What factors affect coupling constants?

Jun 25, 2015

The major factors affecting coupling constants are dihedral angles, substituents, hybridization, and ring strain.

Explanation:

J_("H-H") "Coupling"

The major factors for three-bond couplings between vicinal $\text{H}$ atoms (${J}_{\text{H-H}}$) are the dihedral angle and substituents.

(a) Dihedral Angles

${J}_{\text{H-H}}$ is greatest (7 to 15 Hz) when the dihedral angleis 0 ° (syn) or 180 °(anti), less
(2 to 5 Hz) for a gauche conformation and 0 for a 90 ° angle

(b) Substituent effects

Electronegative substituents decrease the value of ${J}_{\text{H-H}}$.

For example, ${J}_{\text{H-H}}$ = 8.9 Hz for $\text{CH"_3"CH"_2"-Li}$ and 4.7 Hz for ${\text{CH"_3"CH"_2"-"stackrel(+)("O")"R}}_{2}$.

Also, ${J}_{\text{H-H}}$ = 23.9 Hz for trans-$\text{CH"_2"=CH-Li}$ and 12.8 Hz for trans-$\text{CH"_2"=CH-F}$.

J_("C-H") "Coupling"

The major factors for one-bond $\text{C-H}$ couplings are substituent effects, hybridization, and ring strain.

(a) Substituent Effects

Electronegative substituents increase the value of ${J}_{\text{C-H}}$.

For example, ${J}_{\text{C-H}}$ = 125 Hz for ${\text{H-CH}}_{3}$, 149 Hz for $\text{H-CH"_2"F}$, 184 Hz for ${\text{H-CHF}}_{2}$, and 239 Hz for ${\text{H-CF}}_{3}$.

(b) Hybridization

${J}_{\text{C-H}}$ increases as the amount of $s$ character in the $\text{C-H}$ bond increases.

Typical values are 125 Hz for $s {p}^{3}$, 167 Hz for $s {p}^{2}$, and 250 Hz for $s p$ hybridization.

(c) Ring Strain

Strained rings show unusually large $\text{C-H}$ couplings.

This is consistent with the idea that the $\text{C-H}$ bonds in strained rings have a high degree of $s$ character.

Typical values are 127 Hz for cyclohexane, 161 Hz for cyclopropane, and 228 Hz for the alkene $\text{C-H}$ bond of cyclopropene.