J . Phys. Chem. 1984, 88. 4717-4723 WEIGHT RATIO TPWMATRIX
lo-z
Id'
10
100%TPO
i
I
E= 5 x 105 V/CM 0.6-
-
VK MATRIX
4717
The trend, however, is in the right direction. The lower ionization potential material is a trap in the higher I, matrix. As the concentration of TPD is increased to such levels where hopping among TPD molecules becomes feasible, the activation energy begins to decrease. Eventually, it reaches values that are typical for TPD in polycarbonate (dashed line, Figure 5). This also indicates that in this region, hole transport proceeds via TPD with no PVK participation.
Conclusions Hole transport has been measured in poly(N-vinylcarbazole) (PVK) doped with various amounts of N,N'-diphenyl-N,N'-bis% (3-methylphenyl)-[l,l'-biphenyl]-4,4'-diamine (TPD). Charge ;j0.3\ \ carrier mobilities in PVK are drastically reduced (by up to 3 orders \ of magnitude) by the addition of small amounts of TPD which, POLYCARBONATE by itself at high concentration, is a more efficient transport ma0.2MATRIX A, terial than PVK. At higher concentrations of TPD, exceeding I -0.2/1 ratio to PVK, the charge transport proceeds exclusively 0.1 via the additive with no participation of carbazole units in PVK. This behavior is interpreted on the basis of differences in ionization potentials of the two materials. While the lower I, material (TPD) L L I I L l l L l llllIIJ is easily oxidized by the cation radicals of the carbazole groups, IOZ0 IOZ1 5x102' the transfer of charge back to PVK is energetically difficult. Since TPD CONCENTRATION ( MOLECULES/CM3) the distance between molecules of the additive at low concenFigure 5. Hole transport activation energy at B = 5 X lo5 V/cm as a trations is too large for charge transfer to occur, TPD becomes function of TPD concentration in PVK (solid line) and polycarbonate a charge trap. This feature demonstrates the versatility and utility (dashed line). The transport activation energy for PVK is denoted by of amorphous organic systems in studies of trapcontrolled hopping the arrow. and transitions from trap-dominated hopping to hopping transport which is not easily achieved in inorganic systems. The charge drift relation employed to calculate I,, this difference is 0.5-0.6 eV. velocities can be controlled over a wide range by changing the This simplistic picture of correlating the trap depth with the concentration of a low-I, transporting additive. difference in Zp assumes that the ionization potential of an isolated molecule of TPD is independent of the environment, which is Registry No. PVK, 25067-59-8;TPD, 65181-78-4;NIPC, 1484-09-9; N-ETC, 86-28-2. probably not the case. That could account for the discrepancy.
-
0.4
3
'
L
L_i
Long-Range Proton Hyperfine Coupling in Alicyclic Nitroxide Radicals by Resolution-Enhanced Electron Paramagnetic Resonance Magdi M. Mossoba, Keisuke Makino, Peter Riesz,* Radiation Oncology Branch, Clinical Oncology Program, Division of Cancer Treatment, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20205
and Ray C . Perkins, Jr. Varian Associates, Palo Alto, California 94303 (Received: March 8, 1984)
The free radicals 4-R-2,2,6,6-tetramethylpiperidine1-oxy1 (R = OH (Tempol), NH2(Tempamine), H2 (Tempo), and 0 (Tempone)) and 2-R-5,5-dimethylpyrrolidine1-oxy1 (R = COOH,CH,,CD3,OH,and OD)have been investigatedin aqueous solutions at room temperature by EPR using 90' out-of-phase detection. In contrast to in-phase EPR, the present approach provides a means for further identificationof radicals which is particularly useful for species that are otherwise indistinguishable. The superhyperfinecoupling constants of y- and 8-nuclei in piperidine derivatives, which were obtained by computer simulation, are in agreement with those calculated from literature 'HNMR data observed for the same compounds and indicate that Ternpol and Tempamine are stable in chair conformations, while Tempo and Tempone are rapidly interconverting between two identical chair and twist conformations, respectively. The data obtained for the pyrrolidine-1-oxy1 derivatives containing a carboxy or methyl group at C2 were consistent with slightly puckered rings while the derivative with a hydroxy substituent was found to favor a deformed ring with a pucker at C2.
protons that are three or four u bonds away. In the present study, long-range coupling could be obtained for substituted piperidine*Member of NIH ESR Center.
