The rates of reaction of muonium atoms with solutes, ionic and organic, were studied in solvents of wildly differing polarities (water, methanol, and hexane) and their rate constants were compared, where possible. In these reactions — which are those of a highly reactive atom, an isotope of hydrogen — it transpires that the reaction rates are higher in solvents in which the solute is more soluble and muonium diffuses faster. This study leads to various kinetic-solvent-effect ratios and to the observation of the reaction of muonium with free radicals being among the fastest reactions recorded so far between two neutral species in solution.Key words: muonium atoms, kinetic isotope effects, solvent-dependent rates, non-aqueous solvents, muon spin rotation technique.

The mechanism of formation of muonium atoms from positive muons was studied here through measurements of the yield of diamagnetic muon states in dipolar aprotic solvents and for scavenger solutions in hexane and methanol. The results are compared with published data on common solvents covering a full range of the physicochemical properties of liquids that affect an ionic formation mechanism, namely their static dielectric constants, electron mobilities, and radiolysis yields of electrons. It is concluded that muonium is not formed by a thermal charge-neutralization reaction in these chemically-active media, though that mechanism does contribute to muonium formation in inert media like liquefied noble gases. It is clear that muonium materializes on a much shorter timescale than the recently proposed "delayed" mechanism (microseconds) and the earlier "spur" model (nanoseconds). In contrast, the data referring to all these liquids are consistent with the intra-track "hot" model. This is the only Mu-formation model proposed so far in which the immediate precursors of Mu (Mu(hot)) are neither scavengable nor ionic.Key words: muonium atoms, formation mechanism, hot model, spur model, delayed-muonium-formation model, diamagnetic yields.

Studies of the yields of muonium atoms and diamagnetic-species in hexane in the presence of added scavengers have shown, unequivocally, that muonium is not formed by the “delayed” (microsecond) mechanism proposed recently. Instead, the lifetimes evaluated show that if the combination reaction of μ++e−→Mu contributes to muonium formation in “chemically active” liquids, then it is complete within <10−10 s. In fact the results are consistent only with the “hot” model, in which the initial distribution of muons between muonium atoms and diamagnetic molecules is determined at the epithermal level, as the last step of the muon’s charge-exchangecycles and thermalization.

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.

Muon resonance studies show that muonium atoms (Mu) in ethanol add selectively to certain C-sites of aromatic compounds containing -Cl and -OH substituents. The sites chosen seem to be those carrying the lowest electron density. This helps to characterize Mu as a nucleophile in addition reactions and, in this respect, Mu differs from ordinary H-atoms.

A competition was established for the reaction of muonium atoms (Mu) between nitrate ions in water and benzene or styrene solubilized in micelles. The nitrate was 3.3-times more efficient at inhibiting muonated free radical formation with benzene than with styrene as the radical-producing solute. Kinetic analysis of this system indicates that Mu emerges from micelles, on average, at least three times during its short (ns) lifetime, these being medium sized micelles carrying on average 3 benzene molecules. So Mu is certainly not trapped, nor even localized. Its escape rate is estimated to be ∼9×108 s−1, which is commensurate with an ordinary diffusion time. The results were obtained by determining the yield of muonated free radicals formed within the micelles using muon-level-crossing-resonance spectroscopy.

The muon level-crossing-resonance technique has been used to resolve major discrepancies that exist in muon-spin-resonance studies (both free-radical formation and muonium decay rates) in the competition between benzene and styrene. The results, obtained for ∼30 mM solutions in ethanol and for 2.5 mM aqueous micelles solutions, show that muonium atoms (Mu) react 8 (±2) times faster with styrene than with benzene. In the above cases thermalized Mu is unquestionably the reactive species, which is known to show nucleophilic intra-molecular selectivity in the case of styrene. But a similar value, 9 (±2), was also obtained for undiluted mixtures of liquid benzene and styrene (neat mixture) — where the precursor might have been ‘hot Mu’ (which should display weaker selectivity than Mu) or cations derived fromμ+ (which should show higher selectivity). These results support the view that thermalized Mu is the predominant reactive species in liquid benzene and styrene.

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.

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.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTExcited states of tetraoxo complexes of transition metals and their subnanosecond transient absorptionG. B. Porter, A. D. Kirk, and D. K. SharmaCite this: J. Phys. Chem. 1986, 90, 9, 1781–1783Publication Date (Print):April 1, 1986Publication History Published online1 May 2002Published inissue 1 April 1986https://pubs.acs.org/doi/10.1021/j100400a010https://doi.org/10.1021/j100400a010research-articleACS PublicationsRequest reuse permissionsArticle Views70Altmetric-Citations6LEARN 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

The transient absorption seen for Cr(byp) 3+ 3 and Cr(phen) 3+ 3 rises with the integrated laser pulse on excitation with 30 ps pulses of 355 nm radiation, and remains constant out to 400 ps. These observations and other considerations suggest that the doublet state, and not a nanosecond-lived quartet state as earlier claimed, is being observed.

