Proton nuclear magnetic resonance

Simple NMR spectra are recorded in solution, and solvent protons must not be allowed to interfere.

Historically, deuterated solvents were supplied with a small amount (typically 0.1%) of tetramethylsilane (TMS) as an internal standard for referencing the chemical shifts of each analyte proton.

Modern spectrometers are able to reference spectra based on the residual proton in the solvent (e.g. the CHCl3, 0.01% in 99.99% CDCl3).

The spectrum of benzene consists of a single peak at 7.2 ppm due to the diamagnetic ring current.

Chemical shift values, symbolized by δ, are not precise, but typical – they are to be therefore regarded mainly as a reference.

The exact value of chemical shift depends on molecular structure and the solvent, temperature, magnetic field in which the spectrum is being recorded and other neighboring functional groups.

Carbonyl groups, olefinic fragments and aromatic rings contribute sp2 hybridized carbon atoms to an aliphatic chain.

However, such resonances can be identified by the disappearance of a peak when reacted with D2O, as deuterium will replace a protium atom.

The integrated intensities of NMR signals are, ideally, proportional to the ratio of the nuclei within the molecule.

[4] Together with chemical shift and coupling constants, the integrated intensities allow structural assignments.

These considerations are valid only when sufficient time is allowed for full relaxation of the affected signals, as determined by their T1 values.

In addition to chemical shift, NMR spectra allow structural assignments by virtue of spin–spin coupling (and integrated intensities).

When the CH2−CH group is changed to CH3−CH2, keeping the chemical shift and coupling constants identical, the following changes are observed: Something split by three identical protons takes a shape known as a quartet, each peak having relative intensities of 1:3:3:1.

A peak is split by n identical protons into components whose sizes are in the ratio of the nth row of Pascal's triangle:

Because the nth row has n + 1 components, this type of splitting is said to follow the "n + 1 rule": a proton with n neighbors appears as a cluster of n + 1 peaks.

Note that the outer lines of the nonet (which are only 1/8 as high as those of the second peak) can barely be seen, giving a superficial resemblance to a septet.

The analysis of such multiplets (which can be much more complicated than the ones shown here) provides important clues to the structure of the molecule being studied.

[5] Even larger coupling constants may be seen in phosphines, especially if the proton is directly bonded to the phosphorus.

[6] These coupling constants are so large that they may span distances in excess of 1 ppm (depending on the spectrometer), making them prone to overlapping with other proton signals in the molecule.

Other NMR-active nuclei can also cause these satellites, but carbon is most common culprit in the proton NMR spectra of organic compounds.

Example 1 H NMR spectrum (1-dimensional) of a mixture of menthol enantiomers plotted as signal intensity (vertical axis) vs. chemical shift (in ppm on the horizontal axis). Signals from spectrum have been assigned hydrogen atom groups (a through j) from the structure shown at upper left.
1 H NMR spectrum predicted for 1,4-dimethylbenzene. Under ideal conditions, the ratio of integrated signal of protons A and B is related to the structure of this molecule.
Example 1 H NMR spectrum (1-dimensional) of ethyl acetate plotted as signal intensity vs. chemical shift . There are three different types of H atoms in ethyl acetate regarding NMR. The hydrogens (H) on the CH 3 COO− ( acetate ) group are not coupling with the other H atoms and appear as a singlet, but the −CH 2 − and −CH 3 hydrogens of the ethyl group (−CH 2 CH 3 ) are coupling with each other, resulting in a quartet and triplet respectively.
H NMR spectrum of a solution of HD (labeled with red bars) and H 2 (blue bar). The 1:1:1 triplet for HD arises from heteronuclear (different isotopes) coupling.