(1) Perkins. Jr.. R. C.. oaoer oresented at the 5th International EPR Symposium, Denver, CO, 19ki2: Clarkson, R. B. Varian Instrum. Appl. 1979, 13, 4.
This article not subject to U S . Copyright. Published 1984 by the American Chemical Society
4718 The Journal of Physical Chemistry, Vol. 88, No. 20, 1984
b.
PIPERIDINE-1-OXYL
b.
Mossoba et al. due to interactions with y- and &protons. A large number of differently substituted pyrrolidine- 1-oxy1 radicals has been synthesized by the spin-trapping method using the nitrone 5,5-dimethyl-l-pyrroline-N-oxide (DMP0).I6 This procedure consists of converting short-lived radicals into stable 5,5-dimethylpyrrolidine- 1-oxy1 spin adducts by addition to the DMPO spin trap:
PYRROLIDINE-1-OXYL
harmonic of the modulation phase detector. The adopted CY, @, y, and 6 nomenclature is exemplified by the )~ structures of Tempo (2,2,6,6-tetramethylpiperidine- l - o ~ y l and OH-DMPO (2-hydroxy-5,5-dimethylpyrroline-l-oxyl).Highly
I
0.
0
TEMPO
OH-DMPO
I
0DMPO SPIN TRAP
I
0. SPIN ADDUCT
The spin-trapping method allows the stabilization and hence the detection and identification of R. radicals. A serious limitation arises with DMPO when spin adducts with different R groups give rise to the same hyperfine nitrogen and P-proton coupling constants.I6 In such cases the presence of well-resolved SHF structure in the observed spectra becomes essential to distinguish between spin adducts that would otherwise appear identical. Even when different substituents at C2 lead to different aN and aH@ values, the only secondary hyperfine structure used to characterize spin adducts is the @-hydrogen doublet, and further spectral resolution could help identify the stable products. In the present work the following alicyclic nitroxide radicals have been investigated in aqueous solutions at room temperature.
resolved spectral lines with splittings as little as -0.1 G could be observed for alicyclic nitroxide radicals in aqueous solutions at room temperature, and superhyperfine (SHF) coupling constants of y- and &protons were obtained by computer simulation. The resulting SHF coupling constants of 2,2,6,6-tetramethylpiperidine- 1-oxy1derivatives were consistent with those previously ?H R calculated from 'H N M R data observed for the same radicals, while assignments of S H F coupling constants to the different methyl and methylene protons in substituted pyrrolidine-1-oxy1 radicals could be verified by using partially deuterated analogues. The observed out-of-phase spectra were unique and could be used as fingerprints for the corresponding cyclic nitroxide radicals I 1 I I investigated, while the SHF coupling constants were useful in the 0 0 0. 0 conformational analysis of the five- and six-membered alicyclic TEMPOL TEMPAMINE TEMPO TEMPONE radicals. With out-of-phase detection it is possible to differentiate between slow and rapid interconverting chair or twist conformations for piperidine- 1-oxy1 derivatives, as well as between conformations that are planar or readily interconverting through pseudorotations and those that are puckered for pyrrolidinel -oxy1 radicals. 0. 0. 0 Cyclic nitroxide radicals, with tetramethyl substituents on the COOH-DMPO Cb-DMPO CDs-DMPO carbon atoms that are adjacent to the nitrogen, are used in spin-labeling applications and give rise to I4N(Z=1) hyperfine EPR spectra. A significant contribution to the line width in these spectra is the unresolved proton S H F structure besides the intrinsic nitrogen hyperfine line width. Although tetramethylpiperidine1-oxy1 and pyrrolidine- 1-oxy1 radicals have been extensively in~estigated?