Unstable Intermediates in X-Irradiated Clathrate Hydrates: ESR and

X-irradiation of tetramethylammonium hydroxide pentahydrate (TMNOH) at 77 K produces trapped electrons, and CH3• and (CH3)3N+CH2• radicals...
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J. Phys. Chem. 1996, 100, 3910-3916

Unstable Intermediates in X-Irradiated Clathrate Hydrates: ESR and ENDOR of Tetramethylammonium Hydroxide Pentahydrate (TMNOH) Janusz Bednarek,† Anders Lund,‡ and Shulamith Schlick*,§ Institute of Applied Radiation Chemistry, Technical UnVersity of Lodz, 93-590 Lodz, Poland, Department of Physics and Measurement Technology, Linkoping UniVersity, Linkoping S-581 83, Sweden, and Department of Chemistry, UniVersity of Detroit Mercy, Detroit, Michigan 48219-0900 ReceiVed: August 22, 1995; In Final Form: December 1, 1995X

X-irradiation of tetramethylammonium hydroxide pentahydrate (TMNOH) at 77 K produces trapped electrons, and CH3• and (CH3)3N+CH2• radicals. The trapped electrons were detected by bleaching with visible light and by subtraction of spectra from bleached and unbleached samples. The decay of the methyl radicals starts at ≈100 K and that of the (CH3)3N+CH2• radicals at ≈150 K. The ESR spectrum measured at 130 K has the best resolution and is a superposition of contributions from the methyl radicals (13%) and from the (CH3)3N+CH2• radical (87%). The reversibility of the line shapes with temperature variations suggested that the increased resolution observed at 130 K is due to dynamical effects involving the (CH3)3N+CH2• radicals. ESR spectra from these radicals can be simulated by assuming that the hyperfine tensor components for the two R protons are averaged by two types of motions: rotation of the CH2 group about the C2V symmetry axis and precession or wobbling of this axis. The parameters used in the simulation are giso ) 2.0022, two equivalent R protons with axial hyperfine components A|av ) 25.0 G and A⊥av ) 23.0 G, one 14N nucleus with AN ) 3.9 G, and one remote proton with AH,remote ) 4.2 G. The remote proton is identified with a lattice proton. If the precession model is adopted, the precession angle calculated from the principal values of the hyperfine tensor for the R protons used in the simulations is 47°. For the wobbling model, the wobbling angle deduced is 72°. These results suggest that the hydrogen-bonded cage around the guest allows large-scale dynamical effects even at 130 K.

Introduction Peralkylammonium clathrate hydrates belong to a large number of clathrates that have been known for a long time and have been classified into three main groups.1 The most prominent group includes the gas hydrates that have been known since 1811, when Davy discovered and identified crystals of chlorine hydrates; this group also includes hydrates of inert gases. The other two groups are the alkylamine hydrates discovered in 1893 and the peralkylammonium clathrate hydrates discovered in 1940. Although different in composition, the clathrate hydrates share important structural characteristics because the major component, water, is common to all three groups. The clathrate hydrates exist because water molecules crystallize and form a host lattice that can accept other molecules as guests. Polyhedral cavities exist in both ice and the clathrates and are so large in the clathrates that their structure collapses in the absence of guests. The phase diagrams of the clathrate hydrates have been studied since their discovery. X-ray diffraction has become an important method for structural studies and for assessing the effect of guests on cavity size.1-3 More recently, novel spectroscopic methods have been applied: Ripmeester et al. have developed a probe method for the structure of hydrocarbon clathrates hydrates based on 129Xe NMR.4 Clathrates of the type H+(H2O)n, and also mixed clathrates such as H+(H2O)n(TMA)m (where TMA is trimethylamine), have been * To whom correspondence should be addressed. E-mail: SCHLICKS@ UDMERCY.EDU. † Technical University of Lodz. ‡ Linkoping University. § University of Detroit Mercy. X Abstract published in AdVance ACS Abstracts, February 1, 1996.

