Hyperfine Coupling in Methyl Radical Isotopomers

McKenzie, Iain; Brodovitch, Jean‐Claude; Ghandi, Khashayar; McCollum, Brett; Percival, Paul W.

doi:10.1021/jp0746190

Cited by 20 publications

(32 citation statements)

References 64 publications

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“…7,8,10,11 An ALC signal appears as a "dip" in the time-integrated decay asymmetry at a particular value of the LF, corresponding to a resonant transfer of muon spin polarization from the backward to the forward direction as the magnetic field is varied. 8,11,13,14,20,31,32 In the present study, typically several sweeps of the LF range were carried out, in alternating directions, incremented in steps of about 100 G, depending on conditions.…”

Section: Methodsmentioning

confidence: 99%

“…12 Since the electronic Hamiltonian is isotopically invariant within the Born-Oppenheimer (BO) approximation, the effect of isotopic substitution on torsional barriers is due to differences in zero-pointenergy (ZPE) at potential extrema. 12,14,[16][17][18][19][20] Given that the muon mass is only one-ninth that of the proton, much larger changes in ABSTRACT: Reported here is the first μSR study of the muon (A μ ) and proton (A p ) β-hyperfine coupling constants (Hfcc) of muoniated sec-butyl radicals, formed by muonium (Mu) addition to 1-butene and to cis-and trans-2-butene. The data are compared with in vacuo spinunrestricted MP2 and hybrid DFT/B3YLP calculations reported in the previous paper (I), which played an important part in the interpretation of the data.…”

Section: Introductionmentioning

confidence: 99%

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Isotope Effects and the Temperature Dependences of the Hyperfine Coupling Constants of Muoniated sec-Butyl Radicals in Condensed Phases

et al. 2011

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Reported here is the first μSR study of the muon (A(μ)) and proton (A(p)) β-hyperfine coupling constants (Hfcc) of muoniated sec-butyl radicals, formed by muonium (Mu) addition to 1-butene and to cis- and trans-2-butene. The data are compared with in vacuo spin-unrestricted MP2 and hybrid DFT/B3YLP calculations reported in the previous paper (I), which played an important part in the interpretation of the data. The T-dependences of both the (reduced) muon, A(μ)′(T), and proton, A(p)(T), Hfcc are surprisingly well explained by a simple model, in which the calculated Hfcc from paper I at energy minima of 0 and near ±120° are thermally averaged, assuming an energy dependence given by a basic 2-fold torsional potential. Fitted torsional barriers to A(μ)′(T) from this model are similar (~3 kJ/mol) for all muoniated butyl radicals, suggesting that these are dominated by ZPE effects arising from the C−Mu bond, but for A(p)(T) exhibit wide variations depending on environment. For the cis- and trans-2-butyl radicals formed from 2-butene, A(μ)′(T) exhibits clear discontinuities at bulk butene melting points, evidence for molecular interactions enhancing these muon Hfcc in the environment of the solid state, similar to that found in earlier reports for muoniated tert-butyl. In contrast, for Mu−sec-butyl formed from 1-butene, there is no such discontinuity. The muon hfcc for the trans-2-butyl radical are seemingly very well predicted by B3LYP calculations in the solid phase, but for sec-butyl from 1-butene, showing the absence of further interactions, much better agreement is found with the MP2 calculations across the whole temperature range. Examples of large proton Hfcc near 0 K are also reported, due to eclipsed C−H bonds, in like manner to C−Mu, which then also exhibit clear discontinuities in A(p)(T) at bulk melting points. The data suggest that the good agreement found between theory and experiment from the B3LYP calculations for eclipsed bonds in the solid phase may be fortuitous. For the staggered protons of the sec-butyl radicals formed, no discontinuities are seen at all in A(p)(T), also demonstrating no further effects of molecular interactions on these particular proton Hfcc.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Isotope Effects and the Temperature Dependences of the Hyperfine Coupling Constants of Muoniated sec-Butyl Radicals in Condensed Phases

