J . Phys. Chem. 1990, 94, 6589-6594
6589
Proton Hyperfine Tensors in Nitroxide Radicals M. Brustolon,t A. L. Maniero,+ M. F. Ottaviani,t M. Romanelli,s and U. Segre*,+ Dipartimento di Chimica Fisica, Universitii di Padova. Padova. Italy, Dipartimento di Chimica, Universitd di Firenze, Firenze, Italy, and Dipartimento di Chimica, Universitci della Basilicata, Potenza, Italy (Received: June 20, 1989; In Final Form: March 19, 1990)
The proton hyperfine tensors of five nitroxide radicals have been obtained by ENDOR spectroscopy in frozen solution. The spectra are interpreted by computing the dipolar hyperfine interaction and simulating the spectra. EPR spectra in solution of the same radicals have been simulated by taking into account the effects of the proton hyperfine tensors. We have been able to reproduce accurately the line broadening effects of the proton hyperfine structures inside each nitrogen hyperfine component and we have determined the correlation times for the rotational motion. In the case of the radical Tempol, our analysis allows discrimination between the effects due to the protons of the axial and equatorial methyl groups. On the basis of experimental evidence we can attribute the larger isotropic hyperfine coupling constant to the axial methyl protons. The possible use of the present results for interpreting the spectra of other nitroxide radicals is discussed.
Introduction
The knowledge of all the magnetic interactions in the nitroxide radicals is an important condition for a thorough exploitation of the information buried in their EPR spectra. While the main features of the EPR spectrum of a nitroxide radical probe can be accounted for by the g and nitrogen hyperfine (hf) tensor AN, the accurate line shape can be simulated only if the small proton hf tensors are taken into account. In fact the EPR line shape in solution is affected by the dynamical averaging of the anisotropic magnetic interactions of the radicals. These effects are well-known and thoroughly understood thanks to the theory developed by Freed and Fraenkel.Is2 The EPR line shape of a nitroxide radical in dilute solutions is dominated by the rotational averaging of the g and AN tensors. However, the effects on the line shape due to the anisotropic magnetic couplings of the protons may be important, and they can be clearly detected when the proton hyperfine components are well resolved.) To reduce the line broadening due to the proton hf interactions perdeuterated nitroxides have been used. In the case of piperidinone nitroxides, like Tempone (2,2,6,6-tetramethyl-4piperidinone- 1 -oxyl), the deuteron hf interactions are negligible? owing to the small proton hf interaction due to the particular twisted-crossover conformation of the ring. On the other hand, in perdeuterated piperidine and pyrroline nitroxide radicals some of the deuteron hf interactions are not negligible. The proton isotropic contact coupling (hfcc) in a number of nitroxide radicals has been measured in the past by EPR,5-7 NMR,8 and ENDOR in s o l ~ t i o n . ~Hyperfine tensors of the protons in some nitroxide radicals were determined by studying by EPR, and more recently by ENDOR, single crystals of a convenient diamagnetic host doped with the nitroxide g~est.l"-'~ However, this method may be difficult and cumbersome, and moreover the conformation of the radical forced in a crystal matrix might be different from that in s o l ~ t i o n . ' ~ ~ ' ~ Recently we have shown that it is possible, from ENDOR spectra of nitroxides in glassy matrices, to obtain the principal values of the proton hyperfine coupling tensors AH together with some information on their principal directions.18J9 The method is based on the simulation of the partly crystallike ENDOR spectraZoand on the calculation of the proton dipolar tensors.2' In this paper we use the latter method to determine the AH tensors of radicals I and 11, which are pyrroline nitroxyl derivatives with a planar five atom ring, and radicals 111-V, which are piperidine derivatives with a six-atom ring in a chair conformation. We have recorded also the EPR spectra of accurately degassed solutions of radicals I and 111, and we have simulated them by using the values of the proton dipolar tensors obtained from the 'Universiti di Padova. Universiti di Firenze. 8 Universiti della Basilicata.
