J. W. ROGERS AND W. H. WATSON
68
Electron Spin Resonance Study of Some Trifluoromethylnitrobenzene Anion Radicals by J. W. Rogers and W. H. Watson Department of Chemistry, Texas Christian University, Fort Worth, Tezas 76189 (Received April 11, 1067)
The electron spin resonance (esr) spectra of the anion radicals of 3-trifluoromethyl-4-nitrophenol and 2-trifluoromethylnitrobenzene show pronounced line-width alternation. This is interpreted as arising from the hindered rotation of the trifluoromethyl group and the extreme variations in the instantaneous hyperfine constants of the equivalent fluorine atoms. Solvent studies in N,N-dimethylformamide, dimethyl sulfoxide, and acetonitrile indicate that radical-solvent interactions are important. A specific model is proposed to explain the observed line-width alternations. The two-electron reduction of 2-trifluoromethylnitrobenzene yields a paramagnetic species whose esr spectrum is consistent with the loss of coupling t o the nitrogen nucleus. Only the monoanion radical spectra are observed for the high-voltage reduction of 3-trifluoromethyl-4-nitrophenol and 3-trifluoromethylnitrobenzene. These radicals probably are formed by electron transfer from the multinegative anions to the neutral species. The ultraviolet-visible spectrum of 2-trifluoromethylnitrobenzene was recorded as a function of time and electrolysis voltage. Some speculations are made concerning the reduction processes in these systems.
Introduction I n recent years, many investigators have studied the esr spectra of radical anions formed by the reduction of various aromatic molecules.'-6 Through these studies, a considerable quantity of data has been accumulated on n-electron densities, bonding, and structure. More recently, the phenomenon of line-width alternation was recognized and has received considerable attention.6-11 A theory of line widths has been developed by Fraenkel which allows a quantitative description of linewidths to be given.12-14 I n general, these effects are minor and result in only slight line broadening; however, for 2-trifluoromethylnitrobenzene and its derivatives, the effects are so pronounced that the broadened lines frequently are difficult to detect. A study of these highly perturbed systems is of interest, and a study which complements the work described in this paper has recently been reported.15 The experimental technique developed by Geske and Maki, in which an electrode is used as a selective reducing agent to generate radicals in situ, has proven to be useful in the esr study of radical reduction products.16 The major attention has been focused on study of organic anion radicals prepared by the one-electron reduction of the parent molecule.'-5 I n general, the two-electron reduction step results in a diamagnetic product or products; however, some anion radicals generated electrochemically at applied potentials considerably above the first polarographic wave have been mentioned in the literature. 17--20 The Journal of Physical Chemistry
I n the 2-trifluoromethylnitrobenzene system, we have observed the esr spectrum of a paramagnetic species generated a t a potential corresponding to the second polarographic wave. Hyperfine interaction (1) A. H. Maki and D. H. Geske, J . Am. Chem. Soc., 83, 1852 (1960). (2) P. H. Rieger and G. K . Fraenkel, J . Chem. Phys., 37, 2795 (1962). (3) P. H. Rieger and G. K . Fraenkel, ibid., 39, 609 (1963). (4) S. H. Glarum and J. H. Marshall, ibid., 41, 2182 (1964). (5) P. L. Kolker and W. A. Waters, J . Chem. Soc., 1136 (1964). (6) J. H. Freed and G . K. Fraenkel, J . Chem. Phys., 37, 1156 (1962). (7) J. H. Freed, P. H. Rieger, and G. K . Fraenkel, ibid., 37, 1881 (1962). (8) J. H. Freed and G. K. Fraenkel, ibid., 41,699 (1964). (9) J. H. Freed and G. K. Fraenkel, ibid., 40, 1815 (1964). (10) A. Hudson, C. Lagercranta, and G . R . Luckhurst, Mol. Phys., 11, 321 (1966). (11) A. Carrington, ibid., 5, 425 (1962). (12) J. H. Freed and G. K. Fraenkel, J . Chem. Phys., 39,326 (1963). (13) J. H. Freed and G. K . Fraenkel, J . A m . Chem. SOC., 86, 3477 (1964). (14) G . K . Fraenkel, J . Phys. Chem., 71, 139 (1967). (15) E. G. Janaen and J, L. Gerlock, presented at the 152nd National Meeting of the American Chemical Society, New York, N. Y., Sept 1966; private communication. (16) D. H. Geske and A. H. Maki, J . Am. Chem. SOC.,82, 2671 (1960). (17) P. H. Rieger, I. Bernal, W. H. Reinmuth, and G. K. Fraenkel, ibid., 85, 683 (1963). (18) I. Bernal and G. K. Fraenkel, ibid., 86, 1671 (1964). (19) R. D. Allendoerfer and P. H. Rieger, ibid., 88, 3711 (1966). (20) J. Q. Chambers and R . N. Adams, J . Electroanal. Chem., 9, 400 (1965).
