N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
4.1 Alkanes
81
Estimation of 13 C Chemical Shifts of Aliphatic Compounds (δ in ppm)
The chemical shifts of sp 3 -hybridized carbon atoms can be estimated with the help
of an additivity rule using the shift value of methane (-2.3 ppm) and increments (Z)
for substituents in α, β, γ, and δ position (see next pages). Some substituents occupy
two positions. Thus, the quaternary carbon atom c in the example given below is
in δ position relative to the carbon atom a since the sp 3 -hybridized oxygen of the
β-COO group occupies the γ position. This simple linear model needs corrections
in case of strong branching of the observed C atom and/or its neighbors (steric
corrections, S). Substituents for which such corrections are necessary are those with
varying branching, i.e., a varying number of directly bonded H atoms. They are
marked with an asterisk (*) in the Table of Increments (next page). Further correction
terms are needed if γ substituents are in a sterically fixed position (conformational
corrections, K).
The chemical shifts estimated with this additivity rule, in general, differ by less
than ≈4 ppm from the experimental values. Larger discrepancies may be expected
for highly branched systems (particularly for quaternary carbon atoms). For carbon atoms bearing several halogen, oxygen, and/or other strongly deshielding
substituents, additional correction terms are needed [1]. Without such corrections,
deviations can be so large as to render the rule useless.
Example: Estimation of chemical shifts for N-(tert-butoxycarbonyl)alanine
a base value
-2.3
b
base value
-2.3
1 α-C
9.1
1 α-C
9.1
1 α-COOH
20.1
1 β-COOH
2.0
1 α-NH
28.3
1 β-NH
11.3
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
estimated
16.2
estimated
53.8
exp
17.3
exp
49.0
c
base value
-2.3
d
base value
-2.3
3 α-C
27.3
1 α-C
9.1
1 α-OCO
56.5
2 β-C
18.8
1 γ-NH
-5.1
1 β-OCO
6.5
1 δ-C
0.3
1 δ-NH
0.0
3 S(quat,1)
-4.5
1 S(prim,4)
-3.4
estimated
72.2
estimated
28.7
exp
78.1
exp
28.1
O
N
O
H
OH
O
a
b
c
d
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
4.1 Alkanes
81
Estimation of 13 C Chemical Shifts of Aliphatic Compounds (δ in ppm)
The chemical shifts of sp 3 -hybridized carbon atoms can be estimated with the help
of an additivity rule using the shift value of methane (-2.3 ppm) and increments (Z)
for substituents in α, β, γ, and δ position (see next pages). Some substituents occupy
two positions. Thus, the quaternary carbon atom c in the example given below is
in δ position relative to the carbon atom a since the sp 3 -hybridized oxygen of the
β-COO group occupies the γ position. This simple linear model needs corrections
in case of strong branching of the observed C atom and/or its neighbors (steric
corrections, S). Substituents for which such corrections are necessary are those with
varying branching, i.e., a varying number of directly bonded H atoms. They are
marked with an asterisk (*) in the Table of Increments (next page). Further correction
terms are needed if γ substituents are in a sterically fixed position (conformational
corrections, K).
The chemical shifts estimated with this additivity rule, in general, differ by less
than ≈4 ppm from the experimental values. Larger discrepancies may be expected
for highly branched systems (particularly for quaternary carbon atoms). For carbon atoms bearing several halogen, oxygen, and/or other strongly deshielding
substituents, additional correction terms are needed [1]. Without such corrections,
deviations can be so large as to render the rule useless.
Example: Estimation of chemical shifts for N-(tert-butoxycarbonyl)alanine
a base value
-2.3
b
base value
-2.3
1 α-C
9.1
1 α-C
9.1
1 α-COOH
20.1
1 β-COOH
2.0
1 α-NH
28.3
1 β-NH
11.3
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
estimated
16.2
estimated
53.8
exp
17.3
exp
49.0
c
base value
-2.3
d
base value
-2.3
3 α-C
27.3
1 α-C
9.1
1 α-OCO
56.5
2 β-C
18.8
1 γ-NH
-5.1
1 β-OCO
6.5
1 δ-C
0.3
1 δ-NH
0.0
3 S(quat,1)
-4.5
1 S(prim,4)
-3.4
estimated
72.2
estimated
28.7
exp
78.1
exp
28.1
O
N
O
H
OH
O
a
b
c
d