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A level-crossing-resonance study of muonated free-radical formation in solutions of acetone in hexane, water and dilute micelles
Volume 145, number 4
CHEMICAL PHYSICS LETTERS
8 April 1988
A LEVEGCROSSINGRESONANCE STUDY OF MUONATED FREE-RADICAL FORMATION IN SOLUTIONS OF ACETONE IN HEXANE, WATER AND DILUTE MICELLES Krishnan VENKATESWARAN, Robert F. KIEFL, Mary V. BARNABAS, John M. STADLBAUER ‘, Bill W. NG 2, Zhennan WU and David C. WALKER Departments of Chemistry, Physics and TRIUMF, University of British Columbia, Vancouver, British Columbia, Canada V6T I Y6
Received 29 December 1987; in final form 27 February 1988
The (CH,),COMu radical forms when positive muons are stopped in pure acetone and dilute mixtures of acetone in n-hexane or water. Muonium is the precursor of the radical in dilute solution and evidently differs from hydrogen in adding readily to the carbonyl group. In micelles this addition reaction appears to be superceded by enhancement of the abstraction reaction because the radical is not observed.
1. Introduction Until recently, muonium-containing free radicals could only be observed by the high transverse-field muon-spin-rotation technique (TF uSR) [ 11, where the radical had to be formed very rapidly ( *: 10e9 s) so that initial coherence of the muon spin was preserved. This excluded the possibility of observing radicals formed by secondary thermal chemical reactions in dilute solution. It meant that the only detectable radicals were those produced at the end of the muon track in pure unsaturated compounds or concentrated mixtures. However, the recent development of the muon level-crossing-resonance (LCR) technique [ 2,3 1, when applied to organic free radicals [ 4,5 1, allows Mu-containing radicals of the dilute solute to be seen even when they form a microsecond or so after implantation of the positive muon in the solvent [ 6,7]. An t-f resonance technique also now does this [ 8 1. Acetone has been shown by TF uSR to produce the (CH3)$OMu radical in pure liquid acetone [9-121 and in mixtures with water down to 2OW ’ Department of Chemistry, Hood College, Frederick, MD 21701,USA. 2 Chemistry Department, Winona State University, Winona, MN 55987, USA.
[ 13,141. Also, the LCR spectrum in pure acetone has already been reported [ 6 1, where the six equivalent protons lead to a single unresolved multiplet. Acetone is particularly suitable for the present study, because its reactivity towards the muonium atom (u+e-, chemical symbol Mu) is known in water and in micelles [ 151, and the results can be compared with hydrogen atoms [ 16 1. Though the muonium atom normally emulates a light hydrogen isotope in its reaction kinetics, when reacting with aqueous acetone Mu reacts much more quickly than does H [171.
2. Experimental Details of the LCR method applied to muonated free radicals of liquids have been given previously [ 4,5,18 1. In brief: a beam of spin-polarized 4.1 MeV positive muons from the Ml 5 beamline at TRIUMF are stopped in deoxygenated solutions in a thinwalled teflon cell. A superconducting magnet was used to apply magnetic fields up to 30 kG along the initial polarization direction which is parallel to the beam direction. A square-wave modulation field of 55 G was applied to minimize systematic errors. This permitted the LCR signal to be recorded as the difference in opposite modulation fields (A + -A - ),
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )
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where A is the integrated muon-decay asymmetry (normalized backward-to-forward count rates). Best fits were obtained by fitting the data to a difference of two Lorentzian lines using a X2-minimization routine. The conventional pSR measurements were performed with a 240 G transverse field using backward muons on the M20A beamline. The acetone was Analar grade from BDH, the nhexane was Puriss grade from Fluka Chemicals, and the water was triply distilled. Solutions were deoxygenated by bubbling with pure Nz prior to being pumped through a closed system into the irradiation cell. Micelle solutions were prepared as described in ref. [19].
