Addison-Jones, Brenda | DOOR - DOuglas Open Repository

Photograph

Person Preferred Name

Dr. Brenda Addison-Jones

Position

Faculty member

Field of Activity

Analytical and Spectroscopy

Email/Contact

jonesb@douglascollege.ca

Department

Chemistry

Science and Technology

Status

current

Affiliation Date

2002 to open end

Education and Credentials

PhD (SFU) (Physical Chemistry)
BSc (University of Manitoba)

Specializing in Spectroscopy. Teaching Analytical and Spectroscopy.

Related Works

Radio-frequency muon spin resonance studies of endohedral and exohedral muonium adducts of fullerenes

Digital Document

The radio-frequency muon spin resonance technique (RF-μSR) is described, with examples drawn from muon studies of fullerences. Two distinct species can be detected by RF-μSR when solid C60 is irradiated with positive muons. Endohedral muonium (Mu@C60) is characterized by a muon hyperfine constant (A μ) close to the vacuum value. A remarkable feature of the RF-μSR spectrum is the double quantum transition, which appears when the allowed transitions are saturated. The exohedral muonium adduct (C60Mu) is also detected, and has a much smaller value ofA μ typical of a carbon-centred organic radical. It has been studied by RF-μSR in dilute solution, which is not possible for transverse field muon spin rotation (TF-μSR). There is a significant difference inA μ of C60Mu in the solid and in solution, a result of great import to the analysis of avoided-level crossing experiments on13C60Mu.

$Temperature dependence of the muon and proton hyperfine constants of an \alpha‐muonium‐substituted methyl radical$

Digital Document

Muon hyperfine constants A_μ have been measured by transverse field μSR for (CH3)3Si\mbox\.CHMu in hexane from 167 K to 332 K. In addition, avoided level‐crossing resonance was used to determine \alpha‐proton coupling constants Ap over a similar range of temperatures. The two hyperfine constants can be described by a common temperature dependence, d|Ai|/ dT=1.4\times 10-3 MHz\,K-1, where Ai represents Ap or the reduced muon constant A^\prime_μ=0.3141A_μ. There is a small isotope effect (A^\prime_μ is 2.2 % larger than Ap) consistent with zero‐point motion in the anharmonic C–H bond stretch. The common temperature dependence is tentatively attributed to a coupled deviation of the C–H and C–Mu bonds out of the nodal plane of the p orbital containing the unpaired electron.

Intramolecular motion and isotope effects in muonium‐substituted chloroalkyl radicals

Digital Document

Muon irradiation of pure liquid 3‐chloropropene, CH2=CH-CH2Cl, yields a primary radical, \dot\mboxCH2-CHMu-CH2Cl, and a secondary radical, MuCH2-\rm\dot\mboxCH-CH2Cl. 2‐methyl‐3‐chloropropene yields only the tertiary radical, MuCH2-\rm\dot\mboxC(CH3)-CH2Cl. These three chloroalkyl radicals have been characterized by μSR and μLCR, and the hyperfine coupling constants (hfcs) have been determined over a range of temperatures, either in the pure liquid precursor or in concentrated solution. The temperature variation of the hfcs has been analyzed to obtain estimates of the barrier to internal rotation about the C_\alpha-C_\beta axis for various alkyl groups, and also their minimum energy conformations, i.e. their orientations with respect to the axis of the 2p_z orbital of the unpaired electron. The tertiary radical is particularly interesting because all three methyl‐like groups, -CH3,-CH2Cl and -CH2Mu, are represented. The results can be compared to electron spin resonance data for analogous radicals, to provide information on the effects of Mu substitution for H.

Zero frequency resonance: Another way to measure muon-electron hyperfine constants

Digital Document

At a magnetic field determined by the zero crossing of the ω12 frequency of a paramagnetic species like a muonium-substituted radical some muons experience a total magnetic field of zero, simply because the hyperfine field exactly cancels the externally applied field. Since the muon polarization does not rotate under this condition, an integral positron asymmetry is seen even in transverse field if the resonance condition is met. Essentially, the same data acquisition set-up and high beam currents as used in other integral μSR techniques, such as ALCR, can be used to scan for the resonant field, which scales linearly with the hyperfine constant, Aμ. A theoretical treatment of the resonances in the case of isotropic Aμ is presented along with measured spectra demonstrating the technique.

