2122
J. Phys. Chem. 1994,98, 2722-2725
EPR Spectra of Iminoxyl Radicals in Restricted Media: Direct Evidence for the Assignment of 2 and E Isomers Prasad S. Lakkaraju,+Junxiong Zhang, and Heinz D. Roth* Department of Chemistry, Rutgers University, Wright-Rieman Laboratories, New Brunswick, New Jersey 08855-0939 Received: September 7, 1993; In Final Form: January 14, 1994"
A single geometric isomer of benzoyliminoxyl radical (1) is formed upon inclusion of 1-phenylpropa- 1,2dione-2-oxime (2) in the pores of a pentasil zeolite (ZSM-5). The EPR powder pattern observed in this matrix shows unambiguously that the principal axes of the 14N and 'Hdipolar hyperfine couplings, ANJand A H J , respectively, are nearly perpendicular to each other. A comparison of the directions expected for the two geometric isomers identifies the spin Hamiltonian parameters as those of the Z isomer. The direct evidence contained in the dipolar hyperfine interactions (All) is supported by indirect evidence derived from long-range isotropic hyperfine interactions (Aim).
Iminoxy radicals have been extensively investigated'-10 as interesting a-type radicals characterized by relatively large nitrogen isotropic hyperfine coupling constants (hfc) and as early examples of geometric isomerism in paramagnetic species. They can be prepared by a variety of methods, including chemical oxidation, photochemical oxidation, and X-or y-radiolysis. The existence of two geometric isomers is firmly established by virtue of the observation of distinct EPR spectra with sufficiently different magnetic parameters. The assignment of the different magnetic parameters to either the Z or E isomer has been based on (INDO) calculations11 or such indirect evidence as studies on radicals derived from sterically hindered 0ximes.~~J3These assignments have not been without controversy, and direct experimental evidence is thus far lacking. We report the generation of a single geometric isomer of benzoyliminoxyl radical (1) upon inclusion of 1-phenylpropa1,2-dione-2-0xime (2)in the pores of a pentasil zeolite (Na-ZSM,O H
5).14-18 The EPR powder pattern observed in this matrix shows unambiguously that the principal axes of the 14Nand 1H dipolar hyperfine couplings, AN,IIand A H J ,respectively, are nearly perpendicular to each other. A comparison of the directions expected for the two geometric isomers, identifies the spin Hamiltonian parameters as those of the Z isomer. The direct evidence contained in the dipolar hyperfine interactions (All) is supported by indirect evidence derived from long-range isotropic hyperfine interactions (Aim). The geometry of the iminoxyl isomers can be derived from the relative orientation of the principal axes of the dipolar hyperfine couplings for the nitrogen and the azomethine hydrogen. The dipolar hyperfine coupling of the azomethine lH is composed of two contributions, due to the interaction of the nuclear spin with the unpaired electron spin density a t the N and 0 centers. The relative magnitudes of the individual contributions and the direction of the resulting principal dipolar couplings, AH,II and of the azomethine lH can be derived with the help of a f Current address: Chemistry Department, Upala College, Prospect Street, East Orange, NJ 07019-1186. * Abstract published in Advance ACS Abstracts, March 1, 1994.
0022-3654/94/2098-2122%04.50/0
2-isomer AH,I/
E- iso me r
ANJ
Figure 1. Schematic representation of the Z and E isomers of
benzoyliminoxylradical (1). The principal axesof the 14Nand azomethine lH dipolar hyperfine interactions are indicated as solid arrows, whereas the individual contributions to the 'H dipolar hyperfine interaction due to the spin densities on N and 0 are shown as dashed arrows. The angles between the parallel components AN.1 and A H Jare 89.3' and 54.1', respectively, for the 2 and E isomers. point-spin model developed by Hutchison and Pearson19 and previously applied to various triplet m e t h y l e n e ~ . ~The ~ ~ following *~ parameters are used in the adopted model. The bond lengths and angles used in this model were obtained from an ab initio calculation of the methyl-iminoxyl radical at the HF/6-31G* level of theory.2' The principal axis of the 14N dipolar coupling lies in the sigma plane along the bisector of the C-N-0 function; this assignment rests on single-crystal EPR data for diphenyl- or acety1,methyl-iminoxyl, which identify the principal axes u n a m b i g u ~ u s l y .The ~ ~ ~distribution ~~ of electron spin densities between the 0 and N centers has been assigned values ranging from 0.61 vs 0.39 to 0.55 vs 0.45;262* we have chosen values of po = 0.6 and p~ = 0.4. The spin density on oxygen is distributed equally between the two lobes of the pure p orbital; the interaction of the resulting point spins are significantly different for the two isomers of the iminoxyl. For the Z isomer, po can be approximated as a point spin a t the center of the oxygen nucleus. For the E isomer, two point spins of magnitude pol2 at distances, d = 0.55 A, from the center of the 0 nucleus are chosen. The spin density on nitrogen, p ~ is, represented as two point spins in the two lobes of the sp2 hybrid orbital in the ratio 0.3:0.7. This value was derived by Hutchison and Pearson forthe a-spin density of fluorenylidene.19 Thevalues of the coordinates for the point spins were determined by fitting the model to the zero-field splitting parameter, D, of triplet nitrene,Z9 according to the methodology of Wasserman and colleagues;30 this yields a distance, d = 0.59 A, from the center of the nitrogen (Figure 1). 0 1994 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 98, No. 1 I, 1994 2723
- 1
V~8.83715GH~
100
3388.20
t
I
_...
