ESR Studies of Naphthoquinones and Related Compounds. Hyperfine Interactionby the Methoxyl Group as a Conformational ProbeGary Paul Rabold, Ronald T. Ogata, M. Okamura, L. H. Piette, R. E. Moore, and P. J. Scheuer Citation: The Journal of Chemical Physics 46, 1161 (1967); doi: 10.1063/1.1840784 View online: http://dx.doi.org/10.1063/1.1840784 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/46/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in An ESR study of an αbromofluoro πradical: Determination of the relative signs of the hyperfine andquadrupole couplings J. Chem. Phys. 73, 81 (1980); 10.1063/1.439838 Concerning the ESR Hyperfine Interaction of the Methoxyl Group J. Chem. Phys. 48, 1411 (1968); 10.1063/1.1668823 Magnetic Resonance Studies of Hyperfine Interactions in the Anion Radicals of Stilbene and RelatedMolecules J. Chem. Phys. 43, 3183 (1965); 10.1063/1.1697292 Note on Hyperfine Interaction Tensors in ESR Spectra J. Chem. Phys. 36, 117 (1962); 10.1063/1.1732277 Anisotropic Hyperfine Interactions in the ESR Spectra of Alkyl Radicals J. Chem. Phys. 34, 1161 (1961); 10.1063/1.1731715

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THE JOURNAL OF CHEMICAL PHYSICS VOLUME 46, NUMBER .1 1 FEBRUARY 1967

ESR Studies of N aphthoquinones and Related Compounds. Hyperfine Interaction by the Methoxyl Group as a Conformational Probe*

GARY PAUL RABOLD, RONALD T. OGATA, t M. OKAMURA, AND L. H. PIETTE

Department of Biochemistry and Biophysics, University of Hawaii

AND

R. E. MOORE AND P. J. SCHEUER

Department of Chemistry, University of Hawaii, Honoluhl, Hawaii

(Received 1 August 1966)

ESR spectra were run on a large number of methoxy-substituted para-semiquinones. With the help of MO calculations it was deduced that methoxyl hyperfine interaction in these systems is, in part, a function of the spin density on the ether oxygen. By the use of model compounds it was further deduced that only one con­formational isomer is effective in giving rise to splitting. A semiempirical value for I QOCBI I was calculated to be 5.6 G.

I. INTRODUCTION

ALTHOUGH ESR proton hyperfine interactions 1"1. through C-H and C-C-H bonds in organic free radicals are well understoodl.2 and amply documented,3.4 there is at present no satisfactory treatment in the literature for similar interactions through 0-H and O-C-H bonds. A perusal of the literature reveals a large number of isolated examples of the transmission of spin density across both ether and acyl bonds (canonically saturated) in a wide variety of aliphatic and aromatic systems. These reports have appeared with surprisingly little cross reference to each other and we therefore feel it helpful to present a summary of the previous work5--l4 in tabular form (Table I) to compare with the results reported here.

* This work was supported by U.S. Public Health Service NIH Grant GM 12798-02.

t NSF undergraduate research participant. 1 (a) B. Venkataraman and G. K. Fraenkel, J. Am. Chern. Soc.

77, 2707 (1955); (b) J. Chern. Phys. 23, 588 (1955); (c) J. E. Wertz and J. L. Vivo, ibid. 23, 2441 (1955).

2 (a) H. M. McConnell, J. Chern. Phys. 24, 632, 764 (1956); (b) H. M. McConnell and D. B. Chesnut, ibid. 28, 107 (1958).

3 (a) R. Dehl and G. K. Fraenkel, J. Chern. Phys. 39, 1793 (1963); (b) G. Vincow and G. K. Fraenkel, ibid. 34,1333 (1960).

4 G. P. Rabold, K. H. Bar-Eli, E. Reid, and K. Weiss, J. Chern. Phys. 42, 2438 (1965).

6 (a) J. R. Bolton and A. Carrington, Mol. Phys. 6,169 (1963); (b) W. T. Dixon, R. O. C. Norman, and A. L. Buley, ibid. 1964, 3625.