ESRSTUDYOF TRIFLUOROMETHYLNITROBENZENE ANIONRADICALS with the nitrogen nucleus is not observed, and the nitro group either has been eliminated or reduced.
Experimental Section The anion radicals were prepared by electrolytic reduction of a LO-3 M solution of the reductant in N,Ndimethylformamide (DMF) , dimethyl sulfoxide (DMSO), and acetonitrile (CHsCN), using tetra-npropylammonium perchlorate as the supporting electrolyte. All potentials were measured between the mercury cathode in the electrolysis vessel and an aqueous saturated calomel reference electrode. The diffusion of small quantities of water through the salt bridge must be considered, since traces of water can greatly alter the appearance of the spectrum. Spectra were always obtained with and without the calomel reference electrode. The general experimental techniques were similar to those described by Geske and Maki. l 6 Spectroquality solvents were used. DMF was dried over calcium hydride, and DMSO was vacuum distilled after standing over calcium hydride. Acetonitrile was used without further purification. Tetra-n-propylammonium perchlorate was prepared by the addition of an aqueous solution of the hydroxide to an equivalent amount of perchloric acid. The hydroxide was obtained as a 10% aqueous solution from Eastman Kodak Co. The salt was recrystallized from aqueous acetonitrile, 20yo, washed with a large quantity of water, and dried over phosphorus pentoxide. The parent compounds were obtained from K & K Laboratories, Inc. The 2-trifluoromethylnitrobenzene was recrystallized from a 20% solution of ethanol in petroleum ether, mp 32". The 3-trifluoromethylnitrobenzene, bp 202", and 3-trifluoromethyl-4-nitrophen01, mp 78", were used without further purification. The two trifluorornethylnitrobenzene isomers were checked by gas chromatography and were found to be chromatographically pure, i.e., much less than 1% total impurities. The trace contaminants were identified as isomers of the main constituent. Reference compounds 2-aminobenzatrifluoride and trifluorotoluene were obtained from Aldrich Chemical Co., Inc., and 2-trifluoromethylphenol was obtained from K & K Laboratories. Polarographic data at a single concentration and 25" were obtained using a Sargent Model XV recording polarograph. All polarographic measurements were made in the same electrolysis vessel used in the esr experiments, and an aqueous saturated calomel reference electrode was used throughout. No iR correction was made for the electrolysis vessel. A conventional X-band spectrometer, employing a Strand Labs cylindrical reflection cavity operating in the TEollmode, was used. A 9-in. Varian magnet with Field Dial control was used in conjunction with 6-kc field modulation. The sample cell was cooled by a
69
JEOLCO-JES-VT-2 temperature controller adapted to the Strand Labs cavity. Temperature was monitored with a thermocouple placed approximately 2 cm from the sample. The hyperfine lines from potassium peroxylaminedisulfonate in water were used to calibrate the magnetic-field sweep. The ultraviolet-visible spectra of electrochemically generated species were recorded on a Cary Model 15 spectrophotometer. A pool of mercury was placed in the bottom of a regular quartz ultraviolet cell and a platinum wire, sealed in a glass tube, was placed beneath the surface. A platinum electrode was inserted into the upper portion of the cell and the two electrodes connected to a voltage bridge. The upper electrode was masked from the optical path of the instrument. Identical solutions were placed in the reference and sample cells and the spectrum recorded to check for a linear base line. An appropriate voltage could be applied, and transient or new species with absorption bands in the ultraviolet or visible regions of the spectrum could be recorded. Quantitative measurements were not possible because of concentration gradients, diffusion, and reaction. The pure solvents were checked to ensure that they were not responsible for any new absorption bands.
