Westwood. W. D.. Thin Solid Films. 1 5 , 15 (1973). Westood, W. D.. Sadler. A. G . , Trans. J . Brit. Ceram. SOC.,70, 277 (1971). Weyerer, H . , 2. Kristallogr., 136, 282 (1972). Weyerer. H.. Meierding. W . , Acta Phys. Austr., 37, 38 (1973). Whyte, T. E . , J r . , Kirklin, P W.. Gould, R . W., Heinemann. H.. J. Catal., 25, 407 (1972). Wickoff, W G , “Crystal Structures,” Vol. 6. Part 2. 2nd ed.. Wiley-interscience, NewYork, N . Y . , 1971 Wilkes. C. E., Yusek. C S I J . Macromoi.
Sci.. Phvs., 7. 157 (1973). (392) Williams. D E , Acta Crystallogr , Sect A 28, 629 (1972) (393) /bid Sect 8 29. 96 119731 (394) Williams. J. M ,’ Peterson’, s w., Spectrosc. lnorg. Chem., 2, 1 (1971) (395) Willoughby, A . F. W., Driscoll, C. M . H , Bellamy, 8 A , J . Mater. Sci.. 6. 1389 (1971), (396) Wilson, A. J C.. Kristallogratiya, 16, 1127 (1971 ) . (397) Wolfstieg, U , Haerterer-Tech. Mitt., 27 (Spec. No.). 245 (1972). (398) Woolfson, M . , Rep. Progf. Phys.. 34, 369 (1971).
(399) Wu, C. C , Armstrong. R. W . . Lee, C H . , J. Appl. Phys., 43, 821 (1972). (400) Yoichi, I . , KotaiBufsuri. 7, 465 (1972). (401 1 Yvon. K , Allg. Prakt. Chem., 23, 139 (1 972). (402) Zav‘yalova, L L., lvoiiov, A. S , App. Metody Rentgenovskogo Anal., No 6, 56 (1 970). (403) Zege. V., Belarus. SSR. Ser Fiz-Tekh Navrik. (1 ) , 93 (1 972). (404) Zuechner, H . , 2. Phys. Chem. (Frankfurt a m Main), 82, 240 (1972) (405) Zugenmaier, P I Sarko, A , Biopolymers, 1 2 , 435 (1973)
Electron Spin Resonance Edward G . Janzen D e p a r t m e n t of C h e m i s t r y The U n i v e r s i t y of Georgia, A t h e n s . Ga 30602
In the last review of electron spin resonance in this Journal (84), suggestions for topics to be discussed in future reviews were solicited. The only response to this request came from two readers who asked why the topic “clinical applications of ESR” was not included in my review. Thus, in this review, certain clinical applications of ESR will be described, including the immunoassay method for detecting drugs of abuse. Since this method is based on properties of stable nitroxyl radicals, it appeared that a review of nitroxide complexes with inorganic agents would be in order. This area has only recently attracted some attention. Another topic I have chosen to review is the application of ESR to the detection of paramagnetic species in moon rocks. In addition, the computer output for the search of al! papers listed in Volumes 76 to 79 of Chemical Abstracts (1972-73) having electron spin resonance, ESR, electron paramagnetic resonance or EPR in the title (or in the key words or phrases) was scanned and topics of possible interest were selected for comment. A special attempt was made to continue mentioning papers on those topics selected in my first review (84) which covered Chemical Abstracts, Volume 72 to 75 (1970-71). Also in this review, most of the papers dealing with some aspect of the ESR of radical ions which appeared in Chemical A bstracts. Volume 76 to 79 (1972-73) are listed.
SPIN IMMUNOASSAY OF DRUGS OF ABUSE The spin imniunoassay technique for detecting certain drugs depend., on the so-called spin labeling method developed by McConnell and coworkers (123). It is well known that diaikyl nitroxides with tertiary carbon atoms attached to the nitrogen atom of the nitroxyl function are stable free radicals. The unpaired electron is approximately equally shared by the nitrogen and oxygen atom in a T*-orbital.
I In open-chain and 5-membered ring nitroxides, the nitroxyl function is essentially planar. In six-membered ring nitroxides and in strained bicyclic nitroxides, the nitroxyl function is apparently somewhat nonplanar. In nonviscous solvents, nitroxides typically give well-defined three-lined spectra due to the hyperfine splitting of the nitrogen nucleus. The magnitude of this splitting is around 14-15 gauss. Since the stable nitroxides have tertiary carbon atoms attached to the nitrogen atom, the nearest hydrogens are in a ?-position. Splitting is usually not resolved from these hydrogens although they contribute substan478R
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O .