0022-3654/96/20100-3910$12.00/0

studied in the gas phase using mass spectroscopic techniques developed by Castleman et al.5 Renewed interest in the clathrate hydrates has also been sparked by theories that proposed the formation of clathrate hydrates with O2 and N2 as guests at depths below the layer of porous ice,6 by recent strategies to exploit the huge amounts of methane that exist in clathrate hydrate form as a future energy source,7 and by their fundamental importance in the study of lattice effects on guest dynamics.8 We became interested in the clathrate hydrates primarily because pulse radiolysis studies at room temperature in a large number of clathrates hydrates have suggested the presence of highly stable trapped electrons.9 This stability has been related to the high electronegativity of the quaternary nitrogen and to the cage effect. These systems presented to us the unique opportunity to compare ionization and charge separation processes in the clathrate hydrates with systems that contain crystalline or glassy water and with other crystalline systems.10-19 We have initiated a study of the radiation effects on clathrate hydrates, in an effort to identify the main unstable intermediates and to determine their dynamics and reactivity. Two compounds were chosen for this initial study: tetramethylammonium hydroxide pentahydrate, (CH3)4N+OH-‚5H2O, mp 335 K (TMNOH), and tetra-n-butylammonium hydroxide hydrate, (C4H9)4N+OH-‚31H2O, mp 303 K (TBNOH).20 This selection was made because the crystal structure for both hydrates has been determined,1-3 the melting points are above ambient temperature, and the amount of water in the stoichiometric compounds is large, about 50% by weight in TMNOH and 68% in TBNOH. Therefore, the radiolytic behavior of these hydrates can be compared with the well-known results for other forms of water (ice, liquid, and glass). TBNOH has the typical crystal structure of most peralkylammonium hydrates based on the © 1996 American Chemical Society

Unstable Intermediates in Clathrate Hydrates

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Figure 1. Distorted truncated octahedron containing the (CH3)4N+ ion in the permethylammonium hydroxide clathrate. The three disordered CH3 groups are represented by the torus, and the hydrogen atoms are omitted (redrawn from ref 3).

pentagonal dodecahedral unit; TMNOH is unique among the peralkylammonium clathrate hydrates, in that its structure is based on a truncated octahedron, as shown in Figure 1.3 The distances between the nitrogen (N) in the center of the cage and the oxygen atoms of the water lattice in TMNOH range from 4.3 Å (to O1) to 4.97 Å (to O2), and the average is 4.61 ( 0.16 Å. The study of the crystal structure suggests that three of the four methyl groups of each cation are disordered, in the sense that they appear to rotate about the fourth N-C bond, thus forming the torus shown in Figure 1. In a recent publication, we have reported the detection by ESR of trapped electrons in polycrystalline TMNOH and TBNOH irradiated with X-rays at 77 K.20 Formation of alkyl radicals was also detected by ESR, in the temperature range 77-210 K. The novel aspect of these systems is the reappearance of the trapped electrons at 150 K, after bleaching of the corresponding signal by visible light. Evidence for the reappearance of the electrons has been obtained from ESR spectra and from the blue coloration typical of trapped electrons. While these phenomena were detected in both TMNOH and TBNOH, the spectra from the alkyl radicals in TMNOH were more amenable to interpretation, and we proposed the presence of CH3• and NCH2• radicals. The ESR spectra obtained for TBNOH after bleaching are complex but also suggest the formation of two types of radicals, one of which was postulated to contain nitrogen.21 In this paper, we present a detailed analysis of the stability of the reactive intermediates formed in TMNOH and provide an identification of the alkyl radicals formed and their dynamics, based on ESR and ENDOR spectra, and spectra simulations.

Figure 2. X-band ESR spectra recorded at 77 K of TMNOH X-irradiated at 77 K: (A) immediately after irradiation; (B) after bleaching for 10 min; (C) signal obtained by subtracting B from A.

rectangular cavity. Unless specified otherwise, spectra were measured with a microwave power of 0.02 mW (40 dB) and 100-kHz magnetic field modulation of amplitude 1 G. Cr3+ in a single crystal of MgO was used as a g standard (g ) 1.9796), and 2,2-diphenyl-1-picrylhydrazyl (DPPH) in benzene solution was used to quantitate the spin concentration by comparison with the visible absorption. ENDOR spectra were recorded in the EN 801 cylindrical cavity, with a saturating microwave power of 20 mW and with 100 mW of rf power modulated at 12.5 kHz. Normally accumulation of 25 scans was needed to improve the signal-to-noise ratio. ESR spectra at 77 K were obtained in a standard Dewar insert. Temperature variation of the ESR and ENDOR spectra in the range 4-200 K was achieved with the Oxford ESR 900 continuous helium gas flow cryostat. Above 220 K, it was not possible to tune the microwave cavity, most likely because the water molecules in the clathrate lattice become mobile. ESR spectra were simulated with the program KVASEC, which applies the perturbation theory to calculate ESR spectra for S ) 1/2 to second order, for the case of anisotropic Zeeman, hyperfine and quadrupole interactions with noncoincident principal axes systems.22,23 Results

