EPR study of radicals formed from aliphatic nitriles

JOURNAL

OF MAGNETIC

RESONANCE

18,254-264

(I 975)

EPR Study of Radicals Formed from Aliphatic Nitriles* P.

SMITH,

Paul

R. A. KABA,~

M. Gross

Chemical

T. C. SMITH, J. T. PEARSON, AND P. B.

Laboratory, Department Durham, North Carolina

of Chemistry, 27706

Duke

WOOD

University,

Received July 25, 1974 By using the Tic&-H202 radical-generating method within a continuous-flow system at 25 f 2°C a comprehensive EPR study of twelve substrates, acetonitrile, propionitrile, butyronitrile, trimethylacetonitrile, succinonitrile, adiponitrile, glycolonitrile, Iactonitrile, 2-hydroxyisobutyronitrile, cyanoacetic acid, methyl cyanoacetate, and ethyl cyanoacetate, has been carried out. For each substrate except the two cyanoacetates, it was generally possible to characterize the spectrum of every radical that could conceivably be formed by hydrogen-atom abstraction from carbon. For methyl and ethyl cyanoactate, respectively, the absorption observed was consistent with the presence of more than one radical, but only the spectrum of .CHZOOCCH2CN and .CH,CH,OOCCH,CN could be characterized. With the use of their H&-photo-flow method, R. Livingston and H. Zeldes have investigated as substrates some of the nitriles included in this present study, in most cases using non-aqueous solvents. For each substrate, only the a-CN radical, formed by hydrogen-atom abstraction from the C-2 position, could be characterized. Consequently, the results of the present study complement and considerably extend those from their investigation.

INTRODUCTION

An earlier paper from this laboratory reported (2) the first results of a comprehensive EPR study of the radicals formed from saturated aliphatic nitriles in aqueous solution at ca. 25°C with the use of the TiCI,-H,O, radical-generating method (2-4). The present study is an extension of the earlier work to other water-soluble aliphatic nitriles, all data being taken at 25 _+2°C. Some of the nitriles reported on here have been examined as substrates by Livingston and Zeldes (5) employing their H,O,photo-flow method of radical generation and, in most cases, using non-aqueous solvents and reaction temperatures in the range 30-43°C. The HzO,-photo-flow and TiCI,-H,O, methods are superficially similar. However, for each nitrile investigated by both techniques, the study using the H,O,-photo-flow method (5) reports the characterization of only one radical, an a-CN radical; whereas in the present work, * Supported by U.S. Public Health Service Grant No. GM 07653, Division of General Medical Sciences, National Science Foundation Grants Nos. GP-I 7579 and GP-32292, and the North Carolina Board of Science and Technology. Taken in part from the Ph.D. dissertation of R. A. Kaba, Duke University, Durham, N.C., 1974. t National Defense Education Act Predoctoral Fellow. Copyright 0 1975 by Academic Press, Inc. All rights of reproduction in any form reserved. Printed in Great Britain

254

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for all but two of the substrates it was generally possible to characterize the spectrum of every radical that could conceivably arise by hydrogen-atom abstraction from carbon. Consequently, the present results complement and considerably extend those based on the use of the H,O,-photo-flow method (5). In particular, the spectroscopic properties of the cr-CN radicals studied with the use of both radical-generating methods agree reasonably well when allowance is made for likely solvent and temperature effects. EXPERIMENTAL

The experimental arrangement and procedures were essentially as described previously (4) except that, during the latter stages of this present investigation, a Varian E-9 spectrometer fitted with an E-080A digital drive X-Y recorder and an E-23 1/E-232 dual sample cavity assembly was employed. All spectra were recorded as the first derivative using lOO-kHz field modulation. As before (4), g-values were taken against aqueous potassium peroxylamine disulfonate but, in the present work, these values were obtained by using the dual cavity assembly and an electronic counter with a frequency converter (Hewlett-Packard Models 524L and 5255A, respectively). The g-value standard also served as the field standard (4,6). Fresh samples of the following substrates were examined without further purification: propionitrile, adiponitrile, and ethyl cyanoacetate (Eastman, reagent); cyanoacetic acid and lactonitrile (Columbia); trimethylacetonitrile and methyl cyanoacetate (Aldrich); 2-hydroxyisobutyronitrile and succinonitrile (Matheson, Coleman and Bell, practical); butyronitrile (Matheson, Coleman and Bell); and glycolonitrile (Matheson, Coleman and Bell, 702, technical). Acetonitrile (Eastman, practical, and Fisher, reagent) was carefully fractionally distilled before examination since the EPR absorption observed with the use of some batches of this substrate as received (from both sources) contained a significant contribution from other than the single radical expected (I). For a similar reason, some samples of propionitrile had to be fractionally distilled in a like fashion. Samples of ethyl cyanoacetate were carefully purified by bulb-to-bulb distillation within a high-vacuum line, collecting the middle 807;; however, the spectra obtained from the original and the purified material were indistinguishable. Unless specified otherwise, the following reaction conditions were employed: both streams, 0.2 M in sulfuric acid; the reducing stream, 0.006 A4 in titanous chloride; the oxidizing stream, 0.04 M in hydrogen peroxide; total flow rate, 2-4 ml see-‘, equally divided between the two streams, the results not varying significantly over this range; reaction temperature, 25 f 2°C. For glycolonitrile and lactonitrile, the reaction conditions were modified with respect to the sulfuric acid concentration, this ranging from zero to 1.8 M in both streams. Each substrate was included in both streams at a concentration that, if possible, was about the minimum needed to quench the signal from any titanium-radical complex (7) and obtain a suitably intense EPR absorption, as follows: trimethylacetonitrile, glycolonitrile, and ethyl cyanoacetate, 0.2 M; butyronitrile and lactonitrile, 0.3 M; adiponitrile, 0.4 M; 2-hydroxyisobutyronitrile, 0.5 M; propionitrile, 0.7 M; methyl cyanoacetate, 1 M; succinonitrile, 1.5 M; acetonitrile, 2.4 M; and cyanoacetic acid, 3 M. Only for the first-named substrate was there a detectable EPR signal from a titanium-radical complex (7). This was because the substrate had to be used sparingly on account of its expense.

