Muonium atoms react with chloroacetic acid and chloroacetate ions in dilute aqueous solution with rate constants of 2.3 × 106 and 9.1 × 105 dm3 mol−1 s−1 respectively. These are compared with the reactions of 1H atoms (and eaq−) and discussed in terms of a pair of competing kinetic isotope effects. Muonium reacts at least eight times faster than H overall, and probably 28 times faster in forming Cl−. It behaves as a nucleophile, thus resembling eaq− more than H, in reacting faster with the acid than the anion. Muonium's reactions must be governed to a considerable extent by quantum-mechanical effects arising from its very small mass.

The reactions of muonium atoms with the individual species HO2CCO2H, HO2CCO2-, and -O2CCO2- at similar to 295 K in water have absolute bimolecular rate constants of 5.6, 0.71, and 0.054 x 10(8) M(-1) s(-1), respectively. No such 100-fold pH-dependence has hitherto been seen for muonium, although H and e(aq)(-) also show large pH effects in reactions with oxalic acid. Arrhenius plots for the muonium reactions at pH = 1, 3, and 8 show the unusually small activation energies of less than or equal to 17 kJ/mol for all three acid-base forms of oxalic acid. When the reaction rate of muonium is compared with published data on ordinary H atoms at pH = 1 (57% oxalic acid and 43% hydrogen oxalate ions), one obtains a kinetic isotope effect of 850 favoring muonium. For the dianion at pH greater than or equal to 7, the muonium-to-H rate ratio is >300, These seem to be the largest primary kinetic isotope effects ever reported. The reaction is taken to be addition of muonium to one of the C=O groups, and the large isotope effect and small E(a) suggest considerable contributions from quantum mechanical tunneling.

This paper presents data showing that in dilute aqueous solutions of butanone, 3-pentanone, cyclohexanone, di-tert-butyl ketone, and acetaldehyde the addition reaction of muonium atoms occurs with a rate constant close to 1 × 108 M−1 s−1. The same value was obtained previously for acetone. Thus the reaction rate is virtually independent of the group attached to the C=O, be it a methyl, methylene, tert-butyl, or even a hydrogen atom. This is in sharp contrast to the reactivity of ordinary 1H atoms, whose rate constants are much slower and dependent on adjacent groups. In fact muonium and 1H react by different mechanisms, to form different products, so their rate ratio represents a complex kinetic isotope effect. Keywords: kinetic isotope effects, muonium atoms, muon spin rotation, ketones, hydrogen atom reactions.

Two reactions of muonium atoms close to the diffusion-controlled limit were analyzed as a function of solute size and solvent viscosity. With Cr(NCS)(6)(3-) as solute in water, the reaction is an electron spin-conversion process, and the observed rate is taken to be half of the actual encounter rate, because of the quantum mechanical statistical factor and the occurrence of multiple collisions due to the solvent cage effect. The encounter rate deduced is 6.2 x 10(10) M-1 s(-1), which implies a large cross section for the Cr complex. In a second series of experiments, the rate of reaction of muonium with I-2 was compared in water, methanol and n-heptane. The bimolecular rate constants determined are (1.7 +/- 0.3), (7.0 +/- 1.2), and (57 +/- 22) x 10(10) M-1 s(-1), respectively, in these three solvents. This similar to 30-fold change in rate is not matched by the change in the inverse of the solvents' viscosities, which changed overall by a factor of only 2.4. It looks as if quantum tunneling dominates over classical diffusion in less polar media where muonium is unencumbered by solvent clathration.

Muonium atoms add to the O atom of the carbonyl group of acetone to give the muonated free radical (CH3)2Ċ-O-Mu when the reaction takes place in water or hydrocarbons, but not when the acetone is localized in micelles. Micelles have no effect on the formation of muonated cyclohexadienyl radicals when muonium reacts with benzene under similar conditions. The addition reaction with acetone appears to have been subsumed by a faster alternative reaction in the micellar environment. Evidence is presented for this interpretation rather than for an inhibition of the radical or for a shift in the muon level-crossing resonance spectrum with hydrogen (muonium) bonding, though major shifts are seen for the spectrum of this radical in pure solvents of widely different dielectric constant. It is suggested that muonium's "abstraction" reaction takes over in micelles because significant micelle-induced enhancement effects were previously observed in that type of reaction. The data are consistent with a rate constant for the abstraction reaction of muonium with acetone in micelles of >6 × 108 M−1 s−1. Key words: muonium, kinetic isotope effects, micelle enhancement, H/Mu-addition, H/Mu abstraction.

p-Aminobenzoic acid was studied as a solute in water to determine the effect of altering the pH from 1 to 10 on the rate of reaction and on the site of addition of muonium atoms. This solute exists in three different acid-base states ranging from the fully protonated, through neutral, to anionic, and its substituents change from electron-accepters to electron-donors with increasing pH. The overall rate of reaction is barely affected by pH, but there is nearly a 2-fold switch in the site at which muonium adds to the ring. There is also a considerable switch in the magnetic field at which muon level crossing resonance occurs, the radical with muonium attached at C(2) moving upfield by 125 mT and the C(3) one moving downfield by a similar amount. If this compound is representative of aromatic systems, then the differences in pH which usually exist between hydrogen and muonium studies would account for only a small factor in most reported kinetic isotope effects.