et al. 2011

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“…The vibrational properties of the bound muonium atom in excited vibrational states of the MuCH 2 CH 2 • radical were later studied theoretically, in which profound anharmonicity of the C-Mu bond was indicated. Using an adept strategy in which muonium was added initially to pure liquid ketene (H 2 C=CO), the muonated methyl radical was detected (MuCH 2 • ) and also the dideuterated version (MuCD 2 • ), formed in liquid ketene-d 2 (D 2 C=CO) 40 . On the basis of ESR and μSR measurements, analyses have been made of the observed temperature dependences of the proton, deuteron and muon hyperfine couplings in terms of the outof-plane vibrations of the various hydrogen isotopes bound to the central carbon atom on which the unpaired electron is formally contained in a 2p z orbital.…”

Section: Structural and Mechanistic Studies Of Radicalsmentioning

confidence: 99%

Muonium–the Second Radioisotope of Hydrogen: A Remarkable and Unique Radiotracer in the Chemical, Materials, Biological and Environmental Sciences

Rhodes

2012

Science Progress

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Muonium (Mu), may be regarded as a radioactive hydrogen atom with a positive muon as its nucleus, and is formed in a range of media which are irradiated with positive muons. This exotic atom can be considered as a second radioisotope of hydrogen, along with tritium. Addition of this light atom (with a mass 1/9th that of a normal hydrogen, protium, atom) to unsaturated organic molecules forms free radicals, in which the muon serves as a radioactive and magnetic probe of their kinetic and structural properties. Suitable examples are chosen to illustrate the very large functionality of organic radicals which have been measured using muons and various methods of μSR, where μ stands for muon, S for spin and R may refer to rotation, resonance or relaxation. The principal techniques illustrated are transverse-field muon spin rotation (TF-μSR), avoided level crossing muon spin resonance (ALC-μSR) and longitudinal-field muon spin relaxation (LF-μSRx). Structural studies of radicals, the determination of mechanisms for radical formation, the measurement of radical stabilisation energies, the determination of the kinetics of reactions of free muonium atoms and of free radicals have all been accomplished using TF-μSR methods. It is further shown that TF-μSR is most useful in measuring radical reaction rates in non-aqueous media, to provide information of relevance to cell membrane damage and repair. Muonium may further be used as a mechanistic probe since it determines a true pattern of H-atom reactivity in molecules, against which results from similar radiolysed materials may be compared. [In many solid materials that are exposed to ionising radiation, apparent H-atom adduct radicals are detected but which originate from charge-neutralisation of positive holes (radical cations) and ejected electrons, without free H-atoms being formed. DNA is the superlative example of this. Free H-atoms normally feature in the province of radiolysed aqueous media]. The applications of ALC-μSR and LF-μSRx in studying the reorientation of reactive radicals on reactive surfaces forms the substantive proportion of the review: considered specifically are radicals sorbed in zeolites, in clays and in porous silica, in porous carbons and on ice-surfaces, in connection with their role as intermediates in catalytic systems, particularly hydrocarbon cracking and oxidation processes, and in atmospheric aerosol chemistry. The formation of muonium and other muon species in cation-exchanged zeolite-X samples are also considered, according to the evidence of longitudinal field repolarisation measurements. Finally, mention is given of the use of μSR techniques for studying radicals in the gas-phase.

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“…In fact, Mu has long been used to understand nuclear quantum effects in chemical reactions including tunneling and quantized vibrations and there have been reported many studies showing very large isotope effects so far [3,4]. In particular, due to its extremely light mass and very large vibrational amplitudes, Mu-containing molecules frequently show a significantly delocalized character in molecular structures compared to hydrogen-and deuteriumcontaining molecules [5][6][7][8][9][10][11][12][13][14][15]. Alternatively, it has been reported that Mu can be localized in a very different region of the potential energy surface compared to hydrogen and deuterium cases due to multi-dimensional effects.…”

Section: Introductionmentioning

confidence: 99%