Question

What factors affect coupling constants?

Answer 1

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 `"H"` atoms (`J_("H-H")`) are the dihedral angle and substituents.

(a) Dihedral Angles

`J_("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_("H-H")`.

For example, `J_("H-H")` = 8.9 Hz for `"CH"_3"CH"_2"-Li"` and 4.7 Hz for `"CH"_3"CH"_2"-"stackrel(+)("O")"R"_2`.

Also, `J_("H-H")` = 23.9 Hz for trans-`"CH"_2"=CH-Li"` and 12.8 Hz for trans-`"CH"_2"=CH-F"`.

`J_("C-H") "Coupling"`

The major factors for one-bond `"C-H"` couplings are substituent effects, hybridization, and ring strain.

(a) Substituent Effects

Electronegative substituents increase the value of `J_("C-H")`.

For example, `J_("C-H")` = 125 Hz for `"H-CH"_3`, 149 Hz for `"H-CH"_2"F"`, 184 Hz for `"H-CHF"_2`, and 239 Hz for `"H-CF"_3`.

(b) Hybridization

`J_("C-H")` increases as the amount of `s` character in the `"C-H"` bond increases.

Typical values are 125 Hz for `sp^3`, 167 Hz for `sp^2`, and 250 Hz for `sp` hybridization.

(c) Ring Strain

Strained rings show unusually large `"C-H"` couplings.

This is consistent with the idea that the `"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 `"C-H"` bond of cyclopropene.