Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Proton NMR

Proton NMR (also Hydrogen-1 NMR, or 1H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen (H) is used, practically all of the hydrogen consists of the isotope 1H (hydrogen-1; i.e. having a proton for a nucleus).

Simple NMR spectra are recorded in solution, and solvent protons must not be allowed to interfere. A solvent without hydrogen, such as carbon tetrachloride or deuterated chloroform may be used. More commonly, deuterated (deuterium = 2H, often symbolized as D) solvents especially for use in NMR are preferred, e.g. deuterated chloroform, CDCl3, and deuterated dimethyl sulfoxide, (CD3)2SO (DMSO).

Historically, deuterated solvents were supplied with a small amount (typically 0.1 %) of tetramethylsilane (TMS) as an internal standard for calibrating the chemical shifts of each analyte proton. TMS is a tetrahedral molecule, with all protons being chemically equivalent, giving one single signal, used to define 0 ppm. It is volatile, making sample recovery easy as well. 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). Deuterated solvents are now commonly supplied without TMS.

Deuterated solvents permit the use of deuterium frequency-field lock (also known as deuterium lock or field lock) to offset the effect of the natural drift of the NMR's magnetic field $$B_0$$. In order to provide deuterium lock, the NMR constantly monitors the deuterium signal resonance frequency from the solvent and makes changes to the $$B_0$$ to keep the resonance frequency constant. Additionally, the deuterium signal may be used to accurately define 0 ppm as the resonant frequency of the lock solvent and the difference between the lock solvent and 0 ppm (TMS) are well known.

Proton NMR spectra of most organic compounds 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 for molecular structure characterization.

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 Cα and of less than 1-2 ppm at Cβ. 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 Cα.

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. Acidic protons may also be suppressed when a solvent containing acidic deuterium ions (e.g. methanol-d4) is used.



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&mdash;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&mdash;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 to two peaks, one at 2.5 ppm + 3.5 Hz and the other at 2.5 ppm - 3.5 Hz&mdash;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 CHn group. 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.
 * 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.

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   singlet                          1 1  doublet                        1   1 2  triplet                      1   2   1 3  quartet                    1   3   3   1 4  pentet                   1   4   6   4   1 5  sextet                 1   5  10  10   5   1 6  septet               1   6  15  20  15   6   1 7  octet              1   7  21  35  35   21  7   1 8  nonet            1   8  28  56  70  56   28   8  1

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.

With 2-methylpropane, (CH3)3CH, as another example: the CH proton is attached to three identical methyl groups. The C-H signal in the spectrum would be split into ten peaks 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.



It should be emphasized that the simple rules for the spin-spin splitting of NMR signals described above only apply if the chemical shifts of the coupling partners are substantially larger that the coupling constant between them, otherwise there may be more peaks, and the intensities of the individual peaks will be distorted (second-order effects).

Carbon satellites and spinning sidebands
Occasionally, small peaks can be seen shouldering the main 1H NMR peaks. These peaks are not the result of proton-proton coupling, but result from the coupling of 1H atoms to an adjoining carbon 13 atom. These small peaks are known as carbon satellites as they are small and appear around the main 1H peak i.e. satellite (around) to them. Carbon satellites are small because Carbon 13 only makes up about 1% of the atomic carbon content of carbon, the rest of the carbon atoms are predominantly NMR inactive Carbon 12. Carbon satellites always appear as an evenly spaced pair around the main 1H peak. This is because they are the result of 1% of the 1H atoms coupling to an adjoined 13C atom to give a wide doublet (carbon 13 has a spin of a half). Note, if the main 1H-peak has proton-proton coupling, then each satellite will be a miniature version of the main peak and will also show this 1H-coupling, e.g. if the main 1H-peak is a doublet, then the carbon satellites will appear as miniature doublets, i.e. one doublet on either side of the main 1H-peak. Sometime other peaks can be seen around 1H peaks, these are known as spinning sidebands and are related to the rate of spin of an NMR tube. Carbon satellites and spinning sidebands should not be confused with impurity peaks.