Results and Discussion The reduction voltages in all solvents were determined from polarographic studies and are indicated in Table I. The first polarographic wave for 2-trifluoromethylnitrobenzene and 3-trifluoromethylnitrobenzene corresponds to a one-electron addition to form the anion radical. The first polarographic wave for 3-trifluoromethyl-4-nitrophenol involves the rebuction of the phenolic proton, while the second wave corresponds to the addition of an electron to form the phenoxy anion radical. Only 2-trifluoromethylnitrobenzene yielded an additional paramagnetic species at higher polarographic potentials. 3-Trijluoromethylnitrobenzene. The esr spectrum of the monoanion radical of 3-trifluoromethylnitrobenzene was recorded in the solvents DMSO and DMF. The coupling constants are listed in Table I and are in good agreement with those obtained in acetonitrile.16 The solvent effects are consistent with numerous other studies. The coupling-constant assignment is in agreement with molecular-orbital calculations and other investigat o r ~ .The ~ ~minimum line widths are about 130 mgauss in DMF and 240 mgauss in DMSO. Reduction at the second polarographic wave yields the monoanion radical as the only paramagnetic species. This is assumed to occur via electron transfer between a diamagnetic dianion and the neutral species, A2A + 2A-. Prolonged reduction at higher voltages, with the calomel reference electrode in the system, yields a spectrum which differs superficially with the low-voltage spectrum. This spectrum is
+
Volume 72,NurnbeT 1 January 1868
70
J. W. ROGERS AND W. H. WATSON ~~
~~
~
~~~
Table I : Coupling Constantsa for Trifluoromethylnitrobenzene Anion Radicalsb Compound
Q '
CFt
;&" I
;+ on
Solvent
Electrolysis voltage
Ax
Az
Aa
A4
As
A6
DMSO DMF DMF(4HsO)
-1.0 -1.0 -1.0
8.64 8.73 8.85
3.22 3.27 3.27
1.W 1.28 1.16
4.32 4.03 4.43
1.18 1.01 1.16
3.22 3.27 3.27
CH&N DMSO DMF
8.32 7.96 7.67
DMFe
-1.0 -1.0 -1.0 -2.0
...
8.51' 9.34 9.64 6.7"
0.87d 0.85 0.87 1.2'
3.90 4.31 4.36 5.2'
1.23d 1.26 1.26 2.3'
3.20 3.07 3.06 2.3'
CHaCN DMSO DMF
-2.0 -2.0 -2.0
12.62 12.10 12.00
8.63" 9.24 9.67
0. 3gd 0.38' 0.38
...
1.Ogd
2.68 2.71 2.71
... ...
1.09 1.09
a The following coupling constants were obtained by Prof. E. G. Janzen (private communication): 3-trifluoromethylnitrobenzene in CHaCN,'AN = 8.84, AF = 1.2, AOH = 3.01 and 3.21, AmH = 1.00, APH = 4.05; and 3-trifluoromethyl-4-nitrophenol in CH&N (numbered as indicated in the table), A x = 12.1, AF = 8.38, AaH and AbH = 1.06 and 0.32, and AeH = 2.68. ' In gauss and at room temperature unless otherwise specified. Quartet splitting assigned to trifluoromethyl groups. Assignments are consistent but ambiguous. Temperature, -20". Assignment ambiguous. Resolved with low modulation, narrow sweep widths, and long sweep times.
