Applications of Muon Spin Spectroscopy in Chemistry Paul

Applications of Muon Spin Spectroscopy in Chemistry Paul

Applications of Muon Spin Spectroscopy in Chemistry Paul Percival TRIUMF and Department of Chemistry, Simon Fraser University 2 SR in Chemistry Overview Is there any demand? Is chemistry any different to physics? Are the needs of chemists different from physicists? Why do chemists prefer a continuous source? Why do chemists prefer a pulsed source? Why do chemists feel misunderstood/unwelcome? Paul Percival October 2012 3 SR in Chemistry Overview Is there any demand? experts versus users Is chemistry any different to physics? the muon as part of the problem Are the needs of chemists different from physicists? sample environment Why do chemists prefer a continuous source? high frequency precession Why do chemists prefer a pulsed source? coherent excitation Why do chemists feel misunderstood/unwelcome?

cultural issues Paul Percival October 2012 4 SR in Chemistry Overview Is there any demand? experts versus users Is chemistry any different to physics? the muon as part of the problem Are the needs of chemists different from physicists? sample environment Why do chemists prefer a continuous source? high frequency precession Why do chemists prefer a pulsed source? coherent excitation Why do chemists feel misunderstood/unwelcome? cultural issues Paul Percival October 2012 5 CMMS Users: Canadian Groups A. Bianchi J.H. Brewer K.H. Chow J.A. Clyburne D.G. Fleming

K. Ghandi J.E. Greedan T. Imai R.F. Kie D. Leznoff G.M. Luke W.A. MacFarlane A. Mar P.W. Percival J.E. Sonier C.R. Wiebe Paul Percival Physics, U. Montreal Physics, UBC Physics, Alberta Chemistry, St. Marys Chemistry, UBC Chemistry, Mount Allison Chemistry, McMaster Physics, McMaster Physics, UBC Chemistry, SFU Physics, McMaster Chemistry, UBC Chemistry, Alberta Chemistry, SFU Physics, SFU Chemistry, U. Winnipeg October 2012 6 Canadian Major Users completely reliant on CMMS

A. Bianchi J.H. Brewer K.H. Chow J.A. Clyburne D.G. Fleming K. Ghandi J.E. Greedan T. Imai R.F. Kie D. Leznoff G.M. Luke W.A. MacFarlane A. Mar P.W. Percival J.E. Sonier C.R. Wiebe Paul Percival Physics, U. Montreal Physics, UBC Physics, Alberta Chemistry, St. Marys Chemistry, UBC Chemistry, Mount Allison Chemistry, McMaster Physics, McMaster Physics, UBC Chemistry, SFU Physics, McMaster Chemistry, UBC Chemistry, Alberta Chemistry, SFU Physics, SFU Chemistry, U. Winnipeg

October 2012 7 M1172/M1345 Sample Contributors The samples are moisture and air sensitive, and some have limited stability. None are commercially available; they are custom-synthesized by the groups of: Paul Percival Robert West Yitzhak Apeloig University of Wisconsin, Madison Kim Baines Matthias Driess University of Western Ontario Mitsuo Kira Charles MacDonald Tohoku University Philip Power Herbert Roesky UC Davis Chris Russell Dominic Wright University of Bristol

Akira Sekiguchi Reinhold Tacke University of Tsukuba Technion, Israel T.U. Berlin University of Windsor Georg-August University, Gttingen Cambridge University University of Wrzburg October 2012 8 SR in Chemistry Overview Is there any demand? experts versus users Is chemistry any different to physics? Chemical applications focus on Muonium Are the needs of chemists different from physicists? sample environment Why do chemists prefer a continuous source? high frequency precession Why do chemists prefer a pulsed source? coherent excitation Why do chemists feel misunderstood/unwelcome? cultural issues Paul Percival October 2012

9 Muonium is a single-electron atom, just like Hydrogen + e p+ e The chemistry of an atom depends on the ionization potential the radius (charge distribution) These properties depend on the reduced mass mN me mr me mN me Chemically, Muonium is an isotope of H; the positive muon is a light proton. Paul Percival October 2012 10 Muonium chemistry is H chemistry MuH

Mu + H2 Mu + OH MuOH Mu + OH MuOH Mu + + (e-)aq Mu C C

C C H Mu + Mu H . Mu Mu Paul Percival + Mu + October 2012 11 Muonium Chemistry Research How fast does it react? kinetic isotope effects; tunneling How is the reaction affected by the environment? T, P, density What is the chemical reaction? reactivity, mechanism What products are formed? free radical chemistry Structure of free radicals geometry, bonding

