Question 1

Why is one-bond coupling between NMR-active nuclei usually positive?

Accepted Answer

One-bond coupling is positive because nuclei polarize their bonding electrons to opposite spins, making antiparallel nuclear spin configurations lower in energy. When coupling makes these antiparallel states more favorable, the coupling constant J is considered positive. This strong, positive one-bond coupling results in large magnitude values characteristic of directly bonded nuclei.

Question 2

How does orbital s character affect one-bond carbon-hydrogen coupling constants?

Accepted Answer

Spin information transmits more effectively through orbitals with increasing s character. This explains why 1JC–H values increase across ethane (125 Hz), ethene (156 Hz), and ethyne (249 Hz), as s character increases with bond type. Similarly, strained cyclic systems like cyclopropane show larger 1JC–H values due to increased s character in exocyclic bonds compared to unstrained cyclohexane.

Question 3

What effect do electronegative substituents have on carbon-hydrogen coupling?

Accepted Answer

Electronegative substituents bonded to carbon increase 1JC–H coupling constants. Chlorinated methanes demonstrate this trend: CH4 (125 Hz), CH3Cl (150 Hz), CH2Cl2 (178 Hz), and CHCl3 (209 Hz). As more chlorine atoms attach to the carbon, the coupling constant increases due to the inductive effect of the electronegative substituent on the bonding electrons.

Question 4

Why is carbon-hydrogen coupling rarely observed in proton NMR spectra?

Accepted Answer

Although one-bond carbon-hydrogen coupling is very strong, it is rarely observed in proton NMR spectra because of the low natural abundance of carbon-13. Since most carbon atoms in a sample are carbon-12, which is NMR-inactive, the probability of observing C–H coupling in routine proton NMR is extremely low.

Question 5

How does ring strain influence coupling constants in cyclic systems?

Accepted Answer

Ring strain increases the s character of exocyclic C–H bonds, leading to larger 1JC–H coupling constants. Strained cyclopropane exhibits 1JC–H of 160 Hz, cyclobutane 134 Hz, and cyclopentane 129 Hz, all higher than unstrained cyclohexane at 125 Hz. This demonstrates how molecular geometry and strain directly affect spin information transmission through bonding electrons.

Question 6

How is spin information transmitted through bonded nuclei in one-bond coupling?

Accepted Answer

Spin information is transmitted directly through the pair of bonding electrons connecting NMR-active nuclei. Because nuclei polarize their electrons to opposite spins, the bonding electron pair has antiparallel spins. This direct electron-mediated pathway makes one-bond coupling the strongest coupling interaction, with large positive coupling constant magnitudes.

Question 7

Can the sign of a coupling constant be determined from NMR spectra?

Accepted Answer

The sign of the coupling constant cannot be determined from NMR spectra. While the magnitude of coupling is evident from the fine structure observed in spectra, the positive or negative value of J remains invisible in standard one-dimensional NMR. Determining coupling sign requires specialized two-dimensional NMR techniques or additional experiments.

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