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

7- Double Resonance

1. Types of double resonance experiments

2. 1H-{1H} Homonuclear Decoupling

3. 13C-{1H} Heteronuclear Decoupling

07- Double Resonance (Dayrit) 2

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.


Introduction

Parameters of a double resonance experiment:

1. homonuclear or heteronuclear;

2. position of 2 irradiation field;

3. strength of B2 irradiation field; and

4. manner of application of B2 (i.e., continuous or modulated).


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.


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).


The type of double resonance experiment depends on the strength of theirradiating frequency, B2. Irradiated nucleus is 1H. (from: Shaw, Fourier-Transform

NMR Spectroscopy)

Strength of B2* (Hz) Effects observed Type

(1H/2) B2 J or(1H/2) B2 1/T2

additional splitting spin tickling

(1H/2) B2 J selective perturbation of spincoupling

selective populationtransfer or inversion(SPT or SPI)

(1H/2) B2 n.J complete removal of spincoupling

spin decoupling

[(1H/2) B2]2 1/(T1.T2) change in relative intensityproportional to distance fromirradiated nucleus

generalized nuclearOverhauser effect(nOe)

* (1H) = 26.8 x 107 T-1s-1; B2 is in units of T.


Irradiation power levels used in decoupling experiments.

Irradiation power*

Experiment Power (watts) Field (Tesla) Frequency (Hz)

1H-{1H}, homonuclear .005 - .02 1 x 10-7 ~ 4 x 10-7 5 - 20

X-{1H }, heteronuclearcoherent field (cw)

0.1 - 1 2 x 10-6 ~ 2 x 10-5 100 ~ 1000

X-{1H }, heteronuclearnoise modulation

1 - 10 2 x 10-5 ~ 2 x 10-4 1000 ~ 10,000

* Conversion of units 5 x 10-3

11 x 10-7

2 x 10-54

1000


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, AX, AX, AX, and AX, where the first

spin refers to the orientation of the A nucleus, and the second spin refers to the X nucleus.

AX

AX

AX

AX


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: AX > ~ > . (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.

4

3

2

1

X2

X1

A2

A1

E

E

1,2 3,4 1,3 2,4

AX


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.

decouple

4

3

2

1

X2

X1

A2

A1

E

decouple

X A

1,2 3,4 1,3 2,4

E


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 2 with 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 B2 field 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)


Spin-spin decoupling for 1st order, weak coupling

AX spin system

AMX spin system

AX

irr X

X

JAX JAX

A

JAM

MAX

JAM

JMX

JAXJMX

JAM

X A

JAX

JAM

M

irr X


Pulse sequence for homonuclear (1H-1H) decouping and selective heteronuclear (1H-13C) decoupling.

Homonuclear decoupling (hom) Selective heteronuclear decoupling (sel)Homonuclear decoupling, 1H {1H} Heteronuclear decoupling 13C {1H}


3. 13C-{1H} Heteronuclear Decoupling

Jacobsen, 2007

• 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


Jacobsen, 2007

Broadband 13C{1H} decoupled

No decoupling

Selective decoupling

Documents

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