High-resolution electron spin resonance studies of hyperfine

HYPERFINE INTERACTIONS IN 1,4-NAPHTHOQUINONESAND NAPHTHAZARINS

computed spin densities for the anions of toluene and pxylene may be regarded as being fairly reliable. They compare favorably with the calculations of others on substituted benzenes

29

(see, e.g., ref 8). We feel therefore that the given semiempirical estimates of A, which account for the vibronic mixing, are probably not largely in error.

High-Resolution Electron Spin Resonance Studies of Hyperfine Interactions

Downloaded by CENTRAL MICHIGAN UNIV on September 13, 2015 | http://pubs.acs.org Publication Date: January 1, 1967 | doi: 10.1021/j100860a005

in Substituted l,.P=Naphthoquinonesand Naphthazarinsla

by L. H. Piette, M. Okamura, G. P. Rabold, R. T. Ogata,lbR. E. Moore, and P. J. Scheuer Department of Biochemistry and Biophyeice, School of Medicine, and Department of Chemistry, Univereity of Hawaii, Honolulu, Hawaii 96822 (Received September 27, 1966)

The esr studies were performed on polarographically reduced l14-naphthoquinones and naphthazarins in hope of using this technique to determine the structure of unknown echinoderm pigments. Unusual hydroxyl, methoxyl, and chlorine splittings were observed in several derivatives of l14-naphthoquinone and naphthazarin. A rough correlation is made between the spin-coupling constant and quinoidal character of ring hydrogens. A large number of compounds were reduced and structurally identified by their hyperfine patterns.

Introduction A number of investigations of the structural pigments of echinoderms have been carried out over the past few years,2 the echinoids (sea urchins) receiving perhaps the closest attention with the result that the number of pigments identified is far in excess of the actual number of derivatives naturally occurring in the animals. These erroneous identifications stem from the extreme difficulties encountered in isolating, purifying, and recovering sufficient quantities of the pigments for careful structural analysis. I n general, these spinochromes are polyhydroxy1 derivatives of 1,Cnaphthoquinone. At least six derivatives have been isolated and their structures positively established.2b A number of other derivatives have been isolated from two different species of Hawaiian sea urchin, the structures of which are under inve~tigation.~ I n order to determine the structure of these unknown derivatives, we have undertaken a systematic study

of the esr spectra of a number of synthetic derivatives and naturally occurring spinochromes. Absorption spectroscopy is probably the most powerful tool used for structure determination. The most recent contributor to this general technique is magnetic resonance, both nuclear and electron (nmr and esr), the former being the most widely used in organic structure analysis to date. The nmr, however, suffers dearly from its lack of sensitivity, thus requiring high concentrations and large quantities of precious material. The esr, on the other hand, is 1000 times more sensitive, requiring on the order of lo-” mole of materials but limited to only those molecules containing (1) (a) This work was supported by Public Health Service Grants GM 12798 and GM 10413; (b) National Science Foundation under-

graduate research participant. (2) (a) R. H.Thompson, “Naturally Occurring Quinones,” Butterworth and Co. Ltd., London, 1957, pp 128-140; (b) I. Singh, R. E. Moore, C. W. J. Chang, and P. J. Scheuer, J . Am. Chem. SOC.,87,4023 (1966),and references therein. (3) R. E. Moore, H. Singh, and P. J. Scheuer, unpublished results.

Volume 7 1 , Number 1

January 1967


30

PIETTE, OKAMURA,RABOLD, OGATA,MOORE, AND SCHEUER

unpaired electrons. This latter restriction, however, can be rectified by polarographically reducing or oxidizing the molecules under investigation in univalent steps in situ, thus producing the paramagnetic entity necessary for esr detection. The esr spectra of organic free radicals are extremely sensitive to functional group substitutions, especially in aromatic or conjugated molecules, making it therefore a very powerful technique for structural analysis. The work outlined here attempts to illustrate the applicability of using esr as an analytical tool for determining the structure of certain organic molecules. The paramagnetic intermediates of a number of synthesized derivatives of 1,Pnaphthoquinone and naphthazarin were formed in situ by univalent reduction at a static mercury pool electrode contained in the esr cavity. The polarographic reductions were referenced to saturated calomel and the technique is described in detail elsewhere.* The unique feature of this technique is that less than 0.5 mg of starting material is required to give sufficient free-radical analysis. Structural assignments could be made after an analysis of the appropriate hyperfine pattern. I n some cases deuterium substitution of OH groups was used to simplify the interpretation of the spectra. For very complex spectra, interpretation was made possible only by applying a trial and error computer simulation of the spectrum. By comparison of computer spectra with experimental spectra, the correct assignment of the hyperfine pattern could be made. Hyperfine data were also compared with available nmr data in hope of unequivocally assigning some of the nmr absorptions observed.