DOS, .m4
8 April 1988
c
,
1
1
1
,
,
,
,
0
I
003 .m2 DO1 0
1 -
-.Wl
-.oot
._a
-mm
t
-00s
om .005
DO4 ms .cv?. PO1
3. Results Fig. 1 shows the LCR spectra of (a) pure acetone, (b) 30% (v/v) acetone in water, and (c) 30% (v/v) acetone in n-hexane. Spectrum (a) is consistent with that already published by Heming et al. [ 61. Speo trum (b) shows that dilution by water shifts the resonance position by + 280 G and reduces the intensity. Spectrum (c) shows that dilution by n-hexane shifts the resonance to lower fields than pure acetone by - 115 G and is comparable in intensity to that in (b ) . Table 1 lists the fitted values for resonance fields (B,,), and amplitudes (Amp ) as percent reduction in muon decay asymmetry on resonance. The data for 0.1 and 0.01 M acetone in water are also given in table 1. A very dilute micelle solution of acetone was studied by LCR and compared with other unsaturated organic compounds. B,, was determined for the four solutes (acetone, acrylamide, styrene, and benzene) as shown in table 2, column 3. In each case the solute was at 9 x 10m5M and the micelle CTAB (cetyltrimethylammonium bromide) was at 3~ lo-$ M, so, on average, there were three solutes localized in each micelle. Acetone differs from acrylamide, styrene, and benzene in that the yield of the Mu radical is zero under the same conditions in which the others gave substantial yields of their radicals [ 201. Conventional l&R experiments were performed on various solutions in order to provide information about radical yields and their formation mechanism. Table 3, column 2, contains diamagnetic yields ( PD) 290
0
--_-_-_____-___
_--_____
-.601
MO
am
lwo
law
1400
,l!no
1mo
moo ?.wa
a00
a400
I
Fig. I. LCR spectra of: (a) pure acetone, (b) 30% (v/v) acetone in water and (c) 30% (v/v) acetone in n-hexane. (A +-A is proportional to the LCR amplitude. ) Table 1 LCR positions (B,,) and amplitudes (Amp) for solutions of acetone Acetone solution
B,(G)
Amp (%)
&a’
pure 308 in hexane 30% in water 0.1 M in water 0.01 M in water
1540(3) 1425(2) 1820(2) 1820(2) 1820(2)
0.526( 10) 0.376( 10) 0.348(9) 0.208( 13) 0.140( 10)
(0.41) 0.29 0.27 0.16 0.11
a’Derived radical yields based on a Pa of 0.4 l(3) in neat acetone by Roduner [ 111.
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CHEMICAL PHYSICS LETTERS
Volume 145, number 4
Table 2 Data on Mu-radical formation in aqueous solutions containing 9 X 1Om5M solute + 3 X 1O-’ M CTAB micelles Radical’s A, a) (MHz)
Solute
k M(mc)C)
B, b’ (G)
(M-1 s-1)
Enhancement factor eJ kt.qmic,lk~
acetone acrylamide styrene benzene
26.1 317.0 213.5 514.6
d) 13620(3) 9066(3) 20848(3)
2.5X 10’0 2.5X 1O’O 8 XlO’O 8 x 10’”
290 3 9 25
a) Published data (see ref. [ 171). b, The vinyl (HP) for acrylamide and styrene were used, and H (6) in the case of benzene. c, kMcmlc)is the rate constant as determined from Mu-decay measurements in waterwith micelles by the pSR technique, and kWthe value in the absence of micelle [ 151. d, Amp=O, i.e. no LCR signal seen at 1820 G in this micelle solution.
where the Y’sare the magnetogyric ratios of the muon, proton and electron, and A, and & are the muon and proton hyperline coupling constants respectively. For pure acetone our value of B,, = 1540 G equals that reported by Heming et aI. [ 6 I. The shift of +280 G in B,, on dilution of acetone (to 30% (v/v) in water) gives a value A,=55.5(5) MHz (using A,=22.0(4) MHz, as measured by TF pSR by Hill et al. [ 13 ] ). This value of AHgequals within the error bars that obtained for pure acetone [6] showing that there is no solvent effect on A,,. Our observed negative shift of - 115 G on dilution with n-hexane (fig. lc) gives a value of A, = 29.4 MHz in the hydrocarbon medium. These B,, shifts provide strong support for the hydrogen-bonding (Mu-bonding) concept [ 13,141, because the order of increasing polarity is hexane, acetone, water. This can be seen in the following way.
obtained at transverse fields of 240 G. These yields represent the fraction of the incident muons which are observed to have been incorporated in diamagnetic molecular states in times shorter than z 10V9 S.
4. Discussion 4.1. Resonance shifts The LCR field at which the muon spin polarization in the (CH3)&OMu radical is transferred to the six protons via hype&e coupling is given to a very good approximation by [ 4,5 1, B xcs=IO.5I(Al-&,)/(Y,-YP)
-(~,+‘4I,)/~el
I,
(1)
Table 3 Tally of yields. Experimental diamagnetic yields (Pn), radical yields (PR) and inferred sources of radicals 3)
Solution
PO
pure acetone 30% (8 M) acetone in n-hexane 30% (8 M) acetone in water 0.1 M acetone in water 0.01 M acetone in water
0.55(l) 0.63( 1) 0.63( 1) 0.63( 1) 0.62( 1)
PRb)
4”
0.41(3) 0.29 0.27 0.16 0.11
0.04(3) 0.08 0.10 0.2 1 0.27
a) As determined here at 240 G by pSR. For pure hexane PDzO.63 and water 0.62. b, PR as in table 1 (see text). c, PL= 1 -P,,-PR-Phi (P,,,=O with acetone present).
