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Physics and Chemistry of Solvated Electron
Vladimir I. Feldman
Department of Chemistry, Moscow State University, feldman@rc.chem.msu.ru


utline
· What is "solvated electron" ? · Localization of excess electrons in condensed molecular media · Experimental detection and spectroscopic manifestations of hydrated electron · Solvated electrons in other molecular liquids and glasses · Models of solvated electron · Solvation dynamics: "digger" or "seeker" ? · Thermodynamic and transport properties of e-aq · Solvated electron as a chemical reagent · Kinetics and mechanism of reactions solvated electron · Excess electrons in ionic liquids · Some implications and applications


What is "solvated electron" ?
· · Solvated electron ­ a solvent-bound state of excess electron in liquid or glassy media, which is often treated as an anion-like "chemical entity" Known for more than 100 years: blue-coloured solutions of alkali metals in liquid ammonia

first observation: W. Weyl (1864); hypothesis of "electron equilibrium": C. Kraus (1908)

Honored 50 years ago: observation of hydrated electron in 1962 ­ listed among top 50 discoveries in chemistry of the 20th century · Still "unbelievable"...(continuos critisism, sometimes, even ignorance) · Very well studied: more than 2000 papers, ca. 1500 kinetic constants · Still not completely understood, a "hot" topic (see, e.g. Marsalek et al., Acc. Chem. Res., 2012; four papers only in Science during last two years)

·


Why do we care about solvated electron ?
· Basic understanding of electron transport and localization in condensed media, related to molecular electronics, chemical physics and biophysics · One of the key species in radiation chemistry, photochemistry and photoelectrochemistry in solutions · Preparative chemistry and environment-friendly technologies: unique chemical reagent (clean and very efficient reducing agent) · Unique probe for microscopic properties of disordered media (including confined environment, organized assemblies and interfaces) ­ electric, optical and magnetic response


Generation of solvated electron
· · · · 1. High-energy irradiation (fast electrons, X-rays, etc.): · M + . + e-qf (e-qf e-loc e-s) (universal) 2. Photoionization of solutes with low IP (e.g.,Fe(CN) SO32- ( = 220 ­ 500 nm)
6 4-

, amines,

3. Heterogeneous photoelectron emission from metals (electron photoinjection from electrode into solution) 4. Heterogeneous chemical reactions: · Na + NH3 Na+ + e-s · Na(Hg) + H2O Na+ + e-aq · U3+ + H2O U4+ + e-aq ...


Excess electrons in condensed dielectrics
· (excess) ­ , ( , .) · «» (e-qf), : E = V0 (V0 ­ ­ ) V0 · 1) : V0 = liq- vac · 2) : V0 = Ig ­ Iliq ­ P+ P+ = (e2/80)(1 ­
-1

)


V0 values and electron mobility in dielectric liquids
Medium Helium Neon Ethane n-Pentane n-Hexane Methane Benzene Neopentane Tetrametylsilane Xenon Ethanol Water V0, eV 1.0 0.6 ~0.2 ~0 ~0 ~0 - 0.14 - 0.43 - 0.6 - 0.65 - 0.65 - 1.3 (?) u, c2 /(V. s)* 0.02 0.002 0.014 0.15 0.09 400 0.1 70 100 2200 0.0003 0.002

*u = v/E, EA = 0.02 ­ 0.5 ( EA< 0)


Electron autolocalization in a dielectric lattice. Polarons
· Polaron ­ ­ . ( « », ) «» : m* > m : .. , .. ; H. FrÆlich Weak coupling case ( <<1 ­ , - ):

· · ·

=
·

2e 2 m1 2
1/ 2 l

/2

3/ 2

- ( 1 - -1 )

Strong coupling case ( 5): self-localization (rl ~ 1 )

m* 0.023 m

4

· ·

«», , , , ...


