The key difference between geminal and vicinal coupling is that geminal coupling refers to the coupling of two hydrogen atoms that are bound to the same carbon atom. But, vicinal coupling refers to the coupling of two hydrogen atoms that are bound to two adjacent carbon atoms.
The terms geminal and vicinal coupling come under NMR (nuclear magnetic resonance) and they describe the differences in the NMR peaks when hydrogen atoms undergo coupling differently. Hydrogen atom coupling may occur between two hydrogen atoms that are bound to the same carbon atom or two hydrogen atoms bound to two adjacent carbon atoms.
CONTENTS
1. Overview and Key Difference
2. What is Geminal Coupling
3. What is Vicinal Coupling
4. Side by Side Comparison – Geminal vs Vicinal Coupling in Tabular Form
5. Summary
What is Geminal Coupling?
Geminal coupling is the coupling of two hydrogen atoms which are bound to the same carbon atom of the sample compound. Although it is mainly applied to hydrogen atoms in NMR, the term geminal refers to the relationship between two functional groups or atoms that are attached to the same atom. For example, geminal diol refers to an alcohol having the two –OH groups attached to the same carbon atom.
In the NMR technique, the geminal coupling occurs only if the two hydrogen atoms attached to a methylene group differ from each other stereochemically. We can denote geminal coupling as 2J. This denotation states that two hydrogen atoms couple through two chemical bonds (two chemical bonds between hydrogen atoms and carbon atom). The geminal coupling also has a value which we can name as the geminal coupling constant. The value of this constant may vary from -23 to +42 Hz, depending on the other substituents that are attached to the same carbon atom.
What is Vicinal Coupling?
Vicinal coupling refers to the coupling of two hydrogen atoms that are bound to two adjacent carbon atoms of the sample compound. The term vicinal refers to the attachment of two functional groups at two adjacent atoms of the same compound. i.e. 2,3-dibromobutane has two bromine atoms attached to 2nd and 3rd carbon atoms of a butane molecule.
However, in NMR spectroscopy, the term vicinal refers to the coupling of two hydrogen atoms that are attached to two adjacent carbon atoms. Here, we can denote this term as 3J. This is because the hydrogen atoms couple through three chemical bonds (two chemical bonds between hydrogen atoms and carbon atoms plus one chemical bond between two carbon atoms). We can measure the vicinal coupling in NMR as the vicinal coupling constant, which has a value that ranges from 0 to +20 HZ, depending on the other substituents attached to the carbon atoms.
What is the Difference Between Geminal and Vicinal Coupling?
The terms geminal coupling and vicinal coupling come under the branch of NMR or nuclear magnetic resonance. These terms differ from each other according to the coupling pattern of hydrogen atoms in the sample compound. The key difference between geminal and vicinal coupling is that geminal coupling refers to the coupling of two hydrogen atoms that are bound to the same carbon atom, whereas vicinal coupling refers to the coupling of two hydrogen atoms that are bound to two adjacent carbon atoms.
Below infographic shows more comparisons related to the difference between geminal and vicinal coupling.
Summary – Geminal vs Vicinal Coupling
The terms geminal coupling and vicinal coupling come under the branch of NMR or nuclear magnetic resonance. The key difference between geminal and vicinal coupling is that geminal coupling refers to the coupling of two hydrogen atoms that are bound to the same carbon atom. Meanwhile, vicinal coupling refers to the coupling of two hydrogen atoms that are bound to two adjacent carbon atoms.
Reference:
1. “Geminal.” Wikipedia, Wikimedia Foundation, 16 July 2019, Available here.
Image Courtesy:
1. “19F-NMR yne-6AmF5” By Jkwchui – Own work (Public Domain) via Commons Wikimedia
2. “J-coupling Fermi contact mechanism” By Keministi – Own work (CC0) via Commons Wikimedia