Experimental Section Materials. All of the naturally occurring spinochromes used were isolated and ,purified following the procedure of Chang, et aL5 The new synthetic derivatives reported here were synthesized in the laboratories of Professor Scheuer and are reported el~ewhere.~ Methods. Details of the esr polarographic experiments used are outlined in ref 4. The only deviation used here was to use a supporting electrolyte of tetraethylammonium perchlorate in dimethylformamide. Samples were deaerated by passing dry nitrogen through the appropriate solution for about 10 min prior to reduction. The polarographic reductions of naphthazarins at a dropping-mercury electrode, using a Kalousek cell of sodium sulfate-saturated mercurous sulfate-mercury as reference, give split waves that vary for the first half-wave potential from 0.50 to 1.14 v and from 0.93 to 1.45 v for the second half-wave. The Journal Of Physical Chemistry

These variations depend upon the substituent and place of substitution in the naphthazarin molecule.6 I n the esr polarographic reductions it was found that free radicals could only be observed at the first halfwave if the substance was carefully degassed. If the sample had not been degassed, free radicals could still be formed but only at the second half-wave, indicating that the free radical was being formed by the reaction

+ 2e +R2R2- + R +2R.R

where R and R2- are the completely oxidized and reduced states. Labile protons were exchanged by addition of four drops of 99% D20 to the electrolysis cell containing the appropriate naphthazarin solution which was mixed by passing nitrogen through it for about 2 min. The esr equipment used was a Varian V-4502 with Fieldial control of the magnetic field. Hyperfine constants were measured by comparison with a reference solution of Fremy’s salt in water, assuming the nitrogen splitting to be 13.0 gauss. Computed spectra were simulated using a modification of the program of Stone and R9aki7on the IBM 7040.

Results and Discussion The naphthazarin system is capable of rapid tautomerization with the possibility of four different tautomers existing in solution

.&

6 \

yyJ OH 0 I1

;# 0

OH 0 I

HO IV

n

i

0 HO

III where R = H for naphthazarin. The equivalence of these structures is borne out in the esr spectrum of univalently reduced naphthazarin (4) L. H. Piette, P. Ludwig, and R. N. Adams, Anal. Chem., 34, 916 (1962). (5) C. W. J. Chang, R. E. Moore, and P. J. Scheuer, J. Am. Chem. Soc., 86, 2959 (1964). (6) B. I. Shiratori, “Redox Potentials of Naphthazarin Derivatives,” Master’s Thesis, University of Hawaii, 1966. (7) E. W. Stone and A. H. Maki, J. Chem. Phys., 38, 1999 (1963).

HYPERFINE INTERACTIONS IN 1,4-NAPHTHOQUINONESAND

31

r\TAPTHAZARINS


Figure 2. The esr spectrum of electrochemically reduced 1,4naphthoquinone.