Experimental Section Crystalline TMNOH (from Fluka) was melted in glass tubes of diameter slightly less than that of the 4-mm-i.d. Suprasil ESR sample tubes and solidified in the refrigerator. The solid sample (≈20 mm long) was pushed out of the “mold” by gentle heating of the walls and transferred into the ESR sample tubes. After X-irradiation at 77 K, the sample was pushed, under liquid nitrogen, into the unirradiated sample tube end, thus avoiding the background quartz ESR signal. Samples prepared in this way did not crack on rapid temperature changes and did not vibrate in the continuous gas flow cryostat during measurement of spectra. Irradiation was done by an X-ray source equipped with a gold anode operating at 70 kV and 20 mA. All irradiations were carried out in the dark at 77 K for 1 h. Bleaching with visible light was achieved with a slide projector equipped with a halogen lamp and an Oriel IR cut-off filter Type G-776-7100. ESR measurements were performed with the X-band Bruker 200D spectrometer equipped with an ER 4105DR double-

ESR Spectra. The ESR spectrum of polycrystalline TMNOH X-irradiated at 77 K and measured at the same temperature is shown in Figure 2A and consists of a strong central feature flanked by broad signals. Bleaching of the central part of the spectrum is accompanied by resolution enhancement of the remaining signals, as seen in Figure 2B. The bleached component (Figure 2C), obtained by subtraction of spectra 2B from 2A, is centered on g ) 2.0008, a value typical of trapped electrons.19 We notice in Figure 2C the presence of two side bands with inverted phase compared to the main spectrum. Such signals are most likely due to the decrease of the line width and increase of the relative intensity of one of the radicals, after bleaching, as clearly seen by comparing spectra 2A and 2B in Figure 2. This result indicates that magnetic dipolar interactions with the trapped electrons broaden the signals from the other species and suggests localization of the electrons close to the radicals derived from the tetramethylammonium cations, probably in the same water-enclosed cavity. Conspicuous in their absence in the ESR spectra presented in Figure 2 are products

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Figure 3. Spin concentration in TMNOH irradiated at 77 K in the dark, from ESR spectra measured at 77 K. A, as a function of irradiation time; B, as a function of annealing temperature (3 min at each temperature) for an irradiation time of 60 min. Solid lines are drawn as guides to the eye.

of water radiolysis such as hydrogen atoms, •OH and/or O-, in spite of the large molar fraction of water in this clathrate hydrate, 5/6. Bobrowski24 has shown that both •OH and O- react vigorously with tetraalkylammonium cations in the liquid phase; the above results suggest that these reactions occur also in the solid. We have detected, however, H atoms in X-irradiated TBNOH.21 After the initial results, we established the general conditions for radical formation and decay. The total spin concentration obtained by irradiation at 77 K in the dark is plotted in Figure 3A as a function of irradiation time. The thermal stability is presented in Figure 3B and is plotted as the ESR intensity of the unbleached samples measured at 77 K as a function of the annealing temperature (3 min at each temperature). The radicals are stable at 77 K; the decay starts at ≈100 K, and a plateau is reached around 130 K; the final decay occurs above ≈150 K. On the basis of the results presented in Figure 3A, we decided to irradiate the samples for 1 h at 77 K. In this way, we were able to maintain good signal-to-noise ratio in the ESR spectra while still working in the linear portion of the yield vs dose dependence. The temperature dependence of ESR spectra for TMNOH irradiated at 77 K and bleached for 10 min is presented in Figure 4. The signals indicated by circles (quartet with a splitting of ≈23 G) weaken as the temperature increases, compared to the signals indicated by squares, suggesting the presence of at least two types of radicals. At 100 K, the component indicated by circles can still be observed, and the resolved spectrum observed at 130 K suggests the presence of a radical of type RCH2•, where the major triplet, with a splitting of ≈24 G, is assigned to hyperfine interactions from two equivalent protons. The additional, smaller splittings detected at 130 K indicate a hyperfine coupling of ≈4 G. The spectrum at 190 K suggests the reappearance of the electron, as deduced from the blue color of the sample and the central sharp line in the ESR spectra.20 Identification of the quartet indicated by circles in Figure 4 with the methyl radical CH3• is demonstrated in Figure 5. Subtraction of the ESR spectrum annealed at 130 K and recorded at 40 K (Figure 5B) from the unannealed spectrum recorded at 40 K (Figure 5A) results in the quartet shown in Figure 5C, where the splittings and the number of lines are typical of the methyl radical. The intensity ratio of the four signals is not exactly as expected for CH3•, most likely because of line width variations with sample treatment and spin concentration. The

