Correlations Through Space: The Nuclear Overhauser Effect (NOE)
A change in the integrated NMR absorption intensity of a spin when the NMR absorption of another nearby spin is saturated is known as the Nuclear Overhauser Effect (NOE). It is field and mobility dependent in solution of the molecule under study. The NOE is a very important tool for determination of the distance between spins. However, the interpretation of NOE measurement, i.e. a steady-state measurement or transient protocol measurement; requires more care than for those methods that measurement of scalar couplings. The steady-state NOE measurement that is pre-saturation or NOE difference experiments is for small molecule under 1000, or molecule in rapidly motion or in non-viscous solvent. The transient NOE measurement that is 2D NOESY is suitable for molecule smaller than 1000 (NOE is positive) and larger than 2000 (NOE is negative). For midsize molecules (1000 – 2000) may fail. It is here that the ROESY may be employed for those molecules.
The NOE is characterized by an enhancement factor:
Where I0 is the intensity of a peak without irradiation of the other spin, and I with irradiation. The intensity changes brought about by the NOE can be both positive (an increase intensity) or negative (a decrease intensity) as dictated by the motional properties of the molecules and as well its magnetogyric ratios ( see table below). The maximum NOE in liquid is:
Where gi is the magnetogyric ratio of irradiating nucleus; go is that of observed nucleus. For homonuclear, the maximum NOE is 0.5. For heteronuclear, the NOE depends on the value of g and its sign. One very important application of NOE is enhancement of S/N when the go is low. In some case, even if the observing nucleus is not directly connected to protons, it could help to develop a potential intermolecular NOE enhancement by dissolving the compound in a protonated solvent, rather than a pure deuterated solvent.
Table of Magnetogyric Ratio
|
Nucleus |
Natural Abundance (%) |
Magnetogyric Ratio g (107 rad T-1 S-1) |
Larmor Frequency (MHz) |
|
1H |
99.985 |
26.753 |
100.000,000 |
|
13C |
1.108 |
6.7283 |
25.145,004 |
|
15N |
0.37 |
-2.7126 |
10.136,767 |
|
19F |
100.0 |
25.1815 |
94.094,003 |
|
29Si |
4.70 |
-5.319 |
19.867,184 |
|
31P |
100.0 |
10.8394 |
40.480,747 |
|
183W |
14.28 |
1.1283 |
4.151,888 |
|
2H |
0.015 |
4.1064 |
15.351 |
|
17O |
0.037 |
-3.6266 |
13.557 |
|
27Al |
100 |
6.9762 |
26.077 |
|
51V |
99.76 |
7.0492 |
26.350 |
|
95Mo |
15.72 |
1.7514 |
6.547 |
Notes Related to NOEs:
1. Scalar coupling --- Bonds; Dipolar Coupling --- Space (it could be bond, but not necessary)
2. Steady-State NOE Experiments (such as presaturation) ------ Good for MW less than 1000. Molecule tumbling fast, none-viscous solution.
3. Transient NOE Experiments (such as NOESY) ------Good for MW less than 1000 (positive NOE); or larger than 2000 (Negative NOE). Use ROESY for MW between 1000 - 2000.
4. Make sure the sample is degassed for NOE measurement.
5. HOESY Experiment is for heteronuclear NOEs measuremnet (such as 1H to 13C, 19F - 13C). It could provide useful information of sterochemicals. Sensitivity may low, need more samples.
6. EXSY Experiment is for observe these protons are exchange positions. When exchange rates are slow on the NMR chemical shift time scale, That is two separated resonances are observed for each exchanging protons. By using temperature controlled experiments, we could measure the exchange rate.