Electron Spin Resonance Edward G. lanzen, Department of Chemistry, The University of Georgia, Athens,
T
of electron spin resonance in this Journal appeared in the 1968 issue. This review covered the literature up to mid-1967 in the form of a listing of references under general headings set up by the author, Dolan Eargle. A similar compilation was made by the wme author for the 1970 issue but it arrived too late for publication. The editors of this journal subsequently asked me to review the 1968-to-present literature in electron spin resonance for the 1972 issue. Since a computer search of C‘hemical Absffacts for all papers having electron spin resonance, ESR, electron paramagnetic resonance or E P R in the title (or in the key words or phrases) was already stored in my files, I accepted the request, planning to continue the review in the format used by Eargle. Much later, however, I realized how large this search is, the output alone comprising over 1 cubic foot of paper. Nevertheless, I began categorizing the output and found the organization used by Eargle quite workable. However, for a complete listing it was soon apparent that a t least 500 references would have to be added to the 1000 references already listed by Eargle in his unpublished review (which was available to me) for the two-year period from mid-1967 to mid1969. With the addition of two more years’ worth of references the review would contain upwards of 3000 references! The enormity of the task contrasted with the questionable usefulness of such a piece of work overwhelmad me. Even if a review covering the 6-year period from mid-1967 to mid-1973 could be divided into two parts and published in two issues in 1972 and 1974, the fact remained that a review of 1971 would not be available until 1974 and the articles would still be unwieldy. Moreover, the editors of this Journal had already impressed me with the need to stay within reasonable page limits. The result of all this was that a decision was made not to be exhaustive but instead to follow the suggestion of the editor and “write on something of interest to analytical chemists,” although the risk of presuming to select topics “of interest to analytical chemists” was not assumed without some trepidation. [In connection with planning for future reviews of this type, we (EGJ and the editors) would be interested in hearing from readers about the desirability of a complete coverage of the ESR literature in the Eargle format (it should be noted that a new quarterly journal, Magnetic HE LAST REVIEW
Ga., 30601
Resonance R w b edited by Charles P. Poole, Jr., and published by Gordon and Breach has been created; the first volume should be available in 1972) and topics and/or papers in ESR of interest to analytical chemists not included in the present review.] Although ESR is unequalled as a tool for the detection of low concentrations of certain paramagnetic speciesor free radicals, most ESR work in the solid state has been done for theoretical reasons and studies in liquids have mainly concentrated on organic radicals for the purpose of elucidating structural detaib. Relatively little work has been done in the past on quantitative applications of ESR or rates and equilibria of systems of possible interest to analytical chemists. However, a few examples of these are now appearing and these have been chosen for detailed review here. ESR-electrochemical techniques have historically been developed by analytical chemists (16) and interest in this area continues. However, applications of ESR in this area have been adequately reviewed (1) and will not be included here, except to note that in a recent paper Goldberg and Bard (17‘) reported the design and application of a cell wherein simultaneouselectrochemical-ESR (SEESR) experiments could be performed with “precise control of electrochemical variables.” DIRECT DETERMINATIONS OF TRANSITION METAL IONS BY ESR
The use of ESR spectrometry to investigate the magnetic properties of transition metal ions and their complexes has of course been recognized since the initial discovery of the resonance phenomenon. However, most of these studies were designed to provide “the d a t a . . . . . t o test old theories and formulate new ones” (86,86).Of more interest here are the direct analytical applications of ESR to the detection and determination of transition metal ions in the liquid phase. Manganous ion [Mn(II)] was the first to be studied. This choice is understandable because manganese in this oxidation state gives a very strong readily detectable ESR signal which in liquid solution in many solvents gives a characteristic spectrum of six peaks with a spacing of approximately 100 gauss centered a t g = 2. (The 6 peaks are due to nuclear hyperfine splitting: &Mn, 100% abundant, I = 5/2). In solid phases a strong signal is still obtained but whether the hyperfine
splitting is seen depends on the nature of the system. Guilbault and Lubrano (18) first made a detailed study of the feasibility of using a commercial ESR spectrometer (the Varian E-3) for the direct determination of Mn(I1) in aqueous solutions from the recorder response displayed in the normal first derivative presentation typical of 100 kHz modulated systems. They found that, if instead of doubly integrating the response to get the area under the absorption curve, which is directly proportional to the number of spins in solution, the peak height is used instead (as measured off the chart but divided by the amplifier gain X modulation amplitude) the instrument gave a linear relationship between the concentratior, of manganous sulfate and peak height in the range lo4 to lO-*M MnSO,. At concentrations above 10-*M, to lO-IM, the response plot became nonlinear but could still be used. When this plot was used as a calibration curve, a precision of 0.4% and an accuracy of *2% was obtained. The repeatability is mainly dependent on the difficulty of reproducing identical instrumental conditions when one sample is replaced by another. I n a subsequent paper Guilbault and Meisel (19) developed similar techniques for the determination of Cu(I1). The spectrum of copper sulfate in water is a broad structureless signal with a linewidth of approximately 800-900 gauss. The response, measured as before by taking the peak height and normalizing it by dividing by the amplifier gain X modulation amplitude, was found to be linear as a function of the concentration of Cu(I1) in the range 10-5 to 10-1.M. With the use of such a calibration curve, Cu(I1) could be determined with a precision and accuracy comparable to that of Mn(I1) to concentrations as low as 5x 10-5~. The effect of various additives on the intensity of the ESR response was evaluated for both Mn(I1) and Cu(I1). The maximum tolerable concentrations of diverse substances which can be added to these ions without affecting the ESR signal are listed in Table I. I n general nearly all additives have some effect on the shape and intensity of the spectra although this effect usually appears only a t very high concentrations of additives. In the case of Cu(I1) some additives produce a splitting of the broad signal with a simultaneous shift in the center of the spectrum (shift in effective 9value). An example of this kind is
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
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Table 1.
