Introduction to the Interpretation of Electron Spin Resonance Spectra of Organic Radicals Nigei J. Bunce University of Guelph, Guelph. ON, Canada NIG 2W1

Interpretation of spectrosco~ic data is now established as an important part of the undergraduate curriculum. These interpretive skills may be acquired as part of courses in organic chemistry, since full-Gar organjc texts now all in- clude chapters on the major spectroscopic methods (IR, NMR, and mass spectrometry). Alternatively, a separate course in spectroscopic interpretation may be offered, allow- ing the introduction of topics that are not usually covered in standard organic tests. One of these is electron spin reso- nance (ESR).

~ i s t o r i c a l l ~ , ESR was discovered before NMR, but, since most undergraduates are much more familiar with the latter technique, this article assumes some knowledge of proton NMR, and treats ESR comparativel~. This article is written in a style that is intended to be addressed directly to under- graduates, rather than to their instructors.

Principles ol ESR Electron snin resonance (ESR). also known as electron . .

paramapeti: resonance (EPR), is an important spectro- scopic technique for the study of molecules and ions contain- ing unpaired electrons. I t can only be applied to systems in which not all the electrons are paired. Diamaenetic sub- stances therefore cannot he studied by ESR, since all their electrons are paired, hut equally they cause no interference with the observation of a paramagnetic substance by ESR. Commonly encountered paramagnetic species include or- ganic free radicals, metal complexes, and triplet excited states of diamagnetic molecules, as well as molecules such as NO, NOz, and ClOz, which are examples of stable (or "persis- tent") free radicals. In this article we shall restrict ourselves to coverage of organic radicals containing just one unpaired electron.

As we learn in freshman chemistry, the electron has asso- ciated with it two spin states ma = +l/~ and m, = -112, just as the proton has two nuclear win states I = +% and I = -5. In rach case, the two spin states are of equal energy in the ahsenre 01' an applied mnrnetlc field but are split n r m t bv a - . powerful magnetic field (Fig. 1).

The basic equations defining the operating frequencies of ESR and NMR spectrometers are of similar form,

ESR: u = gPH,

In each case. Hn is the streneth of the avolied maenetic field: the operating frequency o r the spectrbketer v related td the seuaration between the enerm levels AE in Fieure 1 through AlZ = hv, making v directly proportional the magnetic field strength.

The parameter ,9 in the ESR equation is called the Bohr magneton. Like the magnetogyric ratio y in NMR it deter- mines the extent to which an applied magnetic field splits apart the m, = +'/z energy levels. As Figure 1 implies, this splitting is larger for the electron (AEJ than for a proton nucleus (aE,,) a t equivalent magnetic field strengths, that is, ,9 >> y. Consequently, ESR spectrometers can he operated

Bask Parameters: ESR vs. NMR

ESR 'H NMR

0 = Bohr magneton: cwresponds t o y for an electron.

Differences between gfactors correspond to different chemical shifts.

Hyperfine constant a (gauss) describes me interaction of the electron with neighboring magnetic nuclei.

y = magnetogyric ratio. Characteristic of the nucleus under study. The larger the value of y, the more the nuclear levels split apart

Chemical shift 6 is the separation between the resonance frequency of me proton under study and that of a reference substance (MS).

Coupling constant J(Hz) describes the interaction of the nucleus under study with other neighboring magnetic nuclei.

Figure 1. Splitting of the electronic and nuclear energy levels by a strong magnetic field. Ths diagram is not drawn to scale: the nuclear s+lining should be even smaller lhan it is shown.

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second derivative

I H r

Figure 2. Shapes of absorption, firstderivative, and secondderivative curves. Most ESR Spectra are recorded as first derivatives.

using less powerful magnets than NMR spectrometers, as shown below.' ESR (X-band spectrometer):

Magnetic Field Strength 3390 gauss Freauencv 9.5 X 108 Hz (9.5 GHz) . . ~~. ~~

4E = srpamrion between energy lev& = :Id .I mol ' Excess of electmns in lower energy rfnfr (ROOK, = 1 in 700

'H NMR (60-MHz spectrometer):

AE = separation between energy levels = 2.4 X 10V J mol-I Excess of 'H nuclei in lower energy state (300 K) = 1 in 100,000

When radiation of a frequency v is incident upon a sample whose energy levels have been separated in an applied mag- netic field, both absorption and emission may he stimulated (the "resonance" condition) when A E = hu. For hoth NMR and ESR the absolute magnitude of U is very small, the excess population of nuclei (or electrons) in the lower enerm state is very small, and correspondingly the net ahsorpti& signal is also small. In ESR spectroscopy it is common to improve the simal-to-noise level of this small net absorntion . ~~~

by-recording 'the spectrum as a first (or less commonly, second) derivative curve, as o~oosed to the direct absorotion curve, which is the conventio&l presentation in high-r;solu- tion NMR (Fig. 2).

