ELECTRON SPIS RESONANCE OF NITROGENOUS FREERADICALS
May, 1962
937
ELECTRON SPIN :RESONANCE OF SOME NITROGEN-CONTAINING AROMATIC FREE RADICALS1l2 BY L. DALLAS TUCKAND DAVID W. SCHIESER Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, Xan Francisco Medical Center, S u n Francisco, California Received January 20, 198.2
The results of electron spin resonance studies a t room temperature on free radicals formed from phenaz.ine, phenoxyine, and phenothiazine] together with two derivatives of the latter, thionine and methylene blue, are reported in sulfuric acid or in acidified ethanol. The g-values for the spectra were determined] and the hyperfine splitting measured. The hyperfine structure observed was interpreted as that due to splitting by the nitrogen-14 nuclei on the center ring and protons attached directly thereto.
We report heire the results of electron spin resonance studies on the positive ion free radicals formed in acid solution from several nitrogen-containing aromatic compounds, via, phenazine, phenoxazine, phenothiazine, thionine, and methylene blue. They are similar in structure to the freeradical ions formed from such hydrocarbons as anthracene and naphthalene, but they are considerably more stable, owing to the greater possibilities of resonance due to the presence of nitrogen in the center ring. Each of the free radicals studied is a semiquinone, intermediate in oxidation state between a reduced (leuco) form and an oxidized (dye) form. H
r
H
-I+
-
Phenothiazine
Semiquinone ion
Phenaaothionium ion
To emphasize the involvement of all three rings in the aromatic resonance, it appears to be realistio to draw the bonding of the radical as though it is the oxidized form to which a single electron is added. Considerations below show that in the conditions of our experiments all ring nitrogens of the free radicals are protonated. The parent forms other than phenothiazine, shown above, are
Michaelis,s and existence of stable examples of this series has been proved by e.s.r. s t u d i e ~ . ~ Hyper-~ fine studies of phenazine in essential agreement with ours have been reported by Matsunaga and McDowell.' Instrumental The spectrometer was an X-band reflective type using 1000-cycle field modulation and a lock-in detector with firstderivative presentation. The spectrometer is similar to that of Abraham, Ovenall, and Whiffen.8 Power incident on the sample usually was of the order of 20 mwatts, depending upon instrumental settings, and was generated by a Varian V-58 Klystron operating at 9300 Rlc. A Varian power supply and six-in. electromagnet spaced at 1.75-in, gap were employed for the steady field. For presentation of the spectra, the steady field was swept over an appropriate range of field strength by injection of a linearly varying voltage into the magnet power supply from a motor-driven helipot in a potentiometer circuit. The resonant cavity operated in the TEloa mode, and the solutions were inserted therein in Pyrex tubing of about 1.5 to 2.0 mm. internal diameter. All measurements were made a t room temperature, and no measurement or control of temperature was attempted. Magnetic Field Strength Measurements.-Values of magnetic field strength for line splittings and g-values were determined by calibration of the helipot dial against the proton magnetic resonance line in water. Frequency measurements of the proton resonance signal were determined by comparison with harmonics of a General Radio frequency calibrator, Type 1213-C. Diphenylpicrylhydrazyl was used as a standard fixed point for g-value measurements.
Experimental Preparation of Solutions.-Solutions for which the most satisfactory spectra were obtained were prepared by dissolving approximately 10-50 mg. of the solid parent compound in 10 ml. of concentrated sulfuric acid, to which then was added, depending upon the oxidation state of the parent com ound, 0.1 ml. of 0.5 M stannous chloride or 0.1 ml. of 30 0 hydrogen eroxide. Variations in technique are indicated below. some instances the sulfuric acid effected oxidation without addition of hydrogen peroxide. 1. Phenazine radical ion: To prevent precipitation of solid phenazyl radical salt, it waa found desirable to dilute the sulfuric acid solution by addition of 3 ml. of ethanol or water. An identical spectrum was obtained from a solution in 95% ethanol to which were added 0.2 ml. of concentrated hydrochloric acid and 0.1 ml. of 0.5 M stannous chloride.