I n the esr measurements, the effect observed is a greater spin density for protons on the quinoidal ring than for those on the benzenoid ring. This is nicely demonstrated by 1,Pnaphthoquinone (see Figure 2) which, when reduced electrochemically, gives a hyperfine pattern consisting of a large triplet of septets. The triplet (3.35 gauss) arises from the two equivalent protons at Cz and C3 and the septet from the two protons on C6 and C7 which have twice (0.64 gauss) the splitting of those on C5 and Cs. This is to be contrasted with previously reported spectral0.'l obtained from naphthoquinone in alkaline media where the protons on CA,s,6,,are all equivalent giving a quintet. Figure 1. The esr spectrum of electrochemically reduced naphthazarin in DMF before (top) and after (bottom) 5,&Dimethoxy-l,4-naphthoquinone shows a triplet addition of DtO to exchange hydroxyl protons. of triplets, one coupling of which is reasonably close !2.94 gauss) to that of the C2 and CBproton coupling at 1,4-naphthoquinone and is therefore assigned to the as illustrated in Figure 1 (top). The main quintet protons on the C2 and C3positions. Preliminary HMO arises from equal coupling to the four ring H's and the calculationis also support the assignments made for 1,4small triplet resulting from coupling of two equivalent naphthoquinone and 5,8-dimethoxy-l,4-naphthoquiOH protons. The OH splitting is verified by exchange none. with D20 and the concomitant disappearance of the I n the naphthazarin series, the preference for tautriplet. This same spectrum was observed by Freed tomer I when R is an electron-releasing group is clearly and Fraenkels upon basic oxidation of 1,4,5,8-tetradetected in the effective spin densities of protons adjahydrox ynaphthalene. cent to R. With the exception of 2-hydroxynaphthaMimosubsti2ution. When R is no longer a hydrogen sarin, the spin couplings of the proton on Cs for 2but is a substituent such as OH, OCH3, OC(=O)CH3, methoxy-, 2-acetoxy-, and 2-ethylnaphthazarin are C1, or C2H5, an equivalence of tautomeric structures is larger than those on the C6 and C, positions. I n no longer possible. The preferential tautomer is I addition, with the exception of 2-ethylnaphthazarin, when R is an electron-releasing group and IV when R the spin coupling for the C3position of these compounds is an electron-withdrawing s ~ b s t i t u e n t . ~The evidence is greater than that of naphthazarin, thus indicating for such a preference comes from the decidedly diaagain the increased quinoidal character of the submagnetic shift in the nmr of the protons on C3 for monostituted ring. %Ethylnaphthazarin, which nmr shows substituted naphthazarins of structure I and the parato be the weakest quinoid-directing substituent, magnetic shift of the C7 proton for monosubstituted exhibits a spin coupling at C, which is 0.02 gauss lower naphthazarins of structure IV. The chemical shift than that of naphthazarin. This, however, is within for the proton on C8for 2-methoxy-l,4-naphthoquinone the limits of the experimental accuracy. The evidence, is identical with that for the proton on C3 of 2-methoxythen, seems to support the nmr data which indicate naphthazarin; the protons on Cg and C7 of tautomer I show a paramagnetic shift (see Table I) and the Ct (8) J. H. Freed and G. K. Fraenkel, J. Chem. Phys., 38, 2040 (1963). and C3 protons of I\' show a diamagnetic shift. The (9) R. E. Moore and P. J. Scheuer, J . Org. Chem., 31, 3272 (1966). nmr data indicate the following order for the attracAll nmr data cited in the present work are from this reference. tion of quinoidal properties to a ring by a substituent: (10) J. Wertr and J. Vivo, J . Chem. Phys., 24, 499 (1956). OH > OCH3 >> OCOCH, > CzHs >> H. (11) R. W. Brandon and E. A. Lucken, J . Chem. Soc., 4273 (1961). ~

~~~~~

Volume 7 1 , Number 1

January 1967


32

PIETTE,OKAMURA,RABOLD, OGATA,MOORE,AND SCHEUER

1

1

3

E

a

m

m

The Journal of Physical Chemietry

m m

X

X

m

$0 8

m

ma

8

a"

m

m m

Em

N

*

N

2

8o

t

N

. 3. 35:

mm m m

00

ma

m N

HYPERFINEINTERACTIONS IN 1,4-NAPHTHOQUINONESAND NAPHTHAZARINS

m

m

2

2