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The exceptionally large isotope effect on substituting Mu for H (A,/&= 9) [ 91 arises from the change in moment of inertia, due to the C-O-Mu bond angle not being 180”. But the muon’s coupling to the unpaired electron density is reduced by Mu bonding to the 0 of water much more than Mu bonding to 0 of acetone in pure acetone [ 131. Hence, A, is reduced by dilution with water (shift to higher B,,;) . In contrast, when acetone is diluted by n-hexane, which provides no hydrogen-bonding, the residual Mu bonds to other acetone molecules are weakened, and the shift is now observed in the opposite direction. 4.2. Yields In muon chemistry there are three distinguishable chemical states in which the muon appears: diamagnetic molecules (fractional yield, PO) , free-radicals ( PR), and free muonium atoms (PM). In each case there is the proviso that the state exists at E 10e6 s, can be seen during the muon’s decay, and that the muon is not depolarized (or dephased by precession in precursor states in rotation studies). Depolarization and dephasing lead to a missing fraction PL= I- PD- PR- PM. Our fractional radical yields ( PR), as given in the last column of table 1, were estimated from the observed LCR amplitudes (Amp ) and by equating the amplitude of 0.526% in pure acetone with Roduner’s absolute determination of P,=O.41 as obtained by TF pSR [ 111. The major assumption involved in this procedure is that any relaxation affects the amplitudes to the same degree (because in each solution the linewidths are about twice the theoretically determined widths). This assumption comes from the expectation that the broadening is not caused by a solvent-induced relaxation. This normalization procedure also seems appropriate because pure acetone has been shown to have a negligible missing fraction ( PLz 0.04) [ 111. The difference in experimental asymmetry at 1425 and 1820 G has been shown to be negligible [ 201. Table 3 records the yields obtained in these different acetone solutions. For pure acetone and the 30% solutions there can be direct radical formation from hot-atom interactions or p+ adducts [ 1l-14,17]; but for the 0.1 and 0.01 M aqueous solutions, the only possible precursor to the radical is Mu, the free thermalized muonium atom. For these 292
8 April 1988
dilute solutions hot-atom insertions would be negligible, and p+ would be solvated and converted to MuOH before it could encounter acetone, so the maximum radical yield will equal the non-depolarized muonium yield at the time of the reaction Mu+ (CH&CO+ k,=8.7~
(CH&COMu,
10’M-l
S-I.
(2)
At 0.1 M the radical will be formed on average about 0.1 ps after Mu formation. This means that most of the Mu atoms constituting the missing fraction in water (PL= 0.18 ) will be lost and only Phi= 0.20 is available as the source of PR via reaction (2 ). Furthermore, not all of the muon polarization in muonium is observable because the longitudinal field (&,= 1820 G) is not large enough to decouple completely the hyperline oscillation (for which Ho= 1585 G). Instead, the number of polarized Mu atoms available is the yield with unpaired spins (0.10) plus the fraction x2/ (1 +x2) of the paired yield (also 0.1 O), where x=B,,,/HO. This total equals 0.157 for B,,= 1820 G. Our observed radical yield for the 0.1 M solution is 0.16. For 0.01 M aqueous acetone solution there is another factor to consider. Here, the mean formation time of the radical (expected to be about 1.1 ps) is comparable to the observation time so that the LCR amplitude is reduced from its value at 0.1 M. In fact, when the theory [7] is applied, it results in a reduction in PR from 0.13 to 0.11. This confirms that the rate of decay of Mu in the presence of acetone, as observed by pSR, is due largely to the addition reaction (eq. (2) ). In other words, we have actually observed the rate of formation of the radical to be equal to the observed rate of decay of Mu. 4.3. Micelle solutions Acetone shows micelle-induced enhancement in reaction with Mu, like many organic solutes [ 191 (table 2, columns 4 and 5, records some relevant data). But Mu radicals are evidently not the products of the enhanced reaction, because they were not observed by LCR under conditions where acrylamide, styrene and benzene showed them (table 2). Furthermore, we have searched for the acetone radical in more concentrated micelle solutions (up to
CHEMICAL PHYSICS LETTERS
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0.002 M micelle and 0.012 M acetone) and found none. The implication is that reaction (2), which dominates the reaction in water, must have been superseded by a competitive reaction which is enhanced much more by localization in micelles. Abstraction reactions have been found to give enhancement factors of 104-fold or more [ 19 1, so it looks as if the reaction Mu+ (CH,),CO+MuH+CH&OCH,
,
(3)
takes over in the micellar phase. This means that micelles can be used to change the distribution of products when more than one reaction channel is available, as they have different micelle-induced enhancement factors. Finally, it is interesting to note in this connection that, for H atoms, the abstraction reaction Ht (CH3)zCO-tH2tCH&OCH2 ki_,=2.4~106M-1~-1,
,
(4)
dominates over the addition Ht (CH&CO-t(CH&COH, kH=1.1x106M-1s-‘,
(5)
in water [ 16 1, and that, in general, abstractions show an isotope effect (k,/k,) of about lo2 [ 17 1. If this also occurs for acetone then kM for reaction (3) would only be x 1 x lo4 M-i s’: in which case micelles would have given rise to a lo”-fold enhancement of the abstraction reaction.