We make use of two kinds of information in interpreting NMR spectra, namely chemical shifts and coupling con- stants. Analogous parameters can he obtained from an ESR -r----

The first piece of information is the "gfactor" which is the ESR parameter incorporating "chemical shift" information. Freely rotating radicals containine onlv one unnaired elec- " tron,as in th& article, exhibit only one g factor, just as a molecule with only one type of proton (e.g., CHCls) has only one chemical shift value.

Reference to the ESR and NMR basic equations above

' Calculation details: AE = hv X Avogadro's constant gives A E i n J mol-': N,,,,IIY,., = e-bE'RT(Boltzrnan distribution).

Fioure 3. Diaaramrnatic view of the solittina of the enerov levels of the " nyarogen atom Tne don& lnes show uhal would be expected 11 lhere were no nxlear hypertino lnleract on T h e expsrlmenl 5 run at Con%anr frequency ( v ) and vmying field Ho.

shows that the chemical shift phenomenon is treated differ- ently in ESR and NMR. In NMR, a shielding parameter c gives the relationship between the actual (Ho) and the effec- tive (Herr) appliedfield, hut because c is very small (parts per million of Ho) the chemical shift is measured relative to a standard substance (TMS for oroton NMR). In ESR this information is all "lumped together2' into theg factor.

Just as chemical shift ranges in hertz are usually small compared with the nominal NMR operating frequency, so also the g factor usually varies little from one radical to another. For carbon-centered organic free radicals, as an example, thegfactor is always close to the free electron value of 2.0036 and is rarely diagnostically important. (For metal complexes the g factor does vary substantially from species to s~ecies. and can he used to give information about the elec~ronic'structure of the metal"ion.)

The second ESR narameter is the hwerfine solittine (or . . .. . hyperfine wupling o r just hyperfine). which cr&rsponds to th( cot~plinr ctmtant in I<blH. I t is aivrn the svmhol a and reported in- units of magnetic field strength-(unlike the NMR coupling constant J, which is reported in hertz).

Like the NMR coupling constant, the ESR hyperfine splitting arises because of small changes in the effective magnetic field strength experienced by the free electron, resulting from the magnetic influences of neighboring mag- netic ( I # 0) nuclei ('H. '9F. etc.. hut not IZC. 160. etc.). The ~, ~~ . compl'exity bf the ESR spectrum thus depends on the numl ber and types of these magnetic neighbors. While we shall defer a detailed discussion to the later section on spectral interpretation, we may note here that the typical ESR spec- trum is like one multiplet of an NMR spectrum; in particular i t will he symmetrical about its midpoint orovided that the spectrum is isotropic (freely tumbling mol&ules).

The simplest possible free radical to show the hyperfine interaction is the hydrogen atom for which aH = 507 G. Two resonance lines are observed, one on each side of the hypo- thetical electron resonance value (Fig. 3).

Just as in NMR, the doublet (or multiplet in more com- plex cases) arises because the observed ESR spectrum is actually the superposition of two spectra of one line each: the low-field line corresponds to the population of electrons whose hydrogen nuclei are oriented so as to reinforce the

908 Journal of Chemical Education

applied magnetic field: likewise the population of hydrogen atoms a,hose nuclei arc oriented so as to oppose the applied magnetir field gives rise to the line at highrr field.

Many detailed texts cover this topic adequately (e.g., ref. I), and only the basic components will be described here. The source of electromagnetic radiation ( u ) is a klystron, which vroduces monochromatic microwave radiation whose wavelength is a few centimeters. A waveguide directs the radiation into a sample cavity that lies between the poles of an electromagnet. The field strength Ha is altered gradually, and the amount of power absorbed by the sample is moni- tored by a crystal detector. The change in the power ah- sorbed is greatest as Ho passes through the value appropriate for resonance. A system employing modulation coils and a phase-sensitive detector aids in sensitivity and makes it most appropriate to present the spectrum as a derivative.

