Electron Spin Resonance (ESR) Spectroscopy

1. 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

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

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 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

4. 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

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 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

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 hu －→ •Mn(CO)5 A‖(55Mn) = 6.5 mT A┴(55Mn) = 3.5 mT A┴(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 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

13. 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

14. Ex. 12 d9 system CuII(TPP) complex (frozen solution in CCl3H) Cu(acac)2 frozen solution

15. multiple resonance ENDOR (electron-nuclear double resonance) Ex. 13 [Ti(C8H8)(C5H5)] in toluene (frozen solution) (a) ESR spectrum (b) 1H ENDOR spectrum