Dynamics of free radicals intramolecular motion, tumbling Free radicals as probes soft matter Muonium as a probe fullerenes, semiconductors Muonium diffusion ice, semiconductors Paul Percival October 2012 12 Muon Spin Rotation, TF-SR Transverse-field SR involves counting the elapsed time between a muon stop in the sample and its decay positron. The muon beam is spin-polarized and a magnetic field is applied perpendicular to the initial spin direction F S beam B e+ P SR histogram start stop

N CLOCK time Muon Asymmetry Divide out muon decay 0.00 Paul Percival 0.05 0.10 time / s Fourier transfor m 0.15 0.20 The positron detection probability oscillates at the muon spin precession frequency. Fourier Power

M + 225 250 275 300 Frequency / MHz 325 October 2012 13 TF-SR of Muoniated Free Radicals The unpaired electron has the largest magnetic moment. At high enough field the muon spin flip transitions are degenerate for a given electron spin orientation. example for 1 electron, 1 muon, 1 proton Energy Fourier Power CH2Mu A 0

50 100 150 200 250 300 350 400 Frequency / MHz A R2 R1 Magnetic Field Paul Percival SR ENDOR October 2012 14 Muon Avoided Level Crossing Resonance e , ,X Energy MuLCR involves counting positrons in directions along and opposite to the magnetic field. The difference is

proportional to the longitudinal muon spin polarization. Mixing of spin levels related by flip-flop transitions of the muon and another nucleus results in loss of spin polarization at a field determined by the muon and nuclear hfcs. e ,,X A+ A Magnetic Field BLCR 6.8 7.0 7.2 7.4 7.6 7.8 Magnetic Field / kG Paul Percival 8.0 8.2 1 A Ap A Ap

2 p e The differential line shape is from field modulation. October 2012 15 Hyperfine constants map unpaired spin in free radicals Mu13C60 Avoided Level Crossing Resonance Mu 1.5 3.3 30 1.4 14 5.4 2.0 % Unpaired Spin Density 10.5 11.0

11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 Magnetic Field /kG Percival, Addison-Jones, Brodovitch, Ji, et al., Chem. Phys. Lett., 245 (1995) 90 Paul Percival October 2012 16 Muoniated free radicals R2 R1 Mu C C

R4 R3 SR: precession frequencies muon hyperfine coupling LCR: resonance fields other nuclear hyperfine couplings hyperfine couplings distribution of unpaired electron spin e.g. cyclohexadienyl H Mu 0.33 -0.10 Temperature dependence of hyperfine couplings intramolecular motion Paul Percival 0.33 -0.10 0.48 October 2012 Muoniated Radicals can be clearly identified by their hfcs

17 e.g. ethyl radical ed reduc c hf muon Hyperfine constant /MHz 160 s n o rot p 140 Mu H H C C H H protons

-30 -40 120 -50 -60 100 -70 -80 80 -90 0 60 200 300 Temperature /K 40 0 100 200 Temperature /K

Paul Percival 100 300 ACH2Mu = 13 A+ 32 ACH2 October 2012 18 Anharmonic potentials result in isotope effects V(r) CMu rAB 0 mCH CH 3CH mCMu C-H C-Mu Dissociation Energy 2 Bond length

v= 43 1 2 Zero-point vibration in an anharmonic potential increases the effective bond length over the equilibrium value. Bending vibrations have similar effects. Vibrational averaging results in isotope effects and temperature dependence of hyperfine constants Paul Percival 1 0 0 r < rH > < rMu > ~ < rH >v=1 October 2012 19 SR in Chemistry Overview Is there any demand? experts versus users Is chemistry any different to physics?

