Question 1

What is geminal coupling in NMR spectroscopy?

Accepted Answer

Geminal coupling, or two-bond coupling, occurs between two NMR-active nuclei bonded to a central atom. Common examples include coupling between diastereotopic protons in chiral molecules and protons in unsymmetrical alkenes. The central atom need not be NMR-active because its electrons are affected by electron polarization from the spin-active atoms.

Question 2

Why are 2J coupling values typically weaker than 1J values?

Accepted Answer

Spin information is transmitted less effectively through two bonds than through one bond, resulting in weaker 2J values. Although the central atom need not be NMR-active, the indirect transmission of spin information through electron polarization is less efficient, making geminal coupling generally weaker than direct one-bond coupling.

Question 3

How does orbital s character affect geminal coupling constants?

Accepted Answer

As s character increases in the involved orbitals, 2J values become more positive, observed as a decrease in coupling magnitude in the spectrum. Conversely, as s character decreases, 2J values become more negative, increasing the extent of coupling observed. This relationship between orbital composition and coupling strength is fundamental to interpreting geminal coupling patterns.

Question 4

What role does bond angle play in geminal coupling?

Accepted Answer

In cycloalkanes, as the H–C–H angle approaches 109 degrees, 2J values become more negative, increasing the extent of coupling observed in the spectrum. Bond angle directly influences electron polarization pathways, making it a key structural factor determining geminal coupling magnitude and sign.

Question 5

How do electronegative substituents affect geminal coupling?

Accepted Answer

Electronegative substituents positioned alpha to the coupled nuclei make 2J values more positive, decreasing coupling magnitude. This effect is observed in molecules like fluoromethane and formaldehyde, where the electron-withdrawing group alters the electron polarization environment and shifts coupling toward positive values.

Question 6

Why is the sign of J coupling not visible in NMR spectra?

Accepted Answer

Although 2J values can be negative or positive depending on spin configuration and molecular structure, the sign of J is not evident from standard NMR spectra. Negative 2J values result when parallel spin configurations are energetically favored, but this distinction requires specialized techniques to determine experimentally.

Question 7

Can the central atom in geminal coupling be NMR-inactive?

Accepted Answer

Yes, the central atom need not be NMR-active for geminal coupling to occur. Its electrons are affected by electron polarization from the spin-active nuclei bonded to it, allowing spin information to be transmitted through the central atom even if it cannot directly participate in NMR transitions.

Couplage spin-spin : couplage à deux liaisons (couplage géminal)

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