Proton NMR

  Proton NMR (also Hydrogen-1 NMR, or ¹HNMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen.

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Simple NMR spectra are recorded in solution, and solvent protons must not be allowed to interfere. Therefore a large range of deuterated solvents exist especially for NMR such as deuterated chloroform CDCl₃ and deuterated dimethyl sulfoxide (CD₃)₂SO. These solvents contain small quantities of undeuterated solvent which may give rise to a signal; CHCl₃ is seen as a single peak at 7.27 ppm. Water may be present as a contaminant, this gives a broad peak whose chemical shift varies greatly with solvent; it occurs around 1.6 ppm in CDCl₃. Spectra are usually recorded against tetramethyl silane as the internal standard, set as zero.

Proton NMR spectra are characterized by chemical shifts in the range +12 to -4 ppm and by spin-spin coupling between protons. The integration curve for each proton reflects the abundance of the individual protons.

Simple molecules have simple spectra. The spectrum of ethyl chloride consists of a triplet at 1.5 ppm and a quartet at 3.5 ppm in a 3:2 ratio. The spectrum of benzene consists of a single peak at 7.2 ppm due to the diamagnetic ring current.

Together with Carbon-13 NMR proton NMR is a powerful tool in structure elucidation in chemistry.

Chemical shifts

Chemical shift values are not precise, but typical - they are to be therefore regarded mainly as orientational. Deviations are in ±0.2 ppm range, sometimes more. The exact value of chemical shift depends on molecular structure and the solvent in which the spectrum is being recorded. Hydrogen nuclei are sensitive to the hybridisation of the atom to which the proton is attached and to electronic effects. Nuclei tend to be deshielded by groups which withdraw electron density. Deshielded nuclei resonate at higher δ values, whereas shielded nuclei resonate at lower δ values.

Examples of electron withdrawing substituents are -OH, -OCOR, -OR, -NO₂ and halogens. These cause a downfield shift of approximately 2-4ppm at C_α and of less than 1-2 ppm at C_β. Carbonyl groups, olefinic fragments and aromatic rings contribute sp² hybridised carbon atoms to an aliphatic chain. This causes a downfield shift of 1-2 ppm at C_α.

Note that labile protons (-OH, -NH₂, SH) have no characteristic chemical shift. However such resonances can be identified by the disappearance of a peak when reacted with D₂O, as deuterium will replace a proton. This method is called a D₂O shake.

Spin-spin couplings

The chemical shift is not the only indicator used to assign a molecule. Because nuclei themselves are little magnets they influence each other, changing the energy and hence frequency of nearby nuclei as they resonate—this is known as spin-spin coupling. The most important type in basic NMR is scalar coupling. This interaction between two nuclei occurs through chemical bonds, and can typically be seen up to three bonds away.

The effect of scalar coupling can be understood by examination of a proton which has a signal at 1ppm. This proton is in a hypothetical molecule where three bonds away exists another proton (in a CH-CH group for instance), the neighbouring group (a magnetic field) causes the signal at 1 ppm to split into two, with one peak being a few hertz higher than 1 ppm and the other peak being the same number of hertz lower than 1 ppm. These peaks each have half the area of the former singlet peak. The magnitude of this splitting (difference in frequency between peaks) is known as the coupling constant. A typical coupling constant value would be 7 Hz.

The coupling constant is independent of magnetic field strength because it is caused by the magnetic field of another nucleus, not the spectrometer magnet. Therefore it is quoted in hertz (frequency) and not ppm (chemical shift).

In another molecule a proton resonates at 2.5 ppm and that proton would also be split into two by the proton at 1 ppm. Because the magnitude of interaction is the same the splitting would have the same coupling constant 7 Hz apart. The spectrum would have two signals, each being a doublet. Each doublet will have the same area because both doublets are produced by one proton each.

The two doublets at 1 ppm and 2.5 ppm from the fictional molecule CH-CH are now changed into CH₂-CH:

• The total area of the 1 ppm CH₂ peak will be twice that of the 2.5 ppm CH peak.
• The CH₂ peak will be split into a doublet by the CH peak—with one peak at 1 ppm + 3.5 Hz and one at 1 ppm - 3.5 Hz (total splitting or coupling constant is 7 Hz).

In consequence the CH peak at 2.5 ppm will be split twice by each proton from the CH₂. The first proton will split the peak into two equal intensities and will go from one peak at 2.5 ppm two peaks, one at 2.5 ppm + 3.5 Hz and the other at 2.5 ppm - 3.5 Hz—each having equal intensities. However these will be split again by the second proton. The frequencies will change accordingly:

• The 2.5 ppm + 3.5 Hz signal will be split into 2.5 ppm + 7 Hz and 2.5 ppm
• The 2.5 ppm - 3.5 Hz signal will be split into 2.5 ppm and 2.5 ppm - 7 Hz

The net result is not a signal consisting of 4 peaks but three: one signal at 7 Hz above 2.5 ppm, two signals occur at 2.5 ppm, and a final one at 7 Hz below 2.5 ppm. The ratio of height between them is 1:2:1. This is known as a triplet and is an indicator that the proton is three-bonds from a CH₂ group.

This can be extended to any CH_n group. When the CH₂-CH group is changed to CH₃-CH₂, keeping the chemical shift and coupling constants identical, the following changes are observed:

• The relative areas between the CH₃ and CH₂ subunits will be 3:2.
• The CH₃ is coupled to two protons into a 1:2:1 triplet around 1 ppm.
• The CH₂ is coupled to three protons.

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:

  n                                    
  0                                    1
  1                                 1     1
  2                              1     2     1
  3                           1     3     3     1
  4                        1     4     6     4     1
  5                     1     5    10     10    5     1

With 2-methylpropane, (CH₃)₃CH, as another example: the CH proton is attached to three identical methyl groups. The peak in the spectrum would be split into ten lines according to the (n + 1) rule of multiplicity. Below are NMR signals corresponding to several simple multiplets of this type. 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.

When a proton is coupled to two different protons, then the coupling constants are likely to be different, and instead of a triplet, a doublet of doublets will be seen. Similarly, if a proton is coupled to two other protons of one type, and a third of another type with a different coupling constant, then a triplet of doublets is seen. In the example below, the triplet coupling constant is larger than the doublet one. The analysis of such multiplets (which can get very much more complicated than the ones shown here) provides important clues to the structure of the molecule being studied.

References

1. ^  R. M. Silverstein, G. C. Bassler and T. C. Morrill, Spectrometric Identification of Organic Compounds, 5th Ed., Wiley, 1991.