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Physical Methods in Inorganic Chemistry Magnetic Resonance
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Physical Methods in Inorganic Chemistry Magnetic Resonance

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  1. Physical Methods in Inorganic ChemistryMagnetic Resonance Lecture Course Outline Lecture 1: A quick reminder A few trends in Inorganic NMR A little more on Chemical Exchange Essential NMR Methods Spin Decoupling Spin Relaxation Measurements (again and more) Lecture 2: NMR Methods continued – 2D and others Correlated Spectroscopy (COSY) Nuclear Overhauser (NOE) Magic Angle Spinning (MAS) Lecture 3: Electron Paramagnetic Resonance The why and when of EPRin Inorganic Chemistry EPR methods (ENDOR, DEER) Physical Methods – Magnetic Resonance

  2. Physical Methods in Inorganic ChemistryMagnetic Resonance Literature H. Friebolin One and Two Dimensional NMR Spectroscopy H. Günther NMR Spectroscopy P. J. Hore Nuclear Magnetic Resonance (primer) A. K. Brisdon Inorganic Spectroscopic Methods (primer) C. P. Slichter Principles of Magnetic Resonance R. Freeman Spin Choreography Physical Methods – Magnetic Resonance Website and e-mail: http://timmel.chem.ox.ac.uk timmel@physchem.ox.ac.uk

  3. Magnetic Resonance

  4. Selected NMR properties of some elements Gyromagnetic ratio (107 rad T-1s-1) 26.75 8.58 6.72 1.93 -2.71 29.18 6.98 -5.31 10.84 7.05 6.35 5.12 -0.85 -1.25 -10.02 6.43 -8.50 1.12 0.50 5.80 4.82 Physical Methods – Magnetic Resonance

  5. Trends in Chemical Shifts Remember: The diamagnetic shielding generally becomes smaller as the electron density at the nucleus decreases. Thus electronegative substituents, positive charge or increase in oxidation state usually result in decreased shielding and increased shift. Physical Methods – Magnetic Resonance Opposite effects may be observed for transition metals (ligand effects).

  6. Effect of Charge, Substituents and Oxidation State Physical Methods – Magnetic Resonance

  7. |sp|* d DE The effect of coordination on the chemical shift of a transition metal • Remember • The paramagnetic shielding contribution sp~ 1/DE • 2. paramagnetic currents AUGMENT the magnetic field (sp is negative, hence a DESHIELDING parameter!) Physical Methods – Magnetic Resonance Typically, shifts follow the spectrochemical series: strong field ligands give small or negative chemical shifts whilst halogens give larger chemical shifts.

  8. Chemical Exchange Remember: Physical Methods – Magnetic Resonance • Examples of Fluxional inorganic systems. • Axial-equatorial exchange in trigonal bipyramidal systems • (PF5, SF4, PF4NMe2 , Fe(CO)5) • Bridging/axial exchange in carbonyls. • Bridging terminal exchange in boranes (B2H6 etc.);borohydrides (Al(BH4)3) • Ring-whizzing in 1-cyclopentadienides (Cu(PMe3)( 1-C5H5) • Interchange of ring bonding modes in compounds with mixed heptacity • ( e.g. (1-C5H5)2(5C5H5)2Ti: (4-C6H6)(6-C5H5)Os

  9. 17O spectrum of Co4(CO)12 Physical Methods – Magnetic Resonance

  10. Fa N(Me)2 Fe P Fe Fa The 31P spectrum of PF4N(Me)2 All 19F equivalent at high Temperature Physical Methods – Magnetic Resonance I(31P) = (19F) = 1/2 19Fe and 19Fea not equivalent at low Temperature

  11. 13C{H} spectrum of [(CH3)3C 6Li]4 Recall: multiplets 2nI + 1 I(6Li) = 1 n = 4 Jav = (5.4 Hz x 3 + 0)/4 = 4.1 Hz Physical Methods – Magnetic Resonance J(13C-6Li) = 5.4 Hz n = 3

  12. Nuclear Overhauser Spectroscopy Electron Nuclear Double Resonance Correlated Spectroscopy Magic Angle Spinning NMR Acronyms Physical Methods – Magnetic Resonance