that monosubstitution with an electron-donating group results in a preferred tautomer of structure I. I n Table I, the variation of spin coupling with substituent is compared with the variation of the respective nmr chemical shifts. 2-Hydroxynaphthazarin, although shown to be strongly quinoid attracting by nmr, does not, however, increase the esr spin coupling a t the adjacent proton. The deviation from the general pattern is most likely due to the increased acidity of the OH when in the 2 position and to the rapid enolization reaction of the type

'909

OH 0


33

+@

&$--#; H O

OH

OH 0

OH 0 0 v)

0

N

h m

0

0 0

'9

'909

'4

@-

OH 0

OH OH

0

OH 0

\

#x o m

OH 0

OH 0

H

*

#J$

H

(2)

OH 0

Such an enolization, if fast compared to the magnitude of the hyperfine coupling, would average out the coupling a t C3. In agreement with a first-order valencebond treatment, the net effect of this enolization is experimentally seen to be an increase in the spin density at Ce. Evidence for this process is clearly seen in Figure 3 of 2-hydroxynaphthazarin where the Ct proton has become deuterated by such an enolization in the presence of D20.The resulting spectrum of two doublets is assigned a large coupling by the proton at c 6 and a smaller coupling at C,. The assignment of the greater spin density to position c6 is supported by the results obtained from 2-hydroxy-6-ethylnaphthaaarin (vide infra). The multiple lines in Figure 3 are the deuterium couplings at the two peri-hydroxyls. This influence of acidity of the hydroxyl in the two positions on the spin density at Cg is also seen in a comparison of the free radicals of 2-hydroxy-3-chloronaphthaz arin and 2-methoxy-3-chloronaphthaz arin (Figures 4 and 5). I n this case the proton for the hydroxysubstituted compound has a much larger spin coupling than the analogous 2-methoxy compounds, going from 2.55 to 1.90. Disubstitution. Symmetrical disubstitution in naphthazarin should give four tautomers analogous to those described above, and the spin densities at the C3 and c 6 positions should be just slightly less than those for naphthaaarin. This is the case as seen in Table I. It was observed, however, that disubstitution inVolume 71,Number 1

Januury 1967

PIETTE, OKAMURA, RABOLD, OGATA,MOORE,AND SCHEUER

34

,

3.34 G A U S S

I


Figure 3. The esr spectrum of electrochemically reduced 2-hydroxynaphthazarin after addition of Dz0 to exchange the hydroxyl protons and the Ca Proton.

substituted compound. However, the rate was considerably slower. This is clearly demonstrated in Figure 6 where four drops of DzO are added to the reduced naphthazarin. It can be seen that initially only the 2-, 5-, 7-, and 8-hydroxyls are deuterated and one is left with a simple triplet due to protons at Ca and Cg. As time progresses, the two outer lines of the triplet diminish relative to the single line in the center indicating that both positions Ca and Cs are slowly being deuterated by the enolization reaction. Unsymmetrically disubstituted 2-hydroxy-6-ethylnaphthazarin illustrated in Figure 7, however, shows a rapid deuteration a t Cs leaving only the proton at C7 and the CH2 protons of the ethyl group to couple to give a triplet of doublets. I t should be noted that the CH2 coupling is 2.34 gauss as compared to the C7 coupling of 0.75 gauss, indicating again that ionization of the 2-OH increases the spin density at the Cg position. From the differences in the symmetry of 2,6- and 2,7-dimethoxynaphthazarin, one would expect these compounds to exhibit different preferred tautomeric structures. I n particular, it seems likely that the 2,6 derivative would favor a tautomeric structure of type

Figure 4. The esr spectrum of electrochemically reduced 2-hydroxy-3-chloronaphthanarinbefore (top) and after (bottom) addition of D20.

I

3.3 GAUSS

Figure 5. The esr spectrum of electrochemically reduced 2-methoxy-3-chloronaphthazarinbefore (top) and after (bottom) addition of DzO.

troduced some additional effects that perturb the spin densities and rate of enolization. Symmetrically substituted 2,7-dihydroxynaphthazarin showed the same type of enolization as the monoThe Journal of Physical Chemistry

A

0 . 0

ADDED

Figure 6 . The esr spectrum of electrochemically reduced 2,i’-dihydroxynaphthazarin before (top), immediately after (middle), and some time after (bottom) addition of D20.

HYPERFINE INTERACTIONS

1,4-NAPHTHOQUINONESAND NAPHTHAZARINS

IN


t

3.3 G A U S S 4

35

* -

IN D20

Figure 8. The esr spectrum of electrochemically reduced 2,3,7-trimethoxyjuglone before (top) and after (bot.tom) addition of D20. Figure 7 . The esr spectrum of electrochemically reduced 2-hydroxy-6-ethylnaphthazarin before (top) and immediately after (bottom) addition of D20.