Muonium atoms were observed to undergo a fast reaction with several chromium(III) complexes in water, with rate constants in the neighborhood of 10(10) M(-1) s(-1). The reaction occurring is attributed to a simple electron-spin-exchange in muonium, catalyzed by the paramagnetic d(3) metal ions. There is a quantum mechanical statistical factor for this process of 62% per collision, which becomes 50% when multicollisional cage effects in solution are included, with the result that the observed rates are close to the diffusion-controlled limit for a muonium reaction in water. The implication is that nearly half of the encounters between muonium and a Cr complex result in electron spin exchange. These muonium rates are faster by a factor of similar to 5 than the corresponding reactions of positronium. They also show a different dependence on the nature of the ligands. These differences are attributed to positronium being larger and more polarizable than muonium, so that spin-spin coupling with the metal's d electrons is weaker and more sensitive to the nephelauxetic effect of the ligands in the case of positronium. The corresponding reactions involving ordinary H atoms are not available for comparison but presumably resemble muonium's.

Using the level crossing resonance technique, it has been possible to identify the measure the relative yields of free radicals formed by addition of muonium across the C = C group at carbon atoms 5 and 6 in uracil, thymine, and 6-methyluracil. The C(5) and C(6) adduct ratio is about 1 in uracil but reduces approximately 0.15 in thymine due to presence of -CH3 on C(5). This considerable steric effect shown by muonium contrasts the situation with ordinary H atoms and underlines the importance of zero-point vibrational motion in determining stereospecific directional effects, even for efficient reactions. Proton hyperfine coupling constants of the free radicals formed from addition across the C(5) = C(6) double bonds in uracil have been determined. For thiouracil, no addition was evident at C(5) = C(6), suggesting that attack by muonium occurs at the C of C = S to produce a thiyl radical.

The presence of heterocyclic N atoms in an aromatic solute enhance its rate of reaction toward muonium, and the free radicals formed are seen to have muonium attached to a C atom of the ring. This contrasts the behavior of H-1 in water, where addition to N-heterocyclic rings occurs an order of magnitude slower and with H attaching to N, at least in acid solution. Muonium evidently shows nucleophilic character while ordinary hydrogen atoms are electrophilic. Pyrazine (1,4-diazine) was used for this comparison with benzene because it has the advantage over pyridine of forming only two possible radicals.

The addition of micelles to solutions of solutes in water with which muonium reacts cause a wide variety of effects. Micelle-induced enhancement ratios have been found to vary from less than 1 to greater than 104, depending on the properties of the solute and the type of chemical reaction involved. A kinetic analysis is presented here that seems appropriate for most of the mixtures studied so far. These are nonhomogeneous systems, with separate phases involved, but the effects do not arise simply from confined diffusion because the mean residence time of muonium is only 2 ns. The possibility that the largest enhancements arise from quantum mechanical tunneling, and therefore peculiar to muonium, cannot be ruled out.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMuonium and free-radical yields as determined by the muon level-crossing-resonance technique in aqueous and micelle solutions of acrylamideKrishnan Venkateswaran, Mary V. Barnabas, Robert F. Kiefl, John M. Stadlbauer, and David C. WalkerCite this: J. Phys. Chem. 1989, 93, 1, 388–392Publication Date (Print):January 1, 1989Publication History Published online1 May 2002Published inissue 1 January 1989https://pubs.acs.org/doi/10.1021/j100338a074https://doi.org/10.1021/j100338a074research-articleACS PublicationsRequest reuse permissionsArticle Views36Altmetric-Citations17LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access options Get e-Alerts

