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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.

zero-field splitting

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

anisotropic systems

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

transition metal complexes

• 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

multiple resonance

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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