Hyperfine coupling constants of muonium in sub and supercritical water

Digital Document

Muonium, like the hydrogen atom, is a hydrophobic solute in water under standard conditions. Molecular dynamics simulations suggest that the free atom exists in a transient clathrate-like cage of hydrogen-bonded water molecules. The hyperfine constants of Mu and H are very close to their vacuum values, supporting the picture of an atom “rattling” around in a hole in the liquid. Muonium has now been studied in water over a wide range of temperatures and pressures, from standard conditions to over 400°C and 400 bar (the critical point is at 374°C, 221 bar). Drastic changes occur in the properties of water over this range of conditions, so large changes in the muonium hyperfine constant might well be expected. Surprisingly, the changes are small. The hyperfine coupling constant goes through a minimum in the subcritical region, and then increases toward the vacuum value under supercritical conditions. [ABSTRACT FROM PUBLISHER]

Production of biofuel from used coffee grounds

Digital Document

Brenda Addison-Jones' explores the effectiveness of energy alternatives using used coffee grounds and a Bomb Clarinometer, a standard apparatus that measures the total heat content of substances. This PowerPoint presentation is in lieu of a live recording as part of the DC Research Café (November, 2020).

Digital Document

Muonium has been studied in muon-irradiated water over a wide range of conditions, from standard temperature and pressure (STP) up to 350 bar and up to 420[degree]C, corresponding to water densities from 1.0 down to 0.1 g cm. This is the first report of muonium in supercritical water. Muonium was unambiguously identified from its spin precession frequencies in small transverse magnetic fields. The hyperfine constant was determined and found to be similar to the published values for muonium in water at STP and in vacuum. Muonium was found to be long-lived over the whole range of conditions studied. The fraction of muons which form muonium was found to vary markedly over the density range studied. Correlation of the muonium fraction with the ionic product of water suggests a common cause, such as the rate of proton transfer between molecules involved in the radiolysis of water and the formation of MuOH, which competes with muonium formation.

Digital Document

Muoniated free radicals have been detected in muon-irradiated aqueous solutions at high temperatures and pressures. Results are presented for the cyclohexadienyl radical, formed by muonium addition to benzene, and for tert-butyl, formed by reaction of muonium with isobutene, itself formed in situ from the dehydration of the starting material, tert-butanol. This is the first report of the direct identification of organic free radicals in near critical and supercritical water.

Digital Document

The radical formed by muonium addition to diketene has been studied by transverse field muon spin rotation (TF-[small micro]SR) and muon avoided level-crossing resonance ([small micro]ALCR). The TF-[small micro]SR spectrum shows that muonium adds to only one site in diketene, and it is clear from the [small micro]ALCR spectrum that the radical product contains two inequivalent sets of protons. The muon and proton hyperfine coupling constants (hfcs) were determined at several temperatures between 280 and 362 K. The muon hfc falls with increasing temperature, one proton hfc increases, and the other remains constant. The magnitude and temperature dependence of the hfcs support assignment of the 4-muonomethyl-oxetan-2-on-4-yl radical. Density functional calculations were performed to model the temperature dependence of the hfcs. The results are consistent with a preferred conformation of the muoniated methyl group in which the C-Mu bond eclipses the orbital containing the unpaired electron.

Digital Document

Muonium is long-lived in pure water and has been studied over a very wide range of temperatures and pressures, from 5°C to over 400°C and from 1 to 400bar. We have determined rate constants for representative reactions of muonium in aqueous solution; equivalent data on H atom kinetics is sparse and stops well short of the maximum temperature and pressure attained in our experiments. The results show remarkable deviations from the predictions of standard reaction theories. In particular, rate constants pass through a maximum with temperature well below the critical point. This seems to be a general phenomenon, since we have observed it for spin-exchange and chemical reactions that are diffusion limited at low temperatures, as well as for activated reactions. We believe that a key factor in the drop of rate constants at high temperature is the cage effect, in particular the number of collisions between a pair of reactants over the duration of their encounter. Whatever the reason, the implications are profound for both the efficiency of supercritical water oxidation reactors and for the modelling of radiation chemistry in pressurized water nuclear reactors.

Subscribe to Addison-Jones, Brenda