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

07- Double Resonance (Dayrit)2 Introduction The basic NMR experiment consists of placing the sample in a static magnetic field, B o, then subjecting it to a second RF pulse of field, B 1, 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 B 2 at a position of 2 which perturbs the spin population and yields information regarding spin interactions. B 2 can be homonuclear (irradiation of a nucleus of the same  ), or heteronuclear (different  ). This comprises a very important group of experiments called double resonance.

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

07- Double Resonance (Dayrit)4 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 B 2 : 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 B 2 : decoupling (homonuclear or heteronuclear): This enables spectral simplification and identification of coupling partners. Broadband 1 H decoupling is standard in 13 C NMR spectroscopy.

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

07- Double Resonance (Dayrit)6

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H-{ 1 H} 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 

07- Double Resonance (Dayrit)9 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.

07- Double Resonance (Dayrit)10 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 A and equalizes the populations 1=3 and 2=4, making X a singlet.

07- Double Resonance (Dayrit)11 Theory of Homonuclear Decoupling, 1 H-{ 1 H} 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 2 with sufficient power, B 2, to excite the n J 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 1 H- 1 H AX doublet with J = 5 Hz, one needs a B 2 field strength of: (  /2  )B 2  n. J (26.8 x 10 7 T -1 s -1 / 2  ) B 2  2 x 5 Hz B 2  2.34 x T (or 2.34 x G)‏

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

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

07- Double Resonance (Dayrit) C-{ 1 H} Heteronuclear Decoupling Jacobsen, 2007 In a standard 13 Cspectrum, we want to decouple all of the protons simultaneously using least energy. We utilize a “broadband” decoupling sequence that covers the entire range of 1 H 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 1 H signals.

07- Double Resonance (Dayrit)15 Jacobsen, 2007

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