The quantum yield for the photoaquation of Cr(bpy)33+ in basic medium decreases with increasing pressure, with an apparent volume of activation of 3.8 ± 1.0 ml mol−1. From this value and that associated with the phosphorescence lifetime, the volumes of activation for non-radiative decay to the ground state and for formation of photoproduct are derived as −1.6 and +2.9 ml mol−1, respectively. The latter value is consistent with either an associative process with water entering from pockets between the ligands or a dissociative process involving one or both bonds to a bipyridyl ligand.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTPhotochemistry of bromopentaamminecobalt(2+) ([Co(NH3)5Br]2+): wavelength, pressure, and medium dependence of redox and aquationA. D. Kirk, C. Namasivayam, Gerald B. Porter, M. A. Rampi-Scandola, and A. SimmonsCite this: J. Phys. Chem. 1983, 87, 16, 3108–3113Publication Date (Print):August 1, 1983Publication History Published online1 May 2002Published inissue 1 August 1983https://pubs.acs.org/doi/10.1021/j100239a031https://doi.org/10.1021/j100239a031research-articleACS PublicationsRequest reuse permissionsArticle Views306Altmetric-Citations11LEARN 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

ADVERTISEMENT RETURN TO ISSUEPREVReportNEXTIntroduction to inorganic photochemistry: Principles and methodsGerald B. Porter Cite this: J. Chem. Educ. 1983, 60, 10, 785Publication Date (Print):October 1, 1983Publication History Received3 August 2009Published online1 October 1983Published inissue 1 October 1983https://doi.org/10.1021/ed060p785RIGHTS & PERMISSIONSArticle Views1774Altmetric-Citations15LEARN 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 InReddit PDF (5 MB) Get e-Alertsclose Get e-Alerts

Ru(bpy)32+ is photodecomposed in 0.3 M NaOH with [Formula: see text]. With methyl viologen also present, electron transfer quenching of the luminescence is accompanied by formation of the one electron reduction product of methyl viologen, MV+. The Ru(bpy)33+ formed in the corresponding oxidation is rapidly reduced by either OH− or MV+ to Ru(II).

Phase transitions in NaZnF3 and NaMnF3 were examined up to 24 GPa and 1100 °C using a multianvil apparatus. NaZnF3 perovskite transforms to postperovskite above 11–16 GPa at 600–1000 °C, and the postperovskite is quenchable at ambient conditions. The NaZnF3 perovskite–postperovskite transition boundary is expressed as P (GPa) = 4.9 + 0.011T (°C). At 8–11 GPa and 900–1100 °C, NaMnF3 perovskite dissociates into two phases of Na3Mn2F7 and MnF2. The latter phase is suggested to have the structure of orthorhombic-I type ZrO2 or cotunnite. Using available experimental data on the perovskite–postperovskite transitions in thirteen compounds of A2+B4+O3 and A+B2+F3, several crystal-chemical characteristics of the transition are elucidated as follows. In the transition, the volume change is between −1% and −2%, and the Clapeyron slope of the boundary is 10–17 MPa/°C. These support reliability of recently determined Clapeyron slope of 13 MPa/°C in MgSiO3 which suggests that the perovskite–postperovskite boundary intersects the temperature profile twice in the D″ layer. Postperovskites of ABX3 whose enthalpies are higher by more than 70 kJ/mol relative to the phase stable at 1 atm are unquenchable, while those by less than 15 kJ/mol are quenchable to ambient conditions. Structure refinements indicate that A+B2+F3 postperovskites quenched at 1 atm are more similar to that of MgSiO3 postperovskite at high pressure, than those of quenched A2+B4+O3 postperovskites. With increasing pressure, octahedral tilt angles of both A2+B4+O3 and A+B2+F3 perovskites increase, resulting in transition to postperovskite at the angle of about 26°, and fluoride perovskites are more rapidly distorted with pressure than oxide perovskites. Covalent character of B–X bonds of ABX3 postperovskite is suggested to be favorable for stabilization of the postperovskite structure. All these features suggest that NaNiF3 is a good quenchable, low-pressure analogue compound to MgSiO3 to investigate the perovskite–postperovskite transition.