Maximum Tolerable Concentrations (M). of Diverse Substances on ESR Spectra of Ions indicated in Aqueous Solution
CU(II)c
Cu-End
0.10
Mn(II)b
0.02
0.20
0.02
0.1 0.2
0.1 0.05
0.01 1.0
0.02 0.04
0.02
0.04
0.10
0.2
0.20
0.02
0.05 0.05
0.02 0.02 0.02
0.5 0.1
0.02
0.2
0.1 0.5
0.1
0.3
0.05 0.005
0.2
0.05
0.2 0.1
0.4
0.002
0.10
0.2
0.0005
0.0005
0.01
0.80
0.02 0.20
0.05
0.08 0.1
0.3 0.5
0.005 5 x 10-6 5 x 10-6 0.0001
O.Ooo1’
0.0002
0.0002k 0.0002i
0.0002 2 x 10-6 0.002
0.005 0.0001~ 0.0001
o.oO01 0.0005 0.0005 0 * 0002
0.029
0,020
0.2
0.002 0.02 0 * 002
0.04
0.0008 0.0005
0.05 0.005 0.005 0.005 0.2 0.2
Acetic acid K*HPO4 KCl NaCl CaCh MgCh
0.10
0.10 0.10
0.10
pg8)Z Ca(NOy)z NaC104
Na acetate NaF
KBr ~~
1.8
~
KI NarSaOs KCN Na citrate K oxalate K, Na tartrate EDTA KSCN NaPSOa ZnSO, NaBOt
0.1
0.02
0.50
0.0001
0.01h 0.0005
0.02 0.009
0.0005 0.0002 0.0002 2 x 10-6
Cr(III)I 0.05 0.1 0.1 0.002 0.02 0.001
0.1
0.0008
0.0008
VO(I1)’ 0.05 (or0.4) 0.06 0.1 0.02 0.02
0.1
0.1 0.2
0.5
0.0004i
0. OOOli 0 * 0003 0.05 0.02 0.02 0.02 0,0005
2 x 10-6 0.001b 0.0001 C&L(NHz)e CH&N 2-3 % 2-3 % 2% 65% CHsOH 3% 2% DMF 10% 10% 1% 1% 0.20 0.02 MnCL Acetone 2-3 % 2-3 % 2% 2% G1 cerine 2-3% 2-3 % 0.20 0.005 O.oO05i N&OH 0 * OOOlA Ethanol 2-3% 2-3 % Dioxane 2-3 7% 2-3 % 1% 2% Maximum concentration that can be added with no effect on the ESR spectra. * 1 X lO-aM Mn(C1O4)2(ref. 19). c 1 X 10-*M CuS04 (ref. 19). d 1 X 10-*M CuS04; 1.28M C2H4(NHr)2(ref. 19). s 1 x 10-2M Cr(N08)((ref. 30). f 1 X 10-z&f voso4(ref. 30). 0 Concentrations greater than this cause precipitation. h Quartet observed. i Reduction of Cu(1I) to Cu(I) occurs and no signal is observed. i Some new features appeared in spectrum in presence of excess of ligand. Unusually narrow spectrum with relatively intense peaks. 0
ethylenediamine. At high concentrations of ethylenediamine (1.28M)a fourpeak spectrum is obtained from a 0.02M Cu(I1) solution which is due to the copper nuclear hyperfine splitting in the ethylenediamine complex (Cu-En) , (“CU, 69%, Z = 3/2, “CU, 31%, I = 3/2). As more and more ethylenediamine is added to a Cu(I1) solution the intensity of the broad structureless signal decreases as the quartet spectrum of Cu-En increases. If the fourth peak of the quartet is used to monitor the concentration of Cu-En, a plot of the ESR response as a function of ethylenediamine concentration between 0.1 and 0.001M for a 0.02M Cu(I1) solution resembles a typical titration curve. It: was further shown that the Cu-En spectrum could be used to determine Cu(I1) in the concentration range 1 X 114R
0
10-6 to 1 X 10-lM with a reproducibility of fO.4% and an error of about 1.5% using the same method as described for the determinations of Mn(I1) and Cu(I1) directly. As before, a calibration plot was made, but in the presence of excess ethylenediamine (1.28M), and either the first, third, or fourth peaks of the copper quartet could be used. As pointed out by these authors (19) the fact that the Cu-En complex gives a stable quartet spectrum can be used to advantage in determining the concentration of this metal in the presence of other components in the solution because this spectrum is not sensitive to the presence of quite large concentrations of other additives (see Table I). Also, the problem of overlap with absorptions due to other ions can either be eliminated or minimized if the ethylene-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
diamine complex is used. Thus, although the aquated Cu(I1) ion gives a very broad signal centered a t approximately 3050 gauss (at 9.437 GHs), the ethylenediamine complex gives a quartet spectrum centered a t 3175 gauss with a spacing of about 170 gauss. By using either the first or the fourth peak of the Cu-En spectrum, the concentrations of Cu-En can be monitored at as widely differing points as -2880 or -3390 gauss (3175 - 170 - (170 X 3/4) = 2877.5 and 3175 170 (170 X 1/4) = 3387.5, respectively) with better accuracy than would be possible by evaluating the intensity of the signal on the side of the broad aquated Cu(I1) signal. (These numbers have no particular absolute accuracies but are used for illustration purposes-in practice these positions should be determined for a given spectrometer and laboratory situation). The use of the Cu-En complex for determining this metal should be particularly useful in the presence of other ions which give sharp signals with either small or no hyperfine splitting and/or with different effective g-values (ie., different centers of the spectrum). A publication a t the same time by Moyer and McCarthy (33) showed a similar detailed evaluation of the ESR method for some of the same metal ions. These workers preferred to improve their signals by varying the microwave power incident on the sample and found the ESR response was linear with metal ion concentration if the peak height was normalized by dividing by gain X modulation amplitude x log power (in mW). The relationship is the same as proposed by Guilbault if the microwave power is fixed for all trials. These authors established usable concentration ranges for the determination of Mn(II), Cu(II), Cr(III), and Gd(II1) in aqueous solution and in ethanol by ESR, see Table 11. Ferric ion could also be determined in ethanol but no signal could be obtained in aqueous or ethanol solutions of salts of lanthanum, cerium, praseodymium, neodymium, samarium, europium, terbium, dysprosium, holmium, erbium, ytterbium, cobalt, or nickel. The Cr(II1) signal was also studied in detail by Meisel and Guilbault (90) In aqueous solutions the signal is broad and structureless, the nuclear hyperfine splitting due to W r (Z = 3 / 2 ) , 9.5570in abundance, not being detected. The “relative intensity” for Cr(III), a comparative term defined by Guilbault and coworkers as the relative intensity of the signal observed as compared to that of equal concentrations of Mn(I1) in aqueous solution, is approximately 1/20. The authors note that calibration curves were linear with concentration up to 0.1M for the perchlorate and nitrate but nonlinearity showed up for the chloride and potassium sulfate at concentrations
+
+
I
above 0.02M. Other “additives” tried all lead to decreases in the Cr(II1) signal. Some additional effects are listed in Table I. Whereas inorganic ions caused changes which were independent of the Cr(II1) concentration, “organic” additives showed effects which were dependent on Cr(II1) concentration leading to precipitation and unusual disappearance and appearance sequences of the Cr(II1) signal. It is clear that the use of ESR for determination of Cr(II1) in the presence of some other components of the type listed in Table I should be preceded by independent studies of the effect of these components on the ESR signal in the system of interest. The summary in Table I should be used as a guide only. Meisel and Guilbault (SO) also examined theVO*+spectrum in detail. At or above pI-1 3 the spectrum consists of eight lines 5lV, loo%, I = 7/2). The spacing between the lines is not constant across the spectrum and the lines are not equally intense but in spite of these characteristics, a linear relationship between peak height and concentration was found in the concentration range of 5 X 10-2-1 x 10-jM if the fourth line from downfield was used. The effects of additives on the spectrum are listed in Table I but are quite complex and difficult to summarize. Again the addition of inorganic ions caused a decreaPe iii signals in general but the addition of escess concentrations of the following additives produced new spectra (Le., spectra with smaller hyperfine splittings) : potassium cyanide, EDTA, potassium osalate, sodium citrate, and sodium tartrate. These spectra are assigned to vanadyl(I1) complexes. The cyanide complex is not stable particularly in the presence of Mn(I1). The EDTA complex, however, gave good stable spectra and in the presence of excess EDTA, a linear relationship between response and vanadyl(I1) concentrations Lould be obtained. Ferric ion is impossible to determine i n aqueous solutions because an extremely broad structureless signal (approximately 1000 gauss) is obtained. Both Jfeisel and Guilbault (SO) and hloyer and McCarthy (33) used nonaqueous solvents to develop a method for Fe(II1) determiation in namely, acetone and ethanol, respectively. The determination is done in the presence of excess chloride (LiC1) and appears to depend on the sharper signal given by the FeC14- ion in these solvents. A calibration plot was linear over the concentration range of 0.1 to 8 X 10-6M, see Table 11. It is worth noting that a recent report showed that the Fe(II1) signal sharpens up considerably in aqueous solution in the presence of excess fluoride ion (28). The signal also shows splitting into seven lines due to the six fluoride nuclei in the
Table II.