'
identical with that obtained upon reduction with a solvent composed of 0.4% water in DMF, Table I. Prolonged electrolysis at the higher voltages permits water to diffuse into the system from the reference electrode system. S-Tri$uoromethyl-4-nif,rophenol. The esr spectra of 3-trifluoromethyl-4-nitrophenol in DMSO, DMF, and CH&N are shown in Figure 1. The paramagnetic species was short-lived and the esr signal disappeared with a few minutes after electrolysis had ceased. The spectrum in DMF contained six groups of four lines with no overlapping components. The four lines were composed of two doublets. Without additional information, these spectra could not be readily interpreted as arising from hyperfine interactions with the ring substituents; however, the spectrum of 3-trifluoromethyl-4-nitrophenol in CH3CN contains additional features. The six groups of four sharp lines are still present, but prominent broad lines are now observed between the six groups of lines. This might suggest the simultaneous existence of two or more paramagnetic species, or a line-broadening phenomenon, which affects a particular class of transitions. Esr spectra recorded as a function of modulation amplitude are consistent with the latter interpretation. The theory of line widths developed by Fraenke1l3 can be used to interpret the esr data. This theory assumes that the total spin Hamiltonian can be divided into two parts
hx
= hX0
+ hXl(t)
(1)
The time-independent zero-order Hamiltonian, XO, yields a sharp lined spectrum, but X,(t) is a fluctuating time-dependent perturbation which causes relaxation and line broadening. The perturbing Hamiltonian12 The Journal of Physical Chemistry
for the modulation of the isotropic interaction can be written as
where a&) is the instantaneous value of the isotropic = (at(t)). The correlation hyperfine splitting and functions g t j ( T ) , which are needed to calculate the line widths, are gtj(7) =
+~e'([at(t)-
at1
[aj(t
+
7)
-~$1)
(3)
where i and j refer to the various nuclei. The spectral densities are calculated from the Fourier transform of the correlation functions J t h ) =
('/3Jrn- m 9 d T ) e x p ( - i 4 dt
(4)
where W / ~ Tis the frequency of the transition induced by the perturbation. The three-jump model for nuclei with spin one-half has been developed by Fraenkel.13 There are a number of possible conformations that are consistent with such a mechanism. It has been suggested that p-T overlap is an important factor in these systems.16 This would involve overlap of a p orbital on the fluorine atom with a orbital of the nitro group. The magnitude or significance of such an interaction is difficult to evaluate, and we feel that the data in this paper are consistent with other interpretations. The line broadening depends upon large differences in the instantaneous hyperfine constants of the equivalent nuclei of the trifluoromethyl group and a jump frequency of the same order of magnitude as the frequency difference between the two instantaneous hyperfine values. A possible conformation might involve two
ESRSTUDYOF TRIFLUOROMETHYLNITROBENEENE ANIONRADICALS
-
.
.
71
all nonsecular contributions are negligible, >> 1, and the only spectral densities of importance are those for which o = 0. I n this model, r0 = '/akl, where kl is the rate constant for the jump, and oo/2ris the Larmor frequency. We have neglected anisotropic intramolecular dipolar interactions between the unpaired electron and the nuclear magnetic moments which may contribute to the line broadening. The spectral densities can be written as Jn(0) = Jn(0) = Jaa(0)
=
+ 2a?) - dZ1/3kl
-r2['/~(a12
(5)
Jiz(0) = Jia(0) = Jza(0) = - P / z ) J n ( O )
(6)
The secular contribution of the modulation of the hyperfine splitting to the line width of the kth line is given by
where m, is the z component of the spin angular momentum of nucleus i. Substitution of the spectral densities, eq 5 and eq 6,into eq 7 shows that, under the above assumptions, there is no line-broadening contribution to the M = lines while there is a contribution of Jll(0) to the lines corresponding to M = =t('/2).
Fiaure 1. The esr wectm of the anion radical of 3-tr~uoromethyl-4-nitrophenolin ( a ) DMSO, (b) DMF, and (e) CHCN.