Bednarek et al.

Figure 4. X-band ESR spectra of TMNOH X-irradiated in the dark at 77 K, bleached with visible light, and measured at the indicated temperatures. The microwave power was 0.02 µW (70 dB) at 4 K and 20 µW (40 dB) at the other temperatures. Signals assigned to methyl and (CH3)3N+CH2• radicals are shown by b and 9, respectively.

Figure 5. X-band ESR spectra of TMNOH X-irradiated in the dark at 77 K, bleached with visible light, and measured at 40 K. A, unannealed sample; B, sample annealed at 130 K; C, result of subtracting B from A; D, sample annealed at 190 K. Arrows in B indicate signals from the methyl radicals.

spectra in Figure 5 clearly suggest, however, that the first stage of the decay shown in Figure 3B, which starts at ≈100 K, involves the partial decay of methyl radicals and that the decay of these radicals is irreversible. The component indicated by squares in Figure 4 is assigned to (CH3)3N+CH2• radicals; additional proof for this assignment will be given in the Discussion section. The spectral changes involving this radical are completely reversible up to 130 K and suggest dynamical effects, as seen by comparing the spectrum at 130 K in Figure 4 with Figure 5B. The second decay stage shown in Figure 3B, which starts at ≈150 K, is therefore due to the decay of these radicals. The spectrum shown in Figure 5B also indicates that the decay of the methyl radicals by annealing to 130 K is not complete: the shoulders at the low and high field edges of the spectrum in Figure 5B (indicated by arrows) are contributions from these radicals. The ESR spectrum in Figure 5D is that of the sample annealed at 190 K and measured at 40 K; the central feature resembles

Unstable Intermediates in Clathrate Hydrates

Figure 6. X-band ESR spectra of TMNOH X-irradiated in the dark at 77 K, unbleached, and measured at 77 K before annealing (bottom spectrum) and after successive annealing at the indicated temperatures for 3 min. The top spectrum is the result of subtracting the spectrum annealed at 180 K from the unannealed spectrum.

that in Figure 2C, which was attributed to the trapped electrons. Its reappearance is accompanied by the blue color of the sample and was attributed to the dissociation of electron pairs formed on bleaching.20 The evolution of the ESR spectra measured at 77 K of unbleached X-irradiated TMNOH is shown in Figure 6, for the indicated annealing temperature. The top spectrum is the result of subtracting the spectrum annealed at 180 K from the unannealed (bottom) spectrum. The central part after subtraction is similar to that assigned to the trapped electron (Figure 2C); the arrows indicate a splitting of ≈23 G, thus reinforcing the idea that the major effects of annealing to 180 K are changes in the total spectral intensity due to the decay of the methyl radicals and of the trapped electrons. In pure polycrystalline ice irradiated at 77 K, trapped •OH radicals start to decay at 100 K14 and trapped HO2• radicals above 150 K,15 and the best resolution of ESR spectra from HO2• radicals is observed at ≈130 K.16 The similarity of the decay ranges and of the spectral resolution with the observations presented above for the clathrate hydrates is probably not accidental and suggests that the hydrogen-bonded lattice in ice and in the clathrates have similar dynamics. ENDOR Spectra. ENDOR was measured at 4, 40, and 80 K. The most intense signals were observed at 40 K and are presented in Figure 7. The doublet centered at 4.9 MHz in Figure 7A is assigned to 14N hyperfine interaction: the separation is 2.024 MHz, as expected for twice the Larmor frequency of this nucleus at ≈3370 G. The center of the doublet, expected to appear at AN/2,25,26 is 4.9 MHz and corresponds, therefore, to an isotropic 14N hyperfine splitting AN of 9.8 MHz (3.5 G). The splittings in the proton region are shown in Figure 7B, on an expanded radio-frequency (RF) scale and with the rf modulation depth reduced by a factor of 10. The maximum proton splitting is 4.3 MHz (1.5 G), which is within the line width of the spectra presented in Figure 4, even for the highest resolution detected at 130 K.