Quantitative Aspects of ESR Quantitative aspects of ESR are pursued much less com-

monly than in NMR. One reason is that there is no reason to integrate the ESR spectrum of a free radical containing just one unpaired electron, unlike the situation in NMR, where we frequently wish to determine the stoichiometric ratio of protons in different chemical environments. Another experi- ment much less common in ESR than in NMR is to deter- mine the absolute concentration of the resonant species. Although this has been done for paramagnetic metal com- ~lexes . for examde. the analvsis of Cu(I1) in seawater (2). the ESR method has both poorer precision and accuracy than competitive techniques such as atomic absorption spectrosco~y. By their very nature, organic free radicds are transient, reactive species and consequently their steady state concentrations are usually very low. In fact, it is some- times difficult even to detect the free radicals, their concen- trations are so low, and a special technique (the spin-trap- ping method: see below) may have to he used to observe them at all. Concentrations of radicals -10-6 mol L-' give strong ESR signals, while the detection limit is usually loTs to mol L-'.

In cases where easily detectable concentrations can be achieved, the number of spins may be estimated by compar- ing the intensity of the ESR signal of the sample with that of a suitable reference substance of known concentration. Pos- sible references include pitchblende, or a solution of a persis- t e n t free radical such a s diphenylpicrylhydrazyl, Ph2NNC6H2(N02)3, 1. Even so, it is the steady state concen- tration that will be detected, and not the stoichiometric concentration of radicals produced over the whole course of the reaction.

ldenllflcation of Odd Electron Specles This is the main avvlication of ESR. The vatterns of ESR

spectra are highly diagnostic of the number and type of magnetic nuclei adjacent to the free radical center, and an unambiguous identification of the radical is frequently pos- sible. In the cases that we will study the hyperfine constants can be measured directly from the spectra; more complex cases require computer simulations in which trial value of the hyperfine constants are used to recreate the experimen- tal spectrum as closely as possible.

Organic Alkyl Free Radicals The ESR spectrum of the methyl radical is shown in Fig-

ure 4 as a second derivative (3) presentation. The single resonance line is split into a 1:3:3:1 quartet by the three adjacent protons (I = f $1, just as in 'H NMR the under- lined proton in CH3CII=O appears as a 1:3:3:1 quartet (the n + 1 rule). Exactlythe same kind of pyramid analysis can be used in the interpretation of ESR as in NMR spectra.

Increasing the complexity slightly, the ESR spectrum of

the ethyl radical (3) (Fig. 5) shows the single resonance line split into a 1:2:1 triplet by the CH2 system and into a 1:3:3:1 quartet by the CHn ~ r o u p adjacent to it, for a total of 3 X 4 = i 2 lincs altoget her. The &m&ated appearance of the spec- trum is herause the two hyperfine constants have compara- ble magnitudes, and the patterns overlap. The two hyperfine spacings acHz (22.4 G) and aC"3 (26.9 G ) are marked on the fieure. The three lines marked 1.2. and 4 show the first 1:2:1 pattern and those marked I . R , 6 , and 9 ahow the first 1:M:I oattern. You should he able to pick out the reveats of these patterns.

We should Dause for a moment to describe the ESR spec- troscopists' jargon for describing hyperfine constant;. In radicals the positions are labelled as shown below

The hyperfines are correspondingly a!, a.;, and a:. Compare these with NMR (nucleus X a being observed):

x-C-C-c

JxH(,, would be described as geminal, J x H ~ ) as vicinal, and JXH(3) as long range. The ar hyperfine thus corresponds to the geminal coupling constant, the 6 to a vicinal (neighboring) and the y to a long-range coupling constant. As with cou-

Ftg.re 4 ESR specnumaltnemelhyl r a d m ia)Fysm!dsnslyas. Wsecond- derwalwe speclrum m llq~no memane, reproduced wllh perrnlsslon lrom re1 3

Figure 5. Secandderlvative ESR specrmm of lhe ethyl radlcal in liquid ethane. Lines 1, 2, and 4 represent the first t:2:1 triplet (aCH>) and lines 1. 3. 6. and 9 me first 1:3:3:1 quartet (P,). Reproduced with permission from ref. 3.