Acknowledgement The assistance of the staff of the nSR group at TRIUMF (J. Worden and K. Hoyle) is, as always, very much appreciated. Financial assistance for these experiments came from the NSERC of Canada.
8 April 1988
References [ I ] R. Roduner and H. Fischer, Chem. Phys. 54 ( 198I ) 26 1. [ 21 A. Abragam, Compt. Rend. Acad. Sci. (Paris) Ser. II 299 (1984) 95. [3] S.R. Kreitzman, J.H. Brewer, D.R. Harshman, R. Keitel, D.L. Williams, K.M. Crowe and E.J. Ansaldo, Phys. Rev. Letters 56 (1986) 181. [ 41 R.F. Kiefl, Hyperline Interactions 32 (1986) 707. [ 5] R.F. Kiefl, S.R. Krcitzman, M. Celio, R. Keitel, J.H. Brewer, G.M, Luke, D.R. Noakes. P.W. Percival, T. Matsuzaki and K. Nishiyama, Phys. Rev! A 34 ( 1986) 68 1; R.F. Kiefl, P.W. Percival, J.C. Brodovitch, SK. Lcung, D. Yu, K. Venkateswaran and S.F.J. Cox, Chem. Phys. Letters 143 (1988) 613. 161M. Heming, E. Roduner, B.D. Patterson, W. Odermatt, J. Schneider, Hp. Baumeler, H. Keller and I.M. Savic, Chem. Phys. Letters 128 (1986) 100. [ 71 M. Heming, E. Roduner and RD. Patterson, Hyperfine Interactions 32 ( I986 ) 727. [8] T. Azuma, K. Nishiyama, K. Nagamine, Y. Ito and Y. Tabata, Hyperllne Interactions 32 ( 1986) 837. [9] E. Roduner, in: Exotic atoms ‘79, eds. K.M. Crowe, J. Duclos, G. Fiorentini and G. Torelli (Plenum Press, New York, 1980) p. 379. [ lo] A. Hill, G. Allen, G. Stirling and M.C.R. Symons, J. Chem. Sot. Faraday Trans. I 78 (1982) 2959. [ 111E. Roduner, Radiat. Phys. Chem. 28 (1986) 75. [ 121S.F.J. Cox, D.A. Geeson, C.J. Rhodes, E. Roduner, C.A. Scott and M.C.R. Symons, Hyperline Interactions 32 ( 1986) 763. [ 131 A. Hill, M.C.R. Symons, S.F.J. Cox, R. de Renzi, CA. Scott, C. Bucci and A. Vecli, J. Chem. Sot. Faraday Trans. I 81 (1985) 433. [ 141 S.F.J. Cox and M.C.R. Symons, Radiat. Phys. Chem. 27 (1985) 53. [ 151 K. Venkateswaran, M.V. Barnabas, J.M. Stadlbauer and DC. Walker, to be published. [ 161R.A. Witter and P. Neta, J. Org. Chem. 38 (1973) 484. [ 171D.C. Walker, Muon and muonium chemistry (Cambridge Univ. Press, Cambridge, 1983). [ 181 P.W. Percival, R.F. Kiefl, S.R. Kreitzman, D.M. Gamer, S.F.J. Cox, G.M. Luke, J.H. Brewer, K. Nishiyama and K. Venkateswaran, Chem. Phys. Letters 133 (1987) 465. [ 191K. Venkateswaran, M.V. Barnabas, Z. Wu, J.M. Stadlbauer, B.W. Ngand D.C. Walker, Chem. Phys. Letters 143 (1988) 313. [20] K. Venkateswaran, M.V. Barnabas, R.F. Kiefl, J.M. Stadlbauer and D.C. Walker, to be published.