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Figure 7. ENDOR spectra of TMNOH X-irradiated in the dark at 77 K, bleached with visible light, and measured at 40 K. A, the doublet assigned to 14N and the 1H signals out of scale; B, horizontally expanded 1H signals recorded with an rf modulation amplitude smaller by a factor of 10. The major proton doublet (between arrows) has a splitting of 4.3 MHz (1.5 G).

Discussion Identification of Alkyl Radicals. The stable species obtained by radiolysis of TMNOH at 77 K can be derived from water and from the guest (CH3)4N+OH-. The products expected from water radiolysis are OH radicals, H atoms, and trapped electrons; of these, only trapped electrons were detected in the system studied, and their signal can be removed by bleaching. Methyl radicals were identified unambiguously by the spectra subtraction presented in Figure 5. Inspection of the spectrum measured at 130 K (Figure 4) together with that in Figure 5B indicates that not all methyl radicals disappear by annealing at 130 K. In Figure 8, we show the deconvolution of the signal measured at 130 K (Figure 4) into the contribution of the methyl radicals (8B, ≈13%) and the second signal (8C, ≈87%); the most likely candidates for the latter signal are (CH3)3N+• and (CH3)3N+CH2•, both generated from (CH3)4N+. The crucial additional information obtained in the study of X-irradiated TMNOH comes from the ENDOR spectra, which indicate an isotropic splitting of 9.8 MHz (3.5 G) at 40 K from 14N (Figure 7A). This splitting can be compared with the principal values of the 14N hyperfine tensor, 7.38, 8.77, and 9.44 MHz, measured at 77 K in the NHCH2• fragment trapped in X-irradiated single crystals of hippuric acid, C6H5C(dO)NHCH3COOH;27 the average, AN ) 8.53 MHz, is similar to the value of 9.8 MHz detected in this study. By contrast, the corresponding isotropic hyperfine splitting for 14N in (CH3)3N+ is 50.4 MHz (18.0 G).28,29 The 14N ENDOR splitting (Figure 7A), therefore, identifies the additional alkyl radical as (CH3)3N+CH2•. The isotropic 14N splitting in (CH3)3N+CH2• is due to spin polarization by the 2pz orbital and can be estimated from AN ) QNNNFπN, where FπN is the spin density in the nitrogen 2pz orbital and QNNN is the σ-π interaction constant for 14N, which has a value of ≈25 G.30 The odd electron density in the 2pz (14N) orbital is 3.5/25 or 0.14. Because one electron in the 2pz orbital

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Figure 8. X-band ESR spectra of X-irradiated TMNOH in the dark at 77 K, bleached with visible light, and measured at 130 K: (A) experimental spectrum; (B) simulated spectrum of the methyl radical (aiso ) 22.9 G, peak-to-peak line width ) 7 G, Lorentzian line shape, and intensity 13% of that in A); (C) signal obtained by subtracting B from A.

Figure 9. X-band ESR spectrum of X-irradiated TMNOH in the dark, bleached with visible light, and measured at 130 K, after subtracting the contribution of the methyl radical signals and simulated spectra with 1, 3, and 5 remote protons, respectively. The simulation parameters are giso ) 2.0022, A|av ) 25.0 G, and A⊥av ) 23.0 G (2 R protons), and isotropic splittings of 4.2 and 3.9 G for the remote proton(s) and the 14N nucleus, respectively. Additional details are given in the text.

