The Journal of
Physical Chemistry
0 Copyright, 1984, by the American Chemical Society
VOLUME 88, NUMBER 15
JULY 19, 1984
LETTERS Molecular Disposition of Inclusion Complexes of Three Aminoxyl Radicals with P-Cyclodextrin in Aqueous Solution Masaharu Okazaki and Keiji Kuwata* Department of Chemistry, Faculty of Science, Osaka University, Toyonaka, Osaka, 560, Japan (Received: February 8, 1984)
Q-Band ESR spectra of three aminoxyl radicals, di-tert-butylnitroxide, 2,2,6,6-tetramethyliperidine- 1-oxyl, and 2,2,6,6tetramethylpiperidine- l-oxyl-4-0l, showed distinct changes of the hyperfine splitting constants and the g factors when they are included in @-cyclodextrinin aqueous solution. These changes of the ESR parameters are qualitatively explained on the basis of the electrostatic interactions between the N-O groups of the aminoxyl radicals and the bulk water. The molecular dispositions of these inclusion complexes are proposed.
Introduction Cyclodextrins are the cyclic oligosaccharides which can form inclusion complexes with various molecules in aqueous solution. Because they catalyze the reaction of guest molecules in some cases, they have received a great deal of attention as an enzyme model.* One of the most important functions of enzyme is the molecular recognition as they form the Michaelis complexes. However, there are only a few studies which deal with the ability of the molecular recognition by cy~lodextrins.~,~ In this study, an example of the molecular recognition by @-cyclodextrin (6-CDX) is presented by observing Q-band ESR spectra of three similar aminoxyl free radicals included in @-CDX. These free radicals are di-tert-butylnitroxide (DTBN), 2,2,6,6tetramethylpiperidine- 1-oxy1 (Tempo), and 2,2,6,6-tetramethyl( 1 ) Bender, M. L.; Komiyama, M. "Cyclodextrin Chemistry"; SpringerVerlag: West Berlin, 1978 and references therein. (2) Tabushi, I. Acc. Chem. Res. 1982, 15, 66. (3) For example: Demarco, P. V.; Thakkar, A. L. J . Chem. SOC.D 1970, 2. Bergeron, R. J.; Channing, M. A. J . Am. Chem. SOC.1979, 101, 251 1 . (4) Atherton, N. M.; Strach, S . J. J . Chem. SOC.,Faraday Trans. 1 1975, 71, 357; J . Magn. Reson. 1975, 17, 134.
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piperidine- 1-0xyl-4-01 (Ternpol), Experimental Section DTBN and 6-CDX were purchased from Eastman Kodak Co. and Wako Pure Chemical Industries, respectively, and used without further purification. Tempo and Tempol were synthesized by oxidation of their precursors which were purchased from Aldrich Chemical Co. The phosphate buffer solution of 0.1 mol dm-3 prepared from the phosphate salts and deionized water was used as the solvent throughout the experiments. The sample solution was filled in a quartz tube of 1.O-mm 0.d. without degassing. ESR spectra were recorded at room temperature with a Varian Q-band accessory. Results At the low concentration of @-CDX,both the central peak and the high-field peak in the Q-band ESR spectrum of DTBN split into two components as shown in Figure 1A. The component at the higher field in each peak has been assigned to the free DTBN! As the concentration of p-CDX increases, the high-field peak as well as the central peak becomes a single line as shown 0 1984 American Chemical Society
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The Journal of Physical Chemistry, Vol. 88, No. IS, 1984
Letters TABLE I: Equilibrium Constants (K) for the Complex Formation for the Three Inclusion Complexes
H0.5 r n T
guest molecule DTBN Tempo Tempol
k, dm3 mol-’ 1500 k 100 2950 & 600 85 k 15 LI In the high-field peak of Q-band ESR spectra.
DTBN
0.12
0.02 0.22
OH
y-axis
Figure 1. QBand ESR spectra of 5.0 X lo4 mol dm-’ DTBN in the presence of (A) 5.2 X lo-’ mol dm-3 p-CDX and (B) 1.27 X IO-* mol dm-’ p-CDX. In both cases the major components are assigned to the included DTBN in the cavity of 0-CDX.
separation of , two comp,‘ mT
TEMPO
TEMPOL
TEMPOL
Figure 3. Proposed dispositions of three aminoxyl radicals included in the cavity of 8-CDX in aqueous solution. The cavity of 6-CDX is de-
picted as a cylinder.
1
TEMPO
-/IF--
Figure 2. Q-Band ESR spectra of 7.6 X
mol dm-’ Ternpol and 7.5 IO4 mol dm-’ Tempo solutions in the presence of 5.3 X IO-’ mol dm-3 p-CDX and 6.2 X lo4 mol dm-3 p-CDX, respectively; A total spectrum; B: enlarged spectrum of the high field absorption line. The simulation spectra are shown with the dotted lines. X
in spectrum B. Q-Band ESR spectra of Ternpol and Tempo solutions in the presence of 0-CDX are shown in Figure 2. Spectrum B of each system represents the high-field peak and its spectrum simulation. The separation between the two components and their intensity ratio in the high-field peak were determined by spectrum simulation in each system. The equilibrium constants of the complex formation were determined by assuming 1:1 stoichiometry of the complex formation. The values of the equilibrium constants are listed in Table I.