I1 or 111, whereas the 2,7 compound would favor tautomers I arid IV. The observed differences in the magnitude of the ring-proton splitting constants may indeed reflect such structural nuances (see Table I). I n addition, it was observed that both the 2,7 and 2,6 compounds show a large coupling by the methoxyl protons which are three bonds removed from the ring. This coupling by a methoxyl group was observed whenever the group was unhindered; i.e., the adjacent p position is unsubstituted. The coupling, however, is not observed when the methoxyl is hindered by an adjacent 0 substituent such as in 2,3-dimethoxy, 2methoxy-3-chloro, or 2-hydroxy-3-methoxy, etc. This same effect was observed in naphthoquinones containing methovyl groups in the 2 and/or 3 positions. However, even nonhindered methoxyl groups attached to the aromatic portion of naphthoquinone failed to show coupling. For example, 2-methoxy-1,4-naphthoquinone showed a methoxyl splitting of 0.75 gauss, whereas in 2,3,7-trimethoxyjuglone (see Figure 8) no splitting by any of the methoxyl groups was observed. 5-Xlethoxy-1,4-naphthoquinoneand 5,8-dimethoxy-1,4naphthoquinone similarly showed no methoxyl splitting. -4similar effect was noted in the nmr studies, which indicate that this nonhindered condition is reflected by a paramagnetic shift when compared with the hindered examples. The nmr resonance for all unhindered methoxyls-except those in the 5 and 8 positions of 1,4naphthoquinone-occur in the region 3.87-3.94 ppm, whereas the hindered methoxyls resonate at 4.00ppm and greater. The explanation for this coupling of methoxyl as a function of hindrance is at present not

entirely clear. It is possible that upon substitution adjacent to the methoxyl group, the p orbital of the ether oxygen is forced to lie in a region of decreased overlap with the ‘IF electrons of the conjugated ring system, thus reducing the coupling. A similar effect which might be considered contrary to the above argument is observed in the chloro compounds. 2,3-Dichloronaphthazarin gives the spectrum in Figure 9 indicating a rather large coupling by the two equivalent chlorines (I = 3/2) of about 0.19 gauss. The monochloro compound, 2-chloronaphthazarin, however, shows no coupling by the chlorine, nor does 2-methoxy-3-chloro- or 2-hydroxy-3-chloronapht haz arin. Tri- and Tetrasubstitution. I n many cases the spectra from the tri- and tetrasubstituted naphthazarins did not afford very useful information. They were either too complex for complete analysis or too simple for meaningful conclusions to be drawn regarding favored tautomers. For example, 2-hydroxy-3-acetyl7-methoxynaphthazarin exhibits a very complex spectrum of 39 lines, while the spectrum of 2,3-dimethoxy6,7-dichloronaphthazarin consists of one broad line. These highly substituted naphthazarins also show numerous anomalies, some of which are discussed below. The nmr data list the following order for quinoidal attraction of two adjacent substituents on a naphthazarin system: OH, COCH, > OH, OCH3 > OCHa, OCH3 > OCH3, CzH5 > OCH,, COCH,. Also, from nmr it is seen that the quinoidal-attracting strength of OCH3, OCH3 is approximately equal t o O C H , H.9 The esr spectrum of 2,7-dimethoxy-6-ethylnaphthazarin is in accord with this. One would expect from the above-mentioned order of quinoid attraction that the CBproton would show a large spin coupling. This is Volume 71I Number 1

January I967

PIETTE, OKAMURA, RABOLD, OGATA,MOORE,AND SCHEUER

36

is due to one (or both) of the hindered methoxyl groups or whether it is caused by conformational isomers of some kind. Temperature studies indicate that the spectrum is quite sensitive to changes in temperature. It is anticipated that further work will shed more light on this interesting aspect. Several of the tri- and tetrasubstituted compounds studied were naturally occurring spinochrome pigFigure 9. The esr spectrum of electrochemically reduced ments which contained acetyl groups. Spinochromes 2,3-dichloronaphthazarin after addition of DzO. A (2,7-dihydroxy-3-acetylnaphthazarin)and C (2,6,7trihydroxy-3-acetylnaphthazarin) and their methyl indeed seen to be the case, for the coupling a t the C8 ethers exhibited spectra which suggest an acetyl splitting at about 0.5 gauss. However, in the previproton (2.40 gauss) is almost twice that for the methylene protons (1.24 gauss). 