tially to the line width of the spectra. Line widths are typically around 1 gauss. In the spin labeling method, a stable nitroxide is attached to a macromolecule at a site where the “freedom of motion” is to be investigated. If the region of the macromolecule where the nitroxide (the spin label) is attached imposes no restrictions on the freedom of motion of the nitroxide, the spectrum will be indistinguishable from that of the same nitroxide in solution in a nonviscous solvent, typically three equally spaced lines separated by about 14 gauss (Figure l a ) . On the other hand if the region of the macromolecule where the spin label is attached severely restricts the freedom of motion of the nitroxide, a very broad spectrum will be observed very much like that observed in a highly viscous solvent or in polycrystalline or powdered samples of the spin label (Figure I b ) . This is called a “strongly immobilized spectrum.” A continuous range of spectra of variously immobilized spin label can be imagined for situations between these two extremes. The broadening is due to the anisotropic gvaiue and nitrogen hyperfine splitting of the nitroxide. Discussions of the method with applications to systems of biological interest can be found in a number of papers (123), reviews ( 6 8 ) , and books 1110). Although the number of papers reporting the study of spin-labeled compounds is steadily increasing. the strongest impression left on the author from talks given by spin labelers is that it is relatively easy to collect spectra of the “how-ever-much immobilized” nitroxide in the system under investigation, but considerable difficulty is encountered in providing an unequivocal interpretation of the observed broadening of the nitroxide signal. An analytical application of spin labeling has, however, been developed where interpretation of the result is straightforward. The diagnosis simply depends on recognizing the difference in spectrum obtained in the limiting cases shown in Figure 1. namely the completely “immobilized” spectrum of Figure 1b and the completely “free” spectrum of Figure l a . Thus, in the spin immunoassay method the drug to be detected is modified by attaching a nitroxide spin label to it. This spin-labeled drug is premixed with a known amount of a n antibody of the drug. The spin-labeled drug is firmly bound by the antibody and thus produces a broad “strongly immobilized” spectrum, The concentrations of antibody to spin-labeled drug are arranged so that essentially all the spin-labeled drug is bound by the antibody-Le., no “free” spin-labeled drug spectrum is seen by the ESR spectrometer in the stock solution. The sample presumed to contain the drug is added to the solution of the antibody and spin-labeled drug. If the total amount of drug whether spin-labeled or not is not grossly overdiluted by this addition, a n increase in concentration of the “free” spin-labeled drug is observed since some of the bound spin-labeled drug molecules are exchanged by unlabeled drug molecules.
5 , A P R I L 1974
h
Edward G. Janzen is associate professor in the Department of Chemistry of the University of Georgia Born in Manitoba, Canada, he received his BSc and MSc degrees in chemistry at the University of Manitoba in 1957 and 1960 and his PhD degree from Iowa State University in 1963 He joined the chemistry staff at the University of Georgia in 1964 as assistant professor (1964-68) after spending seven months on a postdoctoral appointment at Iowa State University His research interests are in the area of electron spin resonance applications to studies of the structure and mechanisms of free radicals
A
n
’VVG a
b
Figure 1
hydrogenase wherein NADH functions as the “electron transport agent.” After mixing in a suitable solution (see reference), the signal due to I1 decays a t a rate which is directly proportional to the enzyme concentration.
Thus the appearance of a sharp-lined nitroxide spectrum provides positive identification of the drug in question. From a calibration curve of the peak height us. the amount of drug contained in known samples, the amount of drug contained in an unknown sample can be determined. This method has been developed and patented by the S W A Corporation (formerly Synvar). Its acronym is FRAT (Free Radical Assay Technique) (177). The first report describing this technique dealt with the detection of morphine (100). The spin-labeled morphine used is shown below. The assay was found to be rapid (1 minute) and fairly reproducible. Morphine derivatives such as codeine and ethyl morphine (Dionin) or a morphine metabolite, morphine glucuronide, could also be detected by the same antibody. However morphine substitutes such as methadone and propoxyphene (Darvon) and unrelated drugs such as barbiturates and amphetamines were not recognized by the antibody. Further details of a comparison between the spin immunoassay method and a thin layer chromatography (TLC) assay for morphine on
Me
I
Me
0.
II Another nitroxide method patented by Ullman and Schneider (141, 176) is based on the fact t h a t the asymmetry of the carbon atom of the functional group bonded to the methylene group in I11 and IV is changed by a reagent-analyte reaction and the resulting change in the ESR spectrum of the reagent is measured to determine the analyte.
0-
L Ad
HO
I
0.
