N
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
174
5 1 H NMR
5.2 Alkenes
5.2.1 Ethylenes
1 H Chemical Shifts and Coupling Constants (δ in ppm, J in Hz)
Geminal and Vicinal Couplings of Alkenes (J in Hz)
The values of the coupling constants strongly depend on the electronegativity
of the substituents (see Table on pp. 176, 177). They decrease with increasing
electronegativity and number of electronegative substituents. The same trend holds
for the signed values of geminal coupling constants but not for the absolute values
because J gem can be positive or negative. Although the total ranges of cis and trans
vicinal coupling constants overlap, J trans > J cis always holds for given substituents.
Typical ranges:
J gem -4 to 4
J cis
4 to 12
J trans 14 to 19
H
CH 3
H 3 C
H
H
CH 3
H
H 3 C
H
CH 3
H
H
2 J gem 2.5
3 J cis 11.6
3 J trans 19.1
4 J ab -1.7
3 J ac 15.1
3 J ad
6.5
5 J bd
1.6
3 J ab 10.3
3 J ac 17.2
3 J ad 6.2
2 J bc 2.0
4 J bd -1.3
4 J cd -1.7
3 J ab 10.0
3 J ac 16.8
3 J ad 6.4
2 J bc 2.1
4 J bd -1.3
4 J cd -1.8
3 J ab 10.9
4 J ac -1.8
3 J ad 6.8
5 J cd 1.2
5.55
4.87
5.78
4.94
2.00 1.00
1.58
a
a
b
c
c
5.37
1.54
a
b
c
d
e
d
d
4.88
5.73
1.72
4.97
a
b
c
d
b
H
H
H
H
H
CH 2
H
H
CH 3