28
Using the same coupling constant of 25 gauss, as was used for the p-xylene anion, one would predict a CH3 The experimental splitting of -0.375 gauss at -70". value is +0.82 gauss. This discrepancy can be solved only if one increases the positive contributions to the spin density. With the experimental data hitherto available, it is impossible to disentangle the thermal from the vibronic contributions. Using the spin densities given in Figure 7 one may calculate that at a temperature of -70" the sum of the thermal and vibronic contributions to the spin densities must amount to 26%. For p-xylene, we found for the vibronic contribution alone 12%. Hobey's'* calculations on mono- and dialkyl-substituted benzene anions demonstrate that vibronic coupling is more important for the monosubstituted than for the disubstituted anions. In view of this, it seems justified to conclude that the vibronic contributions to the spin density in the toluene anion amount to more than 12%, with an upper limit of 26%. The negative spin density on the para proton in ethylbenzene- is in accordance with the spin polarization mechanism. The positive spin densities on the CYalkyl protons in the ethylbenzene and the isopropylbenzene anion are in agreement with the results of toluene-. As for toluene-, both vibronic and thermal effects will contribute to the splittings. An interesting feature in Table I1 is that the spin density on the a-methyl proton changes sign going from p-xylene to 1-t-butyl-4-methylbenzene. This is due to a decreasing AE along the series, so that positive contributions to the spin densities become increasingly important. Koteworthy is the large splitting factor of the a-CH3 group in 1-t-butyl-4-methylbenzene anion compared with the a-CH3 splitting in l-isopropyl4-methylbenzene anion. The signs and magnitudes of the 0-alkyl proton splitting factors will be discussed in a forthcoming paper.
E. DE BOERAND J. P. COLPA
that the major cause of the positive sign of the CHI splitting factor is the vibronic interaction between the two lowest states. The sign of the spin density at the CH, protons in p-xylene anion is negative; furthermore, the methyl splitting is not a function of temperature. The temperature invariance led us to the conclusion that the thermal contributions to the splittings can be disregarded. It was estimated by molecular orbital calculations that vibronic interactions mix the nearby excited state into the ground state to an amount of about 12%. The nmr experiments on p-xylene, 1-ethyl-4-methylbenzene, l-isopropyl-4-methylbenzene, and l-t-butyl4-methylbenzene in the presence of their anions show that the spin density at the protons of the methyl group in position 4 changes sign along this series of compounds. One may conclude from this that the energy interval between the two lowest states decreases from p-xylene to 1-t-butyl-4-methylbenzene anion. Consequently, the positive contributions to the spin densities become more important.
Acknowledgment. We wish to thank Dr. H. van Willigen for many useful comments. We are also indebted to Mr. A. P. Praat and Mr. C. W. Hilbers for carrying out the experiments and to Mr. G. ter Maten for his help in computer programming and in computer calculations.
Discussion A. CARRINGTON (University Chemical Laboratory, Cambridge). Your estimate of the vibronic mixing of the nearly degenerate symmetric and antisymmetric states depends upon the accuracy with which you can calculate configurational admixture of higher electronic states. I n view of the fact that the absolute magnitude of the s p h density a t the l,4substituent positions is very small, what limits would you place on your estimate of the vibronic wave function?
VI. Conclusions
J. R. BOLTON (University of Minnesota). How sensitive are your computed spin densities to the inductive parameter and the parameters in the Coulson-Crawford model?
The nmr spectra of ethylbenzene and isopropylbenzene in the presence of their anions reveal that the sign of the spin density at the CH2 and CHI protons of the ethyl group in the ethylbenzene anion and the spin density a t the CH proton of the isopropyl group in the jsopropylbenzene anion is positive. This is in agreement with the positive sign of the spin density at the methyl group in the toluene anion, deduced from line-width studies by Fraenkel." The esr spectra of the monosubstituted anions show that the splittings are temperature dependent. From a theoretical analysis of the esr data of the toluene anion, it was inferred
E. DE BOER. In a recent article, Lazdin and Karplus [ J . Chem. Phys., 44,1600 (1966)] demonstrate that the spin densities in the ethyl radical are relatively insensitive to the Coulomb parameters for the pseudo-* orbitals in the CH, group. The dependence on the core resonance integrals is more critical. Our calculations for the ions of pyracene, the negative ion of acenaphthene and the cyclohexadienyl radical [Mol. Phys., 7, 333 (1964)], show that with the Coulson-Crawford parameters the hyperfine splittings of the methylene protons are accounted for satisfactorily. Inclusion of an inductive parameter of about -0.lp gives reasonable splittings for the ring protons. Laxdin and Karplus [ J . Am. Chem. SOC.,87, 930 (lY65)l came to the same conclusion by fitting the parameter t o the proper energy level splitting in toluene negative ion. In view of this, the
The Journal of Physical Chemistry
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 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 undergraduate 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