6 J. H. Freed and G. K. Fraenkel, J. Chern. Phys. 38, 2040 (1963) .

7 E. Muller, K. Ley, K. Scheffler, and R. Mayer. Chern. Ber. 91, 2682 (1958).

6 Y. Matsunaga and C. A. McDowell, Can. J. Chern. 38, 1158 (1960) .

9 A. H. Maki and D. H. Geske, J. Am. Chern. Soc. 83, 1852 (1961) .

10 H. Judeikis and D. Kivelson, J. Am. Chern. Soc. 84, 1132 (1962).

11 W. F. Forbes and P. D. Sullivan, J. Am. Chern. Soc. 88, 2862 (1966).

12 M. Adams, M. S. Blois, Jr., and R. H. Sands, J. Chern. Phys. 28,774 (1957).

13 A. Zweig and A. K. Hoffmann, J. Am. Chern. Soc. 85, 2736 (1963) .

14 P. Smith, J. T. Pearson, P. B. Wood, and T. C. Smith; J. Chern. Phys. 43, 1535 (1965).

Attempts have been made to treat this type of hyperfine interaction theoretically,4.16 but unfortunately not enough actual examples, obtained under comparable experimental conditions, have been obtained to allow any deductions of a general nature to be made. In the present paper we discuss the results obtained from an examination of the ESR spectra of the anion radicals of a large number of benzoquinone, naphthoquinone, and naphthazarin (I) derivatives, with particular at-

6~

5

13 H

9

10

2

3

12

tention being paid to the methoxy- and hydroxy­substituted derivatives. Spin densities, obtained from molecular orbital calculations, are used as a guide for the interpretation of coupling through the methoxyl oxygen, and in an ancillary manner for some assign­ments of ring splitting constants.

In addition to the intrinsic interest in these types of systems from a more theoretical point of view,2a.3.16 our interest also has a rather practical bent to it. Certain highly substituted naphthazarins (spino­chromes) have been isolated and identified from echino­derms17; it was our hope that detailed studies on model

16 W. Derbyshire, Mol. Phys. 5, 225 (1962). 16 E. A. C. Lucken, J. Chern. Soc. 1964, 4234. 17 I. Singh, R. E. Moore, C. W. J. Chang, and P. J. Scheuer,

J. Am. Chern. Soc. 87, 4023 (1965), and references therein. 1161

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1162 R ABO L D, 0 GAT A, 0 K A M U R A, PIE T T E, MOO R E, AND S C H E U E R

TABLE I. Summary of previous work involving hyperfine coupling through canonically saturated oxygen.

Radical

Durosemiquinone cation

Naphthazarin cation

Naphthazarin anion

Hydroxyperinaphthenyl

Aliphatic hydroxy acids

Aliphatic ethers

2,6-Di-t-Butyl-4-alkoxyphenoxyl

2, 5-Dialkoxy-1, 4-benzoquinone anion

3,31 ,5,51-Tetramethoxy-p-biphenosemiquinone anion

Methoxyphenoxyl radicals

p-Nitroanisole anion

Tri- (p-methoxyphenyl) -methyl

Dimethoxydurene cation

Di- (p-methoxyphenyl) -ni troxide

Alk y 1 galla te

Diethylazodicarboxylate anion

• CH2C02R, RC02CH2 •

• R is Me, Et, 2-Pr. b R is Me, Et, resp. oRis Me, Et, n-Pr, 2~Pr. d R is H,Me.

Reference

5(a)

5(a)

6

4

5(b)

5(b)

7

8

8

e

9

10

11

f

10, 12

13

14

systems would enable us to deduce the structures of unknown spinochromes.18

II. EXPERIMENTAL

A. Materials

1,4-Naphthoquinone (Eastman Organic Chemicals) and juglone (Koch-Light Laboratories) were purified by sublimation. Other benzoquinone, naphthoquinone, juglone, and naphthazarin derivatives were isolated and purified as naturally occurring pigments or were synthesized in these Laboratories. The detailed pro­cedures utilized are reported elsewhere.17 N,N-Di­methylformamide (DMF) (Matheson, Coleman & Bell spectroquality reagent) and tetraethylammonium per­chlorate (TEAP) (Eastmen Organic Chemicals) were used as the solvent and the supporting electrolyte, respectively. Deuterium oxide was obtained from Inter­national Chemical and Nuclear Corporation.