iL
I
'bII
Figure 2. X-band EPR spectrum of benzoyliminoxyl radical (1) sequesteredinpentasilzeolite (Na ZSM-5). Thespectralfeaturesassigned = 44.1 to the parallel and perpendicular components, gli = 2.0030,AN,II G, AH,! = 8.8 G, and g l = 2.0068, AN,^ = 29.1 G, AH.^ = 2.9 G, respectively, are identified.
Given these parameters, the vectors of the individual contributions are given by the dipolar Hamiltonian, %, formulated for a nucleus, n, as
where 8 is the angle between the external magnetic field and the principal axis of the dipolar interaction; g,,and g, are the nuclear and electron g factors, respectively; 8, and j3, are the nuclear and Bohr magnetons, respectively; and I and S are the nuclear and electron angular momentum operators, respectively. The intrinsic value, Bll, of the dipolar hyperfine coupling, A,,,II (for 8 = 0), can be expressed as
where pi is the point electron spin and ri is the distance between point spin and nucleus. The direction of the principal component for the azomethine 1H is then obtained as the weighted vector sum of the dipolar interactions with the various electron point spins according to eq 2. These considerations result in two significantly different orientations of the principal IH dipolar component for the 2 and E isomers, respectively. For the 2 isomer, the model yields an angleof 89.3O, Le., the principal axes of the 1H and 14N hyperfine tensors are essentially perpendicular to each other (Figure 1). In contrast, an angle of 54O is indicated for the E isomer. Hence, EPR spectra of iminoxyl radicals in frozen solutions or single crystals are expected to allow the unambiguous identification of the isomers. The EPR spectrum of the benzoyliminoxyl radical, 1,generated upon inclusion of 2 in the pores of pentasil zeolite (Na-ZSM-5) shows the characteristic features of an axially symmetric powder pattern (Figure 2) with two overlapping 1:1: 1 triplets. In addition, the signals in the high-field region reveal clearly resolved doublets, with splittings of 2.9 and 8.9 G for the highest-field line and the adjacent feature, respectively. We attribute these splittings to the azomethine IH, since iminoxyl radicals without such a nucleus show features due only to g and AN ani~otropies.1~ In the lowfield region, the two triplets are overlapping, causing some
-
uncertainty in determining the line positions. The individual features of the powder spectrum can be assigned on the basis of the relative magnitudes of All and A L for the I4N and IH nuclei. These components are defined as
The isotropic I4N coupling, A N , ~due ~ , to the 2s unpaired spin density, is positive.3' The isotropic 1H coupling, AH,^^, arises by direct overlap and/or by polarization of the N-C and C-H bonds; this splitting is also expected to bepo~itive.~'Finally, the principal components of the dipolar hyperfine interactions, B,,,ll,are expected to the positive for both 14N and IH.3' Therefore, All > A l for both I4N and IH. These considerations lead to the conclusion that the AN,* component (with the smaller I4Ncoupling, AN,^ = 29 G) has the larger 'Hsplitting ( A H= 8.8 G), which must be assigned to A H J . Conversely, the AN,II component (with the larger I4N coupling, AN,II = 44 G) has the smaller IH splitting (AH= 2.9 G), which is assigned to AH,^. Accordingly, the principal components, AN,II and AH,II, of the dipolar hfc for 14N and IH, respectively, must lie essentially perpendicular to each other. In light of the anisotropic hyperfine interactions expected for the two geometric isomers of 1 (Figure l), the assignment of the powder spectrum unambiguously identifies the species sequestered in the zeolite as the 2 conformer. The unambiguous identification of the isomer preferred in the restrictiveenvironment of the zeolitemay lead to the identification of the geometric isomers represented in the solution spectra, if the anisotropic parameters derived from the powder spectrum could be related uniquely to the isotropic parameters observed in solution. The EPR spectrum of radicals generated by lead tetraacetate oxidation of 2 in benzene solution33 consists of resonance lines originating from two chemically distinct species, assigned to the isomeric 2- and E-benzoyliminoxyl radicals, 1 (Figure 3). The species observed in higher abundance is characterized by six resonance lines