~ spectra with only partially resolved proton structure 0 0dewere reported for a few 2,2,6,6-tetramethylpiperidine-l-oxyl OH-DMPO OD-DMPO rivatives in degassed organic ~ o l v e n t swhile , ~ ~ ~Windle7 could obtain These 90" out-of-phase EPR spectra have previously been observed hyperfine coupling constants for some of them as well as for for di-tert-butyl aminoxyl,' which exhibited coupling to 18 di-tert-butyl nitroxide in H 2 0 and CCll solutions. The spectrum equivalent protons in isopentane at 163 K, as well as for 6of 3-carbamoyl-2,2,6,6-tetramethylpiperidine1-oxy1 in an aqueous aminodopamine semiquinone imine radicals" in aqueous solutions medium showing some SHF structure has been published recently.* at room temperature. Stereochemical information could, however, be deduced from the EPR spectra of those unsubstituted or partially substituted alicyclic Experimental Section nitroxide radicals which generally exhibited coupling to one or (Tempol), more p- and y-protons*I5 and, in rare cases, large s p l i t t i n g ~ ~ ~ J ~ ~4-Hydroxy-2,2,6,6-tetramethylpiperidine-l-oxyl ~~ 4-amino-2,2,6,6-tetramethylpiperidine-l-oxyl (Tempamine), (2) Nomenclature: Nitroxide is used as a class name while individual alkyl or aryl nitroxide radicals, RlN(6)R,, are called RlR2aminoxyl and cyclic nitroxides are named pyrrolidine- or piperidine-1-oxyl. (3) Forrester, A. R.; Hay, J. M.; Thomson, R. H. "Organic Chemistry of Stable Free Radicals"; Academic Press: New York, 1968; pp 180-246. (4) Rozantsev, E. G . "Free Nitroxyl Radicals"; Plenum Press: New York, 1970. (5) Britre, R.; Lemaire, H.; Rassat, A. Bull. SOC.Chim. Fr. 1965, 3273. (6) Kreilick, R. W. J. Chem. Phys. 1967, 46, 4260. (7) Windle, J. J. J. Magn. Reson. 1981, 45, 432. (8) Lai, C.-S.; Hopwood, L. E.; Hyde, J. S.;Lukiewicz, S. Proc. Natl. Acad. Sci. U.S.A. 1982, 79, 1166.
(9) Weil, J. T.; Windle, J. J. Nature (London) 1968, 217, 842. (10) Hudson, A.; Hussain, H. A. J . Chem. SOC.B 1968, 251. (1 1) Espie, J.-C.;Lemaire, H.; Rassat, A. Bull. SOC.Chim. Fr. 1969, 399. (12) Windle, J. J.; Kuhnle, J. A.; Beck, B. H. J . Chem. Phys. 1969, 50, 2630. (13) Janzen, E. G . Top. Stereochem. 1971, 6, 1977. (14) Janzen, E. G.; Evans, C. A.; Liu, J. I.-P. J . Magn. Reson. 1973, 9, 513. (15) Aurich, H. G.; Trosken, J. Chem. Ber. 1973, 106, 3483. (16) Janzen, E. G.; Liu, J. I.-P. J . Magn. Reson. 1973, 9, 510. (17) Perez-Reyes, E.; Mason, R. P. Mol. Pharmacol. 1980, 18, 594.
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4719
Alicyclic Nitroxide Radicals
TABLE I: Hyperfine Coupling Constants of Piperidine-1-oxy1Derivatives radical
Ternpol
Tempamine
Tempo Tempone
obsd primary I4N selitting, G 17.1
simulation parameters 1 0 7 ~ , ,s
17.0
1.8
2.2
17.3
16.1
3.0 3.O
a.' G 0.45 (2 ax CH,; C2, C6) c (2 eq CH,; C2, C6) 0.33 (2 ax H; C3, C5) 0.50 (2 eq H; C3, C5) 0.09 (ax 6-H; C4)
0.48 (2 ax CH,; C2, C6) c (2 eq CH,; C2, C6) 0.33 (2 ax H; C3, C5) 0.50 (2 eq H; C3, C5) 0.1 1 (ax 6-H; C4) 0.14 (eq 6-N; C4)
a.e G -0.45'
-0.02 -0.3 1 -0.48
+0.07 0.436d 0.038 0.315
0.450 0.088
0.195 (4 CH,; C2, C6) 0.375 (4 H; C3, C5) 0.175 (2 6-H; C4)
+0.18
0.03 (4 CH,; C2, C6) c (4 H; C3, C5)
-0.12b -0.02
-0.23b -0.39
"&0.005 G for Tempo and fO.O1 G for the others. bReferences 6 and 28. The chemical shifts were corrected for diamagnetic displacement observed on the corresponding amines. Experiments were carried out in organic solvents. c N o contribution was detected. dReference 7. The solvent used was DzO. A temperature of 90 OC was necessary to achieve resolution. Chemical shifts and coupling constants were corrected to room temperature by assuming Curie behavior. 'Calculated from 'H N M R data.