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RESULTS

For each substrate, with the exception of methyl and ethyl cyanoacetate, the spectrum was fully analyzable in terms of the presence of all radicals that could conceivably arise by hydrogen-atom abstraction from carbon. In general, each substrate’s reactivity, assumed to be related to the minimum substrate concentration necessary to suppress the signal from any titanium-radical complex (7), was in line with the results of other typical EPR investigations using the TiCI,-H202 system with the same or related substrates (1, S-21). In particular, the -CN group was deactivating, its effect being roughly similar to that of the -COOH group. Likewise, for substrates with more than one carbon hydrogen-atom-abstraction site, the effect of the -CN group on the selectivity of such abstraction, assumed to be indicated by the relative steady-state molar concentrations of the radicals observed, determined as previously described (9), was roughly comparable to that of the -COOH group, as might be expected (9). It should be noted that neither the nitrile nor the analogous carboxylic acid substrates have been thoroughly studied over a sufficient range of reaction conditions to establish how, for example, the apparent selectivity of hydrogen-atom abstraction from a given substrate varies (7, 9, 12, 13). Nevertheless, for both types of substrates, tests showed that this apparent selectivity was not very sensitive to small changes to the initial value of [TiCI,]/[H,O,] in the mixed stream (9, 12, 13). I. Unsubstituted Cyanoalkanes Acetonitrile, CH,CN, gave only .CH,CN, [l] (5, 14a). The lines from [l] were particularly broad, ca. 0.9 G peak-to-peak. In the other studies of [l] in solution, this property has been noted (.5,14a) and discussed (14a). Pearson et al. reported (I) acetonitrile in the TiCl,-H202 system yielded an EPR absorption of which the chief component was from [l] plus other lines of unknown origin. These extra lines were not observed here if carefully purified substrate was used. Propionitrile, CHJZH&N, formed two radicals, viz. .CH&H#ZN, [2] (14b), and CH,CHCN, [3] (5, 24a), with [2] predominating. Butyronitrile, CH,CH,CH,CN, produced an absorption interpretable in terms of three radicals. The most abundant radical, .CH2CH2CH2CN, [4], gave a triplet (1:2: 1) of triplets (1 :2: 1). The next most abundant species, CH,CHCH2CN, [5], yielded a quarter (1: 3 : 3 : 1) of triplets (1: 2 : 1) of doublets (1 : I). Present in much lower concentrations was CH&H&HCN, [6] (5). Trimethylaceto[7], its spectrum nitrile, (CH,),CCN, gave the single radical *CH,C(CH,),CN, consisting of a triplet (1:2: 1). The triplet lines were broad (ca. 2.5 G peak-to-peak) and, although fine structure was evident, this could not be satisfactorily analyzed. The poor resolution was because of the low concentration of [7] produced, which, in turn, arose from the limited amount of substrate available; see Experimental. On the basis of y-CH,-proton couplings in other radicals similar to [7] which have been investigated under conditions analogous to those here, e.g., .CH,C(CH&X with -X equal to -OH, -NH:, and -COOH (IO, 15, 16a), it is likely that this fine structure arises, at least in part, from the y-CH, protons. However, it cannot be ruled out that there are other contributing effects, e.g., a small y-CN-nitrogen coupling. Such y-CN-nitrogen couplings are rare, the only known case seeming to be in HOCHCH,CN, generated from hydracrylonitrile in the TiCI,-H,O, system under reaction conditions essentially as employed here and for which a:: equals ca. 0.40 G (26).

