What factors affect coupling constants?

1 Answer
Jun 25, 2015

Answer:

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

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(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.