2,2,6,6-tetramethylpiperidine-l-oxyl (Tempo), 4-oxy-2,2,6,6tetramethylpiperidine-1-oxy1(Tempone), and 5,5-dimethyl-1pyrroline-N-oxide (DMPO) were obtained from Aldrich Chemical Co. DMPO was purified before use according to the procedure of Buettner and Oberley described earlier,'* and the concentrations of aqueous DMPO solutions were obtained by UV spectroscopy (cZz7 8000 M-' cm-l). DMSO and H2Oz(30%) were purchased from Fisher Scientific Co. DMSO-d6 and DzO (99.8%) were acquired from Merck Sharp and Dohme Canada Ltd. Dilute aqueous solutions ( N M) of the stable nitroxide radicals were used, and dissolved oxygen was removed by bubbling nitrogen gas prior to the EPR measurements. Photolyses were carried out in situ in the EPR quartz cells (60 X 10 X 0.25 mm3) placed in the spectrometer cavity using a 1000-W Hg-Xe high-pressure arc coupled to a grating monochromator. The bandwidth employed was 10 nm. The irradiation time was about 10-20 s. The methyl adduct of DMPO was prepared by the 265-nm photolysis of 30% HzOz (3 pL) in an aqueous solution of DMPO (0.1 M, 200 pL) containing DMSO or DMSO-$ (30 pL). COOH-DMPO was formed by the UV photolysis of an aqueous solution containing DMPO (0.1 M), sodium formate (0.1 M), and H2OZ.The O H and O D adducts were generated by the photolysis of HzO or DzO solutions containing DMPO and hydrogen peroxide. EPR measurements were made on a Varian E-9 X-band (9.5 GHz) spectrometer equipped with external modulation detection controls which consist of a step phase shifter and a micrometer phase adjustment dial. The modulation phase detector was set at the second harmonic and operates at 50-kHz sample modulation and 100 kHz detection. The microwave power was maintained at 1 m W or lower to avoid saturation. The out-of-phase position was determined by the so-called "null method". In order to avoid cited failures of the null method on sharp lines,lg.the null method was applied on an in situ ruby crystal. The crystal, developed by the U S . National Bureau of Standards,*Owas oriented so that no EPR lines were observed around g = 2 and was attached directly to the EPR sample cell. Application of the null method to the broad, intense line of the ruby crystal at about 5.4 kG ensured rapid, precise setting of the modulation phase detector. The spectra were simulated by using the program RESIM.','~ In brief, the simulation theory developed for saturation transfer (18) Buettner, G . R.; Oberley, L. W. Eiochern. Eiophys. Res. Commun. 1978, 83, 69. (19) Beth, A. H.; Wilder, R.; Wilkerson, L. S.;Perkins, R. C.; Meriwether, B. P.; Dalton, L. R.; Park, C. R.; Park, J. H. J. Chem. Phys. 1979, 72, 2074. Thomas, D. D.; Dalton, L. R.; Hyde, J. S . J. Chem. Phys. 1976, 65, 3006. (20) Chang, T.; Kahn, A. H. NBS Spec. Publ. (US.) 1978, No. 260-59.
(a)
TEMPOL
Ib) I
I
1st HARMONIC OUT-OF-PHASE
/I
I
2nd HARMONIC OUT-OF-PHASE
(C)
Figure 1. EPR spectra of Ternpol in aqueous solution at room temperature (microwave power 1 mW, modulation amplitude 0.16 G, time constant 1 s, scan time 16 min): (a) first harmonic in-phase, (b) first harmonic out-of-phase,and (c) second harmonic out-of-phase spectra. EPRZ1contains the generalized approach for describing the detection scheme used in EPR spectrometers. WIM adopts the form of the equations which detail the phase-sensitive detection cited above and incorporates the standard approach for prediction of peak positions from the number and types of nuclei.