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Delocalized Organic Free Radicals Typical lsotroplc Values of Hyperllnes In Locallzed Alkyl Radicals Iln aausrl

pling constants, a, and a0 are of similar magnitude, and a, is very much smaller (see table).

We finish this section with a look a t two more complex alkyl radicals. The ESR spectrum of the CHFz radical (4) (Fig. 6) shows different hyperfine interactions for the proton (aH = 22 G, similar to that of CHx) and the two equivalent I9F nuclei (I = '/2, aF = 84 G). The observed pattern is thus a 121 triolet because of the two eouivalent fluorines. with

~ ~. ~~~

each line further split into a douhlkt hy the proton. This is described as a t r i ~ l e t of doublets.

The spectrum k~ Figure 7 was obtained when diethyl ether reacted with hydroxyl radicals (5). I t was assigned to radical 2, which has one a hydrogen, plus 3 p's, and 2 7's.

The spectrum should, and does, show a total of 2 X 4 X 3 = 24 lines. The values of a:, a;, and a" are respectively 13.8,21.9, and 1.4 G. The low value of a ~ ~ c o m p a r e d with that in the table is prohahly the result of some delocalization of the radical center onto oxygen. The spectrum is not compatible with the alternative hydrogen abstraction product CH3CH20CHzCHz which should show 3 X 3 = 9 lines he- cause of the two a and two @ hydrogens.

Figure 6. Secondderivative ESR spectrum of the difluaamethyl radical show- ing splining due to one proton and two flumine nuclei. The original spechum has been retouched by eliminating impurity lines due to CHd. Reproduced With permission fram ref. 4.

Figure 7. Firstderivative ESR spectrum of the radical CH&HeOCHCH, Repr* duced with permission from ref. 5.

The radicals 3-5 each have all equivalent protons and show the expected n + 1 lines in the intensity pattern of a Pascal's triangle.

0 B

0 H H' 'H

3 5 4 n = 4 n = 6 n = 5

1:4:6:4:1 1:5:10:10:5:1 1:6:15:20:15:6:1

The spectrum of 3 is reproduced as Figure 8 (6). Two interesting cases are the ESR spectra of the hutadi-

ene anion radical (CH2=CH-CH=CH2)-., 6, and the ben- zyl radical, 7. The spectrum of 6 (Fig. 9 (7)) appears as a quintet of triplets, resulting from the four equivalent CH2 protons and the two equivalent CH protons. The hyperfine a1.4 (7.6 G) is much larger than az.3 (2.8 G), where the suh- scripts indicate the carbon atoms of 6. Several years ago McConnell concluded that the magnitude of the hyperfine constant is proportional to the electron density of the free electron (the "spin density") a t that position. Thus in the case of the butadiene anion radical we would write struc- tures 6a as more important resonance forms than 6b.

Likewise, the hyperfine constants for the henzyl radical 7 ( ~ c H * = 16.4G,aortho = 5.1 G,amet. = 1.6G,apa,= 6.3G) (8) are entirely in accord with the organic chemists' representa-

Figure 8. Firstderivative ESR spectrum of the benzoquinone radical anion. The lines are in the 1:4:6:4:1 ratio characteristic of four equivalent hydrogens. Reproduced with permission fram ref. 6.

910 Journal of Chemical Education

tion of it. Resonance form 7a is the most important followed by 7b and 7c (X2). Very little electron density appears a t the meta positions.

CH, CH, CH, I Ii -

l e 7b IC A very complex-looking spectrum is that of the naphtha-

lene anion radical 8, Figure 10. There are four n hydrogens and four p hydrogens. We thus expect a total of 5 X 5 = 25 lines. In practice some of the lines are very weak and hard to distinguish. Lines 1, 2, 3, 5, and 7 (arrow) are the first 1:4:6:4:1 pattern for the (3 protons (smaller aH); lines 1, 4,9, 14, and 19 (arrow) are the similar pattern for the four a protons.

