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 CH3COO− (
acetate) group are not coupling with the other H atoms and appear as a singlet, but the −CH2− and −CH3 hydrogens of the
ethyl group (−CH2CH3) are coupling with each other, resulting in a quartet and triplet respectively. In addition to
chemical shift, NMR spectra allow structural assignments by virtue of spin–spin coupling (and integrated intensities). Because nuclei themselves possess a small magnetic field, 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 (3-J coupling), although it can occasionally be visible over four to five bonds, though these tend to be considerably weaker. The effect of scalar coupling can be understood by examination of a proton which has a signal at 1 ppm. 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 amount 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 for aliphatic protons 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 to 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: 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 containing a total of 9 identical protons. The C−H signal in the spectrum would be split into 10 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, smaller coupling constant, then a triplet of doublets is seen. In the example below, the triplet coupling constant is larger than the doublet one. By convention the pattern created by the largest coupling constant is indicated first and the splitting patterns of smaller constants are named in turn. In the case below it would be erroneous to refer to the quartet of triplets as a triplet of quartets. 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. The simple rules for the spin-spin splitting of NMR signals described above apply only if the chemical shifts of the coupling partners are substantially larger than the coupling constant between them. Otherwise there may be more peaks, and the intensities of the individual peaks will be distorted (second-order effects).
Hetero-nuclear coupling If there are other NMR-active nuclei present in a molecule, spin–spin coupling will be observed between the hetero-atoms and the protons. This occurs most frequently in compounds that contain
phosphorus or
fluorine, as they are both spin-1/2 nuclei of 100% abundance. For example, the 1H signals for the protons in
fluoromethane are split into a doublet by the fluorine atom; conversely, the fluorine-19 NMR spectrum of this compound shows a quartet due to being split by the three protons. Typical 2J coupling constants between fluorine and protons are 48 Hz or so; the strength of coupling declines to 2 Hz in 4J coupling. Even larger coupling constants may be seen in phosphines, especially if the proton is directly bonded to the phosphorus. Coupling constants for these protons are often as large as 200 Hz, for example, in diethylphosphine, where the 1J P−H coupling constant is 190 Hz. 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. ==Carbon satellites and spinning sidebands==