B. Methods

All spectra were run in DMF solutions which were O.lM in tetraethylammonium perchlorate and ",lO-3M

18 L. H. Piette, M. Okamura, G. Rabold, R. Ogata, R. Moore, and P. Scheuer, Symp. Electron Spin Resonance Spectry., August 1966, E-1, Michigan State Univ., East Lansing, Mich. (J. Phys. Chem. to be published).

H2SO,

H 2SO,

Solvent

Basic H20 (soln)

2-PrOH

Acid (aq)

Acid (aq)

Benzene

Basic aq pyridine

Basic aq pyridine

Acid (aq)

Acetonitrile

Toluene

CHaN02-AICh

Alkaline polar solvents

Dimethoxyethane

Acid (aq)

Splitting constant (gauss)

I1oH=0.59

I1oH=0.14

I10H = 1.6-2.6

Oo--CHnRa-n = 1-2. 1

I1o_R=1-1.6a

Oo--R = 1. OS, 1.17b

OocH3=0.49

OocHa= 1.5-2.2

OocHa=0.30

OocHa=0.34

I1ocHa=2.76

I10R =0.24-0.38-

OocH"cHa = 0 . 9

I1oR=1.4-2.5d

e T. J. Stone and w. A. Waters, J. Chern. Soc. 1964, 213 . ! E. G. Rozantsev, L. A. Kalashinikova, andM. B. Neiman, J. Appl. Chern.

USSR English transl. 38, 709 (1966).

in the quinone. The solutions were deaerated by passing nitrogen through them for about 10 min, just prior to reduction. The radicals were produced by electrolytic reduction of the quinone in situ at a static mercury-pool electrode. A saturated calomel reference electrode was employed. The detailed procedure is outlined in Ref. 19. The potential required to produce the radicals varied among the different compounds. However, since the reduction potentials of many of these compounds have been measured previously,20 no attempt was made to measure them accurately. Deuterium exchange was performed by making the solution ",10% in deuterium oxide and passing nitrogen through it for about 5 min.

ESR spectra were measured on a Varian V 4502 spectrometer with Fieldial control of the magnetic field. The hyperfine-splitting constants were calibrated by comparison with a sample of Fremy's salt in water, using aN = 13.0 G. For the well-resolved spectra they are believed to be accurate to within 1 %, or 0.010 G, whichever is greater. In the more complex cases the spectral interpretation was verified by a comparison with a computer-simulated spectrum using a modifica-

19 L. H. Piette, P. Ludwig, and R. N. Adams, Anal. Chern. 34, 916 (1962).

20 B. I. Shiratori, "Redox Potential of Naphthazarin Deriva­tives," Master's thesis, University of Hawaii, 1966.

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ESR OF NAPHTHOQUINONES 1163

tion of the Stone-Maki computer program,21 on the IBM 7040. It should be pointed out here that some measure of caution should be exercised in these com­parisons. In some cases near agreement between the experimental and the calculated spectra was obtained over a rather broad range of splitting parameters, whereas in other cases, usually the more complex ones, very small changes in the splitting constants gave simulated spectra which were decidedly different from each other.

Molecular orbital calculations were carried out on a CDC 3100 computer.

C. RESULTS

In Tables II and III we present a summary of the hyperfine splitting constants for 25 semiquinone radical anions, each of which has at least one methoxyl group as a substituent. The data show a number of clear

TABLE II. ESR hyperfine splitting constants for benzoquinone anion radicals.a

ai (Gauss) Benzoquinone

derivative Cz C, Cs

Benzoquinone 2.41 2.41 2.41 2-Methoxy· 0.63h 3.58e 2.28e

2,3-Dimethoxy· N.S.d N.S. 2.86 2,5-Dimethoxy·· 0.75b 0.75 0.75b

2,6-Dimethoxy- 0.61b 1.83 1.83 2,6-Dimethoxy-3-ethyl- N.S. 1.351 1.35

• The solvent is DMF 0.1 in TEAP. b Splitting by three equivalent protons.

