The key difference between chemical shift and coupling constant is that chemical shift describes the shift of the position of NMR absorption that arises due to shielding or deshielding of the protons by the electrons of the compound whereas coupling constant refers to the interaction between a pair of protons.
Both chemical shift and coupling constant are terms which give numerical values related to NMR. NMR is nuclear magnetic resonance. It is a technique that produces a series of signals upon placing a sample in a magnetic field.
CONTENTS
1. Overview and Key Difference
2. What is Chemical Shift
3. What is Coupling Constant
4. Side by Side Comparison – Chemical Shift vs Coupling Constant in Tabular Form
5. Summary
What is Chemical Shift?
Chemical shift is the change in the nuclear magnetic resonance frequency of a nucleus depending on the electronic environment. We can denote this term as δ. A chemical shift describes the shift of the position of NMR absorption that arises due to shielding or deshielding of the protons by the electrons of the compound. We can determine the chemical shift by observing the difference between the absorption position of the sample proton and the reference proton of a standard compound. The chemical shift has a value which we can express by the unit ppm or parts per million. There are some important features we need to consider when choosing an appropriate references standard;
- Has to be chemically inert
- Magnetic isotropy
- Should give a peak that is easily recognizable
- It has to be miscible with a wide range of solvents
- It should give a single, sharp peak
Furthermore, there are some factors that affect the chemical shift; e.g, inductive effect, van der Waals deshielding, anisotropic effect and hydrogen bonding ability of the compound.
- When considering the inductive effect, higher the electronegativity, higher the deshielding effect and higher the chemical shift value
- In the Van der Waals deshielding effect, the presence of bulky groups leads to the repulsion between the electron cloud around the bulky group and protons, which makes the protons deshielded.
- In the anisotropic effect, presence of alkenes causes high chemical shift and the presence of alkynes cause a low chemical shift.
- The deshielding effect depends on the strength of hydrogen bonding.
What is Coupling Constant?
Coupling constant refers to the coupling of any two adjacent lines in NMR peaks of two sets of equivalent hydrogen nuclei. We can denote this term as J. This coupling constant measures this effect numerically, and the unit of measurement for coupling constant is Hertz or Hz. It is a measure of the interaction between a pair of protons.
There are three different types of coupling as germinal coupling, vicinal coupling and long range coupling.
What is the Difference Between Chemical Shift and Coupling Constant?
Both chemical shift and coupling constant are terms which give numerical values related to NMR. Chemical shift is the change in the nuclear magnetic resonance frequency of a nucleus depending on the electronic environment. Coupling constant refers to the coupling of any two adjacent lines in NMR peaks of two sets of equivalent hydrogen nuclei. The key difference between chemical shift and coupling constant is that chemical shift refers to the shift of the position of NMR absorption that arises due to shielding or deshielding of the protons by the electrons of the compound, whereas coupling constant refers to the interaction between a pair of protons.
Below infographic summarizes the difference between chemical shift and coupling constant.
Summary – Chemical Shift vs Coupling Constant
Both chemical shift and coupling constant are terms that give numerical values related to NMR. The key difference between chemical shift and coupling constant is that the term chemical shift refers to the shift of the position of NMR absorption that arises due to shielding or deshielding of the protons by the electrons of the compound whereas coupling constant refers to the interaction between a pair of protons.
Reference:
1. “13.3 Chemical Shifts.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
Image Courtesy:
1. “13C NMR ethanal” By Chris Evans – D:\My Webs\index.htm (CC0) via Commons Wikimedia
2. “NMR J-coupling trees” By Keministi – Own work (CC0) via Commons Wikimedia