© Copyright Hans J. Reich 2017 There are two distinct types of magnetic interaction (coupling) between nuclei (A and X) with a non-zero spin - the direct interaction (dipole-dipole coupling:
The scalar coupling
Coupling constants can be either positive or negative, defined as follows: coupling constants are positive if the energy of A is lower when X has the opposite spin as A (αβ or βα), and negative if the energy of A is lower when X has the same spin as A (αα or ββ). For the Fermi contact mechanism of spin-spin coupling (there are other mechanisms), the bonding electrons for a H- If we continue down the bond sequence, the polarization of the C-H electrons will cause polarization of the C-C electron pair. Again, parallel spins are the more stable orientation (triplets are more stable than singlets -- Hund's rule). Thus the two-bond coupling ( The principal mechanism for A depiction of the perturbation of energy levels of a nucleus A by a neighboring
Consider the NMR spectrum of 3,4-dichlorobenzoyl chloride below. The proton-proton couplings in benzene are typically 7-9 Hz for A slightly more complicated case is 1,1,2-trichloropropane. A simulated spectrum is shown below. The C-2 proton is coupled to one proton at C-1 and three protons of the methyl group at C-3. Naively, one might expect a
1. Nuclei must be chemical shift nonequivalent to show obvious coupling to each other. Thus the protons of CH
2. 3. Two closely spaced lines can be either chemically shifted or coupled. It is not always possible to distinguish
For a simple example see the spectrum of 3-acetoxy-2-butanone below. Here it is pretty easy to identify one of the doublets as the 4-methyl group, the other "doublet" (with a separation of 9 Hz, which could easily be a coupling) actually corresponds to the two CH 4. Chemical shifts are usually reported in δ (units: ppm) so that the numeric values will not depend on the spectrometer frequency (field-independent units), coupling constants are 5. Protons two ( 6. Multiplicity for first order patterns follows the "doubling rule". If 7. 8. More typically,
Protons or groups of protons form simple multiplets only if the chemical shift differences between the protons (Δν) are large compared to the coupling constants between them (
A
First, the chemical shift of the observed proton must be far away from any of the protons it is coupled to (far away means Δν >>
Second, if more than one proton is coupled to the observed one, then these protons must not be "strongly coupled." In other words, if they are coupled to each other A first order multiplet consists of the 1. All truly first order multiplets are
2. If the small outermost peaks are assigned intensity 1, then all other peaks must be an integral multiple intensity of this one (1x, 2x, 3x, 4x in height), and the total intensity of all peaks must be a power of 2 (2, 4, 8, 16, 32, etc). The intensity of each of the two outermost lines is 1/2 3. There is a strict regularity of spacing in a first order multiplet: if you have correctly identified a coupling constant 4. Most first order multiplets integrate to a single proton, a few may be 2 or 3 protons in area. It is rare to have more than 3 protons, unless there is symmetry in the molecule (e.g., (CH
5. The symmetry and intensities of an otherwise first-order multiplet can be distorted by
1. "Take out" the smallest couplings first. The separation between the two lines at the edge of the multiplet is the smallest coupling. Each time you remove a coupling you generate a new, simpler multiplet, which can then be analyzed in turn. Remember that 2. Watch line intensities (i.e., peak areas or peak heights) carefully--when you "take out" a coupling, the intensities of the newly created lines should be appropriate (i.e., each time you "take out" a coupling, also "take out" the proper intensity). When a coupling has been taken out completely, all intensity should be accounted for. Keep track of your analysis by using a "coupling tree". 3. The couplings may be removed one at a time as doublets, or as triplets, quartets and higher multiplets. The intensity ratio of the first two lines signals the number of protons involved in the coupling: 1:1 means there is only one proton, 1:2 means that there are 2 protons (triplet), etc. Be especially careful to keep track of intensities when you "take out" triplets (1:2:1) or quartets (1:3:3:1). Each time you completely remove a coupling you generate a new simpler multiplet which follows first order rules, and can be analyzed in turn.
When you have finished your analysis, all peaks and all intensity in the multiplet must be accounted for. You can check the analysis as follows: the separation of the two outermost peaks of the multiplet is the sum of all the
Some criteria to use:
True higher order coupling patterns (t, q, pentet, septet, etc) result from two, three, four or more symmetry-equivalent couplings to one proton. Such multiplets also arise from the accidental equivalence of two or more different couplings.
An illustrative example is the multiplet in spectrum The gem ( The axial-equatorial and equatorial-equatorial In cyclopentenes, the vicinal and four bond allylic couplings { Other common equivalent couplings are seen in the nearly identical ca 10 Hz coupling of the central protons in conjugated dienes, and coupling to cis protons across the double bond, as seen in Spectrum
The accurate measurement of For first order multiplets a simple "coupling tree" analysis as described in Section 5-HMR-3.9 can directly yield coupling constants within the accuracy of the digital resolution of the spectrum. This includes For J_{AB} and the chemical shifts can be obtained by simple arithmetic manipulations, provided that line assignments can be made correctly. For ABX spectra J_{AB} is accurately measureable by inspection. An approximate analysis, which treats the peaks as AMX, will give values for J_{AX} and J_{BX} that will be in error by varying amounts, depending on the relative size of J_{AB} and ν_{AB} (the smaller ν_{AB} the larger the error), and the relative size of J_{AX} and J_{BX}. To get accurate values for the J_{AX} and J_{BX} coupling constants a proper ABX analysis as described in Section 5-HMR-12 is required. For many simple compounds the symmetry is such that protons are homotopic or enantiotopic, and no coupling constants can be measured directly (e.g., the J_{AA'}. An example is 1,3-butadiene, an AA'BB'CC' system in which all protons are compled to all other ones. Analysis of the complex NMR spectrum gave, among numerous others, values for the following couplings between chemical shift equivalent nuclei: ^{3}J_{AA'}, ^{5}J_{BB'} and ^{5}J_{CC'} (Hobgood, R. T., Jr.; Goldstein, J. H. J. Mol. Spectr. 1964, 12, 76). ^{3}J_{HH} can often be obtained by analysis of the ^{13}C satellites. The ^{1}H NMR signal for the vinyl protons of dimethyl maleate is a singlet. However, the ^{13}C satellites are doublets, with a splitting that is equal to ^{3}J_{HH}. In effect, the A_{2} spin system of the ^{12}C isotopomer has become an ABX pattern in the mono-^{13}C labelled compound, where X is the ^{13}C nucleus, and A and B are the two vinyl protons, one on ^{13}C and the other on ^{12}C. Below is an example of the measurement of a For systems of the X-CH Next Section: Two-bond Coupling · Previous Section: Integration · Home |