C6

2.41 0.93" 2.86 0.75 0.61 0.61b

e Proton assignment is uncertain. Based on an elementary first-order valence bond analysis.

d No splitting . • Compare aocH.~1.05, acH~O.30 in basic aqueous pyridine (Ref. 8). 1 Splitting by two equivalent protons.

examples of splitting by the methoxyl protons, but interestingly, many of these methoxy-substituted semi­quinone radicals do not give rise to any methoxyl hyperfine interaction. It should be emphasized here that in only a few cases was instrumental sensitivity a problem. The majority of the quinones gave high con­centrations of radicals and advantage was taken of the full resolving power of the spectrometer. Thus the absence of methoxyl splittings reflects an inherent property of the systems studied and not an experi­mental artifact.

An examination of the data reveals that a common structural unit, as depicted in Fig. 1, is present in all but one of those compounds which do give rise to methoxyl splitting. The single requirement for coupling appears to be that the methoxyl groups be unhindered, i.e., that there be no more than one nonhydrogen substituent adjacent to it in the ring. The ESR spec­trum for the 2-methoxybenzosemiquinone radical anion

21 E. W. Stone and A. H. Maki, J. Chem. Phys. 38, 1999 (1963).

FIG. 1. Structural fea­tures common to those com­pounds which give rise to methoxyl splitting.

H,

is shown in Fig. 2. This spectrum analyzes clearly for splitting by three ring protons and three methoxyl protons. Similarly, ring-proton and methoxyl-proton splitting is observed for 2,5-dimetboxy-, 2,6-di­methoxy-, and 2, 6-dimethoxy-3-ethylbenzoquinone,22 and for 2-methoxy-, 2-methoxy-5-hydroxy-, 2,7-di­methoxy-3,5-dihydroxy-, and 3, 7-dimethoxy-5-hy­droxy-1,4-naphthoquinone. This is to be compared with 2,3-dimethoxybenzoquinone, and 2,3,7-tri­methoxy-5-hydroxy-1,4-naphthoquinone, in both of which there is no discernible hyperfine interaction from the methoxyl protons. The results with the naphtha­zarin derivatives are too numerous to cite specifically.23 However, all these compounds give rise to spectra which are in accord with the benzoquinone and 1,4-naphthoquinone results. In particular, metboxyl cou­pling is observed only in those naphthazarins possessing the structural unit depicted in Fig. 1. The spectrum of 2-methoxy-(da)-naphthazarin reveals a magnitude of 0.11 G for aOCDa as compared with aocHa=0.725 for the nondeuterated compound.

In addition to splitting by the methoxyl protons, it should be pointed out that most of the radicals bearing -OH groups generally give rise to splitting by hydrogen on oxygen. A frequent exception to this, however, is the absence of splitting from some quinones which bear a hydroxyl in the 2-position. This exception can be attributed to the ionization of this group. IS

III. DISCUSSION

A. Nature of Methoxyl Coupling

It first seemed to us that the presence or absence of methoxyl coupling in these series of compounds could

I 320 GAUSS t

FIG. 2. ESR spectrum of the radical anion of 2-methoxyben­zoquinone.

22 No splitting is observed for the hindered methoxyl protons. 23 In a consideration of the napthazarin system cognizance must

be taken of the fact that several nonequivalent tautomeric structures may exist when the ring is substituted.I8·24

24 R. E. Moore and P. J. Scheuer, J. Org. Chern. 31,3272 (1966).

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1164 RABOLD, OGATA, OKAMURA, PIETTE, MOORE, AND SCHEl'ER

TABLE III. ESR hyperfine splitting constants for naphthoquinone and naphthazarin anion radicals."

Naphthoquinone derivative

N aphthoquinoneb

2-Methoxy_b

5-Methoxy-b

2-Methoxy-5-hydroxy-·

3,7-Dimethoxy-5-hydroxy_f

2 , 7-Dimethoxy-3 , 5-dihydroxy

2,3,7-Trimethoxy-5-hydroxy-

5,8-Dimethoxy_b

Naphthazarin derivative-

2-Methoxy_b

2,3-Dimethoxy-

2,6-Dimethoxy-

2,7 -Dimethoxy_i

2,3,6-Trimethoxy_i

Tetramethoxy-

2-Methoxy-3-chloro-

2-Hydroxy-3-methoxy-

2-Hydroxy-3-acetyl-5, 7 -dimethoxy-k

2-Hydroxy-3-acetyl-7,8-dimethoxy-

2-Hydroxy-3-ethyl-6,7-dimethoxy-

2,3-Dimethoxy-6,7-dichloro-1

2,3, 6-Trimethoxy-7 -ethyl-

2,7-Dimethoxy-3-ethyl-

• In DMF O.lM in TEAP. b Assignments of splitting constants based on MO calcnlations. o Splitting by three equivalent protons. d No splitting observed. • Spectrum not analyzed. Tentatively aoCH3=0.935 G.