Discussion As the solvent polarity decreases, the g factor increasess and the hyperfine splitting constants decrease.s,6 Thus, the changes in both parameters together increase the separation between the two peaks. The separations of the two components in the high-field peaks were 0.22, 0.12, and 0.02 mT for Tempol, DTBN, and Tempo, respectively. By comparison of these values with those calculated from data in the literat~re,~.’ the N - O groups of Ternpol ( 5 ) Kawamura, T.; Matsunami, S.; Yonezawa, T. Bull. Chem. SOC. Jpn. 1966, 40, 1111. Zager, S.A.; Freed, J. H. J . Chem. Phys. 1982, 77, 3344. (6) Knauer, B. R.; Napier, J. J. J . Am. Chem. SOC.1976, 98, 4395. (7) On the basis of the data of Table 1 of ref 5, it is expected that the shift
of the high-field peak of DTBN is 0.23 mT (at Q-band) when the radical is transferred from water to methanol. Nearly the same shift in the high-field peaks is also expected for the other two radicals because of the similarity in the electronic structure of these radicals.
and Tempo are shown to be in the environments of nearly the same polarity as methanol and the bulk water, respectively, while the N-0 group of DTBN is probably of intermediate polarity between methanol and water. Recently, the molecular disposition and the dynamical properties of the inclusion complex of P-CDX with DTBN in aqueous solution have been studied by the present authors.8 According to their results in this system, the guest molecule is included with its y axis (see Figure 3) directed parallel to the axis of the cavity of p-CDX. Thus, the N - 0 group in DTBN is almost fully included in the cavity, which offers a less polar environment than that of the bulk water. The N-0 group in Ternpol should be deeply included in the cavity, since the shift of the high-field peak is even greater than that for DTBN. In the case of Tempo the N - 0 group is supposed to be exposed to the bulk water because of the small shift observed for the high-field peak. Considering the molecular sizes of Ternpol and Tempo and the dimension of the cavity of 0-CDX, it seems to be difficult for p-CDX to accommodate the guest with they axis directed parallel to the axis of the cavity of /3-CDX. Thus, we propose the dispositions shown in Figure 3 for the complexes. The definition of the molecular coordinate system is also shown in the figure. The equilibrium constant for the complex formation may relate to the contact area between the host and the guest molecules if one assumes hydrophobic interactions for the complex f ~ r m a t i o n . ~ -AGO = RT In K = const X S,
(1)
where S,and K are the contact area and the equilibrium constant of the complex formation, respectively, and the other symbols have the usual meanings. The relative magnitude of the contact area can be estimated by using eq 1 from the equilibrium constants listed in Table I. The result is as follows: S,(DTBN):S,(Tempo):S,(Tempol) = 0.9: 1.O:O.S (2) This also gives an intuitive measure to elucidate the molecular dispositions of the inclusion complexes. In the case of Tempo the strongest hydrophobic interaction might be expected when the Tempo molecule is accommodated (8) Okazaki, M.; Kuwata, K. J . Phys. Chem., in press.
(9) Tabushi, I.; Nishiya, T. “Host-Guest Chemistry”; Osa, T., Ed.; Kyoritsu Publishing: Tokyo, 1979; Chapter 1.
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In the case of Ternpol the hydrogen bonding between the hydroxyl group of Ternpol and the bulk water seems to reduce the stability of the molecular complex, and this may be a reason for the small equilibrium constant of complex formation in this system. Recently, the rate of the reduction of these aminoxyl radicals by ascorbic acid in aqueous solution was observed in the presence of 0-CDX. The reaction rates of both DTBN and Tempol lowered to '/2-'/3 of their original values (Le. in the absence of p-CDX) in the temperature range of 273-310 K. There was, however, no change in the case of Tempo.lo These observations are consistent with the models shown in Figure 3, if the effect of protection by the cavity wall of p-CDX on the reaction is considered. It is noted here that formation of inclusion complexes with p-CDX sharply discriminates these three aminoxyl radicals, though they are not so different from each other in both their molecular shape and electronic structures.
in the cavity along its y axis like DTBN. This disposition has been suggested by Rassat et al." as one of the possible structures of the complex; however, it is vitiated by the present results. The disposition depicted in Figure 3 also gives strong hydrophobic interaction because the hydrophobic trimethylene group is fully included in the cavity and the four methyl groups also in contact with the cavity wall of 0-CDX. Thus, the dispositions in the cases of Ternpol and Tempo are inclusive of the molecular sizes and the shapes of both the guest and the host molecules and the hydrophobic interaction between them. In the DTBN complex the disposition shown in Figure 3 gives the strongest hydrophobic interaction with the cavity wall of the host molecule. (10) Okazaki, M.; Kuwata, K.,to be submitted for publication. (11) Martinie, J.; Michon, J.; Rassat, A. J. Am. Chem. SOC.1975, 97, 1818.