R = H, morphine ( b ) R = CH,, codeine (c) R = CH,CH,, Dionin (ethyl morphine) (d) R = CH,CONH, spin-labeled morphine the same samples were published (101). It was found that (a)
the spin immunoassay method is more sensitive than the TLC method in the 1-5 fig/ml morphine range but a t higher concentrations the methods are in good agreement. The advantages of FRAT include (1) a smaller sample size requirement (20 p1 of urine as compared to 10 ml for TLC), which makes testing saliva samples feasible; (2) fast determination (30 sec-1 minute compared to hours or days with TLC); (3) specificity for drugs of the morphine family-ie., no wrong indications of a positive test. The FRAT method has been further developed to include assays for the detection of cocaine metabolites, barbiturates, amphetamines and methadone although only company literature is available on these assays (142). The data in Table I were constructed from this information. SYVA sells a kit for each drug assay which includes the drug antibody, the spin-labeled drug, drug calibrators, and ancillary supplies for 1000 assays. The morphine and methadone assay methods do occasionally give false positives with high concentrations of chlorpromazine or dextromethorphan (see Table I ) , and diphenoxylate by the former and promethazine by the latter. False positives with nonbenzoyl ecgonine substances were not observed with the cocaine metabolite assay method. The only drug which gave false positives with the barbiturate assay method was glutethimide a t high concentrations. The a m phetamine assay method will also detect phenylethylamines, phenylpropanolamine, and ephedrine. Leute and Schneider (99) a t Syva Corp. have also patented the use of stable nitroxyl radicals or their precursors (as the hydroxylamine) for free radical assay of redox enzymes. The example given involves an assay of lactic de-
I11b For example 70 fiM I11 in pH 10.9 buffer was used to determine 10-*M Zn2+. IIIb in pH 8.5 buffer was used to determine alkaline phosphatase. Other variations of these stable nitroxides were used to determine leucine aminopeptidase and cholinesterase. An interesting observation made by Mushak, Taylor, and Coleman (113) was used by these authors to assay alkaline phosphatase. It was found t h a t the resolution of yhydrogen hyperfine splitting in IVa and IVb was better than in I1 (the 4-hydroxy parent) in aqueous buffer (pH 8)
0
II
0
1I
0-C-CH,
0 -P(OH),
Na
Nb
I
I
a t equal concentrations. (Resolution of y-hydrogen hyperfine splitting of either I1 or IVb is better in aqueous medium than in heptane). Moreover when the concentration of nitroxide was increased, good resolution vanished first for 11, then for IVa a t concentrations of nitroxide which still ietained some resolution in IVb. This fact coupled with a small difference in g-value between I1 and IVb allowed these workers to measure the activity of alkaline phosphatase which converts IVb to 11.
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, N O . 5, A P R I L 1 9 7 4
479R
Table I. Relevant D a t a for D r u g Detection b y FRAT M e t h o d n Morphineb
Drug
Alpha-acet ylmethadol Amobarbital Amphetamine Apro barbital Atropine Barbital Benzoyl ecgonine Benzphetamine Butabarbital Chlorpromazine Cocaine Codeine C yclopentamine Dextromethorphan Dextropropoxyphene Diphenoxylate Ecgonine Ephedrine Glutethimide Heroin Homatropine Isoxsuprine Librium Meperidine Mephobarbital Methamphetamine Mephentermine Methadone Metharbital Methoxy phenamine Methylphenidiate Morphine Nalorphine Naloxone Nylidrin Pentobarbital Phenylpropanolamine Phenmetrazine Phenobarbital Phentermine Probarbital Promethazine Secobarbital Scopolamine Talbutal T himylal Thiopental
Cocaine‘
Barbiturated
Amphetaminee
Methadone’
0.5
>loo0 .o >loo0 .o
6.8 3 .O 46 . O
.o 1 .o
200 .o
>1000.0
13 . O
>loo0
35 . O 160 . O
255 . O 0.1 165 . O >loo0 .o 90 .o
23 . O >loo0 .o
>500.0 45 . O 155 . O >500.0
12 .o
70 . O 25 . O
1.5 >loo0
.o
365 .O
>loo0 .o 35 . O
400 . O 0.8
1000 .o
0.5 8.5 985 . O
4.4 5.6 >1000.0
0.5
200 .o
265 . O >loo0
>loo0 .o
>500 . O
>500.0
90 .o
.o
4.5
>loo0 .o
2.8
>loo0
.o
>loo0 .o
75 . O 50 . O 10 .o
200 .o >1000.0 >loo0 .o >1000.0
2 .o 7.6 40 . O 14 . O
a All numbers are typical concentrations in pg/ml of the drug listed when added to synthetic urine which will give a signal of intensity equal to that produced by 0.5 pg/ml morphine, ‘ 1.0 pg/ml benzoyl ecgonine, d 2.0 pg/ml secobarbital. # 3.0 pg/ml amphetamine. f 0.5 pg /ml methadone.
’
An evaluation of the effect of nitrogen oxides (NO and NOz) on tissues ( 5 4 ) was attempted by the use of ESR (“Effect on the human organism of toxic fumes produced by welding operations. 1. Study of the intoxication by nitrous vapors by EPR spectrometry”). An ESR response assigned to absorption by NO and NO2 was found in both fresh and lyophilized tissues. The signal was 5~ more intense in the latter. The search for nitroxides was unsuccessful in lung and tracheal tissue obtained from guinea pigs and rats, even after exposure to 100 ppm of nitrogen oxides for 1 hr. The workers concluded that “the EPR method is not sensitive enough for the detection of pulmonary edema induced by nitrogen oxides.” ESR has also been used to identify Mn2+ in vaginal contents 136. COMPLEXES WITH NITROXIDES Neutral free radicals in general have the capability of being reduced or oxidized by one electron:
R+
+e t
R.