*
0022-3654/90/2094-6589$02.50/0
ENDOR analysis. The quality of the fit is high, and the correlation times obtained by this analysis should be quite reliable. In the case of radical 111the EPR simulation gives for the first time experimental unambiguous support to the assignment of the largest hfcc to the protons of the axial methyl group.s It is worth noting that the tensors we determined are very similar for different radicals with the same conformation of the ring bearing the NO group. This fact allows the use of our results for other pyrroline or piperidine nitroxides. Experimental Section
Nitroxides I (Tempyo), IV (Tempamine), and V were prepared by the methods described by Rozantsev.22 Tempyo deuterated in the CH position was obtained by stirring a basic CH3CH20D solution of the radical for 12 h at 50 OC. The extent of deuteration checked by N M R was of 50%. Nitroxide 111 (Tempol) was purchased from EGA Chemie. Nitroxide I1 and Tempol deuterated in the methylene positions were a gift from Prof. Lucedio Greci, Universitg di Ancona. EPR and ENDOR measurements were carried out on dilute solutions (1 O4 M) of the radicals. Toluene, toluene-de, paraffin oil, ethanol, and di-n-butyl phthalate were used as solvents for the glassy matrices. The ENDOR spectra have been recorded at T = 105 K. The fluid solution EPR measurements were carried ~~
(1) Freed, J. H.; Fraenkel, G. K. J. Chem. Phys. 1963, 39, 326. (2) Fraenkel, G. K. J . Phys. Chem. 1967, 71, 139. (3) Wilson, R.; Kivelson, D. J . Chem. Phys. 1966, 44, 4445. (4) Hwang, J. S.; Mason, R.; Hwang, L. P.; Freed, J. H. J . Phys. Chem. 1975 -. . -, .79 ., 489 .- - . ( 5 ) Windle, J. J. J. Magn. Reson. 1981, 45, 432. (6) Mossoba, M. M.; Makino, K.; Riesz, P. J. Phys. Chem. 1984,88,4717. (7) Trousson. P.: Lion. Y.J . Phvs. Chem. 1985. 89, 1954.
(8) BriEre, R.; Lemaire, H.; Rassat, A.; Dunand, J. J. Bull. SOC.Chim. Fr. 1970, 12, 4220. (9) Kirste, 8.;Kruger, A.; Kurreck, H. J . Am. Chem. Soc. 1982,104,3850. (10) Griffith, 0. H.; Cornell, D. W.; McConnell, H. M. J . Chem. Phys. 1965, 43, 401 I . (11) Lin, T. S. J . Chem. Phys. 1972, 57, 2260. (12) Birrell, G. B.; Van, S. P.; Griffith, 0. H. J . Am. Chem. Soc. 1973, 95. 2451. (13) Snipes, W.; Cupp, J.; Cohn, G.; Keith, A. Biophys. J . 1974, 14, 20. (14) Ozheki, F.; Kispert, L. D.; Arroyo, C.; Steffen, M. J . Phys. Chem. 1982.86. 2909. (15) Tabak, M.; Alonso, A,; Nascimento, 0. R. J. Chem. Phys. 1983, 79, 1176. (16) Brustolon, M.; Maniero, A. L.; Corvaja, C. Mol. Phys. 1984, 51, 1269. (17) Maniero, A. L.; Brustolon, M. J . Chem. SOC.,Faraday Trans. I 1988, 84, 2875. (18) Brustolon, M.; Maniero, A. L.; Segre, U. Mol. Phys. 1985,55, 713. (19) Brustolon, M.; Maniero, A. L.; Segre, U.; Greci, L. J . Chem. SOC., Faraday Trans. I 1987, 83, 69. (20) Kevan, L.; Narayana, P. A. In Multiple Electron Spin Resonance; Dorio, M. M., Freed, J. H., Eds.; Plenum Press: New York, 1979. (21) McConnell, H. M.; Strathdee, J. Mol. Phys. 1959,2, 129; Derbyshire, W. Mol. Phys. 1962, 5 , 225. (22) Rozantsev, E. G. Free Nirroxyl Radicals; Plenum Press: New York, 1970.
0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 1 7 , I990 Ph-,
Brustolon et ai. the microwave frequency v, and the electron spin resonance frequency.25 We assume therefore that the weight factor is given by P(6',@)= C g ( v , - V e W , @ ) ) (3)
/"
M
NI
K 0
0
1
where the summation is over all the hf components of the EPR spectrum. We expect that the smaller the frequency difference 6 = u, - u,, the larger is the value of the function g(6), and we use for it a Gaussian form:18
X II
g(6) = ( 1 /2ru2)1/2e-62/2~2 H OH \ /
0 111
H
NHP
\ /
H NHCO CH, \ /
0 IV
O V
out on toluene-d8, ethanol, and 1 -propanol solutions of Tempyo (I), Tempo1 (Ill), and Tempol-d,. The samples were deoxygenated by bubbling dry and oxygen-free nitrogen for 2 h and stored and handled in a nitrogen atmosphere. The EPR spectra were recorded in the temperature range between 283 and 348 K. EPR and ENDOR spectra were obtained with a Bruker ER 200D spectrometer equipped with a 300-W rf amplifier and a Bruker variable-temperature unit. Typical experimental conditions for ENDOR measurements were as follows: microwave and radiofrequency fields in the rotating frame about 0.1 and 6 G, re~pectively,~~ modulation depth of the frequency modulation of the rf field 70 kHz.
