Protons Attached to Oxygen and Nitrogen

Under normal operating conditions, it is unusual to see any coupling between the protons of a hydroxyl group and protons on the carbon atom to which it is attached. This is exemplified by the proton NMR spectrum of ethanol under ordinary conditions compared 

FIGURE 6.42 Proton NMR spectrum of 2-methyl-2-butyn-2-ol after DjO shake. 

FIGURE 6.43 Proton NMR spectrum of 4-hydroxy-4-methyl-2-pentanone. 

Thus far, we have considered the chemical shifts of OH protons in carboxylic acids and alcohols, acids 5 10-13 and alcohols 5 2-7. Phenols are intermediate between these two (5 4-8), as indeed they are in terms of acidity. This will be discussed more fully in Chapter 8, but for now the approx. piC s of acids, phenols, and alcohols are 5,10, and 15, respectively. Hence, it would seem that the more acidic the proton, the higher the chemical shift. However, this is not the whole story—the extent of hydrogen bonding is critical in determining the observed chemical shift for alcohols and phenols (carboxylic acids are hydrogen bonded under all conditions of temperature and solvation). 

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Chemical shifts, and the observation or otherwise of couplings, for protons attached to oxygen (and nitrogen) depend on the extent of proton exchange and hydrogen bonding, which in turn depend on temperature, solvent, pH, and pK of the proton in question. 

Solid-state NMR spectroscopy has not found as wide an application in soils as it has in other fields. The great advantage of NMR is that it is specific for specific elements that is, it is tuned to a specific element and other elements are not detected. NMR spectroscopy shows the environment or multiple environments in which a particular element exists. For example, in a proton NMR spectrum, primary, secondary, and tertiary protons can be differentiated, as can protons attached to oxygen, nitrogen, and other atoms. 

For protons attached to atoms other than carbon the chemical shifts of protons attached to oxygen increase with increasing acidity of the O-H group thus 5=1—6 ppm for alcohols, 4-12 ppm for phenols and 10-14 ppm for carboxylic acids. Hydrogens bound to nitrogen (1° and 2° amines) are found at 5 = 3—8 ppm. The approximate chemical shift regions are shown in Fig. 29.5. 

Protons that undergo rapid chemical exchange (i.e., those attached to oxygen or nitrogen) can be easily detected by placing the compound in D2O. The protons are rapidly replaced by deuterons, and the proton signal disappears from the spectrum. 

The really important aspect to all of this is that nuclei in similar chemical environments exhibit similar chemical shifts. Thus protons ( H nuclei) attached to a carbon atom bonded to oxygen, H-C-O, show a characteristic chemical shift (3.5-A.5 ppm), while protons attached to a carbon atom bonded to nitrogen, H-C-N, have a different chemical shift range (2.5-3.5 ppm) and, since the carbon is attached to the less electronegative N atom, resonate at lower frequency. We can therefore use chemical shifts to our great advantage when interpreting NMR spectra. 

The heteroatoms most commonly encountered by the organic chemist are oxygen, nitrogen and sulphur. The position of absorption of protons attached to these atoms is not normally sufficiently reliable for interpretative purposes, although there are exceptions to this general rule (e.g. carboxylic acids, enols,... 

The central metal ion has nine coordination sites. It is attached to the three nitrogen atoms and to five carboxylate moieties (oxygen atoms). A single water molecule is able to coordinate at the vacant ninth site resulting in a strong enhancement of the water proton relaxation rate. The chelate can be described as a distorted capped square antiprism according to X-ray analysis [6]. 

Most acidic protons are attached to heteroatoms like halogen, oxygen, and nitrogen. Protons attached to carbon are not normally acidic but there are exceptions. One such exception occurs with aldehydes or ketones when there is a CHR2, CH R or CH3 group next to the carbonyl group (Following fig.). The protons indicated are acidic and are attached to the a (alpha) carbon. They are therefore called as a protons. 

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