Jean-Claude Brodovitch

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.

Free Radical Reactivity of Mono‐ and Dichlorosilylene with Muonium

Silylenes are generally considered highly reactive species, and it was only in 1994 that the first synthesis of an isolable N-heterocyclic silylene (NHSi) 1 (Scheme 1) was accomplished. Other classes of isolable silylenes have since been discovered, spurred by the utility of silylenes as building blocks in organosilane synthesis and as ligands in transition metal complexes. Understanding the reactivity of silylenes is key to both their isolation and their application in synthesis. The chemistry of analogous carbon compounds is a guide, but there are significant differences between carbon and silicon, most obviously evident in the prevalence of long-chain carbon compounds which are the basis of life on this planet. The stability of unsaturated NHSi compounds is a consequence of p-donor stabilisation of the silicon(II) atom by the nitrogen atoms, as well as pseudoaromaticity in the ring. The latter factor is absent from the corresponding saturated NHSi 2 compounds and they are indeed less stable. An additional (kinetic) factor in the reactivity of silylenes is steric protection of the reaction centre by bulky substituents on neighbouring atoms. Thus, silylene 3, synthesised by Kira, is isolable despite the absence of ring nitrogen atoms. We have explored the reactivity of silylenes by studying the free radicals formed by muonium addition. Muonium (Mu) is a single-electron atom, the nucleus of which is the positive muon; it is chemically equivalent to H, but has only one-ninth the mass. At first we could not reconcile the muon hyperfine constants (hfcs) of the radicals formed from silylenes with the predictions of quantum calculations. The explanation lies in a coupling of the primary muonium adduct with a second silylene, so that the disilanyl product radical 4 is observed. This was confirmed in a subsequent experiment on a series of NHSi compounds with different substituents on the nitrogen atoms. The largest substituent (2,6-diisopropylphenyl) served to slow the silyl coupling reaction so that we were able to detect the primary silyl radical 5, as evident from the record high hfc (931 MHz). In a related study we explored the relative reactivity of carbon and silicon atoms in a silene 6, this being the product of trimethylsilyl migration in silylene 3. We now report our investigation of the reactivity of two novel chlorosilylenes, 7 and 8 (Scheme 2). Both offer multiple sites for potential attack by free radicals. The H atom (and by extension, Mu)

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.

Spin depolarization in muonium by hydrated electrons

Abstract The missing fraction of muon polarization in water is shown to originate from encounters between muonium and hydrated electrons. This takes place at ≈1 ns after the primary events in which muonium and hydrated electrons are born.

Partial spin depolarization of muonium in ice

Abstract Muon spin precession signals arising from both muonium and a diamagnetic muon species have been studied in single crystal of ice over the temperature range 90–263 K. Conversion of initial signal amplitudes to fractions of muon polarization reveals that for temperatures above 200 K part of the original muon polarization is unaccounted for. Such a missing fraction is well known for liquid water. but was not found in earlier work on polycrystalline ice. Muonium signals in Polycrystalline ice were reanalyzed using a non-exponential decay function appropriate to the powder pattern spectrum. Smaller muonium fractions were found. consistent with those determined for single crystall, thus confirming the existence of the missing fraction. The origin of the missing fraction is discussed, and it is proposed that non-reactive spin exchange encounters between muonium and hydrogen atoms may be the cause.

Spin relaxation of muonium‐substituted ethyl radicals (MuCH2ĊH2) in the gas phase

The spin relaxation of the muonium‐substituted ethyl radical (MuCH2ĊH2) and its deuterated analog (MuCD2ĊD2) has been studied in the gas phase in both transverse and longitudinal magnetic fields spanning the range ∼0.5–35 kG, over a pressure range from ∼1–16 atm at ambient temperature. The Mu13CH213ĊH2 radical has also been investigated, at 2.7 atm. For comparison, some data is also reported for the MuCH2Ċ(CH3)2 (Mu‐t‐butyl) radical at a pressure of 2.6 atm. This experiment establishes the importance of the μSR technique in studying spin relaxation phenomena of polyatomic radicals in the gas phase, where equivalent ESR data is sparse or nonexistent. Both T1 (longitudinal) and T2 (transverse) μSR relaxation rates are reported and interpreted with a phenomenological model. Relaxation results from fluctuating terms in the spin Hamiltonian, inducing transitions between the eigenstates assumed from an isotropic hyperfine interaction. Low‐field relaxation is primarily due to the electron, via both the nuclear h...

13C hyperfine coupling constants in MuC60

Abstract 13 C hyperfine coupling constants of the MuC 60 radical have been measured by muon level-crossing spectroscopy, using a dilute solution of 99% enriched 13 C 60 in decalin. The signs as well as the magnitudes of the hyperfine constants were determined. The results range from 52.6 to −25.4 MHz and support those calculations which predict an extended distribution of unpaired electron spin density in radical adducts of fullerenes. The hyperfine constants are consistent with published electron spin resonance results for (CD 3 ) 3 CC 60 , but contradict a recent report for HC 60 , where a considerably smaller value is reported for the largest splitting.

Spin relaxation of muonated radicals in the gas phase

We report on recent results obtained for longitudinal field (T1) spin relaxation of the muonium-substituted (“muonated”) free radicals MuCO, MuC2F4, MuC2H3F, and MuC4H8 (t-butyl), comparing with results reported earlier for MuC2H4 (and MuC2D4). Some comparison with transverse field (T2) data is also given. These data are fit to a phenomenological model based on NMR theory of spin relaxation in gases. The parameters of these fits are presented and discussed.