fluorine atoms adjacent to the negative nitro group and the other surrounded by the positive ends of the solvent
dipoles. The solventrradical complexes would fluctuate rapidly and only the time-averaged value would be observed. A solvent dependence should be observed in the spectra, since the solvent polarity should affect the average value of the instantaneous hyperfine constant and possibly the jump frequency. A continuous rotation of the triflnoromethyl group would lead to the same qualitative conclusions as the three-jump mechanism. The threejump mechanism is more convenient for computations and has been used in this article. The three-jump model can be simplified if we assume
The assumption of linewidth alternation permits a reasonable set of coupling constants to be assigned to the spectra of 3-trifluorometbyl-4-nitrophenolobtained in the three solvents, Table I. If the spectra are r e lines corded at the same low modulation, the M = are absent in DMF, noticeable in DMSO, and prominent in CH3CN. The trend can be seen in Figure 1. The line-broadening effects are approximately in the same order as the polarity of the solvent, and this is interpreted as indication of a solvent interaction with the CF, group. The spectrum of 3-trifluoromethyl-4nitrophenol in CH3CN can be reconstructed, and excellent agreement is obtained, except for small frequency shifts due to secular terms which have not been included in the analysis. Under normal recording conditions, the sharpest lines are approximately 450 mgauss wide; however, low modulation and slow sweeps reduce the line widths to about 130 mgauss and a 0.38-gauss coupling constant can be extracted from the spectrum. The spectrum of the 3-trifluoromethyl-4-nitrophenol anion radical in DMF and CH3CN was recorded at several temperatures. I n DMF, the intensity of the sharp lines decreased as the temperature was lowered, and the spectrum was unobservable at approximately -45". No significant broadening of the sharp lines occurred. If electrolysis was discontinued at -45' and the solution immediately warmed, the spectrum reappeared. This is postulated to involve a temperaturedependent equilibrium of transient paramagnetic and diamagnetic species. The diamagnetic species Volume 76. Number 1 Jonuarv 1868
J. W. ROGERSAND W. H. WATSON
72
The esr spectrum of an anion radical of Ztrifluoromethylnitrobenzene generated at - 1.0 v: Figure 3.
(a) superimposed spectra at room temperature in DMF, and (b) spectrum of new species resolved at -20'. The esr spectra of the anion radical oi Ztrifluoromethylnitrobenzenegenerated at - 1.0 v in (a) DMF, (b) DMSO, and ( e ) CH&N.
Figure 2.
might be a short-lived dimer dianion. When 3-trifluoromethyl-4-nitrophenol in CH&N was cooled, the prominent broad lines further broadened until they disappeared. The sharp lines decreased in intensity without broadening until, at -45", the spectrum was not observed. A small sample of DPPH was placed in the cavity along with the usual electrolysis vessel. The anion radical of 3-trifluoromethyl-4-nitrophenol was generated and the esr spectrum recorded as the temperature lowered. The intensity of the DPPH spectrum increased, as expected. Polarograms were run at room temperature and at -8". There was a notice able decrease in diffusion current, but not of sufficient magnitude to account for the disappearance of the spectrum. It was concluded that the disappearance of the spectrum upon lowering the temperature was consistent with the postulate of a shift in equilibrium between the anion radical and some diamagnetic species. d-TrZRuoromelhylnitrobenz~e. The esr spectra of the 2-trifluoromethylnitrobenzeue anion radical generated a t - 1.0 v in the solvents DMSO, DAW, and acetonitrile are shown in Figure 2. If line-width alternation is assumed, the spectra can be fitted by a reasonable set of coupling constants, Table I. The line widths are approximately 280 mgauss. There are no indicsr tions of the broadened lines in any of these spectra. The spectrum of the anion radical of 2-trifluoromethylnitrobenzene in DMF and CHaCN was recorded at several temperatures. The intensity of the lines decreased as the temperature was lowered, and at -4' The Journal of Phydcal Cham+
the spectrum was not observed. The line widths did not increase significantly as the temperature was lowered, and the spectrum did not reappear upon warming the solution. When the reduction potential was raised to -2.0 v, a second spectrum appeared superimposed upon the first, Figure 3. The two spectra could be resolved by lowering the temperature to -4', where only the highvoltage spectrum was observed, although the end lines frequently were lost in the background. The spectrum is simple in appearance, but it is not interpretable in terms of the original ring substituents. The most reasonable set of hyperfine constants is obtained when coupling to the nitrogen nucleus is assumed to be negligible. This phenomenon has been observed in the high-voltage reduction of aromatic polynitro compound~."-~0 It has been shown that the reduction of polynitromesitylenes and -durenes'O can yield amines and a variety of intermediates. These reductions involve proton transfer from an activated methyl group to a nitro group. The reductions were observed to occur between the first two polarographic waves, and the addition of a proton donor increased the formation of amine product. 2-Tritluoromethylnitrohenzene does not contain an activated methyl group, and protons must be abstracted from either the solvent or the ring itself. The solvent protons would be most susceptible to abstraction, but there is no precedent for such a mechanism in nitrogroup reduction. The unknown paramagnetic species is not formed in the presence of a proton donor, and the spectrum is not that of 2-aminobenzotrifluoride. The amine is not the paramagnetic contributor; however,