  13. Methods Continuous wave E hn Physical Methods – Magnetic Resonance B B

  14. p p/2 t t1 t4 t3 t2 p/2 p/2 p/2 p/2 Spin Lattice Relaxation and The Inversion-Recovery Experiment Physical Methods – Magnetic Resonance

  15. Inversion Recovery Method p/2 t1 t2 t3 t4 z z z z y y y y x x x x NMR Signal I(t) Physical Methods – Magnetic Resonance

  16. p t t p/2 t z z y y s y y m x x t z f = p x x t y y y s m x f x x Spin Spin Relaxation and theSpin Echo Experiment echo = Physical Methods – Magnetic Resonance

  17. What is the effect of relaxation on the echo amplitude? Spin spin Relaxation random magnetic fields destroy phase coherence and are not refocused byppulse Physical Methods – Magnetic Resonance NMR Echo of each signal:

  18. Echo Trains Physical Methods – Magnetic Resonance

  19. The Method of Spin Decoupling FACT: Spin–Spin Coupling yields important information but NMR data interpretation complicated by line splittings. A SOLUTION: simplify spectra by removing some (chosen) splittings and learn about whichnuclei couple to which. HOW: apply a second Radiofrequency source (S2) with strength B2 in addition to transmitter S1 used for detection of spectrum (a so-called double resonance experiment). S2 is positioned at the resonance of a particular nucleus. Physical Methods – Magnetic Resonance RESULT: decoupled spectra are less crowded and have much higher sensitivity as all available NMR intensity concentrated into single line (and Nuclear Overhauser).

  20. irrad at nX The Origin of the Spin Decoupling Effect I(X) = I(A) = 1/2 A X J Irradiation of X at its resonance frequency induces rapid transitions from X( ) to X( ) and vice versa. A “sees” a single, averaged field. nA nX X( ) X( ) A( ) A( ) B2 of same order as 2pJAX nX should be sufficiently far away from nA Physical Methods – Magnetic Resonance Notation: A{X} nA

  21. Fluorine Spectrum I(19F) = 1/2 Fa ii) irrad ii) irrad i) irrad i) irrad X Fe A X Fa i) i) irrad ii) The Method of Spin Decoupling Fe{Fa} Physical Methods – Magnetic Resonance Fa{Fe} I(A) = I(X) = 0

  22. 31P(CH3CH2O)3 irrad 31P(CH3CH2O)3 31P(CH3CH2O)3 I(31P)=1/2 irrad Physical Methods – Magnetic Resonance 31P(CH3CH2O)3

  23. Recall: Exercise B = 1.41T Electron: Can we transfer this polarisation? Physical Methods – Magnetic Resonance 1H:

  24. The Nuclear Overhauser Effect 1) Enhancement of Sensitivity ie, the heteronuclear (13C – H) Nuclear Overhauser Effect g(1H) = 26.75 107radT-1 s-1 g(13C) = 6.72 107 rad T-1 s-1 Physical Methods – Magnetic Resonance 2) Information about proximity of two nuclei (ie, protons) 3) Dependent on Cross Relaxation between different spins. Prerequisite for this cross relaxation experiment is that the spin lattice relaxation of the nuclei is dominated by dipole-dipole interaction with the other nuclear spins.

  25. The origin of the Nuclear Overhauser Effect Result: saturated proton transitions, 13Cpopulation difference increased 3-fold Irradiate proton resonances 2 1 0 13C H sat 1 4 3 1 2 4 H sat 13C Physical Methods – Magnetic Resonance 5 4 3 Boltzmann Protons saturated Cross Relaxation Takes spins from top to bottom level, competition with 13C relaxation (restoring Boltzmann in 13C population)

  26. The maximum attainable enhancement (the fractional increase in intensity) hmax = 1/2 gI/gS where I is the saturated spin and S is the observed spin. Physical Methods – Magnetic Resonance • Maximum effect occurs when there is no “leakage” as a result of relaxation mechanisms other than the dipole-dipole interaction (a through space interaction!). • For homonuclear systems, maximum enhancement is 50%. • Remember that 15N and 29Si have negative g.