of equal intensity corresponding to a I4N ( I = 1) splitting of AN = 29.1G, and a IH ( I = I/') splitting, AH= 5.9 G. At high resolution, each of the six lines is split further into a doublet ( A = 0.26 G) of triplets ( A = 0.13 G; Figure 3, bottom right). The isomeric iminoxyl radical is present in much lower abundance. It also consists of six signals with couplings, AN = 31.4 G and A H = 27.6 G; each of the main signals is split further into 1:2:1 triplets (Figure 3, bottom left). The most significant difference between the major and minor isomers in solution lies in the magnitude of the isotropic hfcs for the azomethine 'H nuclei, viz., &,,,in = 27.6 G vs AH,maj = 5.9 G. On the other hand, the I4Ncouplings show lesser differences, viz.,AN,maj= 29.1 G v s A N , ~= ~ ,31.4G. , Theanisotropichyperfine components of the 2 isomer in the zeolite ( A N J= 44.1 G, A N , I = 29.1 G, and A H J= 8.8 G, AH,^ = 2.9 G) allow the assignment of approximate values to the corresponding isotropic hyperfine couplings, according to (5)
The isotropic hyperfine coupling, AN,^^^ = 34.1 G, calculated according to eq 5, is roughly compatible with either isomer. In contrast, the value of &,avg (4.9 G) is compatible with the major species AH,,^ = 5.9 G) but totally incompatible with the minor species (&,,,,in = 27.6 G). This comparison clearly identifies the major species observed in solution as the Z isomer. In view of the pronounced difference between the azomethine hyperfine couplings of the two isomers in solution, we suggest that this coupling can be used to differentiate the iminoxyl isomers. Accordingly, we identify any of three iminoxyls with azomethine
2724
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The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 N>O
3490.20
1 d
I
I
C6H5
C6H5
Z-syn
E-syn
Z-anti
E-anti
Figure 4. Molecular structures of the syn and anti conformers of Z - and E-benzoyliminoxyl radical (1). The approximate molecular dimensions, based on a b initio calculations on methyliminoxyl radical at the HF/ 6-31G* levelz1and on the standard geometry of the phenyl group,35are as follows: E-syn, 4.1 A x 8.9 A; Z-syn, 4.1 A x 8.6 A; E-anti, 5.3 A x 7.7 A; Z-anti, 5.3 A x 1.5 A.
Figure 3. X-band EPR spectrum of benzoyliminoxyl radical (1) generated by lead tetraacetate oxidation of 2 in benzene at ambient temperature." The top trace illustrates the presence of two isomers; both species appear as 1:l:l triplets of doublets. The major isomer has g = 2.0046, AN = 29.1 G, and AH = 5.9 G, whereas the minor species has g = 2.0042, AN = 31.4 G, and AH = 21.6 G . The downfield signal (A) of the minor isomer is expanded 13-fold (bottom, left), showing a 1:2:1 triplet ( A H= 0.45 G). Similarly, one signal (B)of the major isomer is expanded (24fold; bottom, right), showing a doublet (AH= 0.26 G ) of triplets (AH= 0.13 G ) .
IH couplings in the range 26.8 < AH< 27.9 G as the E isomers and any of six iminoxyls with azomethine IH couplings in the range 5.7 < AH < 6.2 G as the Z isomers.34 Model considerations suggest that each geometric isomer (Z and E) of 1may have two rotameric forms (syn and anti)differing
I
C6HS
I
C6H5
2-syn
E-syn
z-ant/
€-ant/
in the orientation about the C-C bond linking the C=N-0 function with the carbonyl moiety (Figure 4). Thesyn conformers are longer and narrower than the anti conformers and may be incorporated into the zeolite more readily. However, the zeolite does not appear to change the preferred geometry: the isomer observed in the zeolite is the predominant isomer in solution. In solution, the minor isomer is observable, even at 1 5 % the abundance of the major isomer, because of the narrow line width. In contrast, the broad lines of the anisotropic spectrum do not allow the observation of the minor isomer. Ultimately, the key to the assignments of Z and E isomers lie in EPR studies of iminoxy radicals derived from aldoximes in
single crystals, as the directions of the various tensor components can be established unequivocally from those studies. These studies are currently in progress in our laboratory. We note further that the procedure for assigning the configurational isomers of sigma type radicals, introduced in this paper, is general in scope. and can be applied, in principle, to other sigma type radicals, including substituted vinyl radicals.