Anderson localization in disordered media
· P.W. Anderson (1958; Nobel prize, 1977): , () , ,

·


Electron energy in a disordered medium
U

(« »
: g ­ ( g < F)

·

(t ), (R) ~ const R<>L () : = 0 T = 0 K; «» T > 0

·


Molecular localization: excess electron capture
· Excess electrons can be captured by solvent molecules or impurities with positive effective electron affinity M (s) + e · · · ·
qf

M-.(s) (1) (2) (3)

O2 (s) + e-qf O2-.(s) CO2 (s) + e-qf CO2-.(s) CH3COCH3 (s) + e-qf CH3COCH

3

-.(s)

The role of solvation: Possible capture of electron by molecules with slightly negative gas-phase EA (ex. 3) |Gs(M)|<< |Gs(M-.)| Capture by dimers or clusters: Mn (s) + e

·

qf

Mn -.(s)


Localization of excess electrons: preliminary conclusions
· Excess electrons become localized in dielectric crystals and disordered molecular media (liquids, glasses) sharp drop in electron mobility · Different mechanisms may be responsible for electron localization in molecular media


Pulse radiolysis

The scheme of experimental set-up used by Boag and Hart (1962) : 1 ­ , 2 ­ , 3 ­ , 4 ­ , 5 ­ , 6 ­ , 7 ­ , 8 ­ )

Pulse duration 2 s, resolution 5 s; spectral range 300 ­ 880 nm Pulse duration 10 ps, resolution 1 ps (Orsay, France, 2006) Pulse duration 100 fs, resolution 250 fs (Osaka, Japan, 2009)
The up-to-date level (pulse-probe) :


Discovery of the hydrated electron
· J.W. Boag, E.J. Hart (1962): Detection of optical absorption in the red region (max ~ 720 nm) upon irradiation of liquid water with fast electrons Assignment: hydrated electron (e-aq) [ ~ 20 s] H2O --/\/\- H2O +., H2O *, eH2O +.+ H2O H3O+ + OH. e- e-aq H2O * H. + OH. (?) Arguments: (1) Spectroscopic (., .OH, H2O2, H3O+, OH-) cannot absorb in the red region (2) Chemical (absorption is suppressed in the presence of elctron scavengers : O2, O2,N2O)
J.W. Boag, E.J. Hart, Nature, 1963, 197, 45; independently J.P. Keene, Nature, 1963, 197, 47


Spectroscopic properties of e
Hydrated electron: z = -1, S = ½ (a charged paramagnetic species) · Optical spectrum (298 ): max = 715 nm (Emax = 1.73 eV) max = 1.85 . 104 M -1-1 1/2 = 0.93 eV F 0.7 (allowed transition)* · EPR signal (283 , lquid) Sinflet g = 2.00043 (close to ge) B < 0.01 mT (dynamic narrowing)

-

aq

Optical spectrum of e-

aq

at 298

EPR spectrum of e-

*)F = 4.315 * 10-9 d ( = 1/)

(Jeevarandan & Fessenden J. Phys. Chem., 1989, 93, 3511) ­ in situ photolysis of sulphite anion Independent of generation method

aq

at 296


Solvated electrons in other molecular media
Medium Methanol Ethanol 2-Propanol EG THF n-Hexane Ammonia (225 ) Water max ,nm 630 700 820 580 ~2100 > 1600 1400 715 E
max

, eV

, D 1.67 1.70 1.65 2.28 1.63 0.08 1.44 1.83

33.6 25.1 19 38 7.3 1.89 22 80

1.96 1.77 1.5 2.1 ~0.6 < 0.8 0.89 1.73

Emax ­ optical trap depth No direct correlation between Emax and molecular (dipole moment) or macroscopic () properties of liquids


Solvated (trapped) electrons in molecular glasses (77 K)
g /l ~ 10 (e-tr)
15

­ 10

30

Emax , eV 2.38 2.28 1.92 2.41 1,0 0.75 1.96

Medium Methanol Ethanol 2-Propanol EG 2-MTHF 3-Methylpentane Water (ice)*

max , nm 520 540 645 500 1250 1650 630

B, (EPR)
~1.4 ~1.2 ~1.0 ~1.5 0.4 0.3 ~1.5

Spectral properties of e-s e-tr are similar. Emax (e-tr) > Emax (e-s) (E ~ 0.2­0.4 eV). B increases with increasing Emax (determined by the HFC constants of unpaired ewith matrix protons)
*


The structure of solvated electron: models
· : · 1. Continual ( ): , . · 2. Configuration-continual (semi-contnual) · 3. Molecular dynamics simulation : - ( ) - () - ? - - () ­


Polaron model (before 1960)
· A. S. Davydov (1948,based on polaron theory as formulated by Pekar) Potential: V = - e2/r Optical transition: 1s
E
max

· ·

( = -1- -1) 2p
2

( ) = E = E2 p - E1s 1.93

m* m

· · · ·

Optimum values m*/m: 1.5 (ammonia); 2.7 (water) (realistic?) (-) no clear physical interpretation of m* (-) no explanation of Emax (T, p) (-) the spectral shape cannot be interpreted unsatisfactory