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CHEMICAL PHYSICS LETTERS
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A LEVEGCROSSINGRESONANCE STUDY OF MUONATED FREE-RADICAL FORMATION IN SOLUTIONS OF ACETONE IN HEXANE, WATER AND DILUTE MICELLES Krishnan VENKATESWARAN, Robert F. KIEFL, Mary V. BARNABAS, John M. STADLBAUER ‘, Bill W. NG 2, Zhennan WU and David C. WALKER Departments of Chemistry, Physics and TRIUMF, University of British Columbia, Vancouver, British Columbia, Canada V6T I Y6
Received 29 December 1987; in final form 27 February 1988
The (CH,),COMu radical forms when positive muons are stopped in pure acetone and dilute mixtures of acetone in n-hexane or water. Muonium is the precursor of the radical in dilute solution and evidently differs from hydrogen in adding readily to the carbonyl group. In micelles this addition reaction appears to be superceded by enhancement of the abstraction reaction because the radical is not observed.
1. Introduction Until recently, muonium-containing free radicals could only be observed by the high transverse-field muon-spin-rotation technique (TF uSR) [ 11, where the radical had to be formed very rapidly ( *: 10e9 s) so that initial coherence of the muon spin was preserved. This excluded the possibility of observing radicals formed by secondary thermal chemical reactions in dilute solution. It meant that the only detectable radicals were those produced at the end of the muon track in pure unsaturated compounds or concentrated mixtures. However, the recent development of the muon level-crossing-resonance (LCR) technique [ 2,3 1, when applied to organic free radicals [ 4,5 1, allows Mu-containing radicals of the dilute solute to be seen even when they form a microsecond or so after implantation of the positive muon in the solvent [ 6,7]. An t-f resonance technique also now does this [ 8 1. Acetone has been shown by TF uSR to produce the (CH3)$OMu radical in pure liquid acetone [9-121 and in mixtures with water down to 2OW ’ Department of Chemistry, Hood College, Frederick, MD 21701,USA. 2 Chemistry Department, Winona State University, Winona, MN 55987, USA.
[ 13,141. Also, the LCR spectrum in pure acetone has already been reported [ 6 1, where the six equivalent protons lead to a single unresolved multiplet. Acetone is particularly suitable for the present study, because its reactivity towards the muonium atom (u+e-, chemical symbol Mu) is known in water and in micelles [ 151, and the results can be compared with hydrogen atoms [ 16 1. Though the muonium atom normally emulates a light hydrogen isotope in its reaction kinetics, when reacting with aqueous acetone Mu reacts much more quickly than does H [171.
2. Experimental Details of the LCR method applied to muonated free radicals of liquids have been given previously [ 4,5,18 1. In brief: a beam of spin-polarized 4.1 MeV positive muons from the Ml 5 beamline at TRIUMF are stopped in deoxygenated solutions in a thinwalled teflon cell. A superconducting magnet was used to apply magnetic fields up to 30 kG along the initial polarization direction which is parallel to the beam direction. A square-wave modulation field of 55 G was applied to minimize systematic errors. This permitted the LCR signal to be recorded as the difference in opposite modulation fields (A + -A - ),
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )
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where A is the integrated muon-decay asymmetry (normalized backward-to-forward count rates). Best fits were obtained by fitting the data to a difference of two Lorentzian lines using a X2-minimization routine. The conventional pSR measurements were performed with a 240 G transverse field using backward muons on the M20A beamline. The acetone was Analar grade from BDH, the nhexane was Puriss grade from Fluka Chemicals, and the water was triply distilled. Solutions were deoxygenated by bubbling with pure Nz prior to being pumped through a closed system into the irradiation cell. Micelle solutions were prepared as described in ref. [19].
