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For a nucleus to cause splitting, it must be close enough to the nucleus being observed to affect its magnetic environment. The splitting technically occurs through bonds, not through space, so as a general rule, only nuclei separated by three or fewer bonds can split each other. However, even if a nucleus is close enough to another, it may not cause splitting. For splitting to occur, the nuclei must also be non-equivalent. To see how these factors affect real NMR spectra, consider the spectrum for chloroethane ( [link] )

The NMR spectrum for chloroethane. Adapted from A. M. Castillo, L. Patiny, and J. Wist. J. Magn. Reson. , 2010, 209 , 123.

Notice that in [link] there are two groups of peaks in the spectrum for chloroethane, a triplet and a quartet. These arise from the two different types of I ≠ 0 nuclei in the molecule, the protons on the methyl and methylene groups. The multiplet corresponding to the CH 3 protons has a relative integration (peak area) of three (one for each proton) and is split by the two methylene protons ( n = 2), which results in n + 1 peaks, i.e., 3 which is a triplet. The multiplet corresponding to the CH 2 protons has an integration of two (one for each proton) and is split by the three methyl protons (( n = 3) which results in n + 1 peaks, i.e., 4 which is a quartet. Each group of nuclei splits the other, so in this way, they are coupled .

Coupling constants

The difference (in Hz) between the peaks of a mulitplet is called the coupling constant . It is particular to the types of nuclei that give rise to the multiplet, and is independent of the field strength of the NMR instrument used. For this reason, the coupling constant is given in Hz, not ppm. The coupling constant for many common pairs of nuclei are known ( [link] ), and this can help when interpreting spectra.

Typical coupling constants for various organic structural types.
Structural type Coupling constant (Hz)
6 - 8
5 - 7
2 - 12
0.5 - 3
12 - 15
12 - 18
7 - 12
0.5 - 3
3 - 11
2 - 3
ortho = 6 - 9; meta = 1 - 3; para = 0 - 1

Coupling constants are sometimes written n J to denote the number of bonds (n) between the coupled nuclei. Alternatively, they are written as J (H-H) or J HH to indicate the coupling is between two hydrogen atoms. Thus, a coupling constant between a phosphorous atom and a hydrogen would be written as J (P-H) or J PH . Coupling constants are calculated empirically by measuring the distance between the peaks of a multiplet, and are expressed in Hz.

Coupling constants may be calculated from spectra using frequency or chemical shift data. Consider the spectrum of chloroethane shown in [link] and the frequency of the peaks (collected on a 60 MHz spectrometer) given in [link] .

1 H NMR spectrum of chloroethane. Peak positions for labeled peaks are given in [link] .
Chemical shift in ppm and Hz for all peaks in the 1 H NMR spectrum of chloroethane. Peak labels are given in [link] .
Peak label δ (ppm) ν (Hz)
a 3.7805 226.83
b 3.6628 219.77
c 3.5452 212.71
d 3.4275 205.65
e 1.3646 81.88
f 1.2470 74.82
g 1.1293 67.76

To determine the coupling constant for a multiplet (in this case, the quartet in [link] ), the difference in frequency (ν) between each peak is calculated and the average of this value provides the coupling constant in Hz. For example using the data from [link] :

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Source:  OpenStax, Basic knowledge of nuclear magnetic resonance spectroscopy ( nmr ). OpenStax CNX. Jun 07, 2012 Download for free at http://cnx.org/content/col11429/1.1
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