9
Phenoxaaine
Thionine (as chloride)
Methylene blue (as chloride)
Phenazine
The stability and conditions for stability of these semiquinone free radicals were predicted by L. (1) Presented before the 138th National Meeting of the Amerioan Chemical Society, New York City, N. Y., September, 1960. (2) Based on a thesis presented by David W . Sohieser in partial fulfillment of the requirements for the Ph.D. degree, University of California, January, 1960.
(3) L. Michaelis, Chem. Revs., 16, 243 (1935). (4) A. N. Holden, W. A. Yager, and F. R. Merritt, J . Chem. Phus., 19, 1319 (1951). (5) Y. Fellion and J. Uebersfeld, Arch. sci. (Geneva), 10, Spec. no., 95 (1957). (6) P. Camagni and G. Lanai, Intern. Conf. Mesons and Recentlv Discovered Particles 49' Conor. Nazl. Fis., Padua-Venice, 1957, Comun., Vol. XII,pp. 4-5. (7) Y. Matsunaga and C. A. McDowell, Proc. Chem. SOC., 176 (1960). (8) R. J. Abraham, D. W. Ovenall, and D. H. Whiffen, Trans. Faraday Sac., 64, 1128 (195g).
938
L. DALLAS TUCKA
w
-
lo G* Y
Fig. 1.-E.s.r. spectra of positive ion free radicals formed from phenoxazine, phenothiazine, thionine, and methylene blue in sulfuric acid.
DAVID w. SCHIESER
TTol. 66
with uniform line spacing of 6.60 gauss, line widths of 5.74 gauss, and relative line intensities of 1 :4:8:10 :8 :4 :1. The latter intensities are consistent with a hyperfine splitting by the two equivalent nitrogen-14 nuclei and an identical or nearly identical splitting by the two protons attached to the nitrogen in the strongly acidic solution. Phenoxazine, phenothiazine, and thionine, as shown in Fig. 1, produce radical ions having uniformly spaced four-line spectra of appreciably asymmetry. Comparisons with calculated spectra having line intensities in the ratio 1:2 :2 : 1show reasonable agreement, and yield the coupling constants and line widths shown in Table I. Measured gvalues also are shown in the table.
Thiourea (saturated aqueous solution) was a satisfactory TABLE I substitute for stannous chloride as a reducing agent. 2. Phenoxazine radical ion: Hydrogen peroxide proCHBRACTERISTICS O F THE E.s.R. SPECTRA duced an e.8.r. spectrum immediately. However, on standNumin for a few days the sulfuric acid solution yielded an identiLine ber Coupling of constant, width, cay spectrum without addition of the oxidant. 9 series of gauss Parent compound U-value lines gaum dilutions in concentrated sulfuric acid produced spectra 5.74 Phenazine 2.0026 7 6.60 which were identical except for intensity. 3. Phenothiazine radical ion: No oxidizing agent other Phenoxazine 8.19 2.0049 4 9.83 than the sulfuric acid was needed. An identical spectrum Phenothiazine 4 7.10 5.69 2.0050 was obtained from an ethanolic solution acidified by hydro6.13 2.0045 4 7.05 chloric acid to which a drop of hydrogen peroxide was added. Thionine 4.92 2.0043 4 6.89 A series of dilutions in concentrated sulfuric acid produced Methylene blue identical spectra except for intensity. 8 Methylene blue in base 2.004 3 7.8 4. Thionine radical ion: Reduction by stannous chloride vas required to produce the free radical ion. A series of The interpretation of the hyperfine splitting from dilutions up to eightfold from the original with concentrated the number of lines and their intensities is similar sulfuric acid produced the same spectrum. 5. Methylene blue radical ion: Reduction mas re uired to that for phenazine, namely, a basic three-line for the production of the semiquinone radical. I n adjition, splitting by the single nitrogen-14 nucleus of the a strong apectrum of different structure is obtained from an ethanolic solution containing potassium hydroxide and center ring and a nearly identical splitting due to the attached acidic proton, The sulfur-32 and oxywithout oxidizing or reducing agent. Solid Radical Salts.-Several of the substances showed a gen-16 nuclei produce no splitting as they have no tendency to precipitate the solid radical salt if the solution magnetic moment. was made sufficiently concentrated. This usually resulted in The large difference between the N14 coupling a spectrum in which a single sharp line is superimposed on constant in the oxygen-containing molecule as comthe more complex spectrum of the solute species. In the case of phenazine, the solid, presumably the semi- pared with the sulfur-containing one should be quinone radical chloride, was separated from the superna- noted. We are tempted to ascribe this effect to the tant, washed, and dried. Its g-value was determined to be 2.0033 in agreement with Holden, Yager, and Merritt,* relative negativities of the two elements, but recogwho observed a "weak line" with g-value of 2.0031. Its line nize that other factors may contribute. This difwidth, measured as the separation of points of maximum ference in terms of the spin density on the nitrogen slope, was found to be 3.3 gauss. The line width increases would be a sensitive point for confirmation in a slowly with time without apparent change in the over-all in- molecular orbital model of these molecules. tensity, no doubt owing to physical adsorption of oxygen.