Upper limit(M)
Ions
(b) (c)
0.1 0.005 0.1 0.1 0.1 0.02
(b )
0.01
Mn(W (a) (b)
(c) Cu(W (a)
Cr(II1) (a)
0.1
(c) VOW) (a)
0.2 0.05
(c)
0.02 0.5
Fe(II1) (d)
0.1
E:#) $1
ESR Determinationof Various Ions Lower Relative limit(M) intensity Precision, % Error, % 1 x lo-’ 1: 1 f0.4 2 5 x 10-8 1 x 101 x 10-6 1:12.5 f0.4 1.5 3 x 10-8 5 x 108X 1:21 10.4 2.2 1 x 10-5 2 x 10-4 1 x 10-6 1:6.4 10.6 1.8 8 x 10-6 1:4.5 10.4 2.5 7 x 10-6 1 x 10-
0.04 4 x 10-6 0.1 1 x 10-4 (C) (a) I n aqueous solution taken from ref. 18, 19, and SO. (b) I n aqueous solution taken from ref. 33. (c) In ethanol solution taken from ref. 33. (d) In acetone solution taken from ref. SO.
FeFB- ion. Perhaps analytical use can be made of this finding. Another transition metal for which direct ESR determination would a t first appear not to be possible is titanium since both Ti4+and Ti3+in aqueoue solution do not give ESR signals (although the latter does have one unpaired electron). However, Johnson, Murchison, and Bolton (24) have found that the addition of alcohol to the aqueous solutions of Tia+ produces an ESR signal wherein the line width narrows and the signal becomes more intense in the series, methyl, ethyl, ieopropyl and tertbutyl alcohol. I n the latter case, the following equilibrium is established :
+
Ti (H20)6a+ l-BuOH e Ti (t-BuO) (H20)4(0H)+
+ 2H+
The structural assignment is based on the resolution of hyperfine splitting from four water molecules [9 partially resolved peaks ( 2 4 1 and the pH dependence of the ESR signal (8). Guilbault and McCarthy could not find a suitable method for determining nickel(I1) by ESR methods. This ion and most of its complexes apparently have too rapid a relaxation time to give ESR signals. However, Brinkman and Freiser (IO)recently reported the use of a paramagnetic ESR active nickel complex in the determination of Ki(I1) in aqueous solution buffered a t pH 6.0. The method involves a 4-hour extraction of nickel out of a 10-ml sample with an equal volume of a chloroform solution of toluene-3,44ithiol also containing tetra-n-butylammonium bromide. The chloroform extract is measured by ESR after filtration. The ESR response was proportional to the nickel content of the aqueous sample in the range of 6.5 to 325.8 micrograms per 10ml sample. The structure of the nickel
complex which gives the ESR signal is (46):
--
CH3
a h , ,
p0w”
0 s’ ‘
DIRECT DETERMINATIONS OF MIXTURES OF TRANSITION METAL IONS BY ESR
The first attempt to develop a method for directly determining the concentration of one transition metal ion in the presence of another dealt with manganese(I1) and copper(I1) mixtures (19). The ESR spectrum of a mixture of 0.01M Cu(I1) and 0.001M Mn(I1) shows overlap a t the low field side of the manganese sextet. The authors show a typical spectrum (Figure 1, ref. 19) but it is difficult to see how seriously the Mn(I1) spectrum is overlapped. They report that Mn(I1) could be determined directly from the spectrum of the mixture containing Cu(I1) although the converse was not true: Cu(I1) could not be determined directly in the presence of Mn(I1) because of the overlap problem. This observation is predictable. The Cu(I1) signal stretches about 400-500 gauss on either side of center (2900-3500 gauss) whereas the first and last maxima of the 6-line Mn(I1) spectrum centered a t 3330 gauss fall a t approximately 3055 [3330 - 2(100) 3/4(100)] and 3555 [3330 2(100) 1/4(100) ] gauss. respectively. Obviously the first maximum of the Mn(I1) spectrum and the Cu(I1) signal overlap seriously but the last maximum of the Mn(I1) spectrum might be free of serious overlap with the Cu(I1) signal. I n fact the authors report that Mn(1I) can be determined in the presence of Cu(I1) using the fourth line from downfield of the hfn(I1) spectrum’ This result is
ANALYTICAL CHEMISTRY, VOL. 44,
+
+
NO. 5, APRIL 1972
115R
Table 111.
Limitations on the Determination of Various Transition Metal Ions in the Presence of Other Ions of the Same Kind Maximum allowable amount of other transition metal ion0 Ion to be
determined -Mn(II) VO’+ Cu(I1) Cr(II1) Co(I1) Ni(I1) Fe(II1) Cu(1I) as Cu-En 2x lox ... lox N.L. N.L. N.L. VO’+ directly lox ... 2x 5x N.L. N.L. N.L. VO’+ a~ VO+-EDTA 1OX 0.1x Cr(II1) dlrectly Listed as the amount times the concentration of the ion to be determined. N.L. = no limit.