of 14N corresponds to a maximum anisotropic splitting of 34 G,31 the 14N dipolar tensor is expected to have principal values of 4.8, -2.4, and -2.4 G. These values will be averaged by motion, as described below. The alkyl radicals obtained in irradiated TMNOH (after bleaching) can be compared to those identified in γ-irradiated single crystals of tetramethylammonium halides (TMNX),28,29 where CH3•, (CH3)3N+•, and (CH3)3N+CH2• were detected, depending on the anion, the irradiation temperature, and the annealing conditions. In the chloride TMNCl, the dominant species detected after room-temperature irradiation is (CH3)3N+•, with isotropic splittings of 18.0 and 26.7 G for 14N and 1H nuclei, respectively;28 all nine methyl protons are equivalent, with an isotropic splitting of 26.7 G. In the iodide TMNI, irradiation at 77 K produces signals that were identified with methyl radicals superimposed on a broad (30-40 G) signal, and no significant changes occurred on annealing to 148 K.29 In γ-irradiated TMNCl and TMNBr, CH3• and (CH3)3N+CH2• radicals were detected on irradiation at 77 K. The ESR spectrum of the γ-irradiated TMNBr at 77 K (Figure 1B in ref 29) is similar to that obtained in TMNOH after bleaching of the signal from the trapped electrons, Figure 2B, and has been assigned to a superposition of CH3• and (CH3)3N+CH2• radicals,29 as in this study. Annealing to 148 K leads to a triplet of broad lines, with a splitting of ≈24 G; the resolution did not allow further characterization. Comparison of the results presented for the X-irradiated TMNOH with those obtained for single crystals of TMNX reinforces the assignments we proposed for the signals presented in Figure 4 and suggests that while the trapped electrons are formed from the water lattice in the clathrate, the alkyl radicals are derived from the guest. Moreover, the higher resolution for the ESR spectra assigned to (CH3)3N+CH2• radicals in TMNOH, compared to the same radical in TMNCl and TMNBr single crystals, must be due to dynamical effects and to motional averaging of the ESR parameters. Spectral Simulations. We failed in our initial attempts to reproduce the line shape given in Figure 8C by assuming the

full anisotropy of the hyperfine splittings from the R protons in (CH3)3N+CH2• (principal values of 33.1, 20.6, 10.132) or even partially averaged tensors due to the C2V rotation about the C-C bond in the ethyl radical (A| ) 29.9 G and A⊥ ) 20.0 G33 or A| ) 29.6 G and A⊥ ) 19.4 G34). These initial simulations established, however, that the g tensor is isotropic, that the degree of motional averaging of the hyperfine anisotropy of the R protons goes beyond a C2V rotation about the C-N bond, and that there must be an odd number of “remote” protons. The most successful simulations are presented in Figure 9, together with the “experimental” spectrum obtained by subtracting the contribution of the methyl radical from the spectrum measured at 130 K, Figure 8C. The parameters used for the simulations are isotropic g tensor, giso ) 2.0022; two equivalent R protons with axial hyperfine tensor A|av ) 25.0 G and A⊥av ) 23.0 G; one 14N nucleus with AN ) 3.9 G; and Lorentzian line shapes with a peak-to-peak line width of 3.1 G. The simulated spectra presented in Figure 9 differ in the number of remote protons (1, 3, and 5), all with isotropic hyperfine constant AH,remote ) 4.2 G. The best fit is obtained with one remote proton. Since the nine methyl protons are expected to be equivalent, we assign the splitting of 4.2 G to the OH- proton and/or the clathrate protons. The splitting of 1.5 G measured in the ENDOR spectra (Figure 7B) is tentatively assigned to the equivalent methyl protons. We plan deuterium substitution experiments to confirm these assignments. Dynamics in (CH3)3N+CH2•. The most striking result of the simulations is the very low anisotropy of the hyperfine tensors for the R protons. These tensor components suggest additional averaging, beyond the rotation of the methylene protons about the C2V symmetry axis, which is along the N-C bond. Two additional motional mechanisms can explain this result: a precessional motion of the N-C axis direction with an angle γ and wobbling of this axis in a solid angle defined by γ, as shown in Figure 10. A precessional motion has been proposed previously, based on X-ray diffraction studies at ambient temperature:3 “the N-C bond is describing a solid angle

Unstable Intermediates in Clathrate Hydrates

J. Phys. Chem., Vol. 100, No. 10, 1996 3915 the methyl radicals after bleaching of the sample (as seen in Figure 2A and 2B) suggests, however, that this reaction occurs to some extent. Conclusions