Temperature and Site Dependence of the Rate of Hydrogen and Deuterium Abstraction by Methyl Radicals In Methanol Glassed T. Doha,$ K. U. Ingold, W. Siebrand,* and T. A. Wildmad Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K1A OR6 (Received: March 9, 1984)
Rate constants are reported for hydrogen and deuterium abstraction by methyl radicals in CH30H and CD30D glasses in theranges 5-89 and 77-97 K, respectively. At each temperature, they show a distribution due to a variation of radical trapping sites. The rate constants of this distribution are analyzed theoretically to yield a quantitative relation between tunneling rate and equilibrium tunneling distance.
Introduction Hydrogen abstraction by methyl radicals in organic crystals and glasses has been studied e~tensively'-'~and is generally regarded as an outstanding example of a reaction proceeding by hydrogen t ~ n n e l i n g . ~ -In ' ~ this paper we report experimental and theoretical results for the reactions CH3. CH30H(D) CH4 .CH,OH(D) and CH3. + CD30D CH3D + .CD20D which together form a relatively simple model system for testing theoretical descriptions of the tunneling process. These reactions can be studied conveniently by generating the methyl radicals in a methanol glass. In earlier studies of this system,'-12 two types of experimental difficulties were encountered: the hydrogen transfer did not follow (pseudo-) first-order kinetics and the rate of deuterium transfer was too slow to be measured accurately. W e have recently shown how the nonexponential decay of the methyl radical can be explained as a distribution of exponential decays.I3 Using this approach, we report here new and well-defined exponential rate constants for the hydrogen transfer reaction between 5 and 89 K. In addition, we report for the first time the corresponding deuterium transfer rate constants from 77 to 97 K. These observations are used to test a recently d e ~ e l o p e d ' ~ - ' ~ two-dimensional tunneling model.
-+
+
-
Experimental Section In our experiments, methyl radicals were generated from methyl chloride by dissociative electron c a p t ~ r e . ~ - ' , 'Methanol, ~ 99.9% pure (dried over barium oxide), and C H 3 0 D , 99.5% pure, from Aldrich Chemical Co., and CD30D, 99.6% pure, from Merck Sharp and Dohme Isotopes, were used without further purification. The glassy solutions, formed at 77 K by rapid immersion into liquid nitrogen, were placed in a Dewar capable of maintaining a preset
f
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0022-3654/84/2088-3165$01.50/0
temperature ( f l K; more accurately at the boiling points of nitrogen and argon). After reaching this temperature, the samples were irradiated in the cavity of a Varian E12 X-band ESR spectrometer with light from a 1-kW mercury lamp for a period short compared to the half-life of the methyl radical signal. The reaction was monitored through the high-field line of the methyl quartet, corrected for background due to the hydroxy carbinyl radical formed in the process. As before,5-',l3 the signal was found to decay nonexponentially at all temperatures used.
Results We recently13 explained the observed time dependence as due to the inhomogeneity of the medium: the presence of inequivalent trapping sites for the methyl radical in the glass leads to a distribution of first-order rate constants. If N(k,t) is the number (1) French, W. G.; Willard, J. E. J. Phys. Chem. 1968, 72, 4604. (2) Sprague, E. D. J. Phys. Chem. 1973, 77, 2066. (3) Neiss, M. A,; Willard, J. E. J. Phys. Chem. 1975, 79, 783. (4) Neiss, M. A.; Sprague, E. D.; Willard, J. E. J. Chem. Phys. 1975, 63, 1118. (5) Bol'shakov, B. V.; Tolkatchev, V. A. Chem. Phys. Lett. 1976,40,468. (6) Stepanov, A. A.; Tkatchenko, V. A,; Bol'shakov, B. V.; Tolkatchev, V. A. In?. J. Chem. Kine?. 1978, 10, 637. (7) Bol'shakov, B. V.; Stepanov, A. A.; Tolkatchev, V. A. Int. J. Chem. Kine?. 1980, 12, 27 1. (8) Wang, J. T.; Williams, F. J. Am. Chem. SOC.1972, 94, 2930. (9) Campion, A.; Williams, F. J. Am. Chem. SOC.1972, 94, 7633. (10) Hudson, R. L.; Shiotani, M.; Williams, F. Chem. Phys. Lett. 1977, 48, 193. (1 1) Le Roy, R. J.; Murai, H.; Williams, F. J. Am. Chem. SOC.1980, 102, 2325. (12) Williams, F.; Sprague, E. D. Arc. Chem. Res. 1982, 15, 408. (13) Doba, T.; Ingold, K. U.; Siebrand, W. Chem. Phys. Lett. 1984,103, 339. (14) Siebrand, W.; Wildman, T. A.; Zgierski, M. Z. Chem. Phys. Lett. 1983, 98, 108. (15) Siebrand, W.; Wildman, T. A.; Zgierski, M. Z. J . Am. Chem. SOC., in press. (16) Siebrand, W.; Wildman, T. A.; Zgierski, M. 2. J. Am. Chem. SOC. in press.
Published 1984 American Chemical Society