C2

3.35

0.75e

2.58

N.S.

N.S.

2.87

C2

0.73

N.S.

0.650

0.610

N.S.

N.S.

N.S.

N.S.

0.50

N.S.

N.S.

f Spectrum not analyzed. The complexity of the spectrum clearly indicates methoxyl coupling.

• The numbering system is that of Fig.!. b Spectrum not completely analyzed. The assignments for the peri-hYdroxyl

be explained by recourse to a simple steric argument. Quite in analogy with the model proposed by Smith, Pearson, Wood, and Smith for splitting by an alkyl group across the acyl bond of an ester,14 a reasonable model in the present system appears to be as in Fig. 3 (b), where the p7r orbital of the pertinent oxygen atom must overlap with the 7r system of the ring for effective splitting to take place. If another substituent were adjacent to the methoxyl group, as in Fig. 3(a), the result would be to twist the methoxyl methyl group out of the plane giving a decreased 7r-orbital overlap and no (or decreased) hyperfine interaction. It should

a. (gauss)

C3 C. C6 C7 Cs

3.35 0.32 0.64 0.64 0.32

1.65 0.48 0.48 1.17 0.29

3.50 N.S.d 0.90 0.50 0.70

N.S. 0.58 0.58 0.15e 0.58

N.S. 0.25 1.38 N.S. 0.90

2.87 N.S. 1.10 1.10 N.S.

ai (gauss)

Ca C6 C7 C6 Cs

0.42 0.57

N.S. 1.89 1.89 0.52 0.52

1.00 0.65e 1.00 0.65 0.65

1. 79 1. 79 0.61e

N.S. N.S. N.S. 0.47 0.47

N.S. 1.90 1. 59 0.52 0.52

N.S. 3.40 1.20 0.37 0.74

N.S. N.S. 2.52 N.S. N.S.

0.67m N.S. N.S. 0.67 0.67

N.S. N.S. 0.84m 0.62 0.62

1.24m 2.44 0.75e N.S. 0.495

splitting constants may be reversed. For 2-methoxy-(d,)-naphthazarin aCCD.=

0.11 G. i Peri-hydroxyls replaced by deuterium. i Spectrum not analyzed. Tentatively aoCH3=O.645 G. k Spectrum not analyzed. Tentatively "OCHa=O.60 G • lOne broad line was obtained, suggesting complete ionization of the peri­

protons. m Splitting by two equivalent protons.

be pointed out that the models are not meant to imply that the methoxyl groups are in a fixed position, they merely depict favored conformers.26 This model is in excellent accord with the experimental results only if the methoxyl group is in the quinoidal ring, but fails

26 In this connection it should be noted that Maki, in the case of the terphthaldehyde anion,26 and Rieger and Fraenkel,27 in the case of the 1,4-diacetylbenzene anion have presented evidence for a conformational preference of substituents bonded to an aromatic system by a single (canonical) band.

26 A. H. Maki, J. Chern. Phys. 35, 761 (1961). 27 P. H. Rieger and G. K. Fraenkel, J. Chern. Phys. 37, 2811

(1962) .

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ESR OF NAPHTHOQUINONES 1165

in some other cases. Thus, although all the benzo­quinone and naphthazarin radicals can be explained by the model above, certain of the naphthoquinones can­not; 5-methoxy-, 5,8-dimethoxy- and 2,3, 7-trimethoxy-5-hydroxy-naphthoquinone, for example. In these three molecules no coupling was observed by any of the methoxy protons despite the fact that each contains an unhindered methoxyl group in the benzenoid ring. From this, it must be concluded that steric consider­ations alone cannot account for the observed splitting.