Theoretical Methods The spin Hamiltonian for a nitroxide radical is 7f = vJZ
- u N I N Z - vHCIiz + S'AN'IN i
+ CS.AH;Ii
(I)
I
where IN, uN, AN are the nitrogen spin, nuclear frequency, and hf tensor, and l i , uH, AHi are the spin, nuclear frequency, and hf tensor of the ith proton. We will recall in the following the theoretical methods to be used to compute ENDOR spectra in frozen solution and EPR spectra in the fast motion limit on the basis of the Hamiltonian given in eq 1. Simulation of Frozen Matrix ENDOR S p e ~ t r a . l ~The , ' ~line shape of a frozen matrix spectrum is computed as the superposition of the contributions from the probes having different orientations (6',C$) with respect to the magnetic field. We consider the ith proton of the radical. Its resonance frequencies vi*(8,C$) are computed from the first-order eigenvalues of the Hamiltonian (1). The f signs correspond to the electron spin states ms = &I/'. The line shape for the f transition of the ith proton is then given by
IAi(ur) = SP(O,C$)h(vr - vif(6',@)) sin 6' d6' d@
(2)
where ur is the radio-frequency value and h ( x ) is a Lorentzian or Gaussian line-shape function. P(6'4~)is an orientation-dependent weight factor. In general, P(8,d) should be affected by the orientation dependence of the hyperfine enhancements, of the relaxation mechanisms, and of the resonance conditions. However, we consider here the case of isotropic relaxation mechanisms and of a limit ENDOR enhancement, which is obtained at high radiofrequency power and is independent of the transition moments.24 Therefore, at a given orientation the weight factor should be different from zero only if the microwave field value matches one of the resonance frequencies of the electron spin v,'"(8,@), where the index M labels the different hf components of the EPR spectrum. However, since the EPR lines have a finite line width, the weight factor will be a decreasing function of the offset between (23) Brustolon, M.; Cassol. T. J . Magn. Reson 1984, 60, 254. (24) Freed, J H. J . Chem. Phys. 1965, 43, 2312.
(4)
The shape of the spectrum depends sensibly upon the actual value of the width u. When u goes to zero, only the spin packets exactly at resonance with the microwave frequency give rise to the ENDOR absorption, and a crystallike spectrum is obtained; for large u values, on the contrary, the line shape is powderlike, since all the probes contribute equally to the spectrum, irrespectively of their orientation. Since the protons contribute additively to the ENDOR spectrum, the total ENDOR line shape is given by I(Y)
=
E[[I+'(u) + I-i(P)] i
(5)
In any case, the ENDOR peak positions for nitroxides are determined by the principal values of the AH,tensor, and these, as a result, can be extracted straightforwardly from the ENDOR spectrum.18 This fact allows the initial assignments of the ENDOR features to the different protons on the basis of the dipolar tensors calculated by the method discussed later. The procedure to simulate the ENDOR spectra in frozen matrix is the following: A first guess of the hf tensor principal values is obtained from the positions of the lines of the experimental spectrum. The principal directions of the hf tensors are computed with the McConnell-Strathdee-Derbyshire (MSD) method,'I starting from the molecular geometry and the spin density of the radical. Then the spectrum is computed by using the eqs 1-5 and the principal values of the tensors are refined to obtain the best fit with the experimental spectrum. In the case of crystallike ENDOR spectra the simulation gives also a check on the reliability of the computed principal directions of the AHi tensors. Simulation of Fast Motion EPR Spectra. When the proton hf interactions are neglected, the EPR line shape in a moderately viscous solution is given by three (derivative) Lorentzian lines, with line widths: T 2 - ' ( m N= ) A
+ BmN + CmN2+ T2,0-'
(6)
The terms A, B, C originate from the modulation of the magnetic interactions by the motions in solution, and they can be computed from the anisotropies of the AN and g tensors and from the value of the correlation times. The term T2,0-'instead takes into account the contribution from other line-broadening mechanisms. The inclusion of the proton hf couplings in the spin Hamiltonian increases considerably the complexity of the line-shape expression, since the protons constitute different sets of equivalent nuclei. The line shape for a degenerate transition is given, in principle, by a superposition of Lorentzian lines, whose line widths must be computed by diagonalizing the relaxation matrix in the transition space.' However, in our case the contribution to the line width arising from the proton hf anisotropies, although not negligible, is small, and one can use an averaged expression for the line shape.2 Let us consider K sets of equivalent protons: then the transitions are specified by K 1 quantum numbers m N ,mH,,( i = 1, K ) . The line shape of each transition is a derivative Lorentzian line, with an average line width given by a quadratic expression in the quantum numbers:
+
( T ~ - I ( ~ N , ~ H , , = 22ersmrMs .a*))
r
The expressions for the coefficients
(7)
s
are given by Fraenkel.'