ESRSTUDY OF
'TRIFLUOROMETHYLNITROBENZENE
73
ANIONRADICALS
the unknown species might be a paramagnetic intermediate in the reduction process. The reduction of 0- and p-dinitrobenzene in wet DMF yielded the corresponding nitrophenols.20 The nitro group was assumed to be displaced by the attack of a hydroxyl ion, which was generated during the reduction of a nitro group in the presence of water. This mechanism does not appear applicable unless small quantities of water remain in the solvent after drying. The unknown paramagnetic species can be generated in wet solvent, but the spectrum does not correspond to that of an authentic sample of 2-trifluoromethylphenol. No paramagnetic species was obtained upon reduction of trifluoro tolu ene. The behavior of the 2-trifluoromethylnitrobenzene system during reduction differs from that observed in systems where amine formation occurs. The paramagnetic species is formed only above the second polarographic wave. The spectrum of the anion radical is also observed above the second polarographic wave. This can be interpreted in terms of a dianion, which either transfers an electron to the neutral species or decomposes with possible loss of a nitro group. Dimer formation with subsequent decomposition to the anion radical and the unknown paramagnetic species is an additional possibility. Reactions involving the trifluoromethyl group also have been considered, but we have no evidence that such reactions occur. A study of the ultraviolet-visible spectrum as a function of reduction potential and time should yield information concerning the reduction process. The electrolysis of 3-trifluoromethyl-4-nitrophenol cannot be monitored in the visible portion of the spectrum due to the intense band obtained upon formation of the phenoxy ion. The compound 2-trifluoromethylnitrobenzene does not present this problem, and the various spectra are shown in Figure 4. When the electrolysis voltage is set a t a value below - 1.0 v with respect to saturated calomel, a small band appears at approximately 420 mp and does not increase in intensity after the first few minutes of electrolysis. This band remains indefinitely after electrolysis has terminated. It is assumed that this corresponds to the reduction of an impurity which is completed after a short period of electrolysis, or some phenomena associated with the increased surface area of the mercury electrode. This material was never observed in the electrolysis cell used in the esr studies. When the electrolysis voltage is raised above -1.0 v, a band immediately appears at approximately 474 mp. When electrolysis is continued at this voltage, an additional band a t 445 mp begins to grow and soon becomes more prominent than the one at 475 mp. Two intense bands appear in the ultraviolet, and the one at 325 mp remains after electrolysis has terminated. The 325-mp band i s assumed to represent product formation. If the electrolysis voltage is suddenly raised, the band
1 330
I
1
400
500
I
I
600
700
I I I 400 500 600 WAVELENG T H IN MI LL I M I C R 0 NS
700
I
Figure 4. The ultraviolet-visible spectrum of 2-trifluoromethylnitrobenzene as a function of reduction potential and time: (I)potential below first polarographic wave, (11) voltage increased to first polarographic wave, 1.0 v, (111) generation a t the first polarographic wave until steady-state concentration obtained, (IV)potential raised to -2.0 v, (V)20 min after electrolysis terminated, (VI)40 min after electrolysis terminated, and (VII)final stable species.
-
at 475 mp is increased in intensity relative t o the 445mp band; however, after a few minutes the band at 445 mp again becomes more prominent. If electrolysis is terminated, the band at 475 mp decreases more rapidly than that at 445 mp, and after a short period they both disappear. The band at 325 mp appears to decrease in intensity; however, the decrease is due to diffusion of this species throughout the cell and the disappearance of a transient species whose absorption band overlaps the one at 325 mp. The band at 475 mp is assumed to be the anion radical, and the band a t 445 mp the diamagnetic intermediate. If electrolysis is maintained at constant voltage for a period of time, a steady-state concentration of each is obtained. Both of these species then disappear and a permanent product is formed. A number of reaction schemes are possible. The formation of an unstable dianion dimer which then reacts or decomposes t o form a neutral product represents a likely path; however, no definite evidence has been presented for this speculation. At the highest voltage, a band also appears a t 575 mp, and this is assumed to be the second paramagnetic species generated above -2.0 v. This band also disappears after electrolysis has ceased. It is impossible to Volume 78, Number 1 January 1068
74
N. L. JARVIS AND M. A. SCHEIMAN
obtain rates from these data because of concentration gradients, diffusion, and other variables. The 2-trifluoromethylnitrobenaene and 3-trifluoromethyl-4-nitrophenol systems have proven t o be interesting. The pronounced line-width alternation can be adequately explained by the theory of Fraenkel, and there is evidence for a significant contribution to the perturbation from a fluctuating radical-solvent interaction. Some general features of the highvoltage electrochemical reduction have been discussed, but the diamagnetic intermediates and products have
not been identified. Additional studies utilizing preparative electrochemical techniques are being made now. The fate of the nitro group during high-voltage reduction is of interest, and we hope that a detailed reaction scheme for the reduction can be developed.