2,3,6-Trimethoxy-7-ethyl- ously mentioned spectrum of a dimethyl ether of spinochrome A, 2-hydroxy-3-acetyl-7,8-dimethoxyjuglone, naphthazarin also supports the given order of quinoid only a doublet (2.68 gauss) is observed. No explanaattraction. Here, the methylene protons show a very tion of this apparent anomaly is presently available. small spin coupling of 0.835 gauss, indicating that the Spectra of three tetrasubstituted chloro compounds ethyl group is attached to a benzenoid ring. Again, revealed no chlorine splitting. These compounds were as in the mono- and disubstituted compounds, the all substituted by two chlorine atoms in the same ring spectra from the 2-hydroxy derivatives suggest an similar to 2,3-dichloronaphthazarin (see Table I). enolization mechanism to be operative. I n 2-hydroxyThe lack of splitting is interesting in view of the very 3-ethyl-6,7-dimethoxynaphthazarina fairly large splitobvious chlorine interaction (0.19 gauss) displayed ting would be expected by the methylene protons if only by 2,3-dichloronaphthazarin (Figure 9). Hollocher, the quinoid-attracting properties of the substituents Tooney, and Adman noted a chlorine splitting of 0.12 are considered. However, the spin coupling by these gauss from the anion radical of 2-hydroxy-3-chloroprotons is seen to be very small, 0.67 gauss, which we naphthazarin and attributed the successful resolution take to be evidence for an enolization reaction, as in of the hyperfine components to a sufficiently low radical eq 2. concentration.12 This concentration effect may thereThe quinoid-attracting properties of the OH, fore be an important factor in the chloro-substituted COCHZ pair could not be extensively studied owing to naphthazarins we have examined. We do note, howthe very complex spectra arising from compounds conever, that 2-hydroxy-3-chloronapht hazarin, structurtaining these groups. However, in the spectrum of 2ally analogous to the naphthoquinone, showed no chlohydroxy-3-acetyl-7,8-dimethoxyjuglone, only one large rine splitting. doublet (2.68 gauss) is observed. This splitting must I n the interpretation of many of the hydroxy-subbe attributed to the C5 proton which is on a benzenoid stituted compounds, it was often found that less than ring. While enolization might again be postulated the number of protons available for splitting would to explain this, nmr data indicate that the 2-hydroxy account for the observed spectrum. I n these cases proton is strongly hydrogen bonded with the acetyl it was assumed that a hydroxyl proton had ionized. group, making it less acidic than a nonhydrogenI n most cases signals for these protons also were not bonded hydroxyl proton. observed in the nmr spectrum of the compound, indiWhereas the esr spectra of the mono- and disubcating another basis for comparison between the esr stituted naphthazarins show quite consistent trends, and nmr spectra of these compounds and also a rough the tri- and tetrasubstituted compounds exhibit measure of their acidity. More complete and quantinumerous anomalies. As was the case in the monotative acidity measurements are planned. and disubstituted derivatives, methoxyl groups in most tri- and tetrasubstituted naphthazarins do not show Conclusion coupling when hindered by a /3 substituent. This It certainly appears feasible that the esr technique holds true in all cases except for 2,3,6-trimethoxycould be a powerful structural tool in the field of natural naphthazarin. I n this compound an explanation of products. The results here demonstrate the sensitivity the observed spectrum is inconsistent with the assumpof the esr spectrum to changes in substitution or steric tion that only the unhindered methoxyl group and three other protons are available for coupling. It (12) T. C. Hollocher, N. M. Tooney, and R. Adman, Nature, 197, is not clear a t this time whether the additional splitting 7 4 (1963).


-

The Journal of Physical Chemistry


HYPERFINE INTERACTIONS

IN