(25) Hoffman, B. M.; Venters, R. A,; Martinsen, J. J . Magn. Reson. 1W5. 62. 537.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6591
Proton Hyperfine Tensors in Nitroxide Radicals TABLE I: Princiwl Values of I and AN (MHz) pyrroline radicals piperidine radicals g, = 2.0082 gv = 2.0075
AN, = 13.4 A N y = 14.8 A N , = 96.3
g, = 2.0021
g, = 2.0093 gv = 2.0062 g, 2.0027
AN, ANy AN,
= 16.2 = 19.6 = 99.4
TABLE 11: Experimental and Calculated TH (MHz) Tensors of Pyrroline Radicals direction cosines proton CH
CHI
a
TexP
Tjal 2.76 3.61 -1.56 (glass) -1.43 -2.14 -1.33 (sol) -1.33 -1.47
xi
Yi
zi
0.9114 -0.4114 0.0000 0.4114 0.9114 0.0000 0.0000 0.0000 1.000
7.07 6.96 0.2710 0.7229 0.6356 0.0732 0.3195 -1.12 (glass) -4.03 -3.43 -0.9448 0.1844 -0.6871 0.7028 -0.64 (sol) -3.03 -3.53
The cross terms with r # s arise from the correlation between the motions of different groups of equivalent nuclei. All the coefficients are expressed through the spectral densities of the correlation functions for the magnetic anisotropies. The spin Hamiltonian can be rewritten in terms of spherical tensor components f l 2 > / ) , I and the spectral density functions are given by
where A, p denote the magnetic interactions, and relation times for rotational diffusion:
are the cor-
(9) R , and RIIare the principal components of the molecular diffusion tensor. It is useful to define the average rotational correlation time iR and the diffusion anisotropy N: 7R
= (6R,R11)’/2, N = RII/R,
(10)
It is worth noting that all the tensor components F;(’*O in eq 8 must be expressed in the same molecular axis system. We have taken for all the radicals the reference frame that diagonalizes the A N tensor, with the z axis along the unpaired 2p orbital and the x axis along the N O bond. In conclusion, to simulate the EPR spectra on the basis of the line-width expression eq 7, the values of the following parameters must be known: (i) The principal values of the g and A N tensors; these are obtained by the fitting of EPR spectra at T = 77 K by the standard method^,^ and are reported in Table 1. (ii) The principal values of the AHi tensors, and their cosine directors (or their Euler angles (ai,pi,yi))which relate the proton and the nitrogen hf interaction principal axis systems; these are obtained by the simulation of the ENDOR spectra in frozen solution, as explained in the following section, and are reported in Tables I 1 and 111. (iii) The values of the diffusion constants R , and RII,or the related values of rR and N, these are obtained by a method of successive approximations. As starting values we used those obtained by applying the simplified expression for the line width eq 6 . Then, they are varied until the fit with the experimental shape becomes satisfactory.
Results Calculations of the Proton Dipolar Tensors. In this section we report on the calculations of the proton dipolar tensors THi = AHi- ai, where ai is the hfcc for the ith proton. The calculations have been performed with the MSD method.21 Pyrroline Nitroxides. We have calculated the ring C H and C H 3 dipolar tensors for radicals I and I 1 on the basis of the geometry obtained from the crystal structure on Tempyo.26 The CH, groups are nearly equivalent, since the ring is planar and (26) Turley. J. W.; Boer, F. P. Acra Crystallogr. 1972, B28, 1641.
TABLE 111: Experimental and Calculated TH (MHz) Tensors of Piperidine Radicals direction cosines proton a TjexP T.a1 vi zi 5.73 5.27 -0.2474 0.6199 0.7444 0.9536 0.2917 0.0740 CH,ax -1.26 -3.55 -2.49 -2.18 -2.78 -0.1714 0.7284 -0.6634 5.79 5.34 -0.0932 0.9043 -0.4167 C H p -0.68 (glass) -3.67 -2.51 0.7536 0.3376 0.5640 -0.08 (sol) -2.12 -2.83 -0.6506 0.2614 0.7130 5.46 5.25 0.8345 -0.3829 0.3963 CH2ax -0.98 (glass) -3.28 -2.54 -0.4380 -0.0235 0.8990 -0.90 (sol) -2.18 -2.71 0.3350 0.9230 0.1870 3.32 2.91 0.8093 -0.5847 0.0560 CH2W -1.67 (glass) -1.89 -1.44 -0.0879 -0.0257 0.9960 -1.32 (sol) -1.43 -1.48 0.5807 0.8108 0.0722
each C-CH3 bond makes a dihedral angle of nearly 30’ with the perpendicular to the ring plane. The spin density has been considered as localized on the N O group, but for the small density on C(3) and C(4) atoms ( p = 0.022), obtained from the isotropic hfcc of the C H proton9 by the usual semiempirical relationship. The C H proton dipolar tensor has been calculated by taking separately into account the contributions due to the local spin density on the a carbon atom and to the spin density on the N O group. The effect of the latter contribution has been calculated with the MSD method, whereas the former has been taken into account in a semiempirical way, by using the usual principal values for the dipolar tensor of an a proton (30,0, -30 MHz) in a C H fragment. In fact, the MSD method gives reliable results only for a dipolar interaction with electron distribution at least two bonds away from the nucleus. In the case of the methyl protons, it is necessary to