Radicals Containing "Heteroatoms" We are familiar with the splittings in a proton NMH spec-

trum due to other orotons. and UI nuclei such as IgF and "P. However, there are other nuclei that do not normally cause snlittine in an NMR soectrum even thouah they have non- - zrro magneticspin quantum nnmhers. Examplesof'theseare "N (I = l) .~ '5CIand"Cl(l~othI =3/2I,Wrand"Br (both1 = 312). and 1271 (1 = 512). These nuclei, along with many others, have in common I values 2 1. Such nuclei have the property of a nuclear quadrupole moment, a consequence of which is that in most chemical environments they cycle ranidlv throneh their snin states. On the timescale of the ~ ~ ~ " e x ~ e r i m e n t (v = several MHz) their magnetic effects average to zero. In other words, in a time of l l u (s) the I4N

nucleus, for example, samples its I = -1, I = 0, and I = +1 s ~ i n states sufficientlv often that a neiahborine nroton sees

A

only the average of ali of these, that is, 7 = 0. Now consider an ESR experiment. The frequency of an

ESR spectrometer is higher (GHz) than that of an NMR spectrometer, and in l l u seconds the I4N nucleus will not have had time to cycle thnnlgh its spin states. An electron resonance line will be split inta, three lines of PQUOI ~ntrnsi t \ by the I4N nucleus, corresponding to the almost equal pop"- lations of 14N nuclei in the I = -1, I = 0, and I = +1 spin states. Therefore, as a generalization, quadrupolar nuclei such as I4N, 35C1, etc., cause splitting in an ESR spectrum even though no such splitting is seen in NMR. For nuclei having I > %, the "n + 1" rule is generalized so that the number of lines in the pattern becomes (2nI + 1).

A similar time-related phenomenon is seen in the ESR spectra of hydroxyalkyl radicals such as CH20H. We com- pare the ESR spectrum of CH2OH with the 'H NMR spec- trum of CH30H. In the NMR spectrum the CH3 protons are not split into a doublet by the OH proton because the mak- ing and breaking of hydrogen bonds occurs so quickly on the timescale of the NMR experiment that several OH bonds have been made and broken during the observation of the CHQ nuclei. Thus all soin-soin information is averaped out. By contrast, the OH group appears to be static on &e ESR timescale. and the doublet s~ l i t t ine is seen (10) (Fie. 11). . . . , , .

Three exnmplm involving quadrupolar nuclei are shown in Figures l?-14. Figure 12sh0ws the spectrum ofthe radical

n - II

P~CH=N--0.(11) which shows six lines of equal intensi- ty; these are due to the 1:l:l triplet of I4N, each line of which is further split into a 1:l doublet by the lone CH proton.

I 6 I O G --3

Figure 9. Flrstderivative ESR spectrum olthe butadieneradicai anion. Llnes 1. 2, and 3 represent lhe 1 2 1 hyperline panern due to the two equivalent hydrogens at C(2) and C(3). The four equivalent hydrogens atC(1)and C(4) can be seen. for example. in the 1:4:6:4:1 pattern of lines 1. 4. 7. 10. and 13. Reproduced with permission from ref. 7.

Figure 10. Firstderivative ESR spectrum of the naphthalene anion radical. Reproduced with permission hom ref. 9.

FlgLre 11 F rEtderlvatlve ESR SpeChLm of the hyaronlmelhyl rascal, show- ~ n g the tr*plet spl nmg due to me CH,group ano the doublet splln~ngaueto me CHI group and the doublet splitting of the hydroxyl group. Reproduced with permission from ref. 10.

Figure 12. Firstderivative ESR specnum of the radical PhCOCHNO. showing the spliiings due to "N (1:l:l) and the underlined proton. Reproduced with permission from ref. 11.

Volume 64 Number 11 November 1987 911

FlgUre 13 F rstderwatw ESR spscrrum 01 CiChCO,r( showlng tne sp lnlngs aJe to chior ne ( 1 1 1 1) and the bnderilnw proton Reproduced wllh perms- sion from ref. 5.

Figure 14. Firstderlvalive ESR spectrum of the pyrazhe anion radicai. show- ing splining due to two "N nuclei (1:2:3:2:1) and four protons (1:4:6:4:1). Reproduced with permission from ref. 12.

Figure 13 is attributed to the radical MCHC02H, formed by the attack of hydroxyl radicals on chloroacetic acid (5). I t shows eight lines: two sets of 1:l:l:l quartets. The quartets are the result of solittine bv CI: both 35C1 and W 1 have I = 312 and give rise io fourusph states ( I = -312, -112, +1/2, + 312). I t happens that the magnetic properties of 35CI and "GI are rather similar and the splitting8 due to these two isotopes are not resolved separately except in favorable cases. The a proton is, of course, the source of the doublet pattern.