modifications of the molecule. The small quantities of material required make it extremely attractive to the natural products chemist. By comparison with nmr, chemical shifts of the order of 0.1 part per loe are observed for variation of the substituting group, whereas these same groups would produce a change of approximately 3 parts per lo4 in the coupling constants of an esr spectrum. The resolving power of the esr method is certainly good to less than 0.3 part per 104. There appears to be a general agreement with the nmr data as far as the quinoidal character of one ring in monosubstituted naphthazarins. The spin density at the quinoidal hydrogens is generally greater than that at the aromatic hydrogens with the exception of 2hydroxy derivatives that are ionized. Preliminary temperature studies on the unhindered and hindered methoxy derivatives indicate the methoxy hyperfine is sensitive to motion of these groups. We are presently engaged in a program attempting to correlate molecular orbital spin density calculations with esr hyperfine splitting constants to determine if a working set of RIO parameters can be obtained for this system.

Acknowledgments. We wish to acknowledge the generosity of Dr. Harjit Singh in furnishing us with pure samples of many of the compounds used in this study. We are especially grateful to NIr. Alvin Katekaru for his valuable and generous assistance with the compilation of data.

Discussion J. R. BOLTON(University of Minnesota, Minneapolis). Are you aware that in the naphthazarin cation

r

37

1,4-r\TAPHTHOQUINONESAND NAPHTHAZARINS

l+

(Cf. J. R. Bolton, A. Carrington, and R. F. Todd, Mol. Phys., in press. ) L. H. PIETTE. This finding is reasonable on at least two counts. I n the first place, the spin-density distribution on the pairs of oxygen atoms should be different, and secondly, each of one pair of protons interacts with two oxygen atoms whereas each of the other pair interacts with just one oxygen atom.

T. E. GOUGH(University of Waterloo, Ontario). I should like to comment on your reported value of 7 for &OH. I t seems, as you suggest, that QOH and Q O M ~ depend upon steric environment, and we have found &OH values as high as 18 for monoprotonated benzosemiquinone (T. E. Gough, Trans. Faraday SOC.,in press). I n contrast, &OH for monoprotonated durosemiquinone varied from 0 to 7, depending upon solvent (T. A. Claxton, T. E. Gough, and M. C. R. Symons, Trans. Faraday SOC.,62, 279 (1966)); one cannot properly describe hydroxy and, presumably, methoxy splittings as being directly proportional to the spin density on oxygen. L. H. PIETTE. We by no means believe that there should in all cases be a direct proportionality between the spin density on oxygen and the pertinent splitting constant. It is our intention at present to draw attention to those cases where such a proportionality seems to be present. Within a given solvent system it is altogether reasonable, in the absence of complicating steric factors, to expect the Q values to be meaningful.

J. J. WINDLE(Western Regional Research Laboratory, Albany, Calif.). The deuterium-hydrogen exchange rates on the benzene nucleus you mention are also known in other dihydroxy compounds such as catechol, for which it is very slow, but in mhydroxy compounds such as orcinol, on the other hand, is very fast (E. S. Hand and R. M. Horowitz, J. .4m. Chem. SOC.,86, 2084 (1964)). We have recently made use of these facts to deuterate selectively the components in a study of the mixed condensation products of catechols with resorcinols. This has been very useful in assigning the hf coupling constants in tthe esr spectra of the products (A. C. Wain, Jr., J. A. Kuhnle, J. J. Windle, and A. K. Wiersema, to be published).

P. H. RIEGER(Brown University, Providence). Are your Q values for methoxy groups consistent with the data of Smith and co-workers on ester radicals? L. H. PIETTE. Using our value of Q O C H = ~ 5.6 gauss and Smith's constants, we obtain an average spin density on the acyl oxygens of about 0.22. This is indeed consistent with their findings. A. OTFENBERG (Lockheed, Palo Alto Research Laboratory). Does the original (unsubstituted) material dimerize?

the OH hyperfine splittings differ by more than a factor of 2 depending on whether or not the proton is hydrogen bonded?

L. H. PIETTE. There is no evidence for such a process taking place. The ultraviolet spectrum of naphthazarin is unchanged over a wide range of concentration as is the esr spectrum of the semiquinone.

V o l u m e 7 1 , Number 1

J a n u a r y 1967

High-resolution electron spin resonance studies of hyperfine

Recommend Documents