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NITRILES

Succinonitrile, NC(CH,),CN, gave only one radical, NCCH$HCN, [8] (5). Adiponitrile, NC(CH,),CN, produced two radicals. The major radical, NCCH$H(CH&N, [9], exhibited a triplet (1:2: 1) of triplets (1 : 2: 1) of doublets (1: 1). The two triplet splittings were about the same size and appear not to be assignable to specific /GCH2 protons. Although only seven lines were completely resolved, the spectrum from the minor species, NCCH(CH2)&N, [lo], was successfully analyzed as a triplet (1 :2: 1) of doublets (1: 1) of triplets (1 : 1: 1). The data for the cyanoalkyl radicals described so far are given in Table 1 together with those for related radicals taken in this laboratory (17). TABLE

1

DATAFORUNSUBSTITUTEDCYANOALKYL

RADICALS"

Coupling constants, gauss Radical

X-H

[l], *CH,CN [2], .CH&H2CN [3], CH,CHCN

20.88 22.44 19.65 22.00 21.90

[41, .CH,(CH,),CN

[5], CH,CHCH,CN [6], CH$.ZH2CHCN .CH,CH(CH,)CN” (CH&CNd,

’

[7], .CH#Z(CH&CN” [S], NCCHZCHCN [9], NCCHZCH(CH2)2CN [IO], NCCH(CH&CN

19.90 22.50 22.30 20.66 22.14 20. I 1

B-H 26.57 23.05 26.82 22.79b, 25.31' 24.98 24.59 20.85 23.77 23.64h, 24.58" 23.22

N

g-Value

3.58

2.0029 2.0027 2.0030 2.0027 2.0028

3.48

3.489 -

2.0030 2.0026 2.0030 2.0025 2.0030 2.0026

3~52~

2.0329

3.54 3.44

Qp,

0.850 0.940 0.781 0.864

gauss

24.6 23.9 25.2 25.3 25.7

25.9

4 At 25 & 2°C; the maximum uncertainty of the u-values is estimated to be 0.1 G and that of the F-values, 0.0001. The p&values are calculated on the assumption that d(-CN), &CH,CN), d(-CH3), and d(-CH,CH,) equal 0.150,0.060,0.08 1, and 0.090, respectively, and are used to calculate the QgH-values; see Discussion. Throughout this report all u- and Q-values are given as absolute quantities. h a;!;. Cl13 c a/,.,, d All experimental data were taken by S. E. Richard (see Ref. (f 7)) in essentially the same way as those for the other radicals in this table. lsobutyronitrile was the substrate. The a-values are considered more reliable than those for this radical reported by Pearson er al. (I, 5). e Many literature data for this radical have been tabulated elsewhere (17). s Other hyperfine splittings were evident but could not be fully analysed; see Results. g On the basis of its size, assigned to a;.\. h It does not appear possible to assign these couplings to specific /$CHZ protons.

II. Substituted Cyanoalkanes Some of the radicals formed from the 2-hydroxynitriles showed a small, pHdependent B-OH-proton coupling. At low pH values, it was absent because of rapid

258

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AL.

proton exchange (18) and, as the pH was progressively raised, the coupling appeared and grew to a constant, maximum value (IX). Such fi-OH-proton couplings are well known in similarly structured radicals (19), e.g. HOCHCOOH and CH&OH)COOH W). Glycolonitrile, HOCH,CN, gave a single radical HOCHCN, [ll], its spectrum at the high pH limit being a doublet (1: 1) of triplets (1: 1 : 1) of small doublets (1: 1) (5). Lactonitrile, CH,CH(OH)CN, produced two radicals. The major radical was CH&OH)CN, [12], the spectrum at high pH values consisting of a quartet (1: 3 : 3 : 1) of triplets (1: 1: 1) (8) of small doublets (1: 1). Besides the lines attributed to [12], only two others could be resolved. Nevertheless, from line-intensity distortions in the spectrum of [12] at high pH values, it was clear that another species was present. These distortions and resolved lines appear to arise roughly from a triplet of doublets that is assigned to .CH,CH(OH)CN, [13]. However, only the quantity (2~7:~2+ a;!$) for [13] could be accurately determined. It was hoped that the spectrum of [13] could be more readily observed under low pH conditions where that of [12] would be simplified by the collapse of the OH-proton splitting (18). However, as the pH was progressively lowered, the signal intensity of both radicals fell, that of [13] reaching noise level before any additional lines could be resolved. 2-Hydroxyisobutyronitrile, (CH,),C(OH)CN, yielded only .CH,C(CH,)(OH)CN, [14], the spectrum consisting of a triplet (1:2: 1) of small quartets (1 : 3 : 3 : l), the latter being assigned to the y-CH, protons. Cyunoacetic acid, HOOCCH,CN, gave only HOOCCHCN, [15] (5). Methyl cyunoacetute, CH,OOCCH&N, produced such a complex absorption that only the predominant component could be successfully analyzed. This consisted of a triplet (1 :2: 1) of small triplets (1:2: 1) and was attributed to .CH200CCH2CN [16]. The TABLE DATA