@-'l Compared to phenothiazine, the very slightly The rate of increase in width is greater in a powdered sample smaller coupling constant in thionine suggests that than in the crystals obtained from the preparation. The hyperfine coupling constants and line widths were the amine group possibly enhances the resonance computed by comparison of the experimental curves with contribution of the side rings, as might be expected. theoretical gaussian curves drawn with appropriate line intensities ( 1 :2 : 2: 1 for phenothiazine-type radicals) and The effect does not seem to be so important, howhaving various ratios of splitting t o component line width. ever, as suggested by the work of Michaelis, A choice of the best model was made 011 the basis of relative Schubert, and Granick.I2 The substitution of heights of peaks. A plot of the positions of the maximum methyl groups on the amines to give methylene and minimum points on the experimental curve (in gauss) against the corresponding points on the theoretical curve (in blue results in a further enhancement of tthe same standard deviation units) ylelded the line width as t&e the type. slope of the resulting straight line. The coupling constant The asymmetry of these spectra is the same type then was calculated from the splitting-to-width ratio. and produced under the same conditions as that
Results and Discussion The phenazine radical ion produces a relatively symmetrical seven-line spectrum in agreement with the results of Matsunaga and McDowell.' The experimental spectrum shows almost quantitative agreement with a theoretical curve constructed
described by Blois, Brown, and Maling,13and may be ascribed to a phase shift by the solvent, sulfuric acid, which produces a spectrum having partial dispersive character. It is possible that in addition there is some asymmetry caused by the presence of an "impurity" free radical, as there is evidence that
(9) N. S. Garif'yanov and B M Koeyrev, Dokladv Aknd. h-auk S S S R , 118,738 (1958). (10) J J Lothe and G. Eia, Acta Chem Scand., 12, 1635 (1958). (11) J. E, Bennett and E. J , EI. Mara;nn, *Vatwe, &BPI 188 (1968)t
(12) L. Midhaelis, M. P. Schubert, and S. Granick, J . Am. Chem. &e., 68, 204 (1940). (13) &I. 9. Blois, Jr., H. W. Brown, and 3. E. Maling, "Free Radicals in B i d , Bpstemra," Prao, Symposium, Stanford, Calif,, 1060, p. 117.
939
NOTES
May, 1962 the oxidation of these compounds does not occur without complicahions.~4 Agreement of each of the spectra is found with a model in which the coupling constant of the nitrogen is equal to that of a single proton, presumed to be the N proton. To the precision of the present measurements, it, is not possible to ascribe individual values to the nitrogen and the proton. This result differs froini the results of Billon, Cauquis, Cambrisson, and Li,I5 who interpreted their fourline spectrum of phenothiazine free radical ion, formed by electrochemical oxidation in acetonitrile solution, as derivable from a nitrogen coupling constant of 7.5 gauss and a proton coupling constant of 4.5 gauss. Their larger value for the nitrogen splitting and smaller value for the proton may be (14) J. C. Craig, RI. E. Tate, F. W. Donovan, and W.P. Rogers, J . N e d . Pharm. Chem., 2, 669 (1960). (15) J. P. Billon, G. Cauquis, J. Cambnsson, and A M. Li, Bull. S O C . chin. France, 2062 (1960).
due to partial acidic dissociation of the radical ion in the absence of excess acid. Models based on their constants gave spectra which n-ere not in agreement with our experimental spectra. The XI4 splitting constant could be determined separately and to a better precision by obtaining the e.s.r. spectra in D2S04,in which circumstance deuterium rather than the proton would be attached to the nitrogen, No hyperfine splitting by the side-ring protons was found in these studies. Preliminary results on phenazine have been reported by Schieser and Zvirblisl6 showing that modification of the solvent yields resolvable splitting by these protons. The envelope of their spectrum corresponds to ours, showing that our line width is the result principally of unresolved hyperfine splitting. (16) D. W. Schieser and (1962).