-e
R:-
The nitroxyl function can readily be reduced to the hydroxylamine anion or oxidized to the nitroxonium ion (12, 53, 173): 480R
I1
-
\.. /
”
I
2\N/
I
+e
-
\&/
A-
O+ O. 0 The study of whether transition metal ions oxidize, reduce, or complex with nitroxides has just begun. In a brief paper, Krinitskaya and Dobryakov (94) first re orted that nitroxide (II) and CuSO4 react so that the E ~ signal R decreases for both species in aqueous solution. The signal of I1 can be completely destroyed by adequate amounts of Cu(I1). The observed concentrations of I1 and Cu(I1) at intermediate amounts were attained immediately and did not change “during the time of making the measurements.” An equilibrium constant of 8.62 liters/ mole a t 20 “C was calculated on the assumption that one nitroxide radical reacts with a Cu(I1) ion. The authors propose t h a t a Cu(I1) complex containing unpaired electrons is formed but supporting evidence for this conclusion is not shown. Also, in this paper, the preparation of the copper salt (V)&u is reported but not described. The salt does not give a n ESR signal in the solid state. In water or ethanol, a weak signal is reported which increases sharply upon addition of acetic acid. However, the nature of the spectrum is not reported. Apparently the following equilibrium is established in these solvents:
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 5, A P R I L 1974
COOH
Bu
v cu The question arises, does Cu(I1) react with nitroxides which do not have carboxy or hydroxy groups for coordination? In an extensive study of a complicated reaction involving the oxidation of methanol to formaldehyde, Brackman and Gaasbeek (21; note p 238) comment that cupric nitrate causes “considerable” decomposition of ditert-butyl nitroxide in methanol or acetronitrile with formation of teyt-nitrosobutane. However a nitroxide-Cu(I1) complex was apparently not considered in this paper. More recently Lim and Drago (98) studied the complex formed between I1 and bis(hexafluoroacety1-acetonato)copper(II). On the basis of NMR, magnetic susceptibility, and ESR experiments, it was concluded that the adduct is diamagnetic but dissociates slightly in CC14. The equilibrium constant for association was estimated to be about 1000. The proposed structure involves bonding through oxygen to copper. Beck and Schmidtner (16, 17, 22) studied the reaction of di-tert-butyl nitroxide with dry cobalt(I1) and palladium(I1) halides. The dry solid cobalt halide was added to the pure liquid nitroxide a t room temperature in mole ratios of 1:2 to 1:8. Upon gentle warming, a green solution results (with the iodide) which upon the addition of benzene produces green crystals after evacuation. The crystals consist of the 2: 1 complex: COX,
+
2t-Bu2NO.
-
CO [ ~ - B u ~ N O ] , X ,
The crystals dissolve in benzene to give only the spectrum of the nitroxide. Since the intensity of the signal increased with dilution, the authors concluded that the above reaction is reversible in benzene, although slight irreversible decomposition of the complex (e.g., by iodide reduction) could not be excluded: t-Bu,SO,
+
I-
+
H+
+
t-BuJ’OH
+
%I2
With moisture, the complexes rapidly decompose. In water or dilute HC1, the complexes decompose with production of the nitroxide. The authors’ main interest centered on the nature of the bonding in the solid state. They concluded t h a t the cobalt was tetrahedrally bonded as in other cobalt(I1) complexes (e.g. with R3P, R3P0, and amine-hi-oxide ligands). Magnetic susceptability and IR measurements were consistent with a net spin of one unpaired electron in the complex ( i e . , two of the three unpaired electrons in Co(1I) pair with the nitroxides). However, the spin relaxation times must be too fast to allow detection of the complex in benzene solution at room temperature. The addition of palladium(I1) chloride or bromide or sodium tetrachloropalladiate(I1) to pure di-tert-butyl nitroxide by the same method produced dimeric crystals of the 1:l complex ( e . g . , [ClPdON(t-Bu)z]z). The addition of water to benzene or chloroform solutions produced a palladium mirror. In aqueous acid or alkali, palladium metal is obtained quantitatively. Both in benzene solution and in the solid state, this complex is diamagnetic. A very weak nitroxide signal can be detected in solution but this is thought to be due to an impurity. The NMR spectrum shows no paramagnestism and only one methyl proton signal. The IR spectrum shows that the palladium atoms are halogen bridged. The following structure is proposed:
Bu
In a recently available paper, Jahr, Rebhan, Schwarzhans, and Wiedemann (83) describe a sizable number of complexes between various nitroxides and a number of transition metal ions. They group the complexes into those wherein the metal coordinates with the NO function of the nitroxide, or those wherein a substituent on the nitroxyl radical coordinates with the metal. A number of examples of each are described and some ESR spectra shown. In 1969, three papers by two groups appeared on the ESR spectra of nitroxides complexed with aluminum or gallium trichloride, tribromide, or triiodide ( 1 , 73, 130). The Lewis acid was added to a solution of nitroxide I, VI, or VI1 under vacuum in benzene or carbon tetrachloride under completely anhydrous conditions. The 3-line ESR 0
