ENDOR Electron Nuclear Double Resonance

2. G. Feher (1956) Phys. Rev. 103, 834  It complements the technique of EPR in identifying the nuclei that are weakly interacting and allows the detailed mapping of the electron wave function. EPR Hamiltonian 

3. ENDOR Solves the ambiguity in the assignment of hyperfine multiplets (i.e., correct identification of the nucleus) Resolves the hyperfine lines, not resolved in EPR due to line broadening or complexity. Gets more accurate values (PRECISION) for hyperfine coupling. Measures nuclear quadrupole coupling constants (when I � 1) Advantages: Sharper EPR lines/precise determination of hfc constants. Two Related Techniques ENDOR is EPR detected NMR

7. S = , I = Case. Isotropic spin Hamiltonian H = g b B Sz - gn bnB Iz + A S. I E = g b B ms � gn bn B mI + A ms mI Transition probability is given by Wif a | < f | v | i > |2 Let us say that the first EPR transition occurs at a field of Bk and second one at Bm, |Bk ? Bm| being the hyperfine coupling constant A/h and the centre of Bk and Bm defining the g ? value of the system.

8. CW ENDOR The sample placed in a microwave cavity is subjected to a low microwave power and the magnetic field is placed at Bk. Optimize the parameter to get maximum amplitude of this signal Achieve Partial saturation by increasing the microwave power (B1e) several fold. Now the sample is subjected to rf magnetic field (B1n) of wide range and large power out put from an rf generator. rf-frequency range will depend on Bk i.e microwave frequency and also the nn of the concerned nucleus. [In the case of proton the range is 2 to 30 MHz since proton NMR frequency corresponds to nn = gn bn Bk/h ] The base line indicative of a constant EPR absorption (horizontal axis is that of n from rf generator) will have two absorptions at nn1 and nn2

11. (nn1 � nn2) = nn = gn bn Bk/h � NMR frequency of the nucleide (bare) responsible for the hfc. Also nn2 � nn1 = A/h. Upper sign � | A | > 2nn Lower sign � | A | < 2nn IMPLIES Identification of the nucleus (gn with in 0.1% accuracy) Precise hfcc. Extremely low linewidth Repeat the experiment at Bm to get another ENDOR spectrum. This plot of rf frequency of changes in the EPR absorption intensity is called the ENDOR experiment

16. Dynamics of ENDOR Rearrangement of level populations from thermal equilibrium (� = gbB/kT) Boltzmann Population on the application of B. By pumping MW power on top of the EPR line; (ii) Turning on the RF pump power; (iii) Tracing out the ENDOR response on a recorder

20. Quadrupole Coupling Constant (QCC) For spin I � 1, consider the quadrupole interaction terms. Due to the additional relaxation paths, more difficult to predict the intensity of the ENDOR lines. In liquids ? isotropic hyperfine coupling In solids, powders, frozen solution ? anisotropic information including QCC.

21. Applications F Centre in alkali Halides. Anion vacancy in KBr. EPR line width 12.5 mT i.e., 125G due to a large number of unresolved lines. First shell and all other odd number shells have 39K (93.26%) or 41K(6.73%) Even numbered shells 79Br(50.69%) and 81Br(49.31%) Six First shell and twelve 2nd neighbour shell nuclei give pj (2njI + 1) = 19x37 = 703 epr lines. ENDOR will be spread over a considerable range of frequency. Anisotropic and isotropic interactions will fall off with distance All are of I = 3/2 i.e., hfcc + Qcc + differing gn Hence EPR give no indication of any structure. ENDOR 0.5 to 26 MHz Narrowest line 10KHz Quadrupolar contribution added nn1 = |1/2[A|| + A^ (3cos2q - 1)] - gnbnB + 3P(3cos2q - 1) (MI - )|/h nn2 = | - [A|| + A^ (3cos2q - 1)] - gnbnB + 3P(3cos2q - 1) (MI - )|/h Where P = e2qQ/4I(2I ? 1)

24. Pulsed ENDOR Uses pulsed MW and RF In a pulsed ENDOR, the intensity of electron spin echo is measured as a function of the radio frequency. Advantages: (i) Entire sequence can be made short enough to exclude unwanted and competing relaxation effects. (ii) Pulsed ENDOR efficiency upto 100% while the CWENDOR is only a few %

28. Comparison of Davies- and Chirp ENDOR spectra of bis(glycinato)copper(II)

29. 2D Chirp ENDOR of bis(glycinato)copper(II)

30. Comparison of ENDOR and ESEEM experiments

31. 23Na ENDOR spectrum of the Fe(CN)63- complex in NaCl, for four different orientations. The contribution of the nuclear Zeeman interaction (around 80 MHz) is subtracted. FIR (ESR) frequency 244.996GHz.

32. ENDOR spectrum of the Fe(CN)63- complex in NaCl in the 18-27 MHz range.B||[100] = 6.0017 Tesla. FIR (ESR) frequency 244.996 GHz.

33. EPR spectrum of 2,2,6,6-tetramethyl-1-piperidinyl-oxy (TEMPO) in toluene-d8 + 10% dimethylformamide-d7 at 285.135GHz and 40K. The spectrum is fitted with the g-values gzz = 2.00214(4), gyy = 2.00620(4) and gxx = 2.00972(4). The hyperfine splitting due to the nitrogen nucleus is only visible along the z-direction (93.5MHz).

34. ENDOR spectra of 2,2,6,6-tetramethyl-1-piperidinyl-oxy (TEMPO) in toluene-d8 + 10% dimethylformamide-d7 at 285.135GHz and 3.0K. The magnetic field varies from 10.132(top) to 10.176(bottom) in 2mT steps.

35. Example 1: EPR and corresponding ENDOR spectrum (recorded at 4K) of a Cu doped MgO catalyst. The ENDOR spectrum clearly reveals the coordination environment of the Cu2+ ions, which are surrounded by 5 distinct OH groups. The weak interactions between the Cu2+ ion and the proton are clearly not visible in the EPR spectrum. ENDOR spectrum

36. Example 2: EPR and ENDOR spectrum (100K) of a surface defect center (an FS+(H) colour center) on a polycrystalline oxide surface. The ENDOR spectrum clearly shows the magnitude of the coupling between the surface trapped electron and a nearby proton. Despite the complex heterogeneous nature of the polycrystalline oxide sample, the resolution of the ENDOR signals is excellent. EPR spectrum ENDOR spectrum

37. Thank you