the muon as part of the problem Are the needs of chemists different from physicists? sample environment Why do chemists prefer a continuous source? high frequency precession Why do chemists prefer a pulsed source? coherent excitation Why do chemists feel misunderstood/unwelcome? cultural issues Paul Percival October 2012 20 Sample environments for chemical samples Low pressure gases Mu kinetics, isolated free radicals large volumes, large detectors surface muons High pressure gases for slow Mu reactions pressure vessel decay muon channel Liquids close to ambient conditions typically noxious and/or reactive surface muons Superheated and/or pressurized liquids sc-CO2, sc-H2O pressure vessel decay muon channel Solids ice, fullerenes, polymers surface muons

standard cryostats and ovens Paul Percival October 2012 21 Sample environments for chemical samples Low pressure gases Mu kinetics, isolated free radicals large volumes, large detectors surface muons High pressure gases for slow Mu reactions pressure vessel decay muon channel Paul Percival October 2012 22 Fundamental Kinetic Studies: Mu + H2 Mu precession in ~7 G TF Muon precession in ~300 G TF Paul Percival October 2012 23

Fundamental Kinetic Studies: H + H2 Experimental tests of reaction rate theory: Mu+H2 and Mu+D2 Fleming et al, J. Chem. Phys. 1987 Quasiclassical trajectory (and variational transition state theory) study of the rates and temperaturedependent activation energies of the reactions Mu+H2 (completely thermal) and H, D, and Mu+H2 (v=0, j=2) Truhlar et al, J. Chem. Phys. 1983 Paul Percival October 2012 Chemical Reaction Rates from Ring Polymer Molecular Dynamics: Zero Point Energy Conservation in Mu+H2 MuH+H 24 Ricardo Prez de Tudela, F. Javier Aoiz, Yury V. Suleimanov, and David E. Manolopoulos J. Phys. Chem. Letters, DOI: 10.1021/jz201702q vibrationally adiabatic classical MEP Mu + H2 is slower than H + H2 and does not involve tunneling. Paul Percival

October 2012 25 Muons extend the range of Hydrogen isotopes Mu H D e e 0.11 Paul Percival H e n p p 1 H

He+ 2 H e He 4.1 H October 2012 26 Kinetics of the He + H2 reaction Fleming et al., J. Chem. Phys. 135, 184310 (2011) Paul Percival October 2012 27 Sample environments for chemical samples Low pressure gases Mu kinetics, isolated free radicals large volumes, large detectors surface muons High pressure gases for slow Mu reactions

pressure vessel decay muon channel Liquids close to ambient conditions typically noxious and/or reactive surface muons Paul Percival October 2012 28 Sample environments for chemical samples Low pressure gases Mu kinetics, isolated free radicals large volumes, large detectors surface muons High pressure gases for slow Mu reactions pressure vessel decay muon channel Liquids close to ambient conditions typically noxious and/or reactive surface muons Superheated and/or pressurized liquids sc-CO2, sc-H2O pressure vessel decay muon channel Solids ice, fullerenes, polymers surface muons standard cryostats and ovens Paul Percival

October 2012 29 Pressure cells for Hydrothermal Studies Paul Percival October 2012 30 Supercritical Water Oxidation There are drastic changes in the physical properties of water close to and above the critical point (Tc = 374C, Pc = 221 bar). organic compounds are miscible in SCW This leads to unusual chemistry: combustion of organic materials is possible SCWO facilities are being developed for the destruction of hazardous waste such as PCBs, sewage sludge, and chemical A Flame in Water! weapons. 30% methane in water It has even been proposed for water at 2000 bar, 450C

recycling on long space flights. Schilling and Franck, Many of these reactions involve free radical intermediates, but there is very little direct knowledge about these transient species under such extreme conditions. Paul Percival Ber. Bunsenges. Phys. Chem. 92 (1988) 631. October 2012 31 Destruction of Chemical Weapons The Blue Grass Chemical Agent-Destruction Pilot Plant (BGCAPP) is a chemical weapons destruction facility under construction. The plant is being built to destroy the chemical weapons stockpile at the Blue Grass Army Depot (BGAD), near Richmond, Kentucky. The plant is dedicated to the destruction of 523 tons of nerve agents sarin (GB) and VX, and mustard agent, which constitute about two percent of the United States chemical weapons stockpile. Abstracted from Wikipedia. Further information at the General Atomics web site http://www.ga.com/hazardous-waste-destruction/supercritical-water-oxidation/... Paul Percival October 2012

32 Surprising Chemistry in Superheated Water Dehydration! Dehydration of t-butanol forms isobutene which can be spin-labelled by adding Mu Power 350C, 250 bar tertbutanol OH - H2O 0 CH3 Mu H3C - H2O 400 CH2Mu isobutene secbutanol Percival, Brodovitch, Ghandi, B. McCollum and McKenzie, J. Am. Chem. Soc. 127 Paul Percival (2005) 13714.