FOR SUBSTITUTED

2

CYANOALKYL

RADICALS“

Coupling constants, gauss Radical [II], HOtHCN [12], CH&OH)CN

a-H

,fI-CH

N

18.10

17.88 -c

3.37 3.50

-

-c

-

[13], .CH,CH(OH)CN [14], .CH,C(CH,)(OH)CN

22.45

-

[El, [16], [17],

19.81

-

3.08

20.77 22.06

24.05

-

HOOCCHCN .CH200CCHZCN

.CH,CH,OOCCH,CN

Other

g-Value

d-c

Q?, gauss

1 .94b 2.0034 l.lOb 2.0033

0.610

-

1.02” .165’ -

0.789 -

25.1 -

2.0024 2.0024 2.00335 2.0028 2.0026

0.664

27.3

0 At 25 + 2°C; the maximum uncertainty of the a-values is estimated to be 0.1 G and that of the g-values, 0.0001. The &-values are calculated on the assumption that A(-CHB), A(-CN), and d(-COOH) are 0.081, 0.150, and 0.072, respectively, and that d(-OH) is 0.219, as calculated from ai. of [12], and are used to calculate the Qg”-values; see Discussion. b a:.:; this is the maximum value observed. ~~ecause of overlap of this spectrum with that of the major radical [12], only the quantity (2&F + &,), found to be 6.0 G, con’d be determined; see Results. d ay. C”* . * aa.”

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small triplet splitting was assigned to the 6-CH2 protons by analogy with results for related radicals, e.g., . CH200CCH, (9). EtZzyZcyanoacelate, CH&H,OOCCH&N, likewise yielded a complicated pattern of lines of which only the predominant component could be reliably analyzed. This was made up of a triplet (1:2: 1) of triplets (1 :2: 1) consistent with .CH&H,OOCCH,CN, [17], the two triplet splittings being assigned by comparison with those in similar radicals, e.g., .CH2CH,00CCH3 (9). The spectrum from each ester substrate had many additional lines of low intensity. For neither substrate could these lines be reliably analyzed even with, in the case of ethyl cyanoacetate, knowledge of the coupling constants for CH3CH200CCHCN (5). These results for the two esters are about as expected on the basis of an analogous study (9) of methyl acetate and ethyl acetate as substrates in the TiCl-H,O, system, bearing in mind the deactivating effect of the -CN group. Table 2 presents the results for the radicals observed with the substituted cyanoalkane substrates. DISCUSSION

For a given substrate, the TiCI,-H202 method produced a spectrum generally more complex than that found by the photo-flow method employing H,O, as the radical source (5). This is because, by the latter method, hydrogen-atom abstraction from carbon tended to take place almost exclusively from the carbon to which a -CN group is attached; whereas, by the former method, such abstraction occurred, with the possible exceptions of the cyanoacetates, from the carbons at every position. Furthermore, for a substrate with two or more nonequivalent carbon hydrogen-atomabstraction positions, the apparent selectivity of hydrogen-atom abstraction from carbon observed by the TiCI,-H202 method was predictable on the reasonable assumption (3,9, 10, 1.5) that -CN and -CO-O- groups are strongly deactivating and alcoholic -OH groups, strongly activating. Of course, other molecular characteristics of substrates will exert their influence on this apparent selectivity (9). Nevertheless, these three group-characteristics are roughly supported by the observed dependence of overall substrate reactivity on molecular structure; see Experimental. The scope of the present data and those obtained by the H,O,-photo-flow method (5) do not allow their kinetic differences to be unambiguously reconciled. The TiCI,H,O, reaction system is clearly kinetically complex (7, 9, 20, 21). Also, because of the relatively high concentration of H,O, typically employed, ca. 0.5 M (5) vs 0.04 M in the present work, the H,O,-photo-flow reaction system may not be free of relevant kinetic complications (22). Differences between the H,O,-photo-flow and TIC],-H,O, methods with respect to the relative proportions of the radicals observed with the use of a given substrate are well known (23) and continue to draw attention (22). On the other hand, in general, the spectroscopic data for the radicals in Tables 1 and 2 agree fairly well with the corresponding results for the same species when available in the literature (5,14,24-26), provided allowance is made for likely so vent and temperature effects (27-29). The deactivating effect of the -CN group on the overall reactivity and the selectivity of hydrogen-atom abstraction in the case of the nitriles was similar to that noticed for the -COOH group in analogous studies of carboxylic acids (9-11). This similarity has been already pointed out and reasonably interpreted in terms of these groups being