Figure 10. Dynamical model for (CH3)3N+CH2• radicals: rotation about the symmetry axis (C-N direction) and wobbling or precession of the symmetry axis with an angle γ.

of about 15°”. The parallel tensor component obtained as a result of the additional motion, A|av, can be calculated from35,36

A|av ) A⊥ + (A| - A⊥)W (1) where A| and A⊥ are, respectively, the hyperfine tensor components of the R protons that result from the rotation of the CH2 group about the N-C bond. For the precession model with a precession angle γ, W ) cos2 γ while for the wobbling model with a wobbling angle γ,

(2)

W ) (1/3)(cos2 γ + cos γ + 1) (3) The tensor component value used in the simulation presented in Figure 8 (A|av ) 25.0 G) together with the tensor elements averaged by rotation about the symmetry axis (A| ) 30.4 G and A⊥ ) 20.3 G37) give a value of 47° for the precessional angle and a value of 72° for the wobbling angle. Both the precessional and wobbling mechanisms suggest large-scale motion of the symmetry axis at 130 K, above that suggested from crystallographic data obtained at ambient temperature.3 We emphasize, however, that the ESR spectroscopic results provide direct evidence for the motional mechanism we proposed above; moreover, the results we obtained are in agreement with the recent NMR studies,8 which have indicated that the activation energy for rotational jumps of benzene in clathrate hydrates is significantly lower compared to water solutions, 22 kJ/mol in solution vs 7.6 kJ/mol in the clathrate cages. The large-scale dynamics deduced from the splittings of the R protons is also responsible for the averaging of the 14N hyperfine interaction anisotropy and the measurement of an isotropic hyperfine splitting by ENDOR. Radiolysis Mechanism. On the basis of the results obtained, we propose the following mechanism for the formation of the intermediates in X-irradiated TMNOH. In the primary stage, electrons (e-), hydrogen atoms, and •OH radicals are produced. The initial processes are followed by trapping of the electron (et-) and by reactions with the guest and the formation of the two types of alkyl radicals, as shown below. H2O f e-, H, •OH V etH/•OH + (CH3)4N+ f (CH3)3N+CH2• + H2/H2O e- + (CH3)4N+ f (CH3)3N + CH3• The last reaction is largely suppressed by the high reduction potential of (CH3)4N+; the slight increase in the intensity of