P. Zvirblis, J .
Chem. Phys., 35, April
NOTES THE HYDROLYTIC DEGRADATION OF SODIUM TRIPHOSPHATE BY A. G. BUYERS~ Colgate-Palmolive Company Received August 49 1061
A brief investigation of the hydrolytic degradation of aqueous sodium triphosphat'e, concentration 14 w/o, in the temperature range 108-121' has been carried out. A second-order kinetic interpretation has disclosed that t:hese data are in reasonable agreement with previous studies carried out in aqueous solution, and using solid sodium triphosphate h e ~ a h y d r a t e . ~ ~ ~ Experimental Accurately weighed amounts of recrystallized sodium triphosphate hexahydrat'e, about 2.74 g.:, were dissolved in 15 ml. of distilled water for hydrolysis studies. Immediately after preparation, 5 ml. of a given solution contained in a platinum crucible w'ere sealed within a 22-ml., 98% nickel, flame ignition, Parr peroxide sulfur bomb. The bombs were completely immersed beneath the surface of an oil-bath maintained a t the desired temperature. After 30 min., the bombs were removed, chilled, and the contents were subjected to analysis for ortho-, pyro-, and triphosphates . 4 J
Discussion of Hydrolytic Data The currentdy accepted first-,order mechanism for the hydrolytic degradation of sodium triphosphate is written HP3OIoc4+ H20 HPz07-3+ HP04-2+ Hf (1) However, this equation is not always in accord with quantitative chemical analysis for react'ion prod-
:=
(1) Hughes Research Laboratories, Malibu Canyon Road, Malibu, California. (2) R. N. Bell, I n d . Eng. Chem., 39, 136 (1947). (3) (a) 0. T. Quimby, J . Phys. Chem., 58, 603 (1984); (b) A. C. Zettlemoyer, C. H. Schneider, H. V. Anderson, and R , J. Fuohc, i b i d . , 61, 991 (1957). (4) R. N. Bell, I n d . Eng. Chem., 19, 97 (1947). (3) A. G . Buyers, A n d . Chim, Acta, 19, Nr, 2, 119 (1968).
ucts, which frequently reveals pyro- to ortho-phosphate mole ratios exceeding one. Quimby has suggested that the solid state degradation of the sodium triphosphate hexahydrate may be represented by the second-order reaction 2Na.5P8010f HzO " ,NadPzO?
+ 2Na3HPz07
(2)
The equation is written for this same process, for sodium triphosphate dissolved in water 2HPsOlo-4 HzO " ,~ H P z O T - ~Hf (3) and either reaction would be followed by
+
HzO
+ HPz07-8
__
+
+ Hf
2HP04~'
(4)
Accordingly, summarized in Table I are approximate rate data in the temperature range 25-150' for the hydrolysis of sodium triphosphate in aqueous solution and for the hydrolytic degradation of solid sodium triphosphate hexahydrate. In all cases pyro/orthophosphate mole ratios are greater than one. It is easily seen that reactions 2 , 3 , and 4 will disclose pyro/ortho-phosphate mole ratios which exceed one when analyses are carried out upon a degraded triphosphate system. In some instances, tabulated values for the decimal fraction Na6P3010remaining at 30 min. mere the product of extrapolation. The time, 30 min., n-as chosen as an interval which permitted measurements as the initial step in the degradation process took place. Data so obtained were used t o yield eq. 5 from the plot of this Arrhenius relation, Fig. 1, and the second-order interpretation represented by eq. 2 or 3. The rate equation is log k = 13.9
- 6061 T ~
(5)
where k is the second-order rate constant, (based upon the disappearance of triphosphate) and the energy of activation is calculated to be 28 & 5 kcal. /mole. Let us shsabtm~that the activation step in the