A+-A- OH 200 Frequency / MHz 9.6 9.8 10.0 10.2 10.4 10.6 Field / kG October 2012 33 Supercritical-Water-Cooled Reactor Canada is one of ten countries (the Generation IV International Forum) working together to lay the groundwork for fourth generation nuclear reactor systems. The priority R & D areas for Canada include improved understanding of radiolysis under supercritical water conditions and the effect of radiolysis products on corrosion and stress corrosion cracking.

The Supercritical-Water-Cooled Reactor (SCWR) system is a high-temperature, high-pressure water cooled reactor that operates above the thermodynamic critical point of water (374C, 22 Mpa) The SCWR system is primarily designed for efficient electricity production. Paul Percival October 2012 34 A fall-off of rate is common for reactions in high T water Mu OH MuOH eaq Mu + Benzene 1.2 1011 350 bar 250 bar > 310 bar 0.8 0.6 0.4 < 200 bar 1 bar (lit.)

109 108 0.2 0.0 245 bar 1010 kMu / M-1s-1 kMu / 1010 M-1s-1 1.0 0 Paul Percival 100 200 300 Temperature / C 400 107 0 100

200 300 o Temperature / C 400 October 2012 35 Diffusion and collision-controlled kinetics Reaction rates in liquids depend on diffusion of the reactants to form the encounter pair, as well as the activated chemical reaction. 1 1 1 Mu + A MuA products { } kobs kdiff kact kdiff 4 RMu RA DMu DA kact f R A exp Ea / RT where

fR PZ coll -1enc PZ coll many collisions per encounter at low temperature The key factor in the falloff with temperature seems to be the drop in collision encounter for gas-like behaviour at the number of collisions high T between a pair of reactants over the duration of their encounter. Ghandi, Percival et al Phys. Chem. Chem. Phys. (2002) Paul Percival October 2012 36 H + OH H2O current PWR reactors next generation Data limited to 200C Buxton and Elliot, JCS Far. Trans. 89 (1993) 485 reactors

8.0 AECL k / 1010 M-1s-1 6.0 M1 M2 M3 4.0 2.0 0.0 0 100 200 300 400 500 Temperature /C Ghandi and Percival, J. Phys. Chem. A 107 (2003) 3006 Paul Percival October 2012

37 SR in Chemistry Overview Is there any demand? experts versus users Is chemistry any different to physics? the muon as part of the problem Are the needs of chemists different from physicists? sample environment Why do chemists prefer a continuous source? high frequency precession Why do chemists prefer a pulsed source? coherent excitation Why do chemists feel misunderstood/unwelcome? cultural issues Paul Percival October 2012 38 Free Induction Decay in Muon Spin Spectroscopy Muon Asymmetry In TF-SR M is already in the xy plane, and precesses in applied and local magnetic fields. 0.15 0.10 0.05 0.00 -0.05 -0.10

-0.15 0.00 0.04 0.08 0.12 0.16 MuCH2H2 from ethene gas at 10 bar, 25C, 14.4 kG. The Fourier Transform conveniently reveals the precession frequencies. Paul Percival Fourier Power time/ s 0 50 100 150 200 250 300 350 400 Frequency / MHz October 2012 39 Reactivity studies on the Driess silylene MuCH2

dipp N Mu could add to the exoCH2 Si N CH3 dipp N Si N dipp N Si Mu or to the silylene N Fourier Power dipp Two pairs of radical precession signals are observed, consistent with the formation of both radicals. A = 149 MHz, 716 MHz

-300 -200 -100 0 100 200 300 400 500 Frequency /MHz Paul Percival October 2012 40 N R N N N Si: Fourier Power The spectrum changes when the silylene is complexed R 0 100 200

300 Frequency /MHz A = 19 MHz, 133 MHz Two radical signals are again observed. One is consistent with the CH2 adduct, and there is matching LCR data, but the other has a much smaller hfc. Is it from Mu addition to Si: or to the carbene? N CH2Mu R N N N Si: R Paul Percival October 2012 41 Assignment of complexed radicals N CH2Mu R

N N Si: R Paul Percival N N R N N Si N Mu R October 2012 42 SR in Chemistry Overview Is there any demand? experts versus users Is chemistry any different to physics? the muon as part of the problem Are the needs of chemists different from physicists? sample environment