We next considered the possibility of a spatial overlap between the quinoidal oxygen and the protons of the adjacent methoxyl group. Such an explanation for long­range hyperfine interactions has been advanced to explain the ESR spectra of some iminoxy radicals, the determinant factor being a suitable steric relation be­tween the radical moiety and the pertinent proton.28a ,b,c

Since the proper steric relation for this type of splitting is present in the quinones, such a hypothesis seemed to merit consideration. However, certain experimental re­sults of other workers indicate this coupling mechanism to be inoperative in the present case. There are at least two previous reports of methoxyl-proton coupling in systems where, since there are no other suitably situated substituents, the mechanism for hyperfine interaction must be a simple transmission of spin density through the saturated methoxyl oxygen. The molecules examined in these instances are the tri-(p-methoxy­phenyl) -methyl radical,1O and the p-nitroanisole anion9

(and others in Table I). Further, we have observed methoxyl coupling at C7 from 2,7 -dimethoxy-3, 5-di­hydroxy-1 ,4-naphthoquinone.

In view of these considerations it appears that two factors must determine whether methoxyl coupling is observed. These are a steric criterion, as outlined above, in conjunction with a sufficient spin density on oxygen (or both on oxygen and the carbon atom of the system

FIG. 3. Representation of the con­formers in methoxy-substituted qui­nones, indicating ..--orbital overlap. (aJ disubstituted and (b) mono­substituted.

28 (a) B. C. Gilbert, R. O. C. NOlman, and D. C. Price, Proc. Chern. Soc. 1964, 234; (b) B. C. Gilbert and R. O. C. Norman J. Chern. Soc. 1966,86; (c) R. O. C. Norman and B. C. Gilbert, Syrnp. Electron S~in Resonance. Spectrl" August 196~, C-l, Michigan State UillV., East Lansmg, Mich. (to be published).

to which it is bonded). We now explore this latter criterion in some detail below.

B. Molecular Orbital Calculations

We have utilized MO calculations first, to enable us to assign splitting constants to particular ring protons, and secondly, to obtain an estimate of the spin densities on the methoxyl oxygen atoms. The experimentally measured splitting constants were compared with cal­culated spin densities in the usual manner, using McConnell's2 equation (1) (vide infra). We have ob­tained best agreement using a value for QCH = -19 to - 21 G, which is considerably lower than values used by other workers.3b ,29 Our justification for using this value is that it does afford agreement between theory and experiment. However, more exact calculations may indicate that some other value of QCH is more suitable. We have made the reasonable assumption that when an acceptable spin-density pattern is obtained for the ring-proton splitting constants with a particular set of MO parameters, then the spin densities on the oxygen atoms are also a valid representation of the actual odd­electron distribution. For 2-methoxynaphthoquinone fair agreement was obtained, but only with the McLachlan MO-SCF (self-consistent-field) theory, which introduces spin correlation.30 The oxygen atoms were accommodated in the usual manner by setting ao=a+h/3 and /3cO=k/3.31 For naphthoquinone the pa­rameters hand k were varied over the range 0.4 to 2.0 which includes all reasonable values used to modify the oxygen integrals in these systems.31 For the other mole­cules a more limited range had to be examined, since two, and in some cases three, sets of hand k parameters were employed. In Table IV we present a summary of the MO results for some molecules which gave good agreement between calculated and experimentally meas­ured splitting constants. However, because the agree­ment is not as exact as we would wish, we do not feel at this time that it serves any useful purposes to discuss the variations in spin densities as a function of the various h's and k's employed. It is anticipated that further calculations will lead to better agreement and we hope to present these results (on methoxy and hydroxy derivatives) at a later time. It should also be noted that as yet we have not found a consistent set of hand k values of the quinone or the ether type, which can be transferred successfully to different molecules in the series.

C. Analysis of the Results

The MO calculations leave little doubt that the spin density on the ether oxygen is a key factor in deter-

29 R. W. Fessenden and S. Ogawa, J. Am. Chern. Soc. 86, 3591 (1964).

30 A. D. McLachlan, Mol. Phys. 3, 233 (1960). 31 A. Streitweiser, Jr., Molecular Orbital Calculations for Or­

ganic Chemists (John Wiley & Sons, Inc., New York, 1961), Chap. 5.