surprising and seems justifiable only when the concentrations of Cu(I1) are low relative to Mn(I1) concentrations. The relatively low intensity of the Cu(11) signal as compared to the Mn(I1) signal at the same concentrations (Table 11) works in favor of this analysis. A word of caution should be introduced a t this point concerning a problem inherent in the ESR method. It is characteristic of the ESR signal (usually of Lorentzian shape) that a relatively large fraction of the total area under the absorption curve is found under the wings-i.e., the absorption curve or the first derivative display “tails off” very slowly. The result is that overlap in the wings of a spectrum with peaks of another spectrum is more serious than immediately expected. This problem is compounded by the difficulty in predicting the sum of first derivative spectra or portions thereof by inspection. Moreover, peaks which appear to be well separated (100-200 gauss) in dilute solutions or where the relative concentrations between the two components is near 1, begin to overlap seriously a t higher concentrations of either of the two components. The problem of detecting and determining the peak height of a small absorption near an extremely intense one is of course not new but appears to be more serious in ESR than in spectrophotometry. An attempt towards solving this problem might be the use of computer simulations of the sums of first derivative spectra for constructing calibration curves of mixtures of ions or the use of direct output (nonmodulated) ESR spectrometers which produce absorption curves where techniques for summing overlapping spectra might be adaptable from spectrophotometry. (For a recent critical evaluation of present-day quantitative ESR methods, see Randolph’s chapter in reference (56). Although the direct determination of Cu(I1) in the presence of Mn(1I) is not possible in aqueous solution, in the presence of an excess amount of ethylenediamine, this determination can be carried out (19). As described earlier the Cu(I1)-ethylenediamine complex, (CuEn), gives a stable quartet spectrum, but the Mn(I1) spectrum decreases in intensity rapidly upon the addition of 116 R
0
ethylenediamine above 2 X 10-6M (Table I). At concentrations above this, the Mn(I1) signal disappears completely and Cu(I1) can be determined from the spectrum of Cu-En as described earlier as long as the concentration of Mn(I1) is less than the concentration of Cu(I1). If the Mn(I1) concentration exceeds the Cu(I1) concentration, the Mn(I1) signal appears. This result could be tolerated a t levels of up to twice Cu(I1) concentrations because the first maximum of the Mn(I1) sextet falls about 175 gauss downfield from the first maximum of the Cu-En complex spectrum (3055-2880 gauss, as shown earlier). If the Mn(I1) concentration is not too high relative to the Cu(I1) concentration, it is possible that overlap might not be too serious. I n a subsequent study Guilbault and Meisel (80) developed an ESR method for determining Cu(I1) and VO*+ iri the presence of the following ions: Cu(II), VO*+, Cr(III), Co(II), Fe(III), and Ni(I1). The procedure is as follows. Cu(I1) can be determined in a sohtion of other ions if a high concentration of ethylenediamine is present, since no ESR signals are obtained from any of the other ions listed above under these conditions. Vanadyl ion does not interfere a t concentrations up to lox higher than Cu(I1) up to 0.05M. Higher concentrations can be tolerated if time is allowed for vanadyl(I1) to be oxidized to vanadyl(V) (10-15 minutes), a process which is accompanied by a color change from turbid brown to clear violet-blue. Cr(II1) and Fe(III), if present, are removed by filtering off the precipitates which form upon the addition of ethylenediamine (“probably hydroxides”). Ni(1I) and Co(I1) do not interfere whether ethylenediamine is present or not. Vanadyl(I1) can best be determined in the presence of excess EDTA. The spectrum is a stable relatively sharp &line pattern. The EDTA complexes of Mn(II), Cr(III), and Fe(II1) do not give ESR spectra although the Cu(I1) signal remains with some quartet structure in the presence of EDTA. However, the difference in centers of spectra (effective g-values) of the VOZf-EDTA and Cu-EDTA complexes is of the order of 230 gauss, the latter being a t lower
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
field than the former, thus permitting determination of VO*+ in the presence of aa much as twice the amount of Cu(11) present. The direct determination of vanadyl(I1) in the presence of Cu(I1) is also claimed to be possible at concentrations of up to 10 times the concentration of VO’+. Chromium (111) could not be determined directly in the same solution as Cu(I1) because of extensive overlap of the signals (except a t very low concentrationsof Cu(I1)). These findings are summarized in Table 111. A following paper by Guilbault and Moyer (81) reported on an analytical scheme which permitted the separation and determination of each of the following ions simultaneously present in the same solution: Mn(II), Cu(II), VO*+, Cr(III), Fe(II1) and Gd(II1). The method is as follows. Iron (111) is first separated from the solution by ethyl ether extraction as the chloride and determined by ESR from a calibration curve obtained in an etheracetone solvent mixture (see the original reference for the specific recipe). Next Cu(I1) and VOZf are extracted with acetylacetone and determined simultaneously in acetylacetone by ESR from calibratian curves using nonoverlapping peaks. Gd(II1) is then precipitated with oxalic acid and after freeing with nitric acid, determined by ESR from calibration curves obtained under similar conditions. Cr(II1) is determined by ESR in the original acidified solution wherein all the other interfering ions are oxidized up to “non-ESR active” oxidation states with KIOI. Gd(II1) signal is not changed by this oxidation process but adds “quantitatively” with the Cr(II1) signal. (However, no data are given to justify this statement). The Cr(II1) concentration is thus obtained by subtracting the Gd(II1) concentration as determined above. Mn(I1) is determined by ESR in a separate sample by passing the sample over a “Jones reductor” to eliminate interference from VO2+. Other ions do not interfere. Cu(1I) could also be determined independently but in a separate sample by reaction with diethyldithiocarbamate [after the extraction of Fe(II1) ] and removal with chloroform (see the original reference for the specific recipe). This method has also been used by Solochenkin, Klassin, and Pupkov (59) with slight modification. Cu(I1)diethyldithiocarbamate or dibutyldithiophosphate was extracted with CClr and determined from calibration plots of the same kind. The sensitivity was 4 rg Cu/ml and the range was up to 20 rg Cu/ml. The relative error was less than 10% and bismuth, manganese, nickel, and iron did not interfere. The detection limits of the above scheme devised by Guilbault and Moyer
(81) are summarized in Table IV.
Also
listed are limits obtained by recording the ESR response at below room temperatures (38). I n general this experimental technique was found to be u s e less for mixtures of ions although somewhat lower limits of detection could be achieved at low temperatures with the separate components. A more promising low temperature technique for the determination of transition metal ions which give very broad ESR signals depends on the use of cross relaxation enhancement of the intensities of the hyperfine components of hydrogen and/or deuterium atoms (46). For example, the intensity of the low field hydrogen atom peak (or deuterium atom peak) is greatly enhanced in the presence of Cu(I1) in the aqueous matrix a t - 196 "C. The ratio of the enhanced line to the normal line is proportional to the concentration of Cu(I1). A 1'inear relationship was shown in the range 8.5 X lo-' to 2 x 10-*M Cu(I1). The same effect was shown for Ti(II1) except in this case the high field hydrogen atom peak was substantially enhanced. The precision for Cu(I1) was about 5% and for Ti(II1) was 8 - 1 0 ~ o . It was suggested that qualitative identification of metal ions could be possible by this technique, but that without computer assistance this method was not applicable to analysis of a multicomponent system. USE OF ESR T O DETECT TITRATION END POINTS
It is entirely obvious that ESR could be used to follow typical redox titrations wherein either an ESR active transition metal ion increases in concentration or disappears during the titration. This application was first demonstrated by Agerton and Janzen (2) in the potassium iodate catalyzed permanganate titration of arsenite a t room temperature. A small volume recirculating apparatus driven by nitrogen bubbles was used to permit continuous monitoring of the solution being titrated. Potassium permanganate was added to a solution of the arsenite and the build-up of Mn(I1) was followed by the ESR response. The recorder was advanced manually for each portion added. After a dilution correction was made, the recorder response was plotted and the end point was obtained from the intersection of the lines before and after the end point with a precision of better than 1% and an accuracy of 0.470,. A similar method gave good end points in dichromate titrations where Cr(II1) build-up is followed by ESR (3). The end point of a photochemical titration involving Cu(I1) has been followed by ESR (IS). The system under investigation is a rather complex one but has historical significance (27). Anthraquinone is photolyzed in iso-
fable IV.
