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Electron Spin Resonance (ESR) SpectroscopyPowerPoint Presentation

Electron Spin Resonance (ESR) Spectroscopy

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Electron Spin Resonance (ESR) Spectroscopy

applied to species having one or more unpaired electrons : free radicals, biradicals, other triplet states, transition metal compounds

species having one unpaired electron has two

electron spin energy levels:

E = gmBBoMs

selection rule DMs = ±1

==>DE = gmBBo

g: proportionality constant,

2.00232 for free electron

1.99 – 2.01 for radicals

1.4 – 3.0 for transition metal compounds

in isotropic systems (gas, liquid or solution

of low viscosity, solid sites with spherical

or cubic environment) , g is independent of

field direction

mB: Bohr magneton

9.274 x 10-24 J T-1 for electron

MS: electron spin quantum number

+1/2 or –1/2

Bo: external magnetic field

commonly 0.34 – 1.24 T

==> corresponding frequency

9.5 (X-band) – 35 (Q-band) GHz

the electron interacts with a neighboring nuclear

magnetic dipole, the energy levels become:

E = gmBBoMS + amBMSmI

mI: nuclear spin quantum number for the

neighboring nucleus

a: hyperfine coupling constant

energy levels and transitions for a single

unpaired electron in an external magnetic field

with no coupling coupling to one nucleus with spin 1/2

spin-lattice relaxation: microwave radiation

transferred from the spin system to its

surroundings

long relaxation time

==> decrease in signal intensity

short relaxation time

==> resonance lines become wide

typical ESR spectrometer —

a radiation source (klystron)

a sample chamber between the poles of a magnet

a detection and recorder system

ESR spectrum

(a) absorption curve

(b) first-derivative

spectrum

standard: DPPH (diphenylpicrylhydrazyl radical)

g = 2.0036,

pitch g = 2.0028

Bstd

gsample = gstd ———

Bsample

for field-sweep, lower field (left-hand) than

standard, higher g value

hyperfine coupling in isotropic systems

interactions between electron and nuclear

spin magnetic moments

==> fine structure in ESR spectrum

couplings arise in two ways:

(i) direct dipole-dipole interaction

(ii) Fermi contact interaction

coupling patterns in ESR are determined by the same rules that apply to NMR

coupling to nuclei with spin > 1/2 are more

frequently observed

hyperfine coupling constant

gmB MHz or cm-1

hyperfine splitting constant

A gauss or millitelsla

• depends on the unpaired electron spin

density at the nucleus in question

• is related to the contribution to the atom of

the molecular orbital containing the

unpaired electron

• unpaired electron can polarize the paired spins in an adjacent s bond

==> there is unpaired electron spin density

at both nuclei

Ex. 1 [C6H6•]- coupling to all 6 H atoms

the electron is delocalized over all

6 C atoms

Ex. 2 pyrazine radical anion

(a) coupling to 2 14N nuclei (1:2:3:2:1

quintet), and split by 4 H atoms

further into 1:4:6:4:1 quintet

(b) Na+ salt, further splitting into 1:1:1:1

quartet

Ex. 3 BH4- + •C(CH3)3

[BH3•]- + HC(CH3)3

Ex. 4 NBut┐• +

S(=NBut)2 + Me2SiCl2 S SiMe

NBut

g = 2.005 A(N) = 0.45 mT

Ex. 7 CrIII(porphyrin)Cl

• the patterns of hyperfine splittings provide

direct information about the numbers and

types of spinning nuclei coupled to the

electrons

• the magnitudes of the hyperfine couplings

indicate the extent to which the unpaired

electrons are delocalized, g values show

whether unpaired electrons are based on transition metal atoms or on adjacent

ligands.

in the absence of magnetic field, 2S + 1

energy states split depends on the structure of

sample, spin-orbit coupling

the appearance of more than one line (S > 1/2) fine structure -- in principle, 2S transitions

can occur, their separations representing

the extent of zero-field splitting

solids, frozen solutions, radicals prepared by

irradiation of crystalline materials, radical

trapped in host matrices, paramagnetic

point defect in single crystals

for systems with spherical or cubic symmetry

g factors

for systems with lower symmetry,

g ==> g‖ and g┴ ==> gxx, gyy, gzz

ESR absorption line shapes show distinctive

envelope

system with an axis of symmetryno symmetry

Ex. 8 Li+ – 13CO2- in CO2 matrix

large 13C and small 7Li (I = 3/2) hyperfine

splitting

Ex. 9 HMn(CO)5 /solid Kr matrix at 77 K

hu

－→ •Mn(CO)5

A‖(55Mn) = 6.5 mT

A┴(55Mn) = 3.5 mT

A┴(83Kr) = 0.4 mT

• the number of d electrons

• high or low spin complex

• consequence of Jahn-Teller distortion

• zero-field splitting and Kramer’s degeneracy

ESR spectra of second and third row

transition metal complexes are often hard to

observed, however, rare-earth metal

complexes give clear, useful spectra

short spin-lattice relaxation times

==> broad spectral lines

low temperature experiments will be needed

to observe spectra

Ex. 10 d3 system

trans-[Cr(pyridine)4Cl2]+

(a) frozen solution in DMF/H2O/MeOH

(b) in trans–[Rh(pyridine)4Cl2]Cl·6H2O

powder

Ex. 11 d6 system

low-spin diamagnetic

Oh tetragonal

high-spin 5D －→ 5T2－－－→ 5B2

short relaxation times

==> broad resonances

large zero-field splittings

==> no resonance observed

ENDOR (electron-nuclear double resonance)

Ex. 13 [Ti(C8H8)(C5H5)] in toluene (frozen

solution)

(a) ESR spectrum (b) 1H ENDOR spectrum

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