Figure 14, a more complicated spectrum (121, is due to the anion radical of pyrazine 9. A total of 25 lines is expected; four protons ( I = 112) cause five lines in the ratio 1:4:6:4:1 (lines 1, 2, 3, 5, and I ) , while two nitrogens ( I = 1) result in the five lines 1.4.9. 14. and 19 hut in the different ratio of 1:2:3:2:1 (see ref.' 1'3). he difference in these intensities allows us to assien the values of the hvverfines with confi- dence. The intensities of the arrowed (lj lines 2 and 4 show this clearly.

The organometallic compound shown in Figure 15 is an interesting case (14) with the odd electron experiencing the magnetic influence of four I4N nuclei (I = 1) and one Re nucleus ( I = 512). The spectrum is therefore composed of six equal patterns due to Re, each of which is a nine-line multi- plet due to the 14N nuclei (2nZ + 1). (Not all the lines of each nine-line multiplet are strong enough to see.)

How To Assign the Lines and the Hyperfines in an ESR Spectrum

I t is only rarely that the spectrum is a complete "un- known." Usually the experimentalist has a t least some idea of what radical helshe is expecting. The trick is to go hack and forth between the spectrum (what you actually see) and the expected structure (what you expect to see) and deter- mine whether what you actually see is compatible with your expectations. As an example (15), consider the spectrum of (CF2hNO. Fieure 16. . ,

At first glance, there appear to be nine resonance lines A t o I. You are expecting a total of 7 (6 equivalent 19F) X 3 (one 14N) = 21 lines. Closer inspection of the experimental spec- trum shows that all but the outermost two lines are actually multiplets.

912 Journal of Chemical Education

Look closely at the resonance B. It is actually a large line (2) and a small one (3). The small one is equal in intensity to resonance A. I t is part of a 1:l:l triplet ('4N). The third line of the triplet (6) is a t the extreme right of resonance C. Likewise, the main line in resonance B(2) is repeated a t equal intensity in resonances C(5) and D(9); the largest line in resonance C(4) is repeated in D(8) and E(12), and so on. Thus the spectrum is, as expected, an (overlapping) pattern of seven 1:l:l triplets: (1,3,6), (2,5,9), (4,8,12),. . . .

Now to evaluate the hyperfine constants. Measure the distance. in millimeters. between resonance A and the larger resonance line of 8. (The two outermost liner always give ;he ~rnnllest hyperfinet.Thid is: rnm. hfeasure the 25-(; marker;

Figure 15. Secondderivative ESR specbum of a rhenium-phenanthroline complex.

showing splining due to Re (I = 512) and four "N nuclei. Reproduced wilh permission from ref. 14.

Figure 16. Firstderivative ESR spectrumof (CF&NO.. The spliningsare due to one "N and six I9F nuciei. Reproduced with permission from ref. 15.

it is 21 mm long. By proportion al = 8.3 G. The intensity ratio shows that this must he the start of a 1:6:15:20:15:6:1 pattern (sixIgF), lines 1,2,4,7,10,13,16. Thus al = aF = 8.3 - Cr.

To evaluate aN, measure from resonance A to the right- hand line of resonance B. This is 8 mm, so aN = 9.9 G. I t is because the magnitudes of the hyperfines are so similar that the spectrum initially appeared to have only nine "lines3'A- I.

A slightly different approach can be useful when you can- not make out the patterns by simple inspection. Find the two outside lines (which give you the smallest hyperfine), and place the edge of a piece of paper against their midpoints. Mark these midpoints on the edge of your paper. Let us apply this method to the spectrum shown inFigure 14. Make your marks corresponding to the separation hetween lines 1 and 2. The same spacing can be seen between lines 2 and 3,3 and 5, and 5 and 7. Youmarklines 1,2,3,5, and 7; they are all one pattern. Therefore line 4 does not fit. Make new marks corresponding to the spacings between lines 1 and 4 (the second hyperfine). This separation is repeated between lines 4 and 9,9 and 14, and 14 and 19, giving the second pattern.

Spin Trapping As already mentioned, the steady state concentrations of

free radicals are often too low for them to be detected direct- ly. This problem is most acute when the free radicals arevery reactive. For example, in the free radical chain chlorination of cyclohexane, no cyclohexyl radicals are seen by ESR. This is because the reaction

occurs so rapidly that the steady state concentration of CsHll. is below the detection limit.

