The signal splitting in 1H-NMR spectra is usually small, ranging from fractions of a Hz to as much as 18 Hz, and is designated as J (referred to the coupling constant).
Coupling Constants (J)
J is the distance between peaks in a splitting signal
the magnitude of J is
- dependent only on fields that are caused by magnetic atoms within a molecule but
- independent of the external field strength (B0)
J is the distance between peaks in a splitting signal the magnitude of J is- dependent only on fields that are caused by magnetic atoms within a molecule but
- independent of the external field strength (B0)
The most common patterns that have been seen are doublet and triplet.
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n = 1
|
n = 2
|
First-Order and Second-Order spectra
Spectra that can be interpreted by using n+1 rule are said to be first-order spectra. In first-order spectra, the difference in chemical shifts (in Hz) between 2 groups of protons is large. When Dn/J is large and we see first-order splitting, the system is said to be weakly coupled.
In some spectra, using the n+1 rule is not enough, therefore, such advanced analysis may be required for interpreting spectra. These spectra are said to be second-order spectra. In second-order spectra, the difference in chemical shifts between 2 groups of protons is similar in magnitude to the J. When Dn/J is large and we see second-order splitting, the system is said to be strongly coupled.
Example of first-order spetra
1. The ethyl acetate spectrum displays the typical quartet and triplet of a substituted ethyl group.
2. The spectrum of 1,3-dichloropropane demonstrates the triplet (B) and the quintet (A).
Example of second-order spetra
If a given nucleus is spin-coupled to two or more sets of neighboring nuclei by different J values, the n+1 rule does not predict the entire splitting pattern. The spectra may be complicated due to the strong coupling.
These are some examples of more complicated spectra that can be 2nd order spectra.
Magnitude of Some Typical Coupling Constants
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