average the dipolar tensors on the rotation angle. The motion is described, as usual, by a thermally activated jumping between the minima of a threefold torsional potential. Radical I has a symmetry lower than C, and the calculated dipolar tensors for the methyl groups in positions 2 and 5 are different. An average of the two tensors has been taken, since it gives the best fit to the experimental results. The principal values and directions obtained with these calculations are reported in Table 11. Piperidine Nitroxides. We have calculated the dipolar tensors for the methyl and methylene protons on the basis of the geometry of radical 111, as obtained from the crystal s t r u c t ~ r e . ~ ’Due to the slow interconversion between the two equivalent chair conformations, there are two different types of CH2 and CH3 groups, Le., axial and equatorial. The spin density has been considered localized on the N O group, equally weighted between the two atoms. For the CH, groups an average tensor has been calculated as explained in the previous paragraph. The results are reported in Table 111. Frozen Matrix ENDOR Spectra. Radicals I, II. The ENDOR spectra of these radicals in different frozen solvents are all very similar and show crystallike effects. In Figure 1 is reported the high-frequency part of the spectrum of radical I, obtained on saturating the central band of the EPR spectrum (mN = 0). In order to make easier the interpretation of the spectrum, we have performed ENDOR measurements also on radical I partially deuterated in the C H position. The spectrum is displayed in Figure lb. The sharp line at 16 MHz shows a strong reduction in its relative intensity in the spectrum of the partially deuterated probe. On the basis of this result and of the calculations of the proton dipolar tensors discussed above, we have been able to attribute the features of the spectra to the principal values of the C H and CH, protons. The experimental values, obtained from the frequencies of the ENDOR features, have been compared with the principal values of the calculated tensors THi,to which the isotropic hfcc (obtained from the literature) has been added. We have then simulated the spectra (Figure 1 ) on the basis of these assignments (27) Berliner, L. J. Arm Crystallogr. 1970, B26, 1198.
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Brustolon et al.
The Journal of Physical Chemistry, Vol. 94, No, 17, 1990
VH
Figure 1. ENDOR spectrum of Tempyo in a frozen matrix of di-n-butyl phthalate at T = 105 K, obtained with the magnetic field on the m N = 0 EPR transition. Only the high-frequency half-spectrum is reported. Full line, experimental spectrum; dotted line, simulated spectrum. (a) Fully protonated radical. (b) Partially deuterated radical.
a
,1
b
A
Figure 3. ENDOR spectrum of Tempamine in a frozen matrix of paraffin oil at T = 105 K, obtained with the magnetic field on the mN = + 1 EPR transition. Only the high-frequency half-spectrum is reported. Full line, experimental spectrum; dotted line, simulated spectrum.
& Figure 2. ENDOR spectrum of Tempol in a frozen matrix of toluene-d8 at T = 105 K, obtained with the magnetic field on the m N = 0 EPR transition. Only the high-frequency half-spectrum is reported. Full line, experimental spectrum; dotted line, simulated spectrum. (a) Fully protonated Tempol. (b) Tempol-d,.
of the spectral features, by using the principal directions obtained from the calculations. The best fitting simulation gives the hf tensors principal values reported in Table 11. Radicals 111, W ,and V. The ENDOR spectra of Tempol-d, and Tempol are reported in Figure 2. The two spectra are very similar, owing to a strong overlapping of the absorption lines due to the methylene protons with those due to the methyl ones. The spectra are obtained on saturating the central line of the EPR spectrum; the ENDOR spectra obtained in other positions of the EPR spectrum are very similar. Therefore, the Tempol ENDOR frozen matrix spectra are quite completely powderlike. The ENDOR spectra of radical IV and radical V in the same solvent obtained on saturating the central EPR line are similar to those of Tempol. On the other hand, they are crystallike, being very different when obtained on the other features of the EPR spectrum; an example is shown in Figure 3. We have attributed the different features of the ENDOR spectra to the different protons on the basis of the hf tensors obtained from the calculated dipolar contribution and the isotropic hfcc. NMR and ENDOR measuremnts of the methyl proton hfcc ~ * ~two different values (-1.3 MHz and for Tempol in s o l ~ t i o ngive -0.1 MHz), due to the two nonequivalent methyl groups. The assignment of these two values to the axial or equatorial methyl group is c o n t r o v e r ~ i a l . ~ We - ~ ~have ~ ~ simulated the spectra of (28) Whisnant, C. C.; Ferguson, S.;Chesnut, D. B. J . Phys. Chem. 1974, 78, 1410.