0 VI
0, VI1
spectrum of the nitroxide immediately changed to a spectrum of 18 lines in the aluminum trichloride complex. The aluminum tribromide and triiodide spectra were more complex. The additional hyperfine splitting is due to the aluminum nucleus (IA1 = /z). Similar splittings are observed in gallium complexes (IGa = 3h).Table I1 gives the hyperfine coupling constants reported. Complexes of indium tribromide and triiodide have also been reported ( 2 ) . These complexes were not as stable as the complexes with aluminum and gallium halides as evidenced by the fact that the spectrum of the parent nitroxide did not disappear even when a large excess of the halide was present. Indium trichloride is apparently too insoluble in the solvent (benzene) and no reaction with the nitroxide was observed. When polar donor solvents were used, no complexation between the nitroxides and the indium halides was observed. Boron trihalides also complex with nitroxides (Table 11) but these complexes were thermally unstable above -40 to -60 “C (42). All of these results can best be accounted for by assuming that the Lewis acid bonds to the oxygen atom of the nitroxide giving a pseudo amine radical cation:
‘/ 4. -0-i
-
+
-
‘N-0-A /
’
This picture is similar to t h a t of the protonated nitroxide (72, 106). H >LO’
In this connection, it is of interest that the nitroxide complex with tin tetrachloride which is reportedly stable enough for ESR detection a t low temperature, decomposes a t room temperature with loss of the ESR signal. From IR and NMR spectra, it was concluded t h a t a salt forms wherein the nitroxyl function has been oxidized (167):
I
0.
r
n
i
L
0
-I,
where X = C1, Br
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 5, A P R I L 1974
481 R
Table 11. Hyperfine Splitting Constants for Nitroxide Complexes with Lewis Acids With VII"'b,C
With D B N O a < * as
'' lief
a g or A I
20.61 21.24 21.58 20.15
B Fa BC1, BBrs AlCIy AlBr3 AlIy GaCL GaBra GabI InBrj InI,
3.04, 4.06,
ax
9.11
12.19 12 .89
11.40
as
aB or A I
20.34 21.18 21.41 19.86
2.67, 3.45,
7.96 10.35 10 .89 8 .80
a.&i,(;:L, 0 1 1ri
19 .o
19.5 18.8 18.1 18.1
17.4 19.4 19.2 ( 4 2 ) ; DRNO isdi-teri-butylnitroxide; lOB and
IlB
splitting.
Ref (73); 2'Alsplitting.
An unpublished paper ( J . Inorg. Chem. in press) by Hoffman ( 3 4 ) describes the complexes between nitroxides and group IV halides (SiC14, GeC14, SnC14) and TiC14 and Ti(OC6H5)4. The complex with silicon tetrachloride is stable only a t low temperatures. Spectra due to Lewis acid type of coordination are also found when nitroxides are adsorbed on alumina--i. e . , aluminum hyperfine splitting can be resolved ( 1 0 3 ) . These observations indicate that nitroxides can be used to probe the nature of surface sites. EQUILIBRIA BETWEEN COiMPLEXES When two paramagnetic species (A and B) are in equilibrium with each other, ESR can be used to obtain the equilibrium constant for the system if the spectra and/or g-values of A and B are different enough for suitable analysis. If the rates of the forward and reverse reactions are relatively slow or if the equilibrium can be rapidly quenched, the intensity of the separate spectra can be converted into concentration units and used to calculate the equilibrium constant for the system. Thermochemical parameters can be obtained from data obtained a t different temperatures. If, however, the forward and reverse rates are very fast so that the difference in field positions o f the separate spectra is of the order of the frequency of the ESR spectrometer. the separate spectra of A and B will not be seen. Instead a broadened spectrum is observed with field position intermediate between A and B. Typical rates of the order of 107-109 sec-l produce such spectra. The first system investigated from this point of view appears to be the addition of cyclic nitrogen bases to a vanadium acetylacetonate complex, VO( acac)s by Rieger and coworkers (19, 181):
R,S + VO(acac)l T--, R,N(V@)(acac)i The addition of pyridine, piperidine, or 2-picoline caused a change in line shape from which the lifetime of the adduct could be determined. With the use of some literature data, these workers arrived at a forward rate constant close to that expected for a diffusion-controlled reaction. For pyridine. h , = 1.0 x 109 M - l sec-l, k,. = 1.3 x lo7 see-l a t 18";for piperidine, k , 5 2.8 x lo9 M - I sec-l, k , 5 2 x lo6 sec-1 at 16": for 2-picoline, k l = 5 x lo7 M - l sec-1, k,. = 7 x 107sec~-lat 17" in benzene. Subsequently Anufrienko and Shklyaev (8, 145) studied the addition of pyridine to the copper diethyldithiocarbamate complex, Cu(DTC)z (later workers called this complex Cu(DDC)2): Py
+
Cu(DDC), f Py(Cu)(DDC)-
The spectrum of the complex consists of four fairly sharp lines in toluene and in pyridine although both the 65Cu hyperfine splitting constant and the g-values are different: in toluene acu = 84 G. g = 2.044 and in pyridine acu = 68.5 G, g = 2.059 at 25 "C. In mixtures of toluene and pyridine at room temperature or above, the lines are broadened and these parameters are intermediate in value. At -40 "C. the spectrum could be interpreted in, terms of a superposition of two spectra since at least one extra line 482R
With V I or VIICmdme
Ref ( I ) ; 69Ga, 71Gasplitting.