Cavity model
Jortner et al. (1964): «» R0 · V = - e2/r (r > R0) V = - e2/ R0 (r < R0) Fitting: energy of the 1s 2p optical transition

Emax increases with increasing R0 · Ammonia R0 = 0.30 ­ 0.34 nm (estimated from volume expansion coefficient upon dissolution of alkali metals in ammonia) · Water R0 = 0.14 ­ 0.15 nm (optimized) · · · · Qualitative dEmax/dT = dEmax/dp = Correlation explanation for e-aq : - 2.9 . 10-3 / (<0, thermal expansion of the cavity) 8 . 10-7 / (>0, baric confinement) between Emax and c B Microscopic sense ?


Configuration-continual (geometrical) models
· Specific geometrical configuration of the first co-ordination sphere ("trap") + continuum nedium : ( Xn- .)solv · Arguments: - Observation of metastable negatively charged clusters (H2O)n - . (n > 4) in the gas phase - Results of pulsed magnetic measurements (ESEEM) in glasses None of these arguments are directly related to the liquid phase


Geometry of e-tr in glassy matrices at 77 K as revealed by ESEEM experiment

L. Kevan, Acc. Chem. Res., 1981, 14, 138


Molecular dynamic simulation

The structure of hydrated electron as revealed by ab initio MD simulation for excess electron embedded in a water cluster (n = 32) at 300 K. Radial density of the excess electron averaged over the angular variables (green) and radial distribution function of water oxygen (red) and hydrogen (black) atoms relative to the center of the excess electron from equilibrium configurations at T = 300 K.

(Marasalek et al., Acc. Chem. Res., 2012)


MD simulation: preliminary conclusions
Effect of temperature:

· · ·

30 K: "cushion-like" state ? 300K: cavity A cavity model can be adopted as a rough approximation Cavity is more flexible and accessible to water molecules (as compared to those containing negative ions) The configuration of solvated electron at low temperature (in frozen ice) may be significantly different (?) (Marasalek et al., Acc. Chem. Res., 2012) ...still far from being clear


Dynamics of electron solvation: "digger" or "seeker" ?
· Digger: «» Seeker: «» ( ), (preexisting trap, pre-trap)

·

· First direct observation of electron solvation dynamics at low temperatures: (J.H. Baxendale, P. Wardman, Nature, 1971, 230, 449) e
loc(IR,



max>1350

nm)

e-s (max= 700 ­ 800 nm)

· · · ·

Solvation time:

ethanol S = 3 ns (166 ) 1- propanol S = 5 ns (178 ); 60 ns (152 ) 2-propanol: S = 6 ns (186 ) 1-butanol : S = 4 ns (184 )


... both "digger" and "seeker" ?

e

-

qf

e-

loc

e

-

s

seek .... dig

2-nd stage: "continuous shift" due to orientational polarization ("digging") or trap-to-trap tunneling ?


Correlation between solvation time and solvent relaxation properties
Electron solvation in alcohols at 300 K (G.A. Kenney-Wallace, 1982)

Alcohol Methanol Ethanol 1-Propanol 1-Butanol 1-Octanol 1-Decanol

S, ps 11 18 24 30 45 51

2,, ps* 12 20 22 27 39 48

0.55 1.10 2.00 2.60 8.95 14.1

*rotational relaxation time for monomeric molecules

No direct correlation with viscosity S (e-aq) = 0.54 ps (Yoshida et al., 2010). No digging ?

S ~2 ("digging" ?)


Thermodynamical properties of e
· · · · · 0 = - 2.87 V G0 = -157 kJ/ mol H0 = -136.4 kJ/ mol S0 = 69.8 J/ (mol. ) S0hydr = 49 J/ (mol.K)

-

aq

G0 (e-aq ) e-aq + H+aq ½ H2 · ½ H2 (aq) ½ H2 (g · ½ H2 (g) Hg · Hg e-g + H+g · H+g H+aq --------------------e-aq e-g

(aq) )


Transport properties of e-

aq

Ionic conductivity and drift mobility: 0- (e-aq) = 185 2/(Ohm . g-eqv)
(cf.: -: 198; Cl-: 70)

u- (e-aq) = 1, 92 .10 -3 c2 /(V. s) Diffusion coefficient D (e-aq) = 4.96 .10 -5 c2 / s
(much higher than for heavy anions non-classical diffusion ?)