DOS, .m4
8 April 1988
c
,
1
1
1
,
,
,
,
0
I
003 .m2 DO1 0
1 -
-.Wl
-.oot
._a
-mm
t
-00s
om .005
DO4 ms .cv?. PO1
3. Results Fig. 1 shows the LCR spectra of (a) pure acetone, (b) 30% (v/v) acetone in water, and (c) 30% (v/v) acetone in n-hexane. Spectrum (a) is consistent with that already published by Heming et al. [ 61. Speo trum (b) shows that dilution by water shifts the resonance position by + 280 G and reduces the intensity. Spectrum (c) shows that dilution by n-hexane shifts the resonance to lower fields than pure acetone by - 115 G and is comparable in intensity to that in (b ) . Table 1 lists the fitted values for resonance fields (B,,), and amplitudes (Amp ) as percent reduction in muon decay asymmetry on resonance. The data for 0.1 and 0.01 M acetone in water are also given in table 1. A very dilute micelle solution of acetone was studied by LCR and compared with other unsaturated organic compounds. B,, was determined for the four solutes (acetone, acrylamide, styrene, and benzene) as shown in table 2, column 3. In each case the solute was at 9 x 10m5M and the micelle CTAB (cetyltrimethylammonium bromide) was at 3~ lo-$ M, so, on average, there were three solutes localized in each micelle. Acetone differs from acrylamide, styrene, and benzene in that the yield of the Mu radical is zero under the same conditions in which the others gave substantial yields of their radicals [ 201. Conventional l&R experiments were performed on various solutions in order to provide information about radical yields and their formation mechanism. Table 3, column 2, contains diamagnetic yields ( PD) 290
0
--_-_-_____-___
_--_____
-.601
MO
am
lwo
law
1400
,l!no
1mo
moo ?.wa
a00
a400
I
Fig. I. LCR spectra of: (a) pure acetone, (b) 30% (v/v) acetone in water and (c) 30% (v/v) acetone in n-hexane. (A +-A is proportional to the LCR amplitude. ) Table 1 LCR positions (B,,) and amplitudes (Amp) for solutions of acetone Acetone solution
B,(G)
Amp (%)
&a’
pure 308 in hexane 30% in water 0.1 M in water 0.01 M in water
1540(3) 1425(2) 1820(2) 1820(2) 1820(2)
0.526( 10) 0.376( 10) 0.348(9) 0.208( 13) 0.140( 10)
(0.41) 0.29 0.27 0.16 0.11
a’Derived radical yields based on a Pa of 0.4 l(3) in neat acetone by Roduner [ 111.
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Table 2 Data on Mu-radical formation in aqueous solutions containing 9 X 1Om5M solute + 3 X 1O-’ M CTAB micelles Radical’s A, a) (MHz)
Solute
k M(mc)C)
B, b’ (G)
(M-1 s-1)
Enhancement factor eJ kt.qmic,lk~
acetone acrylamide styrene benzene
26.1 317.0 213.5 514.6
d) 13620(3) 9066(3) 20848(3)
2.5X 10’0 2.5X 1O’O 8 XlO’O 8 x 10’”
290 3 9 25
a) Published data (see ref. [ 171). b, The vinyl (HP) for acrylamide and styrene were used, and H (6) in the case of benzene. c, kMcmlc)is the rate constant as determined from Mu-decay measurements in waterwith micelles by the pSR technique, and kWthe value in the absence of micelle [ 151. d, Amp=O, i.e. no LCR signal seen at 1820 G in this micelle solution.
where the Y’sare the magnetogyric ratios of the muon, proton and electron, and A, and & are the muon and proton hyperline coupling constants respectively. For pure acetone our value of B,, = 1540 G equals that reported by Heming et aI. [ 6 I. The shift of +280 G in B,, on dilution of acetone (to 30% (v/v) in water) gives a value A,=55.5(5) MHz (using A,=22.0(4) MHz, as measured by TF pSR by Hill et al. [ 13 ] ). This value of AHgequals within the error bars that obtained for pure acetone [6] showing that there is no solvent effect on A,,. Our observed negative shift of - 115 G on dilution with n-hexane (fig. lc) gives a value of A, = 29.4 MHz in the hydrocarbon medium. These B,, shifts provide strong support for the hydrogen-bonding (Mu-bonding) concept [ 13,141, because the order of increasing polarity is hexane, acetone, water. This can be seen in the following way.
obtained at transverse fields of 240 G. These yields represent the fraction of the incident muons which are observed to have been incorporated in diamagnetic molecular states in times shorter than z 10V9 S.
4. Discussion 4.1. Resonance shifts The LCR field at which the muon spin polarization in the (CH3)&OMu radical is transferred to the six protons via hype&e coupling is given to a very good approximation by [ 4,5 1, B xcs=IO.5I(Al-&,)/(Y,-YP)
-(~,+‘4I,)/~el
I,
(1)
Table 3 Tally of yields. Experimental diamagnetic yields (Pn), radical yields (PR) and inferred sources of radicals 3)
Solution
PO
pure acetone 30% (8 M) acetone in n-hexane 30% (8 M) acetone in water 0.1 M acetone in water 0.01 M acetone in water
0.55(l) 0.63( 1) 0.63( 1) 0.63( 1) 0.62( 1)
PRb)
4”
0.41(3) 0.29 0.27 0.16 0.11
0.04(3) 0.08 0.10 0.2 1 0.27
a) As determined here at 240 G by pSR. For pure hexane PDzO.63 and water 0.62. b, PR as in table 1 (see text). c, PL= 1 -P,,-PR-Phi (P,,,=O with acetone present).