Results and Discussion The first harmonic in-phase, the first harmonic out-of-phase, and the second harmonic out-of-phase'EPR spectra of Ternpol in aqueous solution at room temperature are shown in parts a, b, and c of Figure 1, rekpectively. In contrast to Figure l a where (21) Robinson, B. H.; Dalton, L. R.; Dalton, L. A,; Kwiram, A. L. Chern. Phys. Lett. 1974, 29,56. Hyde, J. S.;Dalton, L. R. "Spin Labeling"; Berliner, L. J., Ed.; Academic Press: New York, 1979; Vol. 11.
4120
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 2nd HARMONIC OUT-OF-PHASE
Mossoba et al.
SIMULATION
M1=+l (a1
TEMPOL
I
(b)
TEMPAMINE
TEMPO
Figure 2. MI = + 1 components of the second harmonic out-of-phase spectra: (a) Ternpol (microwave power 1 m W ,modulation amplitude 0.16 G , time constant 1 s, scan time 16 min); (b) Tempamine (microwave power 1 mW, modulation amplitude 0.16 G , time constant 4 s, scan time 30 min); (c) Tempo (microwave power 1 mW, modulation amplitude 0.063 G , time constant 8 s, scan time 2 h).
only the three-line 14N(Z=1) hyperfine structure was obtained, the spectra in Figure 1b,c exhibit considerable superhyperfine (SHF) structure due to further interactions of the unpaired electron with y-protons. The expansion of the MI = 1 component of the second harmonic out-of-phase spectrum is shown, along with its computer simulation, in Figure 2a. The observed nitrogen coupling constant, a, and the aHYvalues obtained by computer simulation are given in Table I. Six equivalent protons as well as one unique proton and two sets consisting of two equivalent protons each were found for Tempol. Correlation between observed EPR parameters and conformation is possible since for nitroxides, a @-protoncoupling constant arises by a hyperconjugativemechanism and its magnitude depends on the spin density on nitrogen, pN, and on the dihedral angle, 0, between the plane defined by the nitrogen p orbital and N-C* bond and the plane of the C*-H@and C*-N bonds according to the relationshipZZ
Assuming that the conformation of Ternpol in solution does not differ from the one in the crystalline state,26it can be shown by inspection of structure I that values of 0 increase in the order axial methyl carbon, methylene ring carbon, and equatorial methyl carbon, and according to eq 2, a decrease in the magnitude of the @-couplingconstants should follow the same sequence. Consequently, stronger interactions are expected from axial than from equatorial @-positions. Calculations based on a complex angular dependence27have indicated that y-protons of methyl groups in axial positions are expected to have coupling constants of greater magnitudes than those in equatorial positions while the y-methylene protons in equatorial positions have been predictedz7to have larger aHYvalues than those in axial positions. The data obtained for Ternpol (Table I) are in excellent agreement with those previously calculated from ‘H NMR chemical shifts.6,28The SHF coupling constant aHY = 0.45 G (six equivalent protons) is due to the protons of the two axial methyl groups at C2 and C6F7 The values 0.50 and 0.33 G obtained for the methylene protons at C3 and C5 are due to interactions with the two equatorial and two axial protons, respectively, while a 0.09 G was found for the axial &proton at C4.z7 The agreement between the data observed in the present work and those found by a different method, namely ’H NMR, demonstrates the validity of the present approach for obtaining long-range coupling constants. The values of aHobtained for Tempamine (Figure 2b, Table I) show different S H F coupling constants for axial and equatorial methyl and methylene protons and hence, as in the case of Tempol, are consistent with a molecule in a chair conformation. It is noted that the coupling constants found in this study also agree with Windle’s7 EPR data found for Ternpol and *HNMR data observed for Tempamine. However, Windle7 assigned the larger hyperfine coupling constants of the protons of the methyl groups at C2 and C6 for Ternpol and Tempamine to the equatorial rather than axial methyl groups. This is inconsistent with previous
conclusion^.^^
