE. Proc. Carllbridge Phil. Soc. 36, 103 (1940).