limits of Detection and Ranges of Analytical Utility for Quantitative ESR Analysis6
Ion to, be deterrmned Mn(I1)
kethod and temperature, "C "Jonea reductor" (R.T.) Ethanol at - 188.' Fe(II1) Ether/acetone/LiCl (R.T.) Ether/LiCl/ - 188' Cu(I1) NH$CN/pyridine/R.T. Salic laldoxime (R.T.) Dietlyldithiocarbamate (R.T.) As above at - 170' Acet lacetonate (R.T.) As agove at - 188' voz+ Acet lacetonate (R.T.) As agove at - 140' Gd(II1) Oxalic method -HNO, - (R.T.) . , As above at - 188" I n He0 at - 188' Cr(II1) oxide (R.T.) KI04 oxidation (R.T.) In yridine (R.T.) In kNO3 (1M)at - 188' Ti(II1)as In H,O (R.T.) chloride As above at -188' In ethanol at - 188" Co(I1) KCN in HIO (R.T.) a Taken from ref. 81 and 38.
propyl alcohol containing Cu(I1) in a basic ammonia. The Cu(I1) ammonia complex gives a quartet spectrum which was shown to produce a linear calibration curve in the range 0.2 to 1 X 10-2M Cu(I1) if the fourth line from downfield was used. Excited anthraquinone abstracts a hydrogen atom from isopropyl alcohol but in the basic medium, this radical goes to the semiquinone radical anion. Either oxygen or Cu(I1) is reduced by the semiquinone. From the time taken for the reduction of the Cu(11) to Cu(1) as followed by ESR, it was hoped that a quantitative method of determining this ion could be developed. This goal was achieved with RSD deviations of about 2.5Y0,apparently within the range of errors found for photochemical titrations. The system was not considered by the authors to be the best demonstration of the usefulness of ESR in photochemical titrations but further investigations are under way. A technique and apparatus for the anaerobic titration of oxidative enzymes, e.g., FMN and cytochrome C with solid sodium dithionite diluted with KC1 adapted to ESR monitoring of the concentration of the protein molecule, has been described (34). Although the method is designed for a specific use, the paper gives many valuable items of information accumulated from experience in Beinert's laboratory. The specific reduction titration described is designed to meet the following conditions for the experiment: reducing the oxygen content of the reactants to acceptable levels; preserving the reducing capacity of dithionite which is autoxidizable and unstable when moist; mixing the reactants a t the proper time; and protecting the reaction mixture autoxi-
Lower limit, ( M ) Upper limit, ( M ) 1 x 105 x lo-' much less satisfactory 4 x 108.0 x 104 4 x lo-' 6 x 10-6 1 x 107.0 x 10-7 1 x 10-6 1.5 X 101 x 10-5 8.0 X 1 x 10-4 2.4 X 10-6 2.4 X 10-6 2 x 10-4 2 x 10-4 2 x 10-4 1.0 x 10-6 1 x 10-8 1.0 x lo-' 7.0 x 10-3 1 x 10-3
1 x 10-1 2 . 0 x lo-' 1 x 10-1 1 x 10-2 1 x lo-' 2.0 x lo-* 1 x 10-2 3.0 x 10-8 5 x 10-3 3.0 X lo-* 1 x 10-1 1.0 x lo-' 4.0 x lo-* 1 x 10-1 1 x 10-1 1 x 10-1 2.0 x lo-' 1 x 10-1 1.0 x 10-1 1 . 0 x 10-1 5 x 10-1
dization during subsequent measure ments. USE OF ESR I N STUDYING DYNAMIC SYSTEMS-EQUILIBRIA VANADYLCARBOXYLIC ACID SYSTEMS
Reeder and Rieger (36) used ESR to detect and identify the various complexes present in aqueous solutions of a-hydroxycarboxylic acids and a-mercaptocarboxylic acids and vanadyl ion a t equilibrium as a function of pH. As many as six different species were indicated over the pH range 1-7. This study was based on the fact that the vanadium hyperfine splitting is sensitive to the nature of the bonding in various complexes as described earlier in this paper. Moreover from the analytical point of view, the differences in splitting are magnified in the vanadyl complexes because of the large value of the nuclear spin (7/2). Thus, if the outermost line is taken for analysis the shift is Aa/2 for a nucleus with Z = 1/2 and 7(Aa/2) for a nucleus with Z = 7/2. Using second derivative presentation of the data, Reeder and Rieger were able to detect differences in line positions as small as 6 gauss for a concentration ratio of 20/1 "or more." Since, as will be shown, the smallest difference in field in the outermost line for different complexes is of the order of 11 gauss, ESR provided a unique way to study this system. A preliminary investigation showed that treatment of the same system spectrophotometrically would be "difficult a t best" because of the relatively small shifts obtained in the absorption bands of the various vanadyl complexes. The following is a brief description of the observed ESR spectra of a solution
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
117R
Table V.
ESR Parameters for Vanadyl Hydroxycarboxylic Acid Complexes
Ligand (acid) Spectrum0 av g-value PH Lactic I 1.3 115.7 1.965 Lactic I1 3 108 Lactic 111 4.7 101.8 1.968 Lactic 7.0 IV 1,972 89.8 Lactic 7.0 IVa 1.972 86.9 Glycolic I 1.2 1.965 115.6 Glycolic 4.3 111 1.968 102.8 Glycolic 1.972 IV 91.2 7.0 Glycolic 1.972 IVa 7.0 88.7 Thiolactic I 1.965 2.8-3.6 116 Thiolactic 111 1.970 102 2.8-3.6 Thiolactic 1.978 IV 2.8-3.6 87.6 Thiolactic 2.8-3.6 IVa 1.978 85.0 Thioglycolic I 1.965 2.8-3.6 116 Thioglycolic 101 I11 2.8-3.6 1.970 Thioglycolic 2.8-3.6 IV 1.977 88.2 Thioglycolic IVa 1.977 2.8-3.6 85.8 a I.is a su erimposed spectrum of V 0 2 + and VO(HA)l+. I1 is VO(HA)2. 111 is a supermposec!spectrum of VO(A) and VO(A)(HA)l-. IV and I V a are trans and cis geometric isomers (respectively) of VO(A)12-. Equilibrium constants between these species are given in original paper.