260

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known to exert about the same inductive effect (9,30). Where data exist for structurally analogous aliphatic nitrile and carboxylic acid radicals (31, 32), this similarity of the -CN and -COOH groups extends to both the a- andg-values. For example, theg-value trend in the series [4], [5], and [6] is like that in the analogous carboxylic acid radicals (31); and in [12] and the corresponding carboxylic acid radical (IO), respectively, particularly low ai-%-values, 17.9 and 17.1 G, and especially large &\-values, 1.1 and 2.0 G,’ are observed. However, since these EPR data depend on a number of factors, such as inductive, resonance, and geometrical effects (33-35), this extension is only crudely satisfactory, e.g., the d-values for -COOH and -CN, based on data taken in aqueous solution under comparable conditions, are reported to be 0.072 and 0.148, respectively (34). For a radical with a -CH3 group at the LY-Cthe equation

HI may be assumed, where plec is the n-electron spin density at the cl-C and QscH3 is 29.3 G, provided isotropic averaging occurs (34). From Eq. [l] and the data in Table 1 for aF-2 in [3], [5], and (CH&CN, pzmcis 0.787, 0.864, and 0.712, respectively. Also, with d(-CH3) equal to 0.081 (34) and these &,-values, d(-CN) is 0.144 and 0.157 in [3] and (CH,),&N, respectively, and d(-CH,CN), 0.060 in [5]. Not surprisingly (34), the last value is less than the other two. The average of these d(-CN) estimates, 0.150, agrees with the most comparable literature report, viz., 0.148 (34) which is based on the a:-?-values for HOCH$(CH,)CN, CH&HCOOH, and HOCH&CH,)COOH at ca. 15°C in aqueous solution (35). In contrast, Brumby (36) gives d(-CN) as 0.176. This discrepancy is not unexpected since his value is based on $2, (CH,),&N at 20°C in benzene (6, 17, 25). In Table 1, it is assumed that d(-CN), d(-CH,CN), d(-CH,), and d(-CH,CH,) are 0.150, 0.060, 0.081, and 0.090, respectively, to calculate & and, hence, Qz” in the equation (37,38) a,“.“H= Q”,” &

PI

for all possible radicals. The value of d(-CH,CH,) cited is calculated here from ~$3, CH,CH&HCH, in butane at -80°C (36,39) and Eq. [l] with d(-CHJ equal to 0.08 1. Different d(-CH,CH,)-values may be derived if other ai-$values are taken instead (36, 40), but their use does not seriously change the conclusions here. The missing entries for pzmcand Q”,” in Table 1 may be dealt with as follows. In all . CH,X radicals, -X not -CN, the data for a:-: fall within a narrow range, 22.0-22.5 G, and it may be reasonably assumed that all d(-X)-values are about equal (34, 35). Hence, & and CHin [4], .CH2CH(CH3)CN, and [7] will be about as in [2]. Similar arguments can QH be applied to the two other radicals in Table 1 for which no pi-c- and QC,“-values are given, [9] and [lo], to show that ~2.~ and QE” in [9] and [lo], respectively, will be about as in [5] and [3] or [6] It will be noted in Table 1 that QG” is 23.9-25.9 G. There seems no obvious trend in these Q$” estimates, in contrast to the findings for some other similar groups of structurally analogous species (9, 39). The only other set of reported a-values for 1 It seems that the dependence of this coupling on pH was not investigated (IO), so this value may not be the maximum.