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The exceptionally large isotope effect on substituting Mu for H (A,/&= 9) [ 91 arises from the change in moment of inertia, due to the C-O-Mu bond angle not being 180”. But the muon’s coupling to the unpaired electron density is reduced by Mu bonding to the 0 of water much more than Mu bonding to 0 of acetone in pure acetone [ 131. Hence, A, is reduced by dilution with water (shift to higher B,,;) . In contrast, when acetone is diluted by n-hexane, which provides no hydrogen-bonding, the residual Mu bonds to other acetone molecules are weakened, and the shift is now observed in the opposite direction. 4.2. Yields In muon chemistry there are three distinguishable chemical states in which the muon appears: diamagnetic molecules (fractional yield, PO) , free-radicals ( PR), and free muonium atoms (PM). In each case there is the proviso that the state exists at E 10e6 s, can be seen during the muon’s decay, and that the muon is not depolarized (or dephased by precession in precursor states in rotation studies). Depolarization and dephasing lead to a missing fraction PL= I- PD- PR- PM. Our fractional radical yields ( PR), as given in the last column of table 1, were estimated from the observed LCR amplitudes (Amp ) and by equating the amplitude of 0.526% in pure acetone with Roduner’s absolute determination of P,=O.41 as obtained by TF pSR [ 111. The major assumption involved in this procedure is that any relaxation affects the amplitudes to the same degree (because in each solution the linewidths are about twice the theoretically determined widths). This assumption comes from the expectation that the broadening is not caused by a solvent-induced relaxation. This normalization procedure also seems appropriate because pure acetone has been shown to have a negligible missing fraction ( PLz 0.04) [ 111. The difference in experimental asymmetry at 1425 and 1820 G has been shown to be negligible [ 201. Table 3 records the yields obtained in these different acetone solutions. For pure acetone and the 30% solutions there can be direct radical formation from hot-atom interactions or p+ adducts [ 1l-14,17]; but for the 0.1 and 0.01 M aqueous solutions, the only possible precursor to the radical is Mu, the free thermalized muonium atom. For these 292
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dilute solutions hot-atom insertions would be negligible, and p+ would be solvated and converted to MuOH before it could encounter acetone, so the maximum radical yield will equal the non-depolarized muonium yield at the time of the reaction Mu+ (CH&CO+ k,=8.7~
(CH&COMu,
10’M-l
S-I.
(2)
At 0.1 M the radical will be formed on average about 0.1 ps after Mu formation. This means that most of the Mu atoms constituting the missing fraction in water (PL= 0.18 ) will be lost and only Phi= 0.20 is available as the source of PR via reaction (2 ). Furthermore, not all of the muon polarization in muonium is observable because the longitudinal field (&,= 1820 G) is not large enough to decouple completely the hyperline oscillation (for which Ho= 1585 G). Instead, the number of polarized Mu atoms available is the yield with unpaired spins (0.10) plus the fraction x2/ (1 +x2) of the paired yield (also 0.1 O), where x=B,,,/HO. This total equals 0.157 for B,,= 1820 G. Our observed radical yield for the 0.1 M solution is 0.16. For 0.01 M aqueous acetone solution there is another factor to consider. Here, the mean formation time of the radical (expected to be about 1.1 ps) is comparable to the observation time so that the LCR amplitude is reduced from its value at 0.1 M. In fact, when the theory [7] is applied, it results in a reduction in PR from 0.13 to 0.11. This confirms that the rate of decay of Mu in the presence of acetone, as observed by pSR, is due largely to the addition reaction (eq. (2) ). In other words, we have actually observed the rate of formation of the radical to be equal to the observed rate of decay of Mu. 4.3. Micelle solutions Acetone shows micelle-induced enhancement in reaction with Mu, like many organic solutes [ 191 (table 2, columns 4 and 5, records some relevant data). But Mu radicals are evidently not the products of the enhanced reaction, because they were not observed by LCR under conditions where acrylamide, styrene and benzene showed them (table 2). Furthermore, we have searched for the acetone radical in more concentrated micelle solutions (up to
CHEMICAL PHYSICS LETTERS
Volume 145, number 4
0.002 M micelle and 0.012 M acetone) and found none. The implication is that reaction (2), which dominates the reaction in water, must have been superseded by a competitive reaction which is enhanced much more by localization in micelles. Abstraction reactions have been found to give enhancement factors of 104-fold or more [ 19 1, so it looks as if the reaction Mu+ (CH,),CO+MuH+CH&OCH,
,
(3)
takes over in the micellar phase. This means that micelles can be used to change the distribution of products when more than one reaction channel is available, as they have different micelle-induced enhancement factors. Finally, it is interesting to note in this connection that, for H atoms, the abstraction reaction Ht (CH3)zCO-tH2tCH&OCH2 ki_,=2.4~106M-1~-1,
,
(4)
dominates over the addition Ht (CH&CO-t(CH&COH, kH=1.1x106M-1s-‘,
(5)
in water [ 16 1, and that, in general, abstractions show an isotope effect (k,/k,) of about lo2 [ 17 1. If this also occurs for acetone then kM for reaction (3) would only be x 1 x lo4 M-i s’: in which case micelles would have given rise to a lo”-fold enhancement of the abstraction reaction.