of vanadyl perchlorate containing lactic acid as a function of pH. For treatment of the data and justifications for H
I
CHa-C-COOH bH lactic acid CHt-COOH
I
OH glycolic acid H
I CHa-C-COOH I
SH thiolactic acid CHz-COOH
I
SH thioglycolic acid the assignments, the reader is advised to read the original paper. In a solution with a ligand-to-metal mole ratio of 4, a single 8-line spectrum with a v = 115.7 G, g = 1.965 is observed (spectrum I) in the pH range 1-2. At pH 3 a superposition of spectrum I and new spectrum (11) with a uv = 108 G is seen. At pH 3.2, three spectra appear: I, 11, and I11 where uv = 101.8 G, g = 1.968 for spectrum 111. At pH 3.5 the intensities of the lines due to I, 11, and I11 are approximately equal. At pH 4, spectra I and I1 have almost disappeared, I11 is predominant, and two new sets of lines, labelled IV and IVa, uv = 89.8 and 86.9 G, respectively (9 = 1.972 for both species), have appeared. At pH 7, I11 has now disappeared and IV and IVa are the dominant spectra. In addition, 12 additional lines which prove to be part of a 15-line set are observed. The latter spectrum was assigned to a vanadyl dimer wherein two equivalent vana118R
0
dium nuclei provide the set of hyperfine lines observed. For reasons difficult to summarize here, the following assignments were made for I through IVa (HA = CH&H(OH)COO-) :
I is a superposition of the aquovanadyl ion and the 1:1 complex with lactic acid: V02+andVO(HA)l+ I1 isdue to VO(HA)z I11 is assigned to VO(A) and VO(A)(HA) -I IV and IVa are attributed to geometric isomers (“probably trans and cis, respectively”) of VO (A)z2Similar results were obtained for vanadyl ion and glycolic acid except that spectrum I1 was barely detectable and IV and IVa appeared a t slightly higher pH. With thiolactic and thioglycolic acid, no trace of spectrum I1 was observed. The hyperfine splittings and g-values are given in Table V. Equilibrium constants for these species were calculated to fit the ESR data. For the following compounds, solutions containing the vanadyl ion and the ligand gave an 8-line spectrum with a 116-gauss spacing. The intensity diminished during pH change from 1 to 4. Above pH 4, a precipitate formed and the ESR spectrum disappeared : glycine, o-, p-, and y-aminobutyric acid, N-acetylglycine, N-acetyl-L-cysteine, 0mercaptopropionic acid, ethylene glycol, and acetoin. In the case of methoxyacetic acid and p-hydroxypropionic acid, some evidence was found for complexation in the pH range 1-4 from an increase in line width of the resulting spectrum. OSCILLATING Mn(ll) CONCENTRATION
An oscillating redox equilibrium catalyzed by Mn(I1) has been studied by ESR and preliminarily reported by Fruhbeis and Roder (14). The follow-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
ing components a t typical concentrations are mixed outside the ESR cavity and pumped through the cell: 20 ml 10-aM Mn(I1) in 2N HzSO1, 50 ml 2N HzSO,, 40 ml 1M malic acid, and 40 ml 025M aqueous KBrOa. The concentration of Mn(II), as monitored by ESR, decreases initially upon the addition of KBrOs to a solution of the remaining components. However, shortly thereafter the Mn(1I) concentration increases again to a value which is somewhat less than the starting Mn(I1) concentration, then increases again, decreases, increases, etc. in regular repetition, the cycles having a period which depends on the acid concentration (as fast as 30 sec. and as slow as 10minutes). The scheme proposed to account for this result is the following: (1) The Mn(I1) spectrum decreases initially because of BrOl- oxidation :
+
+
5Mn2+ 6H+ + 5Mna+ l/pBr* not ESR active
+
+ 3Hz0
(2) Next the Mn(I1) concentration increases because the oxidation of the alcohol function of malic acid by Mn(II1) produces Mn(I1) : 2Mna+
+ HOzCCH(0H)CHzCOzH * 0
2Mn2+
I~ + HOZCCCHZCO~H + 2H+
(3) The ketoacid however reacts rapidly with bromine to produce bromide ion:
+
HOzCCOCHzCOzH Brz HOzCCOCHBrCOzH
-.
+ H + + Br-
(4) Since bromate can oxidize bromide to bromine, the above reaction produces an inhibitor for the oxidation of Mn(I1) in that it diverts bromate from that reaction : Br03-
+ 5Br- + 6H+ e 3Brz + 3Hz0
This is supported by the finding that the system begins oscillating again after it has ceased, by the addition of more bromate or the precipitation of bromide by silver ion. The fact that during oscillation the highest level of Mn(I1) concentration detected is below that of the starting concentration shcws that a finite concentration of a non-LbR-detected oxidation state of manganese, presumably Mn(I1) exists in the “oxidation pool.” The authors draw analogies to enzymatic systems. COBALT-OXYGEN COMPLEXES
Cobalt(I1) complexes are known to react with oxygen to form 1:2 adducts wherein the oxygen molecule bridges between the two cobalt atoms of the complex. Recently, relatively stable
"peroxy" radical intermediates have been detected by ESR which can be formed reversibly from the starting material or the final adduct :
+
e CO(L)"OO' CO(L).OO' + CO(L), e CO(L),
0 2
c o (L).OOCo (L)n Since ESR again is a unique analytical tool for the detection and monitoring of the various species involved in these equilibria, even including peroxy radicals, the subject appears worthy of review here. Bayston, Looney, and Winfield (7) appear to be the first to have suggested that a peroxy radical intermediate might account for the ESR spectra observed when certain Co(I1) systems react with oxygen. I n this study CoC12 in the presence of excess KCN and KOH was reacted with oxygen and the solution frozen for ESR study. Also Co(CN)sa- was reacted with oxygen in aqueous basic solution below its freezing point. From results obtained under various conditions, these authors made the following structural assignments to the reaction intermediate detected:
peared when the sample cell was evacuated. This experiment could be repeated through several cycles successfully. The signal a t 2.02 was attributed to the Co(I1) complex coordinated to oxygen : c o (11)-O*' C-C co (111)-02(-).
trogens. Thus the following equilibria were postulated :
(DzH2)Co
- 163 "C
E
PY
I
(DzH~)CO-CO(DZHZ)
I
PY no ESR at -163 "C Oxygen is absorbed to produce a highly colored but unstable peroxide which can be decomposed with loss of oxygen reversibly in anhydrous media by flushing with nitrogen or argon or by warming to 40 "C. However, no peroxy radical intermediate is detected by ESR a t this stage.