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comparable c(-CN radicals in aqueous solution appear to be those in the radical-pair series YCH,C(CH,)CN/ YCH,CHCN having Y- equal to HO-, CHX-, NH2-, and HOCH,- (3.5, 4/-43). By using Eqs. [I] and [2], assuming d(-CHJ equals 0.081, and employing the a;-%- and a~M’&valuesfor each pair, QsH may be calculated to be 25.6, 27.3, 24.6, and 25.5 G for Y-equal to HO-, CH,-, NH*-, and HOCH,-, respectively. Again, there seems to be no obvious trend. Extension of this d-function treatment to the a-values for [ll], [12], and [15] in Table 2 gives Q’,” as 27.3 and 25.8 G for [ll] and [15], respectively, in calculating the latter, A(-COOH) being taken as 0.072 (34). These Q$H-valuesfall in the range described for the a-CN radicals already discussed. However, the validity of this treatment when applied to the [11]/[12] radical pair seems limited since both -OH and CN are strongly perturbing x-substituents (34) and, in addition, cz-CH, and -OH groups would be expected to tend to favor deviations from planarity at the 2-C (44). In Table 2, the missing entries for & and Q kH in [14], [16], and [17] may be estimated as follows. Radicals [14] and [17] are clearly structurally analogous to the other hydrocarbon-like radicals in Table 1 and, not surprisingly, their &‘-values fall within the range 22.022.5 G previously mentioned for such species. Consequently, their QC,H-valuesshould be ca. 23.9 G. Radical [16] has no structural analog in Table 1. However, it is similar to sCH,OOCCH, (9) and, hence, should have a Qs”-value of ca. 22.8 G (9). The only nitrogen couplings observed were readily assigned to the a-CN group since they are about the size of such couplings already reported for the same or related c(-CN radicals in the liquid phase (45). The azN-values in Tables I and 2 fall within a narrow range, 3.4-3.6 G, except for [15] where uN CN is 3.1 G. It would be useful to reliably establish a connection between azN and p,“.c in these radicals. For example, Rieger and Fraenkel (46) suggest that, for a series of aromatic nitrile anions, a$” is approximately proportional to pk. If this were the case for the radicals here, then ug” would be expected to be related to p’j.c. Unfortunately, it does not appear possible to establish such a connection because, in Table 1, although most of the relevant p;*cvalues are reasonably well established, the ugN-range is small; and in Table 2, although the ukN-range is larger, the &-values are generally not as reliably known. The size of uGN, [15], is noteworthy since it is clearly appreciably smaller, for example, than that of a:” in [3] and [6], although [3], [6], and [15] have about equal &-values. Few other uzN-values for aliphatic c(-CN radicals in solution have been reported to be outside the range 3.4-3.6 G. The only available data seem to be for CH,CH,OOC~HCN (5), CH,COC(CH,)CN (47), .CH(CN), and CH,COCHCN (47), uz” being 3.06 + 0.03,’ 3.0,3 2.9,4 and 2.8 G,3 respectively. The first uc$value is about equal to that in [15], as might be expected (9, f7,34). The other three are for radicals structurally similar to [15] but, on the basis of the d-function approach, having pz-,-values smaller ’ Taken in t-butyl alcohol at 34.5”C. 3 Taken at room temperature in aqueous solution (47). 4 Radical .CH(CN)2 was studied by using malononitrile as substrate and following procedures similar to those given in the Experimental section. As expected, this compound proved oflow reactivity so that even when both streams were saturated with substrate, ca. 2 M, the spectrum showed a noticeable contribution from the inorganic species (7) and high modulation amplitudes were necessary. The lines, a doublet (1: l), 19.6 G, of quintets (1: 2: 3 :2: I), 2.9 G, were broad (ca. 1.5 G peak-to-peak), the maximum error limit for these a-values being ca. 0.2 G.

262

SMITH ET AL.

than that in [W, since A(-COCHJ) may be calculated to be 0.164,5 i.e., greater than A(-COOH) and A(-COOCH,CH,) (34). Radical [ll] and [12] are interesting since they each have a rather large pH-dependent OH-proton coupling of maximum value 1.9 and 1.1 G, respectively. Restricted rotation about the C-O bond in 2-hydroxynitriles, such as the parents of [ll] and [U], is well documented (48) and arises from the electronic repulsion of the oxygen lone-pairs with the n-electron cloud of the -CN group and, perhaps, to a lesser extent from formation of a weak intramolecular hydrogen bond between the OH proton and the n-electron cloud of the -CN group. The EPR evidence for [ll] and [12] is also consistent with restricted rotation about the C-O bond. The P-OH-proton coupling is generally given in a form (49-53) similar to a::

= -A(sin2 0) + B(cos2 0)

