N
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
172
5 1 H NMR
172
5 1 H NMR
5.1.2 Coupling Constants
Geminal Coupling Constants ( 2 J in Hz)
C
H
H
2 J HCH -8 to -18
Electronegative substituents cause a decrease in |J| gem while a double or triple bond
next to the CH 2 group causes an increase. The latter effect is strongest if one of the
C–H bonds is parallel to the π orbitals:
H
H
Influence of Substituents on the Geminal Coupling Constant
Compound
J gem
Compound
J gem
CH 4
-12.4
CH 3 COCH 3
-14.9
CH 3 Cl
-10.8
CH 3 COOH
-14.5
CH 2 Cl 2
-7.5
CH 3 CN
-16.9
CH 3 OH
-10.8
CH 2 (CN) 2
-20.3
CH 3
-14.3
CH 2 CN
-18.5
Vicinal Coupling Constants ( 3 J in Hz)
conformation not fixed: 3 J HCCH ≈ 7
fixed:
3 J HCCH ≈ 0–18
Influence of Substituents on the Vicinal Coupling Constant
Compound
J vic
Compound
J vic
Compound
J vic
CH 3 CHF 2
4.5
CH 3 CH 2 OH
6.9
CH 3 CH 2 CN
7.6
CH 3 CHCl 2
6.1
(CH 3 CH 2 ) 3 O + BF 4
– 7.2
(CH 3 CH 2 ) 2 S
7.4
CH 3 CH 2 F
6.9
(CH 3 CH 2 ) 3 N
7.1
(CH 3 CH 2 ) 4 Si
8.0
CH 3 CH 2 Cl
7.2
(CH 3 CH 2 ) 4 N + I –
7.3
CH 3 CH 2 Li
8.4
H
H