N
X
C
O
N
S
Hal
Solvents
Natural
Products
P Si
C
C
C C
C
C
84
4 13 C NMR
84
4 13 C NMR
Example: Estimation of the chemical shifts for the carbon atoms a and b in N-(tertbutylcarbonyl)alanine (left) using the chemical shifts of valine (rigth) as base values
(a', b'):
4.1.2 Coupling Constants
13 C- 1 H Coupling Constants
Coupling through one bond ( 1 J CH in Hz)
The 13 C- 1 H coupling constant of 125 Hz in methane increases in the presence of
electronegative substituents and can be estimated by using the following additivity
rule:
Example: Estimation of the 13 C- 1 H coupling constant in CHCl 3 :
J = 125.0 + 3 × 27.0 = 206.0 Hz (exp: 209.0 Hz).
H 2 N
OH
O
O
N
O
H
OH
O
a
a'
b
b'
c
d
J CHZ 1 Z 2 Z 3
= 125.0 + ∑ Z i
i
Substituent
Increment Z i
Substituent
Increment Z i
–H
0.0
–Br
27.0
–CH 3
1.0
–I
26.0
–C(CH 3 ) 3
-3.0
–OH
18.0
–CH 2 Cl
3.0
–O–phenyl
18.0
–CH 2 Br
3.0
–NH 2
8.0
–CH 2 I
7.0
–NHCH 3
7.0
–CHCl 2
6.0
–N(CH 3 ) 2
6.0
–CCl 3
9.0
–C– – – N
11.0
–C – – – C
7.0
–S(O)CH 3
13.0
–phenyl
1.0
–CHO
2.0
–F
24.0
–COCH 3
-1.0
–Cl
27.0
–COOH
5.5
a
base value (a')
61.9
b base value (b')
30.3
1 β-COO
2.0
1 γ-COO
-2.8
1 δ-C
0.3
1 S(prim,3)
-1.1
1 S(tert,2)
-3.7
-2 α-C
-18.2
- 2 β-C
-18.8
-1 S(tert,3)
8.5
-1 S(tert,3)
8.5
estimated
16.7
estimated
50.2
exp
17.3
exp
49.0