  27. irrad Integration Selective Nuclear Overhauser enhancements g a b d Difference Spectrum Physical Methods – Magnetic Resonance d g a b

  28. 29SiH(Ph)3 gSi = - 5.31 107 rad T-1s-1 Magnitude:1+hmax = 1+1/2 gI/gS ~ -1.5 gH = 26.75 107 rad T-1s-1 29Si{1H} Proton Decoupled Physical Methods – Magnetic Resonance Coupled

  29. Principles of 2-Dimensional NMR Father of 2D NMR: Jeener, Belgium Main Developers: RR Ernst (Switzerland), R Freeman (UK, Oxford) Physical Methods – Magnetic Resonance

  30. What we know from FT NMR p/2 FT Physical Methods – Magnetic Resonance

  31. 2D NMR is a domain of FT and pulsed spectroscopy Physical Methods – Magnetic Resonance

  32. Principles of 2-Dimensional NMR The time-intervals of 2D NMR Physical Methods – Magnetic Resonance

  33. A 2-Dimensional Experiment evolution Series of one-dimensional NMR spectra must be recorded t1 evolution Physical Methods – Magnetic Resonance t1 evolution t1

  34. Amplitude Modulation Phase Modulation t1 t1 Physical Methods – Magnetic Resonance

  35. Fourier transformation of FID signal, S(t1, t2) must be performed to obtain 2D spectrum as function of two frequency variables S(F1, F2) Physical Methods – Magnetic Resonance Spin-spin coupling was active during t1, hence F1 contains coupling constant Larmor precession active during t2, hence F2 contains chemical shift

  36. p/2x p/2x t2 t1 z z z t1 p/2x p/2x y y y x x x What happens during the pulse sequences? Pulse Sequence ?

  37. p/2x Pulse z z Pulse does not affect x-component! y y x x What happens during the secondp/2x Pulse?

  38. p/2x p/2x t1 z t1 p/2x p/2x z z y y y x z x x t2 y y y x x x Pulse Sequence: t2 ? Physical Methods – Magnetic Resonance =

  39. A Simple 2D NMR Spectrum results F2 F1 W Physical Methods – Magnetic Resonance W

  40. Correlated Spectroscopy (COSY) Pulse Sequence p/2x p/2x Aim : To discover spin-spin couplings in a molecule. Answer: Which resonance belongs to which nucleus? t2 t1 Physical Methods – Magnetic Resonance Schematic COSY spectrum of an AX system

  41. Physical Methods – Magnetic Resonance

  42. Use of COSY to assign 11B NMR of B10H14. (no couplings via H-bridges) 2 2 2 4 a 3=4 1=2 5=6=7=8 9=10 d b c a: 2B coupled to all kinds of B = 3,4 Physical Methods – Magnetic Resonance b: 4B coupled to 2 kinds of B = 5,6,7,8 c: 2B coupled to 1 kind of B = 9,10 d: 2B coupled to 2 kinds of B = 1,2

  43. p/2x p/2x t2 p/2x p/2x tm t1 t2 p/2x tm t1 2D-Nuclear Overhauser Spectroscopy D I S WI WS

  44. And the resulting spectrum D I S WI WS Physical Methods – Magnetic Resonance Cross Peaks tell us about interacting spins.

  45. 2D NOESY vs 1D NMR 69 amino acids, M = 7688 Physical Methods – Magnetic Resonance

  46. 2 D NOESY – Why? • Advantages wrt 1D 1H{1H} NOE: • Simplification of crowded spectra • No need for selective excitation of individual resonances • Higher efficiency Physical Methods – Magnetic Resonance

  47. Problems: • Through Space dipolar coupling not averaged out (broadened spectra) • Hence, long spin lattice relaxation times T1 (lack of modulation of dipolar coupling) and therefore restriction of pulse repetition rate, consequently, poor S/N • Fast spin-spin relaxation times T2 (line broadening) • Chemical Shift anisotropy not averaged out (line broadening) NMR in Solids Distance dependent – information on spin separations! Physical Methods – Magnetic Resonance Often broad, structureless resonance

  48. Temperature dependence of line width Proton resonance line Physical Methods – Magnetic Resonance Solid complex adduct

  49. Every nucleus with non-zero I, has a magnetic dipolem=gI z Bmz Bmx r y q x m The Dipolar Coupling-Through Space Coupling S N N S N S N S attraction repulsion Physical Methods – Magnetic Resonance Anisotropic quantity

  50. In a single crystal, this is simple: Recall: D A X Physical Methods – Magnetic Resonance KAX: splitting in spectrum of X caused by dipolar coupling to A