Solvated electron as a chemical reagent
·

e

-

aq

­ «» ( , , «» )


Basic reactions of e-

aq

Reduction of metal cations · e-aq + Mn+ M(n-1)+ (k kdiff) e-aq + Ni2+ Ni+ e-aq + Ag+ Ag0 e-aq + Cu+ Cu0 Reactions with inorganic anions e-aq + NO3NO32e-aq + MnO4MnO42Non-dissociative attachement to neutral molecules e-aq + O2 O2-. e-aq + CH3COCH3 CH3COCH3-. e-aq + C6H6 C6H6-. Dissociative attachemnt to neutral molecules: e-aq + N2O N2 + O-. e-aq + RBr R. + Br-


Reaction kinetics: rate constants for eMolecule or ion k, M-1s 5.1.10 3.9.10 3.5.10
10 10 10 10 -1*

aq

Classificasion of reactions: · - fast (diffusion-controlled) · - «ultrafast» · - slow

Cd(II) Ag (I) Cu(II) MnO4 O2 N2O CO2 Acetone Benzene Methanol Chloroform

3.3.10 3.10
10

1.9.10 9.1.10 7.7.10 6.6.10 < 104

10 9 9 9

1.2.107( pH=11- 13)

*pH = 7, unless otherwise stated


Diffusion-controlled reactions of e
k
dif

-

aq

4rDN A = (t ) 1000
r = RA + RB; D = DA + DB (reactions with neutrals)

k

dif

=

4reff DN 1000

A

exp(U / kT ) reff = [ dx] 2 x r



-1

(general case, taking into account interaction potential)

Hydrated electron: kdif ~ 1010 M-1c-1 R ~ 0.25


Reaction kinetics of e-aq: other cases
· 2. Slow reactions : k << kdif e-aq + C6H6 e-aq + CH3 OH Complex multi-step mechanisms C6H
6 -.

(k =1.2 .107 M-1c-1)
.

CH3 O- + H

(k < 104 M-1c-1)

·3. Ultrafast reactions: k > kdif e-aq + CHCl3 CHCl2 + Cl- (k = 3 .1010 M-1c-1) Formal "reaction radius" r > RA + RB (up to 1 ­ 1.5 ) distant transfer (contribution of tuneling) (novel interpretation may come from recent MD findings)


Applications of slolvated electron: inorgaic chemistry and technology
· Elucidation of the mechanisms of inorganic reactions with electron transfer · Studies on metal ions in unusual oxidation states: Cd(I), Zn(I), Hg(I), In (II), Eu (II), Yb (II), Sm (II), Am (II), etc. · Obtaining spectral properties of neutral atoms in solutions (e.g., Pb0 , Ag0 ) · Structure and reaction kinetics of inorganic radicals · Preparation of metal clusters and nanoparticles by ion reduction (especially, from non-noble metals) · Purification of waste water


Application of solvated electron: organic chemistry and biochemistry
· Mechanisms of organic electron-transfer reactions · Direct characterization of unstable organic radicals and radical anions in solutions · Selective preparative reduction of organic compounds · Modeling electron transfer in biology, in particular in organized systems (photosynthesis, enzymatic reactions, etc.) · Investigation of non-oxidative radiation damage of biomolecules, mecanisms of radioprotective and antioxidative activity


Excess electron in ionic liquids
·What happens to an excess electron injected to an IL: ·(i) going to cation to yield radical? ·(ii) going to anion to yield dianion ? ·(iii) creates a "cavity" ?

e (s)

_

afrer irradiation with X-rays, 77 K after photobleacing (> 400 nm)

320

330

340

350

B, mT

First indication of optical spectrum of "solvated electron in R4NNTf2 (Wishart & Neta, JPC B, 2003, 107, 7261)

An EPR evidence for a "cavity-like" species in 1-butyl-1-methyl pyrrolidinium bis(trifluoromethane sulfonyl)imide (P14) (Saenko, Takahasi & Feldman, 2012)


Conclusions....
· Solvated electron is · a well-documented and extensively studied species, · an useful concept for differnet fields of condensedphase chemistry and chemical physics · coming to technology · However... · We still do not understand completely, what is it...


...and outlook
· Challenges: · - ultrafast dynamics of electron solvation · - solvated electrons in novel media (IL, supercritical water, etc.) · - electron solvation in confined media (nanopores), surfaces and interfaces (structural and dynamic aspects) · - rigorous theoretical treatment of kinetics in view of recent MD findings · - development of "electron solvation probing" · - biological implications · ...development of a compehensive realistic model