Figure 4. EPR spectrum of Tempyo in I-propanol at T = 298 K. Full line, experimental spectrum. Dashed line, simulated spectrum.
Tempol-d, by making the two possible assignments. The simulated spectra are very similar and do not allow one to choose between the two hypotheses. However, as discussed later, the simulations of the EPR spectra of Tempol-d4 in solution allow us to assign the larger coupling constant to the axial methyl group. Solution EPR Spectra. We have confined our analysis to the EPR spectra of Tempyo and Tempol. In fact the EPR spectra of Tempyo are more resolved and with narrower lines than the bulky radical 11, and, on the other hand, radicals 111-V show similar line shapes. Radical I . The experimental and simulated spectra of Tempyo in 1-propanol at T = 298 K are reported in Figure 4. The computation was carried out by using the A N and g tensors reported in Table I, and the proton dipolar tensors obtained from the analysis of frozen matrix ENDOR spectra reported in Table 11. The proton hfcc was instead obtained directly from the EPR spectra in solution. The fitting of the spectrum gives the values of the average correlation time T~ and of the rotational anisotropy N . As starting values we used T~ = 3.5 X lo-” s and N = 1, as obtained by applying the simplified expression for the line width eq 6. The final value of the correlation time is smaller, T~ = 3.0 X lo-” s. Any deviation from the isotropic rotational diffusion gave a worse fitting. Radical I l l . The analysis of the EPR spectra of this radical is complicated since the hyperfine structure is given by five different groups of protons. Moreover, the methylene and methyl protons have only slightly different hf tensors. Therefore, we used Tempol-d+ In Figure 5 is reported the experimental and simulated central EPR hyperfine component of Tempol-d,, obtained in I-propanol at T = 298 K. The computation was carried out by using the A, and g tensors reported in Table I and the methyl
Proton Hyperfine Tensors in Nitroxide Radicals
Figure 5. Central line mN = 0 of the EPR spectrum of Tempol-d,. Continuous line, experimental. Dashed line, simulated with the higher hfcc to the axial methyl group. Dotted line, simulated with the lower hfcc to the axial methyl group.
Figure 6. EPR spectrum of Tempol in I-propanol at T = 298 K. Full line, experimental. Dashed line, simulated.
proton dipolar tensors obtained from the analysis of frozen matrix ENDOR spectra. As discussed in the previous section, the two possible assignments of the hf tensors to the axial and equatorial methyl groups give rise to similar ENDOR spectra. On the other hand, the EPR spectra simulated in the two different hypothesis are quite different, as one can see in Figure 5 . The better simulation is obtained with aHu = -1.26 MHz and a$ = -0.08 MHz. Since no signals arising from the hydroxyl proton were detected in the ENDOR spectra, in the EPR simulation we used the hf tensor obtained by the MSD calculations. On the basis of the results obtained from the analysis of Tempol-d, EPR spectra, we have performed simulations of the spectra of the fully hydrogenated radical. In Figure 6 the experimental and simulated spectra in I-propanol at T = 298 K are reported. The best fit for the simulation was obtained with r R = 3.5 X IO-" s and N = 3, allowing a faster rotation about the direction of the NO bond. A similar value for N a n d a smaller correlation time r R = 3.2 X IO-" s result from the simplified analysis of the line widths. The same rotational anisotropy was obtained previously by EPR spectra of Tempol in ethanol.29
Discussion Proton Hyperfine Tensors. The proton hf tensors of the radicals with the same type of ring are very similar, as is shown by their ENDOR spectra. In fact, the spectral features are located at the same frequencies for each group of radicals, whereas the shapes (29) Ottaviani, M. F. J . Phys. Chem. 1987, 91, 779.
