NUCLEAR OVERHAUSER

EFFECTSUBMITTED BY

RESHMA FATHIMA.K

FIRST YEAR M.PHARM

DEPT.OF.PHARMACEUTICS

WHAT IS NOE ?

INTRODUCTION

EXPLANATION FOR NOE

CONCLUSION

REFERENCE

CONTE

NTS

INTRODUCTIO

N The nuclear overhauser effect is of

great value in studying the

molecular geometry of the

compounds.

It tell whether the two protons are

in close proximity within the

molecules or not.

An important consequence of this

effect is that the line intensities

observed in the normal spectrum

may not be the same as in the

decoupled spectrum.

INTRODUCTION (Contd)…..

• Consider a molecule in which two protons are close enough to allow through space interactions of the fluctuating magnetic vector for this effect, the number of intervening bonds between the two concerned protons have no significance

C C

Ha Hb Consider a hypothetical molecule in

which two protons are in close proximity.

In such a compound if we double

irradiate Hb then this proton gets

stimulated and the stimulation is

transferred through space to the

relaxation mechanism of Ha.

Thus due to the increase in the spin

lattice relaxation of Ha, its signal will

appear more intense by 15-50%.

Thus we say that if the intensity of

absorption of Ha signal is increased by

double irradiating Hb then the protons

Ha and Hb must be in close proximity in

a molecule.

EXPLANATION FOR NOE

• Nuclear OVERHAUSER Effect (NOE): resonance line intensity changes caused by

dipolar cross relaxation from neighboring spins with perturbed energy level

populations.

• To understand the nature of the NOE, we have to look at a two-spin system I1 and

I2.

• Since NOE does not involves coherences, but merely polarization, i.e. population

differences between the a and b states, we can use the energy level diagram here:

The possible transitions for this two-spin system can be classified into three

groups:

- W1 transitions involving a spin flip of only one of the two spins (either I1 or

I2), corresponding to relaxation of the spin.

- a W0 transition involving a simultaneous spin flip ® a b for one spin and ® b a

for the other one

(i.e., in summa a zero-quantum transition).

- a W2 transition involving a simultaneous spin flip of both spins in the same

direction, corresponding to a net double-quantum transition.

E

W0β

β

α α

I1I2

β

α

β

α

I1I2

E

W2

• With the I1 polarization going back from saturation to the BOLTZMANN equilibrium, the

W0 mechanism will cause the neighboring (so far unperturbed) spin to deviate from its

BOLTZMANN equilibrium towards a decrease in , a b population difference. After a 90°

pulse, this will result in a decrease in signal intensity for I2 — a "negative NOE effect".

• On the other hand, the W2 mechanism will cause the population difference of the

undisturbed spin I2 to increase, corresponding to an increase in signal intensity: a "positive

NOE effect".

• These effects can be directly observed in a very simple experiment, the 1D difference NOE

sequence

CWAQ

1H

One spin is selectively saturated by a long, low-power CW (continuous wave) irradiation. As

soon as the spin deviates from its BOLTZMANN population distribution, it starts with T1

relaxation. Via the W0 or W2 mechanisms it causes changes in the population distribution of

neighboring spins. After a 90°pulse, these show up as an increase or decrease in signal intensity

• Usually, the experiment is repeated without saturation,

giving the normal 1D spectrum.

• This is then subtracted from the irradiated spectrum, so that

the small intensity changes from the NOE effects can be

easier distinguished spins with a positive NOE (i.e., higher

intensity in the NOE spectrum than in the reference 1D)

show a small positive residual signal, spins with a negative

NOE yield a negative signal, spins without an NOE cancel

completely.

NOE

• The nuclear overhauser effect is of

great value in studying the molecular

geometry of the compounds.

• An important consequence of this

effect is that the line intensities

observed in the normal spectrum may

not be the same as in the decoupled

spectrum.

• Dipole dipole relaxation occurs when

two nuclei are located close together

and are moving at appropriate

relative rate.

CONCLUSION

• Irradiation of one of these nuclei with a B2 field alters the Boltzmann population distribution of the other nucleus and therefore perturbs the intensity of the resonance.

• No J coupling need be present between nuclei.

REFERENCES

• Textbook of organic spectroscopy by Y.R SHARMA

• Textbook of nuclear magnetic resonance spectroscopy by JOSEPH.B.LAMBERT

THANK YOU