Acknowledgment. Support of this workby theNational Science Foundation through Grant NSFCHE-9110487 andan equipment grant, N S F CHE-8912238, is gratefully acknowledged. We are indebted to Drs. V. Ramamurthy and D. Corbin of the duPont Co. for a generous sample of N a ZSM-5. References and Notes (1) (2) (3) (4)
Thomas, J. R. J . Am. Chem. Soc. 1964.86. 1446. Symons, M. C. R. Adv. Phys. Org. Chem. 1963, I , 333. Symons, M. C. R. Mol. Phys. 1971, 22,551. Norman, R. 0. C.; Gilbert, B. C. Adu. Phys. Org. Chem. 1967, 5,
83. ( 5 ) Gilbert, B. C.; Norman, R. 0. C. J . Chem. SOC.1968, 123. (6) Gilbert, B. C.; Gulick, W. M., Jr. J. Phys. Chem. 1969, 73, 2448. (7) Gilbert, B. C.; Malatesta, V.; Norman, R. 0. C. J. Am. Chem. Soc. 1971, 93, 3290. (8) Just, G.; Dahl, K. Tetrahedron 1968, 24, 5251. (9) Fischer, V.; Mason, R. P.Chem.-Biol. Interact. 1986, 57, 129. (10) Jezierski, A. Magn. Reson. Chem. 1989, 27, 130. (1 1) Chiu, M. F.; Gilbert, B. C.; Sutcliffe, B. T. J. Chem. Phys. 1972,76, 553. (12) Bethoux, M.; Lemaire, H.;Rassat, A. Bull. SOC.Chim. Fr. 1964, 1985. (13) Fox, W. M.; Water, W. A. J. Chem. Soc. 1965,4628. (14) The ZSM-5 zeolite contains active sites that may generate radical cations from substrates with E n p 5 1.6V.I5J6 We have shown that oximes can be converted to iminoxyl radicals upon inclusion in ZSM-5.17J* (15) Ramamurthy, V.; Casper, J. V.;Corbin, D. R. J. Am. Chem. Soc. 1991, 113, 594. (16) Casper, C. V.; Ramamurthy, V.; Corbin, D. R. J. Am. Chem. Soc. 1991, 113, 600. (17) Lakkaraju, P. S.;Zhang, J.; Roth, H. D. J . Chem. Soc., Perkin Tram. 2 1993,2319. (18) ESR samples are prepared by stirring 3-5 mg of the oxime with 70 mg of zeolite (Na-ZSM-5, thermally activated by calcination at 500 OC for 12 h and stored under argon) in 10 mL of 2,2,4-trimethylpentane for 2 h. The loaded zeolite is collected by filtration, washed with n-hexane, and dried under vacuum (0.001 Torr). (19) Hutchison, C. A., Jr.; Pearson, G. A. J . Chem. Phys. 1%7,47,520. (20) Hutton, R. S.;Roth, H.D. J . Am. Chem. SOC.1982, 104, 7395. (21) Lakkaraju, P. S.;Roth, H. D., unpublished results. (22) Miyagawa, I.; Gordy, W. J. Chem. Phys. 1959, 30, 1590. (23) Lin, T. S . J . Chem. Phys. 1975,63, 384. (24) Symons, M. C. R. J. Chem. SOC.1965,2276. (25) Fox, W. M.; Symons, M. C. R. J. Chem. SOC.A 1966, 1503. (26) Kurita, Y.; Kashiwagi, M.; Saisho, H.Nippon Kagaku Zasski 1965, 86, 578.
Letters (27) Muto, H.;Iwasaki, M. J. Chem. Phys. 1973,58, 2454. (28) Lin, T. S. J . Chem. Phys. 1975,63,384. (29) Dixon, R. N. Can. J. Phys. 1959, 37, 1171. (30) Wasserman, E.; Hutton, R. S.;Kuck, V. J.; Yager, W. A. J. Chem. Phys. 1971, 55, 2593. (31) Kurreck, H.; Kirste, B.; Lubitz, W. Electron Nuclear Double Resonance Spectroscopy of Radicals in Solution; VCH Publishers: New York, 1988; p 83.
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2725 (32) Carrington, A.; McLachlan, A. D. Introduction to Magnetic Resonance; Harper and Row Publishers: New York, 1967; p 111. (33) Iminoxy radicals are prepared by condensing thoroughly degassed benzene onto a solid mixture of oxime and lead tetraacetate at -78 O C . After warming the sample to room temperature to allow the formation of the iminoxyl radicals, the EPR spectrum is recorded either at room temperature or at 77 K. (34) Zhang, J.; Lakkaraju, P.S.;Roth, H.D. Unpublished results.