Acknowledgement The assistance of the staff of the nSR group at TRIUMF (J. Worden and K. Hoyle) is, as always, very much appreciated. Financial assistance for these experiments came from the NSERC of Canada.
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References [ I ] R. Roduner and H. Fischer, Chem. Phys. 54 ( 198I ) 26 1. [ 21 A. Abragam, Compt. Rend. Acad. Sci. (Paris) Ser. II 299 (1984) 95. [3] S.R. Kreitzman, J.H. Brewer, D.R. Harshman, R. Keitel, D.L. Williams, K.M. Crowe and E.J. Ansaldo, Phys. Rev. Letters 56 (1986) 181. [ 41 R.F. Kiefl, Hyperline Interactions 32 (1986) 707. [ 5] R.F. Kiefl, S.R. Krcitzman, M. Celio, R. Keitel, J.H. Brewer, G.M, Luke, D.R. Noakes. P.W. Percival, T. Matsuzaki and K. Nishiyama, Phys. Rev! A 34 ( 1986) 68 1; R.F. Kiefl, P.W. Percival, J.C. Brodovitch, SK. Lcung, D. Yu, K. Venkateswaran and S.F.J. Cox, Chem. Phys. Letters 143 (1988) 613. 161M. Heming, E. Roduner, B.D. Patterson, W. Odermatt, J. Schneider, Hp. Baumeler, H. Keller and I.M. Savic, Chem. Phys. Letters 128 (1986) 100. [ 71 M. Heming, E. Roduner and RD. Patterson, Hyperfine Interactions 32 ( I986 ) 727. [8] T. Azuma, K. Nishiyama, K. Nagamine, Y. Ito and Y. Tabata, Hyperllne Interactions 32 ( 1986) 837. [9] E. Roduner, in: Exotic atoms ‘79, eds. K.M. Crowe, J. Duclos, G. Fiorentini and G. Torelli (Plenum Press, New York, 1980) p. 379. [ lo] A. Hill, G. Allen, G. Stirling and M.C.R. Symons, J. Chem. Sot. Faraday Trans. I 78 (1982) 2959. [ 111E. Roduner, Radiat. Phys. Chem. 28 (1986) 75. [ 121S.F.J. Cox, D.A. Geeson, C.J. Rhodes, E. Roduner, C.A. Scott and M.C.R. Symons, Hyperline Interactions 32 ( 1986) 763. [ 131 A. Hill, M.C.R. Symons, S.F.J. Cox, R. de Renzi, CA. Scott, C. Bucci and A. Vecli, J. Chem. Sot. Faraday Trans. I 81 (1985) 433. [ 141 S.F.J. Cox and M.C.R. Symons, Radiat. Phys. Chem. 27 (1985) 53. [ 151 K. Venkateswaran, M.V. Barnabas, J.M. Stadlbauer and DC. Walker, to be published. [ 161R.A. Witter and P. Neta, J. Org. Chem. 38 (1973) 484. [ 171D.C. Walker, Muon and muonium chemistry (Cambridge Univ. Press, Cambridge, 1983). [ 181 P.W. Percival, R.F. Kiefl, S.R. Kreitzman, D.M. Gamer, S.F.J. Cox, G.M. Luke, J.H. Brewer, K. Nishiyama and K. Venkateswaran, Chem. Phys. Letters 133 (1987) 465. [ 191K. Venkateswaran, M.V. Barnabas, Z. Wu, J.M. Stadlbauer, B.W. Ngand D.C. Walker, Chem. Phys. Letters 143 (1988) 313. [20] K. Venkateswaran, M.V. Barnabas, R.F. Kiefl, J.M. Stadlbauer and D.C. Walker, to be published.
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