1

Electron Spin Resonance (ESR) Electron Spin Resonance (ESR) SpectroscopySpectroscopy

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 = gBBoMs

selection rule Ms = ±1

==> E = gBBo 

g: proportionality constant, 2.00232 for free electron 1.99 – 2.01 for radicals 1.4 – 3.0 for transition metal compoundsin isotropic systems (gas, liquid or solution

of low viscosity, solid sites with spherical or cubic environment) , g is independent of

field directionB: Bohr magneton

9.274 x 10-24 J T-1 for electronMS: electron spin quantum number

+1/2 or –1/2

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 = gBBoMS + aBMSmI

mI: nuclear spin quantum number for the neighboring nucleusa: 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

3

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 magneta detection and recorder system

ESR spectrum(a) absorption curve

  

(b) first-derivative spectrum

 

   standard: DPPH (diphenylpicrylhydrazyl radical) g = 2.0036,

pitch g = 2.0028

Bstdgsample = gstd ———

Bsample

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

4

hyperfine coupling in isotropic systemsinteractions between electron and nuclear spin magnetic moments ==> fine structure in ESR spectrumcouplings 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 gB 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 bond

==> there is unpaired electron spin density at both nuclei

5

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

 

6

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

7

Ex. 5 S(=NBut)2 • - g = 2.0071

A(N) = 0.515 mT          

Ex. 6 (MeO)3PBH2•

            

8

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.

9

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

10

anisotropic systemssolids, 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 envelopesystem with an axis of symmetry no symmetry

11

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 h －→ • Mn(CO)5

A‖(55Mn) = 6.5 mT

A┴(55Mn) = 3.5 mTA┴(83Kr) = 0.4 mT

12

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 rowtransition metal complexes are often hard toobserved, however, rare-earth metal complexes give clear, useful spectrashort spin-lattice relaxation times ==> broad spectral lineslow temperature experiments will be needed to observe spectra

Ex. 10 d3 system

13

trans-[Cr(pyridine)4Cl2]+

(a) frozen solution in DMF/H2O/MeOH

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

Ex. 11 d6 systemlow-spin diamagnetic Oh tetragonal

high-spin 5D －→ 5T2 －－－→ 5B2

short relaxation times ==> broad resonances

large zero-field splittings ==> no resonance observed

14

Ex. 12 d9 system         

CuII(TPP) complex (frozen solution in CCl3H)       

Cu(acac)2 frozen solution     

15

multiple resonanceENDOR (electron-nuclear double resonance)

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

 

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

  

16

17