Why do chemists prefer a continuous source? high frequency precession Why do chemists prefer a pulsed source? coherent excitation: RF or light Why do chemists feel misunderstood/unwelcome? cultural issues Paul Percival October 2012 43 Reaction of Muonium with Vibrationally Excited H2 Bakule, Fleming et al., J. Phys. Chem. Lett. 3 (2012) 27552760. Muonium decay with and without laser excitation of H2 Difference of the decay curves shows the reaction with H2{1} Paul Percival October 2012 44 Reaction of Muonium with Vibrationally Excited H2 Bakule, Fleming et al., J. Phys. Chem. Lett. 3 (2012) 27552760. Mu + H2{1} D + H2{1}

D + H2 Mu + H2 Paul Percival October 2012 45 SR in Chemistry Overview Is there any demand? experts versus users Is chemistry any different to physics? the muon as part of the problem Are the needs of chemists different from physicists? sample environment Why do chemists prefer a continuous source? high frequency precession Why do chemists prefer a pulsed source? coherent excitation Why do chemists feel misunderstood/unwelcome? cultural issues Paul Percival October 2012 46 Challenges Facing a Future for CMMS as a User Facility What keeps potential Users away? High Activation Barrier A new group needs 3 or 4 experts to mount an independent program Approval Procedures

designed for long-running subatomic physics experiments High Level of Expertise Required User-friendly controls and procedures require stable set-ups. Insufficient Support Personnel TRIUMF CMMS has far fewer support personnel than PSI and ISIS. Attitude The casual User is a stranger in a strange land! Paul Percival October 2012 47 END Paul Percival E945: Mu radicals from carbenes and carbene analogues October 2012 Paul Percival, June 2007 48 Molecular vibrations affect hyperfine constants 197, 545 cm-1 385 cm-1 Mu

H Mu H Chem. Phys.142 (1990) 229-236. muon hfc 530 Isotope effects increase with the level of anharmonicity. Temperature dependence is enhanced by low frequency modes. 525 hfc /MHz An accurate calculation of hyperfine constants requires vibrational averaging over all normal modes. 520 515 510 0 100 200 300

400 Temperature /K UB3LYP/6-31G(d) Paul Percival October 2012 49 Radicals with Delocalized Pi Orbitals In a 1-electron LCAO model, the SOMO is described by c11 c22 ... 2 i c 1 ci2 is a measure of the unpaired electron density on atom i Ai Qi H H Q 75 MHz H Mu H

H H H H H H H H Mu H H McConnell, 1956 H H H H H H 1 positive and 5 negative

proton couplings 1 positive and 3 negative proton couplings Paul Percival Percival et al, 1999 = positive spin density October 2012 50 Mu diffuses along the c-axis channels of ice-Ih Mu side view Paul Percival view along c channel October 2012 51 Endohedral Muonium [email protected] Muonium in a universe of its own Paul Percival

October 2012 52 The Curious Case of C60 (as studied by SR) Is it a bird? is it a plane? The signals are characteristic of both muonium and a free radical. No other single phase material had shown such behaviour. 12 Mu 34 23 0 50 14 100 150 200 250 300 Frequency / MHz First report: Ansaldo, Niedermayer et al., 1991. Paul Percival October 2012

Mu Formation in Water with Spin-dependent Chemistry 53 based on Leung, Brodovitch, Percival, et al. (1987) + e- H2O Mu H 2 O + Mu 3 Paul Percival [ Mu .. eaq ] 1 [ Mu .. eaq ] time 10 -12 s 10 -9 s (Mu)

Mu MuH, MuOH missing detectable diamagnetic fraction muonium signal PL P Mu PD 10-6 s October 2012 54 Muonium Kinetics Muonium reactions are pseudo-first order because only a few million Mu atoms are needed for an experiment. Mu + A products

[A] is constant d [Mu] = kM [A][Mu] = [Mu] dt Muon Asymmetry - The Mu signal decays exponentially [Mu] = [Mu]t =0 e- t with = kM [A] 0.0 0.2 0.4 0.6 0.8 time / s Paul Percival 1.0 1.2 1.4 second-order rate constant units: M-1s-1

October 2012 55 The Ergodic Principle Complaint: What does [Mu] mean when we only have one Mu atom at a time? Answer: It doesnt matter if the atoms are present at the same time or spread over an interval. The average of a parameter over time and the average over the statistical ensemble are the same Mu survival probability et 37% Paul Percival time October 2012

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