Chemical shifts hydrogen nucleus effect

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

I hope that by now your interest has been piqued. Why do equivalent hydrogens give rise to singlets in some cases and multiplets in others To understand the phenomenon of spin coupling, recall how the magnitude of the effective magnetic field experienced by a nucleus determines its precessional frequency and thereby its chemical shift. If that is not second nature to you by now, perhaps you should review Section 6.1 before proceeding further. 

Secondary isotope effects through two or three bonds are sometimes helpful in structural elucidation, particularly when an exchangeable hydrogen can easily be replaced by deuterium and its effect on the chemical shift of a nearby carbon nucleus observed. 

X (D)], where n is the number of intervening bonds between the deuterium and the observed nucleus, X . The most studied of these effects is the two-bond isotope effect on chemical shifts, AC(XD), produced upon H/D exchange at XH. This isotope effect has been correlated with d(XH) and, when XH takes part in an intramolecular hydrogen bond, to the hydrogen-bond enthalpy The magnitude of... 

Electronic effects of substituents at the C-5 position influence the chemical shift of the NH protons in 5,5-di- and 1,5,5-trisubstituted derivatives, and the possibility of an interaction of the barbituric acid nucleus with a solvent, usually through a hydrogen bond, was confirmed by NH chemical shifts as shown in Table III. 

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