The spin trapping experiment is carried out by adding to the solution containing the expected free radicals a diamag- netic substance with which the radicals react readily. This added substance is called the "spin trap" (T). The spin trap

Figure 17. (a) Firstderivative ESR spectrum of the spin adduct of CH? and (CH&CNO. The spectrum has been retouched to eliminate impurily lines which were present in the original figure. (b) Firstderivative ESR spectrum of the spin adduct of COr and (CH3)sCN0. Reproduced with permission ham ref. 76.

reacts with (traps) the radicals R. to form a "spin adduct" (TR)..

The spin adduct is itself a free radical, which is less reactive than R.. Because it is less reactive, it can attain a higher, and it is hoved detectable, steadv state concentration.

~ o m k o n l ~ used spin trips are substances containing N=O functional groups, such as 2-methyl-2-nitrosopropane (lo), nitrosodurene (111, and phenyl tert-hutyl nitrone (12)

In each case, the spin adduct is a nitroxide radical RR'NO.. Two examples are shown (16,17).

The spectra of 13 and 14 are reproduced as Figures 17a and 18. The ESRspectraof spin adducts always show the charac- teristic 1:l:l pattern due to the @ nitrogen, but there is a severe price to pay for the benefit of trapping the undetect- able R. in that most of the structural information in R. is lost. In adducts of structure 13 only they hyperfine constant shows any dependence on the structure of R., while adducts 14 show only the a" indicated (17). Fortunately the magni- tude of this hyper&ne does vary somewhat from one spin adduct to another, and can he used to help identify a radical R. provided that the experimentalist can obtain a spectrum of the authentic R. spin adduct.

Isotopic Substitution This technique can he very useful for providing additional

information when a radical observed by ESR is of unexpect- ed structure. Common substitutions are the incorporation of deuterium (%:I = 1) in place of protium (I = 1/2), and replacement of 14N (I = 1) by I5N ( I = 112).

Fimre 18. Firstderlvative ESR soecbum of me soin adduct of the ~ b u t v l rad ca worn pheny I-owl nntrone The sp 8 n q s aTe a ~ e lo "h am me proton abelled 7 n struadre 14. Repraaxed wlth Perrnlsslon from ref 17

Volume 64 Number 11 November 1987 913

The radical obtained when the potassium salt of trinitro- methane is reduced in aq. NaOH has the ESR spectrum shown in Figure 19a; Figure 19b is obtained when the reac- tion is carried out in DzO. Each spectrum shows a main 1:2:3:2:1 pattern, due to twoI4N nuclei (aN = 9.6 G). In Hz0 a doublet splitting (a = 4.1 G ) is observed, whereas a 1:l:l triplet splitting (a = 0.6 G) is seen in D20. This shows that the doublet splitting in Figure 19a is due to a proton, since it changes to a typical deuterium pattern in Figure 19h. The radical thus contains two 'W and one 'H nuclei close to the radical center; the author (18) attributed the spectrum to CH(N02)22-.

Figure 17h is the CDB adduct to 2-methyl-2-nitrosopro- pane (161, corresponding to the CHB adduct of Figure 17a. The value of aN is identical for hoth spin adducts. However three deuterium atoms (I = 1) give a seven-line multiplet of smaller aD than the four line aHmultiplet, and so the appear- ance of the two spectra is quite different.

Nitrogen substitution has recently been used to good ad- vantage in identifying the product of the reaction of azide radicals (N3) with nitrosodurene. The ESR of the adduct (Fig. 20a) is a 1:2:3:2:1 pattern with each line further split into a 1:l:l triplet (19). Thus three nitrogen atoms are present, two of them equivalent. The assignment of the spectrum to the expected spin adduct 15a

presented difficulties (why were two nitrogens equivalent? and which two?). When the experiment was repeated using I5N in the azide radical source, the mystery was solved. The spectrum (Fig. 20b) showed the presence of two I 4 N from nitrosodurene and only one 15N atom from the a i d e (20). Consequently, the spin adduct had reacted with a further molecule of nitrosodurene, affording 15b as the radical actu- ally o h s e ~ e d .