9.3 9 .o 7.2
30.7, 38.9 2 7 . 7 , 34.9 19.1, 24.3 44.8 29.4
Ref (130); e Ref. ( 2 , ; l'sIn splitting.
appeared to be present in the full trace. From a n analysis of the line width as a function of M(I) in both solvents, it was concluded that because the "hydrodynamic" radius calculated for the complex was larger in pyridine than in toluene ( r N 4.0 A in pyridine as compared to 3.6 A in toluene), the spectrum in pyridine was indeed due to a pyridine complex probably bonded in a n axial position. From solutions of modified Block equations, the observed values of the 65Cu hyperfine splitting constants in various compositions of pyridine and toluene could be used to calculate an equilibrium constant for the system. This was found to be 0.488 a t 25" for M Cu(DDC)z. An investigation of this constant as a function of temperature gave the enthalpy of formation of the complex: AW = 4.9 kcal /mole. From inspection of the line width, the lifetimes and, hence, the rates of decomposition of the pyridine complex could be calculated near the fast and slow exchange limits. At 27" for example, the lifetime of the pyridine complex is 2.9 x sec. An activation energy of 8.6 kcal/mole for the decomposition of this complex was calculated from the temperature dependence of the line width. Corden and Rieger, apparently unaware of the previous Russian work investigated essentially the same system in great detail (36). These workers studied the di-n-butyl copper complex in methylcyclohexane because this solvent has a wide liquid range and is less likely to coordinate with the copper complex. The agreement between the two studies is very good. For example, for pyridine and Cu(DBDC)z, the equilibrium constant was found to be approximately 0.4 a t 25 "C in methylcyclohexane. Data were also obtained with piperidine and n-hexylamine. Farmer, Herring, and Tapping ( 5 1 ) also published work on the pyridine-Cu(DDC)z system in benzene. They found at 27 "C: K = 0.38 l./mole; AW = -4.8 kcal/mole; A S = -16 eu; k,. = 5.8 x los sec-I; AHf* = -2.9 kcal/ mole; A S l * = -30 eu; AH,.* = +2.1 kcal/mole; AS,.* = -12 eu. In a continuation of their work in these systems, Anufrienko and Shklyaev (9) studied the equilibrium between pyridine and copper(I1) acetylacetonate in toluene: Cu(acac),
+
pyridine
Cu(acac), . pyridine
The rate constant for the forward reaction k , was again near the diffusion controlled limit: k , = 1 x 109 M - l sec- at -45" and 3 x lo9 M - l sec-l a t 35 "C. The equilibrium between copper(I1) acetate and acetylacetone in acetic acid and methand has been measured by ESR by Grasdalen (67). Further work on the metal-complexed hydroperoxyl radical has been published (111, 140). USE O F E S R T O DETECT PARAMAGNETIC COMPLEXES OF TRANSITION METALS PRODUCED BY ELECTROCHEMICAL MEANS Hexavalent molybdenum compounds (NazMo04 and ("4)~MOoh) were reduced in D M F in the presence of alizarin, benzoin, furoin. and 2,3-dihydroxynaphthaleneat
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, N O . 5, A P R I L 1974
= -1.2 volt (relative to mercury pool) (79). The ESR spectrum consisted of an intense central line from 96Mo(V) isotope ( I = 0) and of six additional lines due to 95Mo ( I = 512 16.7%) and 97Mo ( I = 512 9.4%). However it was not clear to the reader on what basis the assignment o f the uncomplexed us. the complexed Mo(V) spectra was made. Hyperfine splitting constants and g-values are reported for the above ligands and for the following taken from previous papers by these authors: salicylic acid, salicylsulfonic acid, thiosalicylic acid, pyrocatechol, and 3.4 -dihydroxybenzophenone. Similar complexes were reported by Lee and Spence (97). In this work (NH4)2MoOC15 was added to solutions of 8-hydroxy-, 8-mercapto-, and 8-aminoquinoline and 3,4-dimercaptotoluene in anhydrous DMF. A 1:l complex formed immediately which had a different ESR spectrum than the Mo(V) solution itself. The formation constants were found to be log K f = 4.13, 4.95, 3.61, and 4.16, respectively, for the ligands listed above. The molybdenum hyperfine splitting was 48.8. 40.3, 50.0, and 30.6 G, respectively, for the complexes with ligands listed above. The g-values were also reported. Zero-valent nickel-2,2’-dipyridyl complexes were electrochemically reduced on mercury in hexamethylphosphoramide at -0.6 to -0.8 volts cs. Ag wire (112). An ESR spectrum was observed which is assigned to the anion radical of the complex. At higher potential. the dipyridyl radical anion was observed. The spectra were not well resolved and hyperfine coupling constants were not report,ed. The R-value was 2.0036. Attempts to study bis(dipyidyl) iron were mentioned. An ESR search for free radicals was made during the electrooxidation of triphenylacetic acid at a platinum electrode in acetonitrile (93). An unstable radical was detected but the signal decreased in intensity during electrolysis and disappeared completely after the current was cut off. Some hyperfine splitting was reported but the abstract did not say whether a structural assignment was made. It is probable that the spectrum was due to the triphenylmethyl radical. The search by ESR for free radicals in the electrooxidation of carboxylic acids (the Kolbe electrolysis) is an old problem and has been attacked with limited success by numerous other workers (Private communication from D . T. Sawyer. Department of Chemistry, University of California, Riverside, Calif.). The difficulty would appear to rest with the fact that a t the potential needed to remove an electron from a carboxy group, the radical resulting from the rapid decarboxylation of the carboxy radical is itself easily oxidized to the carbonium ion : E1.2
0
0
/I
/I
--P
R-C-OH
--+
R-C-0.