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1166 R ABO L D, 0 GAT A, 0 K A M U R A, PIE T T E, MOO R E, AND S C H E U E R

TABLE IV. Comparison of HMO-calculated splitting constants with experimental values.··b

Radical 2 3

Naphthoquinone· 3.35 (3.34)

5,8-Dimethoxynaphthoquinoned 2.89 (2.94)

2-Methoxynaphthoquinonee 0.750 1.65 (1. 65)

5-Methoxynaphthoquinone f 2.58 3.50 (2.46) (3.50)

• Equation (1) was used to obtain the calculated splitting constants. Calcu-lated values are in parentheses.

b In the absence of a suitable Qco, Po. the oxygen spin density is presented. e h=1.0, k=1.80, Q=20.6. d hc_o=0.8, kc_o=1.6, kc_o=1.2, kc_o=1.4, Q=19.3.

mining whether the methoxyl protons will give rise to an observable hyperfine interaction. The 5-methoxy­and 5, 8-dimethoxynaphthoquinone radical anions have only 1 % and 2%, respectively, of their spin densities on the methoxyl oxygens, whereas 2-methoxynaphtho­quinone has 13.4% of its spin density on the ether oxygen. As noted above, the former two compounds display no methoxyl splitting, while the latter deriva­tive gives rise to a significant hyperfine interaction. Although the McLachlan-SCF treatment was used to obtain an acceptable spin-density pattern for 2-meth­oxynaphthoquinone, we note that the simple HMO treatment also consistently places "-'10% of the total spin density on the methoxyl oxygen.32

A comparison of the results from 2,3,7 -trimethoxy-5-hydroxy-1 ,4-naphthoquinone and 2,7 -dimethoxy-3, 5-dihydroxy-1,4-naphthoquinone is especially helpful. The former compound gives rise to no methoxyl splitting, whereas the latter does (assigned to C7).

Other studies in this system18 show that a hydroxyl substituent at C2 or Ca induces a larger splitting from groups at C6 and C7, respectively. This is in complete accord with the above finding, for it predicts that the derivative with a hydroxyl group at Ca should have a greater spin density at C7, and should therefore couple more strongly. It thus appears clear that both a steric criterion and a spin-density criterion must be met in those instances when methoxyl coupling occurs.

One very interesting possible exception to this expla­nation is the methoxyl splitting recently observed from the radical cation of dimethoxy-durene. ll If the C-O-C bond angle is essentially unchanged in the oxidized form of this molecule then there is a very serious steric crowding, which should result in a small overlap of the p oxygen orbitals and the ring 7r-orbital system. The

32 Of uncertain significance, but of definite interest in this context, is the fact that the spin densities on the carbon atoms to which the methoxyl oxygens are attached are in all cases (with or without methoxy splitting) ~2% or less of the total spin density.

a. (gauss) Po

5 6 7 8

0.32 0.64 (0.35) (0.68)

1.10 0.018 (1.11)

0.48 0.48 1.17 0.29 0.134 (0.43) (0.43) (1.18) (0.27)

0.90 0.50 0.70 0.010 (0.87) (0.50) (0.64)

e hc-o=1.2, kc_o=1.6. hc_o=0.8, kc.o=1.24, Q=16.2. The lower value of Q here is probably a result of the use of MO-SCF calculations. A=1.2.

f For 011; hc_o=1.2, kc_o=1.6; for 0,,: hc_o=1.0, kc_o=1.8; for 013: hc_o=2.0, kc_o=0.7, Q=19.2.

reported methoxyl splitting of 2.76 G is quite large and suggests that other factors may be operating here. If the C-O-C bond angle is greater in the cation this might account for the large splitting. The positive charge could also account for a greater involvement of the oxygen and an increased value of QOCH 3' We have generated the radical cation of 5,8-dimethoxy-1,4-naphthoquinone by the method of Forbes and Sullivan,ll and obtain a complex spectrum which is not inconsistent with methoxyl-proton interactions of ""0.5 G. It should be recalled that the corresponding radical anion clearly gave no methoxyl coupling.