The Journal of Physical Chemistry, Vol. 94, No. 17, 1990 6593
of the spectra are different because of the unequal crystallike character of the ENDOR spectra. Therefore, we suggest that the AH tensors we obtained in this work can be quite typical for other radicals with the same ring conformation. This information could be very useful in the analysis of ENDOR spectra. For example, in the nitroxide radical N-( I-oxyl2,2,6,6-tetramethyl-4-piperidinyl)maleimide, which is often used as a spin label for proteins, the ring bearing the NO group has the same conformation as that of radicals 111-V. Thomann et aL30 reported the matrix ENDOR spectra of the latter spin probe bound to oxyhemoglobin, showing a pair of strong features symmetric with respect to the free proton frequency at a distance of 3.7 MHz. The authors attributed this value to the largest principal value of the hf tensor of the methylene or methyl protons bound to the six-atom ring. On the other hand, our results for radicals 111-V (see Table I11 and Figures 2 and 3) show that there are three hf tensors with an intermediate value in the range (absolute value) 3.3-3.7 MHz. We believe that the features observed by Thomann et al. can be attributed to a piling up of intensity due to these very similar principal values of the hf tensors, whereas the features due to the largest principal values (5.8-5.5 MHz) are not observed in their spectra because they are broadened by the residual motions of the probe. The crystallike character of the ENDOR spectra depends on the mobility of the probes in the glassy matrix, which in turn is determined by the dimensions of the probes, but also by the microstructure of the glassy environment of the radical. For example, in the case of Tempol and Tempamine in toluene the probe dimensions are very similar, but their ENDOR spectra have quite different crystallike properties (see Figures 2 and 3). Controversial assignments of the two hfccs measured in solution to the equatorial and axial CH3groups in Tempol have been made from different authors on the basis of theoretical calculations.28 From the present study, it is apparent that the dipolar tensors of the axial and equatorial methyl protons have similar principal values but different sets of principal axes. Therefore, the two possible assignments of the hyperfine tensors give rise to different simulations of the frozen matrix ENDOR and EPR spectra. The ENDOR simulations for the two sets of principal axes are, however, too similar, while the simulated EPR spectra fit well only by attributing the largest isotropic hyperfine coupling constant to the axial methyl group protons (see Figure 5). This is the first experimental evidence that the original assumption of Britre et a].* was in fact the correct one. The information on the dipolar tensors of the protons obtained by the ENDOR simulations is accurate for the principal values and broad in the best case for the principal directions. In fact the former are obtained by a best fit of the simulated spectra, and the latter are simply the calculated ones. The simulated spectrum is independent of the values of the principal directions cosines for powderlike spectra. On the other hand, in the case of spectra with crystallike character the quality of the fit between calculated and experimental spectra shows that the principal values calculated by the MSD are quite reliable. The experimental isotropic hf couplings obtained from the traces of the tensors are in general slightly different from the values in solution. For a comparison in Tables I1 and 111 are reported the hf couplings obtained from the ESR simulations in fluid solution, where they are quite independent of the solvent. Therefore, we attribute the differences between the rigid matrix and the liquid solution to a slightly different geometry of the radicals in the glassy phase. In fact, as mentioned in the Introduction, the conformation of some nitroxide radicals in solid crystalline matrix has been found J ~ less tight matrix to be quite different from that in s o l ~ t i o n . ' ~The of the glass may give rise to a smaller distortion. Finally, it should be noted that the ring proton in radicals 1-11 gives rise to two strong features distant about 3 MHz, which are unusually sharp for an a proton. In a previous work they have been attributed to the methyl protons.I8 From the present results (30) Thomann, H.; Robinson, B. H.; Dalton, L. R.; Beth, A. H.; Perkins, R. C.: Park, J. H. Chem. Phys. Lett. 1980, 73, 131.
6594
J . Phys. Chem. 1990, 94, 6594-6598
we deduce that their unusual sharpness and relative intensity are due to the quasi axiality of the tensor of the CH proton (see Table 11).
Correlation Times. The rotational correlation times of nitroxides in the fast motional regime are usually obtained from the spectra by the means of a simple expression for the line width as eq 6. This expression does not allow for the relaxation effects of the proton interaction anisotropies, and therefore it is interesting to compare these approximate values with those obtained by the means of the full theoretical expression eq 7. The more accurate value of T~ turns out to be smaller than the approximate one for Tempyo and larger for Tempol. This difference is due to the
different signs of the line-width coefficients in eq 7, and the sign of these coefficients is determined by the relative orientations of the A, and A, tensors. Therefore, the influence of the proton interaction anisotropy on the accurate determination of the average rotational correlation time T~ depends upon the geometrical structure of the radical. On the other hand, we found that the rotational anisotropy parameter N = R , , / R , is less sensitive to the accuracy of the simulation. In fact the same value was obtained by both analysis. Registry No. I , 3229-73-0; 11, 106367-37-7; 111. 2226-96-2; IV, 14691-88-4; V, 14691-89-5.
Raman Spectroscopic Study of MethanolCc Electrolyte Solutions in Liquid and Glassy States Shigeru Yamauchi and Hitoshi Kanno* Department of Chemistry, The National Defense Academy, Hashirimizu, Yokosuka, Kanagawa 239, Japan (Received: October 26, 1989; In Final Form: March 7 , 1990)
Raman OH stretching spectra were measured for glassy methanolic solutions of alkali-metal halides. The Raman spectra for the glassy solutions at liquid nitrogen temperature exhibited fine structures in contrast to the spectra of the solutions at room temperature, which showed only a single featureless envelope. Two relatively narrow Raman peaks (one at 3385 cm-' and the other at 3320-3340 cm-I) appeared and grew with an increase in halide concentration in glassy methanolic LiCl solutions. The higher frequency peak was assigned to the OH stretching Raman band arising from methanol molecules weakly hydrogen bonded to halide ions and the lower frequency one to the OH stretching Raman band due to methanol molecules not only hydrogen bonded to halide ions but also coordinated to cations by their oxygen lone pairs. It is shown that ionization of some electrolytes (NaBr, Lil, and NaI) is greatly suppressed in glassy methanolic solutions.