Tempo exhibited a spectrum (Figure 2c) that is different from those of Ternpol and Tempamine since a larger number of narrower lines was observed and the separation between them was half as large (-0.2 G). Only one S H F coupling constant was found for each of the three sets, the four methyl groups, the four methylene y-protons, and the two methylene &protons (Table I), in agreement with previous r e s ~ l t s . ~ * ~The q ~ S~ H F coupling constants obtained for Tempo are close in magnitude to the average values of the corresponding axial and equatorial aHY found p+. 10 for a molecule such as Tempol. For example, aHT 0.20 G for the protons of the four methyl groups in Tempo while the two axial and two equatorial methyl protons in Ternpol gave ( a H Y ) 4 0.23 G. Since line widths and coupling constants depend on the rate of ring inversion, the observed averaging of the coupling B (2) aH = C B+~ BZ cos2 e ) to the axial and equatorial positions in Tempo indicates that the molecule is interconverting rapidly on the EPR time scale at room B, and 82 are constants temperature between two identical chair conformations of equal A similar cosz 0 dependence has also been assumedSfor coupling energy, while Ternpol and Tempamine appear “frozen” in one of constants of @-carbons, a$, and supported by I3C EPR data the two chair conformations that are not equally populated. observed for T e m p ~ l - d ~ X-ray ~ . ~ ~diffraction s t ~ d i e son ~ ~ * ~ ~ The first-derivative spectrum observed for Tempone (Figure Ternpol have shown that the molecule. has a deformed chair 3a) consisted of three narrow lines (AH,, 0.5 G) due to 14N conformation and that the group C2N(O)C6 is pyramidal with which could not be resolved further. Simulation of the second an out-of-plane angle, 4, between the N-O- bond and the C2NC6 plane, of 1 5 . 8 O .
-
N
N
(22) Heller, C.; McConnell, H. M. J . Chem. Phys. 1960, 32, 1535. (23) Chiarelli, R.; Rassat, A. Tetrahedron 1973, 29, 3639. (24) Lajzerowicz-Bonneteau,J. Acta Crystallogr., Sect. E 1968, B24, 196. (25) Berliner, L. J. Acta Crystallogr., Sect. B 1970, B26, 1198.
(26) Lajzerowicz-Bonneteau, J. “Spin Labeling”; Berliner, L. J., Ed.; Academic Press: New York, 1976; pp 239-249. (27) BriBre, R.; Lemaire, H.; Rassat, A.; Dunand, J.-J. Bull. SOC.Chim. Fr. 1970, 4220. Luz, Z. J. Chem. Phys. 1968,48, 4186. (28) BriBre, R.; Lemaire, H.; Rassat, A.; Rey, P.; Rousseau, A. Bull. SOC. Chim. Fr. 1967, 4479.
The Journal of Physical Chemistry, Vol. 88, No. 20, 1984 4721
Alicyclic Nitroxide Radicals TEMPONE N-PHASE
2nd HARMONIC OUT-OF-PHASE
2nd HARMONIC OUT-OF-PHASE
(b)
Figure 3. EPR spectra of Tempone in aqueous solution at room temperature: (a) first harmonic in phase (microwave power 1 mW, modulation amplitude 0.2 G, time constant 0.25 s, scan time 4 min) (I3Cand I5Nsatellites are also observed); (b) second harmonic out of phase (MI = +1) (microwave power 1 mW, modulation amplitude 0.063 G, time constant 0.5 s, scan time 8 rnin). (a)
IN-PHASE
(b)
IN-PHASE
1
Figure 4. In-phase EPR spectra in aqueous solutions at room tempera-
ture (microwave power 1 mW, modulation amplitude 0.2 G, time constant 0.25 s, scan time 4 min): (a) CH3-DMPO, (b) OH-DMPO. The lines labeled with arrows are expanded in parts b and d of Figure 5 , respectively. harmonic out-of-phase spectrum (Figure 3b) yielded uHY = 0.03 G due to interactions with the 12 protons of the four methyl groups. The small magnitude of the coupling constant is consistent with a nonchair conformation. An X-ray diffraction study has shown that Tempone has a twist conformation (structure 11) with O=+N-O
II a twofold axis of symmetry.29 The equivalence of all the methyl protons indicates that Tempone is rapidly inverting between two identical twist conformations in which the y-protons are not in a favorable position for interacting with the pz orbital on nitrogen. The magnitudes of hyperfine coupling constants are known to depend on the nature of the solvent used.30 Since our measurements were carried out in aqueous solutions while the N M R studies6*2swere done in nonpolar solvents, a difference in coupling constants must be expected even for y-protons (see Table I). Values of uHY were found to increase with decrease in solvent polarity for an aIicyclic nitroxide r a d i ~ a l . ~ ’Such dependence could explain the differences in the coupling constants obtained (29)Bordeaux, D.;Lajzerowicz, J. Acta Crystallogr., Sect. B 1974,830, 790. (30) Janzen, E.G . “Free Radicals in Biology”; Pryor, W. A., Ed.; Academic Press: New York, 1980; p 116. (31)Janzen, E. G.;Coulter, G . A.; Oehlen, U. M.;Bergsma, J. P. Can. J . Chem. 1982,60,2125.