Applications of ESR We have seen that ESR can be a powerful method for the

detection and identification of free radicals. For many tran- sient radical species, i t is one of the very few methods avail- able for their study. Consequently i t finds wide application in both chemistrv and bioloev. In mechanistic studies. the intermediacy of free radicalycan he confirmed by act"ally observing the exvected ESR svectrum. This is not alwavs as straightfkard a s i t sounds because, as we have see; the most reactive radicals achieve the lowest steady state con- centrations and are hardest to detect. In this situation, no ESR signals are detected even though the reaction proceeds through free radical intermediat,es. Such a reaction would be a good candidate for a spin-trapping reaction. Alternatively, a minor reaction pathway may afford a stable free radical which is detected easily. The major pathway either involves avery reactive free radical intermediate or is an ionic mecha- nism. In this case ESR gives a misleading picture about the reaction mechanism.

Despite these problems, ESR is very useful in mechanistic studies. The technique of flow ESR, in which flowing solu- tions are studied as they pass through the cavity of the ESR spectrometer, has given a wealth of information about the kinetics of free radical reactions. The technique of spin trap- ping is allowing the detection for the first time of free radical

- L

H

13 GAUSS

Figure 19. Firstderivative ESR spectra of the reduction product of C(NO&(a) in H20. (b) in D,O. Reproduced with permission from ref. IS.

Figure 20. "Stick diagram" of the ESR spechum af the product of reaction of azide radicals (N,.) with nitrosodurene: (a) using l4N-aride. (b) using 'SN-aride.

species in biological systems, and this is of great current interest with regard to the possible role of free radicals in processes such as aging and carcinogenesis. Another tech- nique, spin labelling, involves the covalent attachment of naramaenetic nrobes (ex.. nitroxides) to hioloeical macro- - . - . - molecules such as proteins in order to study conformational effects. Research vuhlications related to the use of ESR in organic chemistry and biochemistry now numher several hundred per year.

Llterature Clted I. B ~ ~ ~ ~ . H. H.: christian, Gn.; O 'R~~IIY,J . E. ~ ~ n ~ ~ ~ . ~ t ~ ~ A ~ G I Y ~ ~ ; B*w":

Boston, 1978:Chapter 13. 2. Virmani.Y. P.: Zel1er.E. J. A n d Chem. 1974.46. 324. 2. Fernenden. R. W.:Sehuler,R.H. J.Chem. Phys. 1963.39.2147, 4. Ferrenden. R. W.;Schuler,R. H. J.Cham.Phys. 1966,43,2704. 5. Diron, W. T.:Norman.R. O.C.;Buley.A.L. J.Chem. Soc. 1964.3625. 6. Wertr. J. E.: Bolton, J. R. Electron Spin Resononce: Elementary Theory and Practi-

cal A p ~ l i ~ o t i o w i MCGTBW-Hill, New York, 1972. 7. Lev, D. H.;Myers.R. J. J. Chpm Phya. 1964,11.1062. 8. Diion, W.T.; Norman.R.0.C. J.Chem. Soc. 1964,4857. 9. carrington. A,; MeLaehlin. A. D, Introduction to Mognefic Resounca; Harper and

Row: New York 1967; p 77. lo. Liuinmton, R.; Zeldas, H. J. Chrm. Phys. 1966.44. 1245. 11. Thomas, J.R. J. Am. Chcm.Soc. 1964.86,1446. 12. Carrington. A : Dos Santca-Veiga. J. Mal. Phya. 1962,5.21. 1%. Kor1er.D. F.:Jone% W. J. Chem.Edue. 1982.59.289. 14. Creber.K.A.M.:Ho.T-I:Depew,M.C.:Weir.D.;Wan.J.K.S.Con. J,Chem. 1982.60,

ISM. 15. Seheidler, P. J.:Bolton, J.R. J.Am Chem.Soc. 1966,88,371. 16 Rosenthal.1.: Mo3oba.M. M.:Rlesr.P. Con. J. Chem. 1982.60.1486 17. .lanren,E.G.;Bisckhurn.B. J. J. Am. Chem.Soe. 1969,91,4481. 18. Lagercrank, C. Acto Chem. Scnnd. 1964,18,1384. 19. Reh0rek.D.: Hennig.H.2. Chem. 1979,263. 20. Kremers. W.; Koroll. G. W.: Singh. A. Con. J. Chem 1982.60,1597.

914 Journal of Chemical Education