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7- Double Resonance

7- Double Resonance. 1. Types of double resonance experiments 2. 1 H-{ 1 H} Homonuclear Decoupling 3. 13 C-{ 1 H} Heteronuclear Decoupling. Introduction.

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7- Double Resonance

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  1. 7- Double Resonance 1. Types of double resonance experiments 2. 1H-{1H} Homonuclear Decoupling 3. 13C-{1H} Heteronuclear Decoupling

  2. 07- Double Resonance (Dayrit) Introduction • The basic NMR experiment consists of placing the sample in a static magnetic field, Bo, then subjecting it to a second RF pulse of field, B1, at a position 1 in order to excite a particular set of nuclei of a given magnetogyric ratio, . • We can increase the number of elements in this experiment by imposing a second RF field, with strength B2 at a position of 2 which perturbs the spin population and yields information regarding spin interactions. B2 can be homonuclear (irradiation of a nucleus of the same ), or heteronuclear (different ). This comprises a very important group of experiments called double resonance.

  3. 07- Double Resonance (Dayrit) Introduction Parameters of a double resonance experiment: 1. homonuclear or heteronuclear; 2. position of 2 irradiation field; 3. strength of B2irradiation field; and 4. manner of application of B2(i.e., continuous or modulated).

  4. 07- Double Resonance (Dayrit) 1. Types of double resonance experiments Depending on how these parameters are selected, one can observe any or a combination of the following phenomena: 1. weak B2: selective population transfer (SPT) or selective population inversion (SPI): These are used to determine energy level arrangements and are important building blocks for more complex pulse sequences. 2. strong B2: decoupling (homonuclear or heteronuclear): This enables spectral simplification and identification of coupling partners. Broadband 1H decoupling is standard in 13C NMR spectroscopy.

  5. 07- Double Resonance (Dayrit) Types of double resonance experiments 3. solvent suppression (1H): Used especially in aqueous solution. 4. nuclear Overhauser effect, nOe (1H): Very important for the determination of spatial relationships. 5. saturation transfer (1H): Technique for studying chemical exchange (migration of protons in solution).

  6. 07- Double Resonance (Dayrit)

  7. 07- Double Resonance (Dayrit)

  8. 07- Double Resonance (Dayrit) 2. 1H-{1H} Homonuclear Decoupling • Spin-spin coupling is a localized interaction due to the proximity of nuclei possessing spin quantum number I >0. • For a coupled two spin system, AX, four spin energy levels are generated, AX, AX, AX, and AX, where the first spin refers to the orientation of the A nucleus, and the second spin refers to the X nucleus. AX AX AX  AX

  9. 07- Double Resonance (Dayrit) Spin population In a coupled spin system, the spin population will be distributed according to the Boltzmann distribution. In the figure below, the populations are approx: AX >  ~  > . (The bars represent the spin populations.) The magnitude of the energy differences depends on the strength of the coupling, and whether the system is homonuclear or heteronuclear.

  10. 07- Double Resonance (Dayrit) Decoupling Decoupling involves irradiation at the center of a multiplet with enough power to cover the J coupling width. Here, the transitions A1 and A2 are being irradiated. This removes the signal due to Aand equalizes the populations 1=3 and 2=4, making X a singlet.

  11. 07- Double Resonance (Dayrit) • Theory of Homonuclear Decoupling, 1H-{1H} • In the decoupling experiment, one shortens the lifetime of spin-spin contact, , (i.e., there is an increase in the rate of spin flipping) by the simultaneous imposition of a second RF field at 2with sufficient power, B2, to excite the nJ multiplets of one nucleus. This has the effect of removing this particular spin-spin interaction from its coupling partner. • For example, to completely decouple a 1H-1H AX doublet with J = 5 Hz, one needs a B2field strength of: • (/2)B2 n . J • (26.8 x 107 T-1s-1/ 2) B2 2 x 5 Hz • B2 2.34 x 10-7 T (or 2.34 x 10-3 G)‏

  12. 07- Double Resonance (Dayrit) Spin-spin decoupling for 1st order, weak coupling AX spin system AMX spin system

  13. 07- Double Resonance (Dayrit) Homonuclear decoupling, 1H {1H} Heteronuclear decoupling 13C {1H} Pulse sequence for homonuclear (1H-1H) decouping and selective heteronuclear (1H-13C) decoupling.

  14. 07- Double Resonance (Dayrit) 3. 13C-{1H} Heteronuclear Decoupling • In a standard 13Cspectrum, we want to decouple all of the protons simultaneously using least energy. • We utilize a “broadband” decoupling sequence that covers the entire range of 1H chemical shifts (typically from -5 to 15 ppm, a width of 8000 Hz on a 400 MHz instrument). • Current methods employ a composite pulse sequence which is a series of several pulses designed to give an overall rotation that is not dependent on the resonance offset of the 1H signals. Jacobsen, 2007

  15. 07- Double Resonance (Dayrit) Jacobsen, 2007

  16. 07- Double Resonance (Dayrit) Broadband 13C{1H} decoupled No decoupling Selective decoupling Jacobsen, 2007

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