[31

where 0 is the dihedral angle between the 2p,-orbital and the O-H bond and ( ), the time-averaged value. Term -A(sin20) represents the spin polarization of the O-H a-bond by the unpaired spin density on oxygen and will be a minimum when the OH proton lies in the nodal plane of the 2p,-orbital, 0 = 90”; and term B(cos2 0) symbolizes the hyperconjugative interaction of the OH proton with the unpaired spin on the U-C and will be maximum when the OH proton eclipses the 2p,-orbital, 0 = 0’. The preferred conformations of [ll] and [12] should have the OH proton in the nodal plane of the 2p,-orbital and cis to the -CN group. This is because the maximum stabilization of these radicals, resulting from delocalization of the unpaired spin onto the oxygen, should occur when the nodal plane of the 2p,-orbital and C-O-H plane coincide, since in this conformation the oxygen lone-pairs will have maximum overlap with the 2p,-orbital. Also, the cis isomer should be favored over the tram isomer because, in the latter, the electronic repulsion between the oxygen lone-pairs and the n-electron cloud of the -CN group would be at a maximum; and, in the former, further stabilization would be possible by the formation of an intramolecular hydrogen bond. The conclusion that the O-H bond lies in the 2p,-orbital nodal plane and, thus, the interaction of the OH proton arises via a spin polarization mechanism agrees with the literature on P-OH-proton couplings (49-54). One reason that aj& [12], is less than a:;, [ll], might be because of increased torsional motion in 1121 relative to that in [ll], this arising from the c(-CH, group in the one being more bulky than the cc-H in the other. Another possible contributing effect might be that these radicals are not planar about the a-C (44) and replacement of a-H in [ll] by a -CH, causes an increased departure from planarity (44~). NonCH3,17.9 G (44). If nonplanarity is planarity in [l2] is supported by its low value of aS-,, significant in these radicals, then Eq. [3] would have to be modified to include such changes in the radical geometry about the cc-C. Molecular orbital calculations are now being carried out in the hope of gaining extra insight into this and related problems. C”3 for CHBCOeHCHx in aqueous solution, 22.5 G, Eq. [I], and 5 Based on the reported (8) value a,+” the assumptionthat d(-CH3) is 0.081. As expected,this value of d(-COCH3) agrees with that given (34) for A(-COCH,CH,), 0.162. An indication of the validity of the d-function treatment here is shown as follows: by assumingd(-CH3), d(-CN), and &COCK) to be 0.081, 0.150, and 0.164, respectively,pzecin CH,COc(CH,)CN may be calculatedto be0.653 ; whereas,basedon the measured &.H,3-value in this radical, 21.8 G (47), and Eq. [II, this p:.c-valueis 0.744.

RADICALS

FROM

ALIPHATIC

NITRILES

263

ACKNOWLEDGMENTS In the early stages of this work, Mr. S. E. Richard gave assistance. The hydrogen peroxide used was a gift from FMC Corp., Inorganic Chemical Division. These contributions are much appreciated. REFERENCES 1. J. T. PEARSON,P. SMITH, AND T. C. SMITH, Can. J. Chem. 42,2022 (1964). 2. For example, P. B. AYSCOUGH, “Electron Spin Resonance in Chemistry,” Methuen, London, 1967. 3. R. 0. C. NORMAN AND B. C. GILBERT, A&an. Phys. Org. Chem. 5, 53 (1967); R. 0. C. NORMAN, Chem. Sot. Spec. Publ. NO. 24, 117 (1970). 4. P. SMITH, R. A. KABA, AND P. B. WOOD, J. Phys. Chem. 78,117 (1974). 5. R. LIVINGSTON AND H. ZELDES,J. Magn. Resonance 1, 169 (1969). 6. P. SMITH, D. W. HOUSE, AND L. B. GILMAN, J. Phys. Chem. 77,2249 (1973). 7. For example, G. CZAPSKI, A. SAMUNI, AND D. MEISEL, J. Phys. Chem. 75,327l (1971). 8. A. L. BULEY, R. 0. C. NORMAN, AND R. J. PRITCHETT,J. Chem. Sot. B 849 (1966). 9. P. SMITH, J. T. PEARSON,P. B. WOOD, AND T. C. SMITH, J. Chem. Phys. 43,1535 (1965). 10. W. T. DIXON, R. 0. C. NORMAN, AND A. L. BULEY, J. Chem. Sot. 3625 (1964). II. H. TANIGUCHI, K. FIKUI, S. OHNISHI, H. HATANO, H. HASEGAWA, AND T. MARUYAMA, J. Phys. Chem. 72,1926 (1968). 12. P. SMITH AND P. B. WOOD, Can. J. Chem. 45,649 (1967). 13. P. SMITH AND P. B. WOOD, 151st National Meeting of the American Chemical Society, Abstract N 90, Pittsburgh, 1966. 14. (a) H. G. BENSON, A. J. BOWLES, A. HUDSON, AND R. A. JACKSON, Mol. Phys. 20, 713 (1971); (b) A. HUDSON AND R. A. JACKSON, Chem. Commun. 1323 (1969). 15. W. T. DIXON AND R. 0. C. NORMAN,.~. Chem. Sot. 3119 (1963). 16. (a) R. A. KABA, Ph.D. dissertation, Duke University, Durham, N.C., 1974; (b) P. SMITH, R.A. KARA, AND J. T. PEARSON,J. Mugn. Resonance 17,20 (1975). 17. P. SMITH AND R. D. STEVENS,J. Phys. Chem. 76,314l (1972). 18. For example, H. FISCHER, Mol. Phys. 9, 149 (1965); R. POUPKO AND A. LOWENSTEIN,J. Chem. Sot A 949 (1968). 19. For example, Refs. (9, IO, 15, 18); P. SMITH, J. T. PEARSON,AND R. V. TSINA, Can. J. Chem. 44, 753 (1966). 20. B. C. GILBERT, R. 0. C. NORMAN, AND R. C. SEALEY, J. Chem. Sot. Perkin II 2174 (I 973). 21. A. SAMUNI, D. MEISEL, AND G. CZAPSKI, J. Chem. Sot. Dalton 1273 (1972). 22. For example, G. CZAPSKI, J. Phys. Chem. 75,2957 (1971). 23. For example, Refs. (4, 12, 13). 24. P. SMITH, R. D. STEVENS,AND L. B. GILMAN, unpublished results. 25. P. SMITH, L. B. GILMAN, AND R. A. DELORENZO, J. Magn. Resonance 10,179 (1973). 26. See also L. B. GILMAN, Ph.D. dissertation, Duke University, Durham, N.C., 1975. 27. For example, Refs. (5, 17, 25). 28. For example, H. FISCHER,J. Phys. Chem. 73,3834 (1969). 29. For example, R. LIVINGSTON AND H. ZELDES,J. Chem. Phys. 44,1245 (1966). 30. For example, J. MARCH, “Advanced Organic Chemistry: Reactions Mechanisms, and Structure,” Chap. 1, McGraw-Hill, New York, 1968. 31. H. TANIGUCHI, H. HASUMI, AND H. HATONO, Bull. Chem. Sot. Jap. 45,338O (1972). 32. For example, Refs. (9-11, 17; 24-26). 33. For example, Refs. (2,3,17); A. J. DOBBS, B. C. GILBERT, AND R. 0. C. NORMAN, J. Chem. SOC. A 124 (1971). 34. H. FISCHER,2. Naturforsch. A 20,428 (1965). 35. H. FISCHER,2. Naturfbrsch. A 19,866 (1964). 36. S. BRUMBY, 2. NaturJbrsch. A 25, 12 (1970). 37. J. E. WERTZ AND J. R. BOLTON, “Electron Spin Resonance: Elementary Theory and Practical Applications,” Chap. 6, McGraw-Hill, New York, 1972. 38. H. M. MCCONNELL AND D. B. CHESNUT,J. Chem. Phys. 28,107 (1958). 39. R. W. FESSENDENAND R. H. SCHULER,J. Chem. Phys. 39,2147 (1963).

