Proton NMR spectra and rate processes

Roberts (Emeritus Professor, California Institute of Teehnology), a pioneer in the application of NMR spectroscopy to problems of organic chemistry, has compared the NMR spectrometer to a camera with a relatively slow shutter speed. Just as a camera with a slow shutter speed blurs photographs of objects that are moving rapidly, the NMR spectrometer blurs its picture of molecular processes that are occurring rapidly. 

What are some of the rapid processes that occur in organic molecules Two processes that we shall mention are chemical exchange of hydrogen atoms bonded to heteroatoms (such as oxygen and nitrogen), and conformational changes. 

If we were to examine a H NMR spectrum of very pure ethanol, however, we would find that the signal from the hydroxyl proton was split into a triplet and that the signal from the protons of the —CH2— group was split into a multiplet of eight peaks. Clearly, in very pure ethanol the spin of the proton of the hydroxyl group is coupled with the spins of the protons of the —CH2— groups. 

Whether coupling occurs between the hydroxyl protons and the methylene protons depends on the length of time the proton spends on a particular ethanol molecule. 

Chapter 9 Nuclear Magnetic Resonance and Mass Spectrometry 

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This section deals with the quantitative description of the proton transfer processes (denoted by Eqs. (4) and (6) in Scheme 1), identified by the qualitative NMR experiments on the acid/base behavior of the Mo(IV), W(IV), Re(V), Tc(V), and Os(VI) systems as described in Section II. The data obtained on the signal behavior from these similar complexes were used to simulate spectra and model the proton exchange processes to finally obtain rate constants associated therewith. 

It is important to emphasize that the presence of two or three basic centres of comparable basicity in a conjugated molecule may lead to protonation processes at all of them. These processes may occur at various rates, but if all the rates are fast, nmr spectra do not enable us to say what are the relative amounts of variously protonated species present at equilibrium. Increasing the acidity of the medium and lowering the temperature in order to observe the resonances of the captured proton may have the effect of shifting the tautomeric equilibria in favour of one or other of the protonation sites. Information on the position of tautomeric equihbria in protonation processes is thus not obt iinable from nmr spectra under the conditions of rapid proton exchange. 

Protons attached to the C atoms of the 1,2,4-trioxolane moiety of FOZs have chemical shifts at distinctly lower field than alcohols, ethers or esters. For example, the chemical shifts of the ozonide product in equation 100 (Section Vin.C.b.a) are S (CDCI3) 5.7 ppm for the H atoms of the trioxolane partial structure, and 4.1 ppm for the protons at the heads of the other ether bridge . Measurement of the rate of disappearance of these signals can be applied in kinetic studies of modifications in the ozonide structure. The course of ozonization of the methyl esters of the fatty acids of sunflower oil can be followed by observing in H and C NMR spectra the gradual disappearance of the olefinic peaks and the appearance of the 3,5-dialkyl-1,2,4-trioxolane peaks. Formation of a small amount of aldehyde, which at the end of the process turns into carboxylic acid, is also observed . 

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