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 = g

m

B B o M s

selection rule D M s ==> = ± 1 D

E = g

m

B B o

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 m B : Bohr magneton 9.274 x 10 -24 J T -1 for electron M S : electron spin quantum number +1/2 or –1/2 1

B o : 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 = g

m

B B o M S + a

m

B M S m I

m I : 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

surroundings : microwave radiation transferred from the spin system to its 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

g sample = g std B B std sample

for field-sweep, lower field (left-hand) than standard, higher g value 3

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 g m B 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 [C 6 H 6 •] coupling to all 6 H atoms the electron is delocalized over all 6 C atoms Ex. 2 pyrazine radical anion (a) coupling to 2 14 N 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 BH 4 + •C(CH 3 ) 3 [BH 3 •] + HC(CH 3 ) 3 Ex. 4 S(=NBu

t

) 2 + Me 2 SiCl 2 NBu

t

S SiMe NBu

t

┐• + g = 2.005

A(N) = 0.45 mT 6

Ex. 5 S(=NBu

t

) 2 • g = 2.0071

A(N) = 0.515 mT Ex. 6 (MeO) 3 PBH 2 • 7

Ex. 7 Cr III (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.

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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 9

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 ┴ ==> g xx , g yy , g zz ESR absorption line shapes show distinctive envelope system with an axis of symmetry no symmetry 10

Ex. 8 Li + – 13 CO 2 in CO 2 matrix large 13 C and small 7 Li (I = 3/2) hyperfine splitting Ex. 9 HMn(CO) 5 /solid Kr matrix at 77 K h u － → •Mn(CO) 5 A ‖ ( 55 Mn) = 6.5 mT A ┴ ( 55 Mn) = 3.5 mT A ┴ ( 83 Kr) = 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 d 3 system 12

trans

[Cr(pyridine) 4 Cl 2 ] + (a) frozen solution in DMF/H 2 O/MeOH (b) in

trans

–[Rh(pyridine) 4 Cl 2 ]Cl·6H 2 O powder Ex. 11 d 6 system low-spin diamagnetic O h high-spin 5 D － → 5 T 2 tetragonal －－－ → 5 B 2 short relaxation times ==> broad resonances large zero-field splittings 13 ==> no resonance observed

Ex. 12 d 9 system Cu II (TPP) complex (frozen solution in CCl 3 H) Cu(acac) 2 frozen solution 14

multiple resonance

ENDOR

(electron-nuclear double resonance) Ex. 13 [Ti(C 8 H 8 )(C 5 H 5 )] in toluene (frozen solution) (a) ESR spectrum (b) 1 H ENDOR spectrum 15

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