/py 2(DzHz)Co
+ Oz-+ PY
PY
I
I
(DzH2)Co-0 -0 -Co(DzHz) purple On standing, however, the purple color fades and a 1.5-line spectrum develops in a brown solution. This signal is ascribed to a radical cation of the peroxide :
PY+
PY
I
I
(D~HZ)CO-O--O-CO(DZHZ) +
-+
+
PY
(DzH2)Co very low concentration
Py
ESR a t
+
+
/
\
These observations have close analogy to similar studies with triphenylmethyl radical by Ayers, Janzen, and Johnston (4) wherein the triphenylmethylperoxy radical trapped in powdered triphenylacetic acid was monitored as a function of temperature and pressure by ESR :
From the peak intensities of the triphenylmethyl and triphenylmethylperoxy radical measured simultaneously as a function of temperature, AH = 9 kcal was calculated. Bayston, King, Looney, and Winfield (6) followed up their earlier work with a study of the Co(I1) containing vitamin cobalamin (vitamin B12?). In this case the peroxy radical intermediate could be formed reversibly also. I n a typical experiment 0 2 was admitted to an aqueous solution of B I a ~t room ~ tem[Co(CN)sla- 0 2 e perature for 1 minute and subsequently broad structureless frozen in liquid nitrogen. The known signal a t g = 2.12 signal of B12r itself was mostly replaced only detected by a new signal with g = 2.02. I n below 0 "C methanol a t suitably low temperatures, [(CN)sCoOO']'the new signal gave an 8-line spectrum %line spectrum, essentially the same as was observed for aco = 9.8 gauss, [CO(CN)~]*-as described previously. g = 2.007 a t -45 "C Since the 1.5-line spectrum characteristic of the dimer peroxide was not observed, [(CN)sCoOO'Ia[Co(CN)slathe coupling reaction was apparently [ (CN)sCo-O-O-Co(CN) 6 1'substantially slowed down in the case of lbline spectrum, B12r, thus facilitating the detection of g = 2.02 the intermediate peroxy radical. If the I t is interesting to note that C O ( C N ) ~ ~ - volume above the sample was replaced by argon, the 8-line spectrum disapalso behaves as a radical in other ways. peared and the Bizr signal appeared with Swanwick and Waters (40) have reabout 90% recovery. These observaported that the reaction with nitrosotions provided good evidence for the benzene gives a relatively stable nitroxreversibility of the reaction with oxy ide, an example of a process of some gen : generality we have called "spin trapping" (23): B12r 02 e B1200' 8-line spectrum 0 uco = 12 gauss // g = 2.02 a t 174 OK 6-N [Co(CN)sla-+ 0' ESR parameters were also given for I [#-N-CO(CN)~]~- cobinamides which behaved in the same way. Subsequently (after 1968) a large Schrauzer and Lee ($8) also found number of examples of peroxy radicals peroxy radicals in the reaction of of Co(I1) complexes have been detected bisdimethylglyoximatocobalt(I1) comand studied by ESR. Misono, Koda, plexes (see chart for structure) with and Uchida (91) reported the detection oxygen in the presence of certain bases, of a signal a t g = 2.02, with some hypere.g., pyridine. The 1: 1 pyridine adduct fine resolved, when bis-salicylaldehydwas shown to be dimeric and did not ethylenediimine cobalt(I1) (see chart for give an ESR signal whereas the 1:2 structure) or bis-salicylaldiminato copyridine adduct was monomeric and balt (11) in poly-4-vinylpyridine was gave an ESR signal showing hyperfine exposed to oxygen. This signal disapsplitting from one cobalt and two ni-
+
YY
+
PY
I
(DzH2)Co-0-0' On further aging or upon the addition of more pyridine, this spectrum gives way to the 8-line spectrum typical of the Co(I1) complex peroxy radical. Some reaction sequences are suggested to account for these results. Further ESR work by Hoffman, Diemente, and Basolo ($8) established the structure of the peroxy radicals of Co(11) complexes more firmly. From work with the Co(I1) complexes of the diimine of acetylacetone [Co(acacen), see chart for structures] these workers concluded that the structure of the Co(11) peroxy radicals are better described as complexes of Co(II1) and the superoxide anion (OZ;), a characterization eluded to by earlier workers (6,$1, 98). In an independent study by Busetto, Neri, Palladino, and Perrotti (11) on the Co(I1) salicylaldehyde diimine complexes (see chart), similar results were obtained except that a new signal with very narrow line width a t g = 2.00 was detected. Their results were interpreted in terms of two 1:1 cobalt-oxygen adducts differing only in the oxida-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
-.
.
_ _ _ I _ _ _ _
__
119 R
of the latter is the uncertain aspect of this work. From known chemistry of Co(acac)2, it is assumed that the complex exists as a dimer in donor solvents and that the peroxy radical adduct is coordinated with a dimer of Co(I1) or Co(II1) acetylacetonstea also complexed with water and/or tertbutyl hydroperoxide.
Structures of Some Co(ll) Complexes Studied with 0 2
B
R
HF--cH* Co(II\ a m p k x of bisealicyladehyde thylenediimine R-H Co(ealen) R=C&O Co(ealen.SCH@)
ROO'
Co(II) complex of bieacet ylacetonethylenediih CO (acacen)
+
Co(II)(acac)z-Co(II1)(acac)a-(OH, ROH) abbreviated Co/Co
co/co--o -0' e R
P
R
I
P
I
c o / c o -0 -0 -0 -oco/co The reversible disappearance of the signal is attributed to tetroxide formation by analogy with the known dimerization of tertbutyl peroxy radical (6). A large amount of excellent work which has produced a whole new generation of ESR spectra of organic radicals for structural study has been provided by the rapid production of hydroxy radicals (at least so assumed) in the mixing of hydrogen peroxide with reducing metal ions in aqueous acidic solution :
M"+
+ Ha02 * M(n+l)+
Bidimethylglyoximatcobalt(II) cobaloximea (II) CO(D2H2) tion state of cobalt. Similar observations were made by Yang and Oster (48) for Co(I1) complexes of diethylene triamine, triethylenetetramine, tetraethylenepentamine, polypropylenimine, and polyethylenimine (number-average molecular weight 40,OOO) in aqueous solutions. These authors also investigated the usefulness of Co(I1) complexes and their peroxy radicaIs as polymerhatibn initiators (47). TRANSITION METAL ION-PEROXY
RADICAL COMPLEXES
If Co(I1) complexes react with oxygen to form relatively stable 1 :1adducts detectable by ESR, it is reasonable to expect that peroxy radicals might do the same. Moreover, if the peroxy radical simply acts as a donor ligand by virtue of its unshared electron pairs, that adduct could retain the properties of a pemxy radical and might be expected to still have a g-value in the range known for p r o x y radicals: 2.014-2.015. Exhmples of complexes presumed to be of the above type have been found for h t b u t y l peroxy radical and cobalt acetylacetdnate complexes, although the structure of these adducts has not b&n too well established (41, 4g). 120R
Both tert-butylperoxy and cumylperoxy radical are also preaumed to complex with manganese, cobalt, and vanadium naphthenates and acetylacetonates (9). A fairly detailed study has been re ported by Tkac, Vesely, and Omelka (41, 4.8) who found that the reaction of tertbutyl hydroperoxide with cobaltous acetylacetone [Co(acac)~]gave a symmetrical ESR signal (AH = 20 gauss) a t g = 2.0147 which increased iii intensity with increase in hydroperoxide added but went through a maximum concentration as a function of time and essentially disappeared as the temperature was lowered below -100 "C. The decomposition was first order with an activation energy of 282 kcal/mole. The signal decreased drastically when polar compounds were added to a solution in toluene in the order: water < tertbutyl alcohol < ethanol = methanol 3 acetone < cyclohexylamine = pyridine. In a 1:1 ratio (with tertbutylhydroperoxide), the latter two additions caused a decrease of the signal to 90% of ita original value. This datum is interpreted in terms of ligand-ligand displacement, the additive displacing the peroxy radical from a bonding site of the cobalt complex. The exact structure
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
+ OH- + 'OH
The hydroxy radicals so produced react rapidly with organic substrates to provide short lived radicals by hydrogen abstraction or addition to double bonds which are detectable in rapid flow sys-
tem. RH+~H+R'+H~O The study of the species giving the ESR spectra observed in the absence of organic substrates has produced confusing interpretations which only recently have been critically unscrambled by Czapski, A. Samuni, and Meisel (12) As a result good ESR evidence is now available for hydroperoxy radical complexes with various aquated transition metal ions. Samuni and Czapski (37) followed the change in ESR spectrum as various transition metal ions were added to a rapidly flowing solution of hydroperoxy radicals produced from the reaction of ceric ion and hydrogen peroxide : Ce4+ + H201+ Ce*+ HOO'
+ H + + HOO'
+ M"+ -c (HOO')M"+
The linewidths and g-values of a variety of these complexes are listed in Table VI. Of particular interest is the finding that a rapid metal ion exchange can be affected by the addition of a different transition metal ion. Thus for example: U,:+(HOO')
+ ThW4+e u,:+ + T~,~+(Hoo*)
Table VI.