All chemical states of the muons in a μSR experiment have now been determined in toluene, allylbenzene and styrene. There are no “missing fractions” because the sum of the various muon-containing free-radicals equals 1-PD, where PD is the directly formed diamagnetic fraction. Use of the new technique of level crossing resonance spectroscopy has enabled yields to be determined and identification of individual isomeric radicals. For toluene, there is a total radical fraction of 0.77 and a distribution of 2.5:2:1 for ortho: meta: para addition within the ring. For allylbenzene, ≈ 70% of the muonated radicals are side chain addition products and of these nearly 40% have Mu on the second C; and, for the 30% adding to the ring, there is virtually no selectivity of site as the o: m: p ratio is the statistical ratio 2:2:1. Toluene and allylbenzene, however, differ dramatically from styrene. In styrene, 82% of the muons form radicals and 85% of these arise from formal addition of muonium to the end C of the side chain to give muonated phenylethyl radicals. The remaining 15% are seen to be distributed (2:1) between the ortho and para positions of the ring, with no addition at the meta position. The high degree of preference shown by styrene indicates strong selectivity in achieving the most stable radical. Proton hyperfine couplings for all of these radicals have also been determined.

Enhancements in rate constants from 106 M−1 s−1 to more than 1010 M−1 s−1 have been found for the reaction of muonium atoms with 2-propanol in water when micelles are added.

The (CH3)2COMu 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.

Muonium-radicals resulting from insertion into the benzene ring are found to be much more prevalent in allyl benzene (C6H5CH2CH=CH2) than in styrene (C6H5CH=CH2). The salient implication of this comparison is that intramolecular rearrangements preceeded the μSR observation for the case of styrene. In turn, this suggests that muonium-containing free radicals, as seen directly by kilogauss transverse field μSR, are not necessarily theprimary radicals. Therefore, the elucidation of mechanism (and identification of the precursor) of Mu-radical formation is further complicated by the fact that the observations may refer to thermodynamically more stable secondary radicals-those resulting from a variety of intra-or inter-molecular relaxations or exchanges. Primary kinetic selectivities of thermalized muonium atom addition reactions can be determined, however, through the substituent effect on the Hammett linear free energy parameter in dilute solution. Results have been obtained for substituted benzenes and benzoic acids. Muonium apparently has a mild nucleophilic character. And, most interestingly, this is opposite to that of its heavy isotope hydrogen.

Kinetic isotope effects and Hammett reaction parameters ϱ have been determined for the interaction of muonium atoms (Mu) in water with substituted benzoic acids as reactive solutes. The ϱ parameter for muonium has a value of +0.27 ± 0.05, which indicates a weakly nucleophilic attack by Mu to give free-radical intermediates. In contrast, the analogous H-atom reactions show slight electrophilic character and are slower than Mu by more than the mean thermal velocity effect. These results were obtained in “muon spin rotation” studies.

Muonium radicals were observed through theirμSR precession frequencies in high transverse magnetic fields in pure benzene, pure styrene and their mixtures, all as liquids at room temperature. In benzene-styrene mixtures, the radicals obtained in each pure liquid are both present, so no slow (10−9−10−5 s) intermolecular exchange occurs; but strong selectivity was found with the formation of the radical from styrene being about eight-times more probable than the radical from benzene.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMuonium addition reactions to aromatic solutesJ. M. Stadlbauer, B. W. Ng, R. Ganti, and D. C. WalkerCite this: J. Am. Chem. Soc. 1984, 106, 11, 3151–3153Publication Date (Print):May 1, 1984Publication History Published online1 May 2002Published inissue 1 May 1984https://pubs.acs.org/doi/10.1021/ja00323a015https://doi.org/10.1021/ja00323a015research-articleACS PublicationsRequest reuse permissionsArticle Views77Altmetric-Citations24LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMuon spin rotation studies involving muonium at high pHB. W. Ng, J. M. Stadlbauer, and D. C. WalkerCite this: J. Phys. Chem. 1984, 88, 5, 857–860Publication Date (Print):March 1, 1984Publication History Published online1 May 2002Published inissue 1 March 1984https://pubs.acs.org/doi/10.1021/j150649a007https://doi.org/10.1021/j150649a007research-articleACS PublicationsRequest reuse permissionsArticle Views46Altmetric-Citations8LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access options Get e-Alerts

Using Muonium Spin Rotation (MSR) techniques the aqueous solution kinetics of several muonium addition reactions and spin conversion interactions have been studied. The addition reactions show both diffusion and activation-controlled reaction rates with isotope effects between 1 and 3 for diffusion-control and between 7 and 31 for activation-control reactions. Barrier energies are typically ≈15 kJ/mole and ≈30 kJ/mole, respectively, for these processes in water. Spin-conversion interactions involving Ni(aq+2 and Ni(cyclam)+2 complexes showed that spin-conversion of “triplet” Mu by a paramagnetic solute occurs at or near the diffusion-controlled limit while the chemical reaction with the diamagnetic configuration of Ni(cyclam)+2 occured some 100 times slower at kM ⩽ 5×108M−1s−1.