X-irradiation of tetramethylammonium hydroxide pentahydrate (TMNOH) at 77 K produces trapped electrons and CH3• and (CH3)3N+CH2• radicals. The trapped electrons were detected by bleaching with visible light and by subtracting the spectrum of bleached from that of unbleached samples. The decay of the methyl radicals starts at ≈100 K and that of the (CH3)3N+CH2• radicals above ≈150 K. The best spectral resolution was observed at 130 K. Temperature variations suggest that the increased resolution observed at 130 K is due to dynamical effects involving (CH3)3N+CH2• radicals. Spectral simulations suggest that the hyperfine tensor components of the R protons in this radical are averaged by two types of motions: rotation of the CH2 group about its symmetry axis and precession or wobbling of the symmetry axis of the CH2 group about the direction of the C-N bond direction. If the precession model is adopted, the precession angle calculated from the simulated spectra is 47°. If the wobbling model is assumed, the wobbling angle deduced is 72°. These results suggest that the hydrogen-bonded cage around the guest allows large-scale dynamical effects even at 130 K. Acknowledgment. This study was supported by the Linkoping University, the Swedish Research Council (NFR), and in part by the U.S. National Science Foundation. References and Notes (1) Jeffrey, G. A. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D., Eds.; Academic: New York, 1984; Vol. 1, p 135. (2) McMullan, R.; Jeffrey, G. A. J. Chem. Phys. 1959, 31, 1231. (3) McMullan, R.; Mak, T. C. W.; Jeffrey, G. A. J. Chem. Phys. 1966, 44, 2338. (4) Ripmeester, J. A.; Ratcliffe, C. I. J. Phys. Chem. 1990, 94, 8773. (5) (a) Yang, X.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1989, 111, 6845. (b) Yang, X.; Castleman, A. W., Jr. J. Phys. Chem. 1990, 94, 8500. (c) Wei, S.; Shi, Z.; Castleman, A. W., Jr. J. Chem. Phys. 1991, 94, 3268. (6) Price, P. B. Science 1995, 267, 1802 and references therein. (7) Chem. Eng. News 1995 (March 6), 40. (8) Nakahara, M.; Wakai, C.; Matubayasi, N. J. Phys. Chem. 1995, 99, 1377. (9) Zagorski, Z. P. (a) Nucleonika 1981, 26, 869; (b) Chem. Phys. Lett. 1985, 115, 507; (c) J. Phys. Chem. 1987, 91, 734; (d) J. Phys. Chem. 1987, 91, 972; (e) J. Inclusion Phenom. Mol. Recognit. Chem. 1989, 7, 569. (10) Hart, E. J.; Anbar, M. The Hydrated Electron; Wiley Interscience: New York, 1970. (11) Electron-SolVent and Anion-SolVent Interactions; Kevan, L., Webster, D., Eds.; Elsevier: New York, 1976. (12) Kevan, L. Chem. ReV. 1980, 80, 1. (13) Kevan, L.; Schlick, S.; Narayana, P. A.; Feng, D. F. J. Chem. Phys. 1981, 75, 1980. (14) Bednarek, J.; Plonka, A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3725. (15) Bednarek, J.; Plonka, A. J. Chem. Soc., Faraday Trans. 1 1987, 83, 3737. (16) Bednarek, J.; Plonka, A. Radiat. Phys. Chem. 1994, 44, 485. (17) Box, H. C.; Budzinski, E. E.; Freund, H. G. J. Chem. Phys. 1978, 69, 1309. (18) Box, H. C.; Budzinski, E. E.; Freund, H. G.; Potter, W. R. J. Chem. Phys. 1979, 70, 1320. (19) Lund, A.; Schlick, S. ReV. Chem. Intermed. 1989, 11, 37. (20) Bednarek, J.; Erickson, R.; Lund, A.; Schlick, S. J. Am. Chem. Soc. 1991, 113, 8990. (21) Bednarek, J.; Schlick, S.; Lund, A. To be published. (22) Thoumas, K.-Å.; Lund, A. J. Magn. Reson. 1976, 22, 315. (23) Claesson, O.; Lund, A. The Studsvik Science Research Laboratory Report NFL-24, 1980. (24) Bobrowski, K. J. Phys. Chem. 1980, 84, 3524. (25) Kurreck, H.; Kirste, B.; Lubitz, W. Electron Nuclear Double Resonance Spectroscopy of Radicals in Solution; VCH: New York, 1988; pp 182, 183.

3916 J. Phys. Chem., Vol. 100, No. 10, 1996 (26) Piekara-Sady, L.; Kispert, L. D. In Handbook of Electron Spin Resonance; Poole, C. P., Farach, H. A., Eds.; AIP: New York, 1994; Chapter V, p 312. (27) Chacko, V. P.; McDowell, C. A.; Singh, B. C. J. Chem. Phys. 1980, 72, 4111. (28) Tench, A. J. J. Chem. Phys. 1963, 38, 593. (29) Tench, A. J. J. Phys. Chem. 1963, 67, 923. (30) Carrington, A.; McLachlan, A. D. Introduction to Magnetic Resonance; Harper & Row: New York, 1967; p 94. (31) Ayscough, P. B. Electron Spin Resonance in Chemistry; Methuen: London, 1967; App. 3, p 438. (32) Horsfield, A.; Morton, J. R.; Whiffen, D. H. Mol. Phys. 1961, 4, 327. In this study, two sets of principal values of the hyperfine tensor for

Bednarek et al. the R protons in the radical •CH2COOH were presented, and the values quoted in this paper are the average of the two sets. (33) Shiga, T.; Lund, A. J. Phys. Chem. 1973, 77, 453. (34) McDowell, C. A.; Raghunathan, P.; Shimokoshi, K. J. Chem. Phys. 1973, 58, 114. (35) Griffith, O. H.; Jost, P. C. In Spin Labeling: Theory and Applications; Berliner, L. J., Ed.; Academic: New York, 1976; Vol. I, p 453. (36) Harvey, R. D.; Schlick, S. Polymer 1989, 30, 11. (37) The tensor components for the R protons in the ethyl radical given in ref 33 were scaled so that the sum of the principal values is 71 G, as for the simulated spectra given in Figure 9.

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