As yet, we have not determined what the minimum spin density is which still gives rise to an observable splitting. However, it is possible from the data of Table IV to obtain a semiempirical value for QOCH 3'

Assuming a direct proportionality between the spin density on oxygen and the splitting constant of the CHa protons, as in Eq. (1), we obtain a value of 5.6 G for I QocH.I. It is interesting to compare this value to the value for I QOH 1=7 G,4 which was obtained semiempirically from the hydroxyperinaphthenyl radi­cal, and also from a configuration-interaction calcu­lation on the OH radical. Fessenden and Schuler,aa in a treatment of the splitting from simple unsubstituted alkyl radicals, have concluded that Q", the proportion­ality constant relating the a-proton splitting constant a" and the a-carbon spin density p is approximately constant [Eq. (1)]. A similar relation [Eq. (2) ] was put forth for radicals having a (3-methyl group and in which isotropic averaging occurs:

a,,=1 Q" I p,

aJ3= I QJ3(CHa) I p.

(1)

(2)

In this equation Q{i(CH3) is the proportionality con­stant relating a{j, the {3-proton splitting constant, and

33 R. W. Fessenden and R. H. Schuler, J. Chern. Phys. 39, 2147 (1963).

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ESR OF NAPHTHOQUINONES 1167

p, the a-carbon spin density. In general the i3-proton coupling constants are larger than those of the a protons by 3 to 5 G giving a ratio of Q~(CH3) /Qa slightly greater than unity. If we assume a similar set of equations for the present system, which is the corresponding oxygen analog, and use the notation QOa and QO~(CH3), we obtain a ratio of QOIl(CH3)/Qoa=0.8. In view of the several approximations involved in such a treatment the agreement seems quite gratifying. One significant difference here is that Fessenden and Schuler are treat­ing a saturated aliphatic system, whereas we are treating an alkoxyl group on an aromatic carbon. Perhaps a more valid comparison is the ratio of the coupling constants for a proton and a methyl group attached to an aromatic 11" system. The results of Venkataraman and Fraenkel2b show that here, too, the ratio QCH/QCCHa is close to unity, and it thus seems that our results are in general conformity with those of the other systems. The data in Tables II and III show that the methoxyl-proton splitting constants are all quite close to each other ("-'0.70±0.10 G). Such a small deviation can be accounted for very nicely by the low value of QOCH 3. Additionally, we obtain a value of QOH = 6.5 G by the semiempirical treatment using the naphthazarin spin-density parameters and splitting constants in conjunction with Eq. (2), where a is now replaced by OH. This value is in quite good agreement with the earlier reported value of 7 G.'

A deuterium splitting constant of 0.11 G was ob­served for the radical anion of 2-methoxy- (d3) -naphtha­zarin (see Table II) which is to be compared with 0.725 for the methoxyl protons in the undeuterated compound. The ratio aOCH a/aocD a=6.6 is in essential

agreement, within experimental error, with the pre­dicted value of 6.513.34

For those compounds in Table IV, assignments of the ring-proton splitting constants could be made on the basis of the MO calculations in conjunction with Eq. (1). We note nothing unusual in the spin-density patterns. The carbon atoms in the quinoidal ring have the greater spin density, which has been shown to be quite general for members of these series of compounds.1S

The results from trimethoxynaphthazarin are as yet not completely analyzed. The expected pattern, by analogy with the other members, would be splitting by one ring hydrogen, three methoxyl protons, and the two peri-hydroxyl protons. The observed spectrum clearly shows methoxyl splitting, but it is more complex than would be expected from the total number of protons which should couple. Preliminary temperature studies indicate that there may be some sort of con­formational isomerization involved, a point which is being examined in some detail.

ACKNOWLEDGMENTS

We wish to acknowledge the generosity of Dr. Harjit Singh and Mr. N arasimham Vadlamani for furnishing us with pure samples of many of the compounds reported herein, and to Mr. William· Soong for help with the computer programming. We-are especially grateful to Mr. Alvin Katekaru for his valuable assist­ance with the compilation of data.

The MO calculations were carried out using the Pacific Biomedical Research Center computer facilities.

34 B. Venkatararnan and G. K. Fraenkel, J. Chern. Phys. 24, 737 (1956).

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