+
H+
n
centration us. peak height plot. The plot was linear in the range 1.0-0.1 y/ml a t different settings. The signal was found to be stable for a t least 24 hr and constant as a function of pH 1-7. MgZ+, Fez+, Ba2+, Mnz+, ZnZ+, C d + , Pb2+, Ni2+, I - , Br-, and NO3- did not interfere, but Hg2+ and CN- ions caused interference. It was shown that Cu(I1) could be determined in aluminum as well by ESR as by spectrophotometric methods but the former method needed less sample. The above method for determining Cu(I1) was not possible in the presence of Hgz+ apparently because of the rapid replacement reaction: Hg(DDC)> + CU” C U ( D D C ) ~+ HgL* Yamomoto and Ikawa (192) used this reaction to determine the Hg2+. The decrease is Cu(DDC)z signal height was used in this case to develop a calibration curve which was found to be linear in the concentration range 1.0-0.1 y ml and 0.1-00.1 y / m l a t different spectrometer settings. he signal height changed slightly with higher p H in the range p H 2-6. Mg2+, Fez+, Ba2+, M n 2 + , Zn2+, Co2+, Pb2+, NiZ+, I - , Br-, NO3-, and C1- ions caused no interference. Cyanide ion did interfere. It was shown that the ESR determination compared well with standard spetrophotometric methods. Ag can be determined after its conversion into a paramagnetic state by oxidation with tetraethylthiuram disulfide (148, 193). Ag and Pd can also be determined by a n exchange reaction and extraction with Cu dithiocarbamate. Cu2+ paramagnetism is used as an indicator. Sensitivity of the EPR method is commensurable with photometric methods. Analysis of iron(II1) was successful using (Bu0)3PO to extract the ferric ion out of a 1.8 M HC1 solution (168). Concentrations were obtained from a working curve using D P P H to calibrate the signal. The relative intensity is proportional to concentration in the range 10-200 ppm iron(II1) per ml (Bu0)sPO used. The extracted species is assumed to be HFeC14.2((Bu0)3PO). Co(II), Cr(LII), Cu(II), K, Mn(II), Xi(II), and VOz- do not interfere. HzO is determined by its effect on the conversion of C U ( O A C )from ~ the dimeric to the monomeric form which gives an ESR signal (156). Excess Cu(0Ac)z is added to the organic solvent ( e . g . , acetone, dioxane, and ethylene glycol) and the water content found from a calibration curve. The calibration curves are linear for 1 x to 5 x 10- volume 70H z 0 . A series of free radicals have been patented as p H indicators by Becher and Ullman ( 1 5 ) . The method for pH determination is based on pH-dependent variations in the ESR spectra of imidazolinyloxy radicals of the type VI11 and IX (IIIa and IIIb are specific examples of type VIII) which have an asymmetric carbon atom bonded through a methylene group to the imidazolinyl ring. Spectra differ in +
4
--P
R. --t R+ Furthermore, it is doubtful that more long-lived carboxy radicals would be detected by ESR even if a high enough concentration were present in the cavity (see 165 for a discussion on the oxy radical dilemma). Two papers reported studies on the phenomenology of electrochemistry in ESR cells (65, 135).
IX VI11 range p H 2.24-10.02 with VI11 where R1 = H, RZ = CH3, R3 = cr-pyridyl.
