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
1
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
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) than3
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
4
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
5
Ex. 3
BH4- + •C(CH3)3
[BH3•]- + HC(CH3)3
Ex. 4
S(=NBut)2 + Me2SiCl2
NBut
S
┐• +
SiMe
NBut
g = 2.005
A(N) = 0.45 mT
6
Ex. 5 S(=NBut)2 • -
Ex. 6
g = 2.0071
A(N) = 0.515 mT
(MeO)3PBH2•
7
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.
8
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
9
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 symmetry
no symmetry
10
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
11
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
12
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
13
==> no resonance observed
Ex. 12
d9 system
CuII(TPP) complex (frozen solution in CCl3H)
Cu(acac)2 frozen solution
14
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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