Introduction Raman spectroscopy is a useful technique for characterizing solvation structures of dissolved ions in aqueous and alcoholic solutions.'-7 Alcoholic solutions of electrolytes have been extensively studied by Raman4-7 and infrared8-" techniques. However, most of these studiese9 are confined to the investigations of alcohol solutions at ordinary temperatures. Strauss and SymonsloJ1 have made extensive infrared studies of alcoholic solutions of various electrolytes at low temperatures, which have revealed that fine structure appears in infrared O H stretching spectra for glassy alcoholic solutions. Kanno and H i r a i ~ h i 'have ~ , ~ ~reported that Raman spectra for glassy aqueous solutions usually give better resolved Raman bands than those for the corresponding solutions at room temperature. These observation^'^'^ lead us to expect that a Raman spectrum for a glassy alcoholic solution should have better resolved features, which may be more informative about solvation than that for the solution at ordinary temperatures. In fact, a well-resolved Raman O H stretching spectrum has been obtained for a glassy ethanolic solution of lithium ch10ride.I~ Thus, as a continuation of our studiesi2-I4of the Raman OH stretching spectra of glassy aqueous and alcoholic electrolyte solutions, we investigated the Raman spectra of glassy methanol solutions of alkali-metal halides as a funtion of salt concentration. As alcohols behave as a hydrophilic solvent and have solvation structures analogous to aqueous solutions, Raman results for glassy alcoholic solutions should give us deeper insight into solvation in both alcoholic and aqueous solutions. Experimental Section All alcoholic solutions were prepared by dissolving commercially available anhydrous salts in absolute methanol (>99.5%). Here *To whom correspondence should be addressed
0022-3654/90/2094-6594$02.50/0
the salt concentration was denoted by R (= moles of alcohol/moles of salt). The sample solution was placed in a 3-5 mm i.d. Raman cell with a flat bottom and then rapidly cooled by being immersed into liquid nitrogen. The overall cooling rate was approximately 4 X IO2 K/min. Glass formation was checked visually. Methanolic solutions are generally vitrifiable, even with a low salt concentration, despite the fact that pure methanol is almost impossible to vitrify with a cooling rate of 4 X IO2 K/min. Raman spectra were obtained with a JASCO NR-1100 spectrometer using -300 mW of the 514.5 nm line of a NEC argon ion laser as an exciton source. The glassy sample was kept at liquid nitrogen temperature during Raman measurements by using a (1) Lilley, T. H. In Water: A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1973; Vol. 3, Chapter 6. (2) Irish, D. E.; Brooker, M. H. In Advances in Infrared and Raman Spectroscopy; Clark, R. J . H., Hester, R. E., Eds.; Heydon: New York, 1976; VOI. 2, pp 212-311. (3) Brooker, M. H. In The Chemical Physics of Solvation; Ulstrup, J . , Dogonadze, R. R., Kalman, E., Karnyshev, A. A,, Eds.; Elsevier: The Netherlands, 1986; Chapter 4. (4) Kecki, 2.Spectrochim. Acta 1962, 18, 1155 and 1165. (5) Minc, S.; Kurowski, S. Spectrochim. Acta, 1963, 19, 330. (6) Hester, R. E.; Plane, R. A. Spectrochim. Acta 1967, 23A, 2289. (7) AI-Baldawi, S. A.; Brooker, M. H.; Cough, T. E.: Irish, D. E. Can. J. Chem. 1970, 48, 1202. (8) Adams, D. M.; Blandamer, M. J.; Symons, M. C. R.; Waddington, D. Trans. Faraday Soc. 1971, 67, 61 1. (9) (a) Symons, M. C. R.; Waddington, D. Chem. Phys. Lett. 1975, 32, 133. (b) Strauss, I. M.; Symons, M. C. R. Chem. Phys. Lett. 1976,39,471. (10) Strauss, 1. M.; Symons, M. C. R. Chem. Phys. Lett. 1976, 39, 471. (1 I ) Strauss, I. M.; Symons, M. C. R. J . Chem. Soc., Faraday Trans. I , 1971, 73, 1796; 1978, 74, 2146. (12) Kanno, H.; Hiraishi, J. Chem. Phys. Lett. 1979, 68, 46. ( I 3) Kanno, H.; Hiraishi, J. Chem. Phys. Lett. 1980, 72, 541. (14) Yamauchi, S.; Kanno, H. Chem. Phys. Lett. 1989, 154, 248.
0 I990 American Chemical Society