-0.K
Figure 5. Low-field lines of the second harmonic out-of-phasespectra: (a) COOH-DMPO (microwave power 0.1 mW, modulation amplitude 0.063 G, time constant 2 s, scan time 30 min); (b) CH3-DMPO (microwave power l mW, modulation amplitude 0.063 G, time constant 4 s, scan time 1 h); (c) CD3-DMPO (microwave power 1 mW, modulation
amplitude 0.063 G, time constant 1 s, scan time 16 rnin); (d) OH-DMPO (microwave power 1 mW, modulation amplitude 0.08 G, time constant 4 s, scan time 1 h); (e) OD-DMPO (spectrometer settings as for (d)). in the present study and those previously f o ~ n d . ~ , ~ , ~ ~ The fivemembered alicyclic nitroxide rings investigated in the present study will be discussed next. DMPO adducts usually exhibit a 14Ntriplet and a secondary doublet due to the @-proton at C2. Parts a and b of Figure 4 show the spectra of 2,5,5-trimethylpyrrolidine- 1-oxy1 (CH3-DMPO) and 2-hydroxy-5,5-dimethylpyrrolidine-1-oxy1 (OH-DMPO), respectively. The observed splittings of adducts listed in Table I1 agree with the literature value^.^**^^ The low-field lines in’parts a and b of Figure 4 (labeled with arrows) were expanded and displayed as second harmonic out-of-phase spectra of parts b and d of Figure 5, respectively. The low-field line of the spectrum observed for 2-carboxy5,5-dimethylpyrrolidine- 1-oxy1 (COOH-DMPO) (structure 111)
bc/ I 0.24G
\N/
I
0
’COOH
H/
\N/
\COOH
’ I 16.88G 0.
H/
1
6.9G
N ‘’
1
0
\H 1
6.9G
a& = 0.13G PHI, a& = 0.46~ QH), 0.24G(HI 0.25G (HJ, 0.28G (HI
m
m
P
exhibited a second harmonic out-of-phase spectrum (Figure 5a) (32)Harbour, J. R.;Bolton, J. R. Photochem. Photobiol. 1978,28, 231. (33)Harbour, J. R.;Chow, V.; Bolton, J. R. Can. J. Chem. 1974,52,3549.
4722 The Journal of Physical Chemistry, Vol. 88.No. 20, 1984 TABLE II: Hyperfine Coupling Constants of DMPO Adducts
simulation parameters
obsd
spin adduct COOH-
splittings aN,G
aw@,G
15.6
18.7
107T,, s 13.0
CH3-
16.1
23.0
2.6
CD3-
16.1
23.0
2.6
OH-
14.9
14.9
3.3
OD-
14.9
14.9
3.3
ar/ G
0.236 (2 CH,) 0.130 (2 H) 0.243 (H) 0.275 (H) 0.473 (CH3) 0.237 (2 CH,) 0.140 (2 H) 0.238 (H) 0.302 (H) 0.072 (CD3) 0.237 (2 CH3) 0.140 (2 H) 0.238 (H) 0.302 (H) 0.227 (OH) 0.224 (2 CH,) 0.135 (2 H) 0.229 (H) 0.370 (H) b (OD) 0.224 (2 CH3) 0.135 (2 H) 0.229 (H) 0.370 (H)
‘f0.005 G. bThe contribution of the OD deuterium was negligible
(