Acknowledgment. We wish to express our appreciation to the Robert A. Welch Foundation and the Texas Christian University Research Foundation for their financial support. We wish to acknowledge NASA for a NASA Traineeship (J. W. R.) and Mr. A. E. Nobles for obtaining the ultraviolet and visible spectra.
Surface Potentials of Aqueous Electrolyte Solutions by N. L. Jarvis and M. A. Scheiman Surface Chemistry Branch, Chemistry Division, (Received April 17, 1967)
U.S. Naval Research Laboratory, Washington, D . C.
80390
The effect of added electrolytes on the surface potential of water was determined using the radioactive-electrode technique. Changes in surface potential, AV, were found to vary from 64 mv for Na2SOr a t 1.8 m to -180 mv for NaSCN a t 7.5 m. The group I a chlorides in water gave surface-potential differences that decreased in the order K + = NH4+ > Na+ > Li+, while the surface potentials of the group IIa cations decreased in the order Ba2+ > Sr2+ > Mg2+. At a constant anion concentration of 2 m, the surfacepotential differences due to the sodium salts were in the order Sod2- > C032- > CH3COO> C1- > NO3- > Br- > I- > SCN-. In general, the anion with the smaller hydration energy gave the greatest decrease in surface potential. The magnitude of each surfacepotential change, however, does not appear to be a simple function of the hydration energy. The surface-potential changes must also involve the orientation and structure of the water molecules a t the water-air interface, which may be only partially dependent upon the ionic properties as determined in bulk solution.
Introduction
almost 3 A for 2 M solutions. Several theoretical explanations of the negative surface excess have been proposed,gJO but none will satisfactorily explain or pre-
Inorganic electrolytes are known to have a marked effect on the surface tension of water;'V2 less well known is their influence on the surface potential of The surface tensions of aqueous electrolyte solutions (1) G. Jones and W. A. Ray, J . Am. Chem. Soc., 59, 187 (1937); increase linearly with concentration, except perhaps 63, 288 (1941). at very low concentrations where Jones and Ray' re(2) W. Drost-Hansen, Ind. Eng. Chem., 57, 18 (1965). ported an apparent surface tension minimum. On the (3) A. Frumkin, 2. Physik. Chem., 109, 34 (1924). basis of the Gibbs adsorption equation, Langmuira (4) J. W. Williams and V. A. Vigfusson, J . Phys. Chem., 35, 345 (1931). concluded that the increase in surface tension indicates (5) J. E. B. Randles, Discussions Faraday Soc., 24, 194 (1957). a deficiency of solute in the surface layer, in the case (6) I. Langmuir, J . Am. Chem. SOC.,39, 1848 (1917). of a KCl solution the layer of pure solvent being about 4 A thick. Calculations by Harkins and c o - w ~ r k e r s ~ ~(7) ~ W. D. Harkins and H. M. McLaughlin, ibid., 47, 2083 (1925). (8) W. D. Harkins and E. C . Gilbert, ihid., 48, 604 (1926). substantiated this value and indicated that for chlorides (9) C. Wagner, Physik. Z . , 25, 474 (1924). of the group I a cations the thickness of this pure sol(10) L. Onsager and N. N. T. Samaras, J . Chem. Phya., 2, 528 vent layer will decrease from 5 A for 0.1 M solutions to (1934). The Journal of Physical Chemistry
