Krishnan Venkateswaran

Muon level-crossing spectroscopy of organic free radicals

Abstract Muon level-crossing spectroscopy has been applied to the study of muonium-substituted radicals formed in liquid benzene, hexadeuterobenzene, furan, 2-methylpropene, 2,3-dimethyl-2-butene, and gaseous ethene. The magnitudes and signs of the proton and deuteron hyperfine constants are reported, and are discussed in terms of isotope effects and intramolecular motion.

Hyperfine constants for the ethyl radical in the gas phase

Abstract Muon spin rotation and level-crossing spectroscopy have been used to measure the muon, proton, deuteron and 13C hyperfine coupling constants for the isotopically substituted ethyl radicals CH2CH2Mu, CD2CD2Mu and 13CH213CH2Mu in the gas phase.

Structure and intramolecular motion of muonium-substituted cyclohexadienyl radicals

Abstract Hyperfine coupling constants of isotopically substituted cyclohexadienyl radicals have been measured as a function of temperature by muon spin rotation and level-crossing spectroscopy. Data are presented for the muon, proton and deuteron hyperfine couplings of the methylene groups in C6H6Mu and C6D6Mu, and also for all the 13C hyperfine couplings of 13C6H6Mu. Comparison of the results with semi-empirical calculations supports a planar ring configuration with complex motion of the methylene substituents.

Intramolecular motion in the tert-butyl radical as studied by muon spin rotation and level-crossing spectroscopy

Abstract Muon spin rotation and muon level-crossing spectroscopy have been used to determine muon ( A μ ) and proton ( A p ) hyperfine coupling constants for the muon-substituted tert-butyl radical (CH 3 ) 2 CCH 2 Mu over a wide range of temperature in isobutene. A p (CH 3 ) is almost constant, but A μ (CH 2 Mu) falls and A p (CH 2 Mu) rises with increasing temperature, consistent with a preferred conformation of the methyl group in which the CMu bond is coplanar with the symmetry axis of the radical orbital. The A μ data cover the temperature range from 297 K down to 43 K, where the solution is frozen. There is a discontinuity in A μ at the melting point, as well as a change in temperature dependence. It is suggested that the potential barrier for methyl group rotation is lower in the liquid due to simultaneous inversion at the radical centre, and that the inversion mode is somewhat inhibited in the solid. The best fit of the liquid-phase data indicates a V 2 barrier of 1.8 kJ mol −1 , and is consistent with a long CMu bond and a tilt of the CH 2 Mu group in the direction that brings the Mu atom closer to the radical centre.

Measurement of the 13C hyperfine constants of the cyclohexadienyl radical using muon level-crossing resonance

Abstract The 13 C hyperfine constants of the cyclohexadienyl radical, formed by muonium addition to 13 C-enriched benzene, have been measured using a novel muon level-crossing resonance technique. The constants are compared with a recent ab initio calculation, Karplus-Fraenkel theory, and with ESR results on the 1,2,3,4,5-pentacarboxyl cyclohexadienyl radical.

A level-crossing-resonance study of muonated free-radical formation in solutions of acetone in hexane, water and dilute micelles

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.

Spin relaxation studies of the muonium substituted ethyl radical in the gas phase

This short communication draws attention to the power of μSR and related measurements in providing an unusually complete characterisation of muonium substituted organic radicals in the gas phase. Spectroscopic information is available from muon spin rotation and muon level crossing resonance, giving all the nuclear hyperfine coupling constants, just as in the liquid phase. In addition, measurements of the relaxation time of the muon Zeeman energy become possible; these are potentially informative on the molecular collision dynamics. Demonstration results are presented in summary for the muonium substituted ethyl radical, ĊH2CH2Mu, in ethene gas.

Muon level crossing resonance study of radical formation in allylbenzene, styrene and toluene

Abstract 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- P D , where P D 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.

Micelle-induced enhancement of the reactivity of muonium atoms in dilute aqueous solution

Abstract Enhancements in rate constants from 10 6 M −1 s −1 to more than 10 10 M −1 s −1 have been found for the reaction of muonium atoms with 2-propanol in water when micelles are added.

Muonium reactions with chloroacetic acid in water: Contrasts with H atoms and hydrated electrons

Abstract 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. Muoniums reactions must be governed to a considerable extent by quantum-mechanical effects arising from its very small mass.