264 40. 41. 42. 43. 44.

45. 46. 47. 48.

49. 50. 51. 52. 53. 54.

SMITH

ET

AL.

He/v. Chim. Actu 56, 1575 (1973). Chem. Ser. 91, 125 (1969). H. FISCHER AND G. GIACOMETTI, J. Polym. Sci. Part C 16,2763 (1967). C. CORVAJA, H. FISCHER, AND G. GIACOMETTI, Z. Phys. Gem. (Frankfurt am Main) 45, 1 (1965). (a) R. W. FESSENDEN, J. Phys. Chem. 71, 74 (1967); (b) Ref. (33); (c) A. J. DOBBS, B. C. GILBERT, AND R. 0. C. NORMAN, J. Chem. Sot. Perkin ZZ 786 (1972); (d) G. P. LAROFF AND R. W. FESSENDEN, J. Chem. Phys. 57,5614 (1972). For example, Refs. (I, 5, 6, 8, 14a, 16, 17, 25, 35, 4/-43); H. HEFTER AND H. FISCHER, Ber Bunsenges. Phys. Chem. 73,633 (1969). P. H. RIEGER AND G. K. FRAENKEL, J. Chem. Phys. 37,2795 (1962). G. A. RUSSELL AND J. LOKENSGARD, J. Amer. Chem. Sot. 89,5059 (1967). For example, A. ALLERHAND AND P. VON R. SCHLEYER, J. Amer. Chem. Sot. 85, 866 (1963); M. OKI AND T. YOSHIDA, Bull. Chem. Sot. Jup. 44, 1336 (1971); N. MORI, S. OMURA, H. YAMAKAWA, AND Y. TSUZUKI, Bull. Chem. Sot. Jap. 38, 1627 (1965); G. L. BENDAZZOLI, F. BERNARDI, AND P. PALMIERI, J. Chem. Sot. Faraday II 69, 579 (1973); G. CAZZOLI, D. G. LISTER, AND A. M. MIRRI, J. Chem. Sot. Faraday 1169,569 (1973). P. D. SULLIVAN, J. Phys. Chem. 73, 2790 (1969); 75, 2195 (1971). P. J. KRUSIC, P. MEAKIN, AND J. P. JESSON, J. Phys. Chem. 75,3438 (1971). P. B. AYS~OUGH AND R. E. D. MCCLLJNG, Mol. Phys. 20,35 (1971). P. B. AYSCOUGH, M. C. BRICE, AND R. E. D. MCCLUNG, Mol. Phys. 20,41 (1971). A. T. BULLOCK AND C. B. HOWARD, Mol. Phys. 27,949 (1974). W. DERBYSHIRE, Mol. Phys. 5,225 (1962). For example, K. TAKAKURA

Ref. (28); H. Paul and AND B. RANBY, Advan.

H. FISCHER,