ESR Parameters of (HOO‘)Mw*+Complexes Linewidth (gauss)
gvalues
27 0.74-0.66s 0.7, 0 . 5 1.3 2.7 1.1 1.2 0.73
2.0140 2.0109 2.0140, 2.0120 2.0180 2.0158 2.0190 2.0209 2.0140
HOO.
(HOO.) Ti,’+ (HOO‘) Ce.,*+ (HOO.) Zr.,4+ (HOO.) Th.,4+ (HOO) U.,6+ (HOO‘) M o d + (HOO.) a Eight lines; linewidth from low to high field. Vaq6+
could be demonstrated from either side of the equilibrium. Other pairs studied were ThW4+-Zraq4+, Zrw4+-Ua2+, ThW4+-Tiaq4+,U,8+-Tiaq4+ and V,6+Tiaq‘+. TRANSITION METAL ION-OXY RADICAL COMPLEXES
chemistry at the University of Western Ontario as a gueat of the photochemistry unit (September 1 to December 31, 1971). Grateful acknowledgement is hereby made. REFERENCES
It is reasonable to question whether oxy radicals also complex with transition metal ions or their complexes. Czapski and coworkers conclude (12) that hydroxy radical complexes with transition metal ions are not detected by ESR in those systems where hydroperoxy complexes weredetectable. However, the stable phenoxy radical, tri-tert-butylphenoxy radical has been shown to complex with Co(acac)s. Thus a t room temperature, the strong triplet due to the phenoxy radical and the 8-line spectrum of the phenoxyCo(acac)zcomplex can be seen in toluene:
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S in Resonance” in “S,t,andard Methods o f Chemical Analysis F. J. Welcher,
Ed., D. Van Nostrand, Princeton, N.J., 1966, p 616-635; Canquis G., Bull. SOC.Cfzm. Fr., 1968, 1618; kastening, B., C h . Zng. Tech., 42, 190 (1970). (2) Agerton, M., Janzen, E. G., Ana. Lett., 2, 457 (1969). (3) Agerton, M., Janzen, E. G., S. E.
Regional ACS Meeting, New Orleaqs, La., November 1970. (4) A ers, C. L., Janzen, E. G., Johnston, F. j., J . Amer. Chem. SOC.,88, 2610 (1966); Janzen, E. G., Johnston, F. J., Ayers, C. L., ibid., 89, 1176 (1967). (5) Bartlett, P. D., Guther, P., zbid., 88,
3288 (1966). (6) Bayston, J. H., King, N..K., Looney F. D., Wmfield, M. E., zbzd., 91, 2775 11 969 I).. \ - _ - -
Mine spectrum g 2.006 a: = 2 gauss
(7) Ba ston, J. H., Looney F. D., Winleld, M. E., A u t . J . dhem., 16, 557 1963). (8) Bo ton, J. R., University of Western
\
dimer
Ontario, private communication,October 1971. -.
I
e%
0-Co
(acach
%
8-line spectrum g 1996 aC0 =13 gauss This equilibrium is temperature d e pendent; the 8-line spectrum disappears a t 60 “ C but reappears reversibly upon cooling. ACKNOWLEDGMENT
This article was written during the author’s visit a t the department of
(9) Brandon, R. W., Elliott, C. S., Tetrahedron Lett., 1967, 4375. (10) Brinkman, W. J., Freiser, H., Anal. Lett., 4, 513 (‘1971). (11) Busetto, C., Neri, C., Palladino, N., Perrotti, E., Znorg. Chim. Acta., 5 , 129 11971). ,--.(12) Czapski G., Samuni, A,, Meisel, D. J . Phw. d‘hem.. 75, 3271 (18711, . .. and
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.
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-
\
-
G. F. J.. Laneb. W.. Walker. G.. Steigman,’G. A., Titulaer, C., S c h c e ;
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L. H., Ludwig, P., Adams, R. N., ANAL.CHEM.,34, 916 (1962). (17) Goldberg, I. B., Bard, A. J., J . Phys. Chem., 75, 3281 (1971).
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6.
(23) Janzen, E. G., Accovnts Chem. Ree., 4, 31 (1971). (24) Johnson, R., Murchison P. W., Bolton, J. R., J . A m . Chem. Soc., 92, 6354. (19702.
(25) Koenig, E.$, Landolt-Bornstein New Series, Gr 11, Vol. 1, Springer-qerlag, Berlin, 1965 (Engish and German). (26) Kuska, H. A. Rogers, . M . T. in
“Radical Ions,” h. T. Kalser and L. Kevan, Ed., Interscience Publishers, John Wiley and Sons, New York, N.Y., 1968 Chap. 13, p 579. (27) duwana, T., ANAL.CHEM.,35, 1398
( 1963). (28) Levanon, H., Stein, G., Luz, Z., J . Amer. Chem. SOC., 90,5292 (1968). (29) Manatt, S. C., Elleman, D. D.,
Vanghan, R. W., Chan, S. I., Tsay, F-D., Huntress, W. T., Jr., Science,
167, 709 (1970). (30) Meisel, T., Guilbault, G. G., Anal. Chim. Acta., 50, 143 (1970). (31) Misono, A., Koda, S., Uchida, Y., Bull. Chem. Soc. Jap., 42 580 (1969). (32) Moyer, E., Guilbadt, G., Anal. Chim. Acta, 52, 281 (1970). (33) Moyer, E. S., McCarthy, W. J., Anal. Chim. Acta, 48, 79 (1969). (34) Orme-Johnson, W. H., Beinert, H., Anal. Bwchem., 32, 425 (1969). (35) Randolph, M. L., in “Biological Applic$ions of Electron S in Rem-
nance, H. M. Swartz, J. R. iolton and D. C. Borg, Ed., John Wiley and Sons, New York, N.Y., 1972, Chap 3. (36) Reeder, R. R., Rieger, P. H., Inorg.
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