USE OF ESR IN DETERMINATION OF TRANSITION METAL IONS, WATER AND pH The copper complexes Cu(DDC)2 can be used t o determine Cu(I1) directly by ESR (191). The strong signal allows a sensitiLity limit of 0.005 ppm Cu(I1) t o be attained. The known samples were prepared by adding 1 ml of 0.2% aqueous solution of DDCNa to 1 ml of Cu(I1) solution. After the pH was adjusted to p H 1-2, with HCl, 1 ml of benzene was added. The complex is completely extracted into the benzene layer after a few minutes and 0.1 ml of the benzene solution is used for the ESR determination. The 2nd line scanning upfield seems to show the best line shape and was used for the determination. These workers simply used the peak height to construct a working con-
MOON ROCKS Lunar material from Apollo 11 was examined by ESR by three groups. The Weeks group (184) reported a strong broad signal a t 9 and 35 GHz a t 4”, 80” and 300 OK. For sample 10065, the line width measured between the inflection points of the derivative curve was 900-980 gauss and the g-value was 2.01-2.11. Some temperature effect was noted on the intensity and shape of the signal. Sample 10047-49 was separated visually into three parts using a microscope with x30 magnification. Sample 10047-49-1 consisting of “fragments with an amber color in transmission” gave a very broad signal a t room temperature with g = 2.1 but a t 80 “K another peak was observed a t g = 1.88. The overall intensity of the resonance increased with de-
I
0.
I
0.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 5, A P R I L 1974
*
483R
Table 111. Radical Anions of Less Common Compounds in Structural Form
Ph ‘C=N
ph’
‘NH-Ph (122)
R\
N-N=O
R ’ R = Me, Et. i-Pr.i-Bu
0
yo .Vo
(197)
0
0
$
0
Ph Ph
Ph
cl+R c1
0 (70)
C1
R i105)
R = Ph, i-Pr (26.119)
4
0 (60)
(127)
(49)
0 2 NG N 0 2 M e 0 OMe i1881
Ph
I
Ph
a;: I
I
4
=
I
Ph (24)
484R
N-P-N
Phj
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO.
c=o O
I Ph (7)
5, A P R I L 1974
I
Ph,-P
NIP“ I
Ph,-P+
I
I
N-P=N Ph,
II
N ,P-Ph, (5)
II
P-Ph,
I
Table 111. Radical Anions of Less Common Compounds i n Structural Form
0
II
S
0
@-(COOH)"
a g : o 1I 0 (50)
or (COO-), n = 1,2,3,4,6 m 2n (116)
(115)
(3.81)
(170,171)
x = co,so,
(104)
(90)
&
&-j
\s
S&
x = co,so, (90)
CN 1
(Continued) A N A L Y T I C A L C H E M I S T R Y , VOL. 46, N O . 5 , A P R I L 1974
485R
Table 111. Radical Anions of Less Common Compounds in Structural Form
Et
crease in temperature. Samples 10047-49-2 (“lustrous, opaque, black fragments with conchoidal surfaces“) gave a broad signal with g = 2.1 which decreased ip intensity with decrease in temperature. Sample 10047-49-3 (“colorless, white, or transparent fragments”) gave a similar signal with a similar temperature dependence a t g = 2.1. In addition a sharp peak a t g = 4.28 was seen a t 80 OK. On closer examination, this peak was also present a t room temperature although very weak. A t 35 GHz, these samples gave a relatively intense absorption in the region of g = 2. The first and second portions also showed six equally spaced lines superimposed on the central part of the signal. The lines were separated by 90 gauss and centered a t g = 2.00. Weeks and coworkers concluded that the most probable source of the intense resonance was Fe3+ although contributions from other types of resonance such as from small metallic particles of iron could be present. The g = 1.89 absorption was tentatively assigned to Ti3+. The six line multiplet with 90 gauss spacing was clearly due to MnZ+. The Manatt group (107) also reported a broad signal a t g = 2.12 f 0.05 from lunar samples 10062-27 and 1008710, but no temperature dependency on the spectrum was detected in the temperature range 77”-298”K. Because they were able to simulate the spectrum by computer simulation of lineshapes using a model consisting of ferromagnetic centers of the order of 1 Fm in diameter distributed throughout the sample, this group concluded that the magnetic resonance was due to metallic iron. The Geake group (58) found a very broad strong signal from their samples a t room temperature and at 900960 “C. They concluded that the observed signal was most likely due to Fe3+. Further details on ESR studies of the above were published in the “Proceedings of the Apollo 11 Lunar Science Conference,” volume 3. In a lengthy preamble, the Weeks group (185) discuss the features of the spectrum expected for Fe3+ ions as compared to iron particles. For the latter, the frequency of resonance absorption is calculable for sphere and cylindrical, ellipsoidal, or planar shapes whose diameters are less than “skin depth.” (In the case of iron, this is cm at frequencies 1 9 GHz). For spherical particles, resonance should be observed a t geff = 2 and for other shapes, geff > 2 with absorption a t H = 0. However geffdecreases with increasing particle diameter. Since g e f f (9 GHz) < geff (35 GHz) for-any given particle size, the resonance spectrum of a sample of particles having a distribution of diameters will always have geff (9 GHz) less than ge!, (35 Ghz). The relaxation time of the particle magnetization is also a function of particle size and shape. The relaxation time, T , is estimated as 10-l sec for a particle 115 8, in radius and T N lo9 sec for a particle 150 A in radius. 486R
A crude model of lunar material with metallic iron was prepared by mixing iron particles with Si02 powder. The particles had a variety of shapes with particle sizes
