N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
4.1 Alkanes
83
Steric Corrections, S j
Conformational Corrections, K k , for γ Substituents
One can also use the chemical shifts of a reference compound as the base value if
its structure is closely related to that assumed for the unknown. The increments corresponding to the structural elements missing in the reference compound are then
added to the base value, while those of structural elements present in the reference
but absent in the unknown are subtracted (see example on next page).
Observed 13 C center
S for number of substituents at the α atom a
1
2
3
4
primary (CH 3 )
0.0
0.0
-1.1
-3.4
secondary (CH 2 )
0.0
0.0
-2.5
-6.0
tertiary (CH)
0.0
-3.7
-8.5
-10.0
quaternary (C)
-1.5
-8.0
-10.0
-12.5
a To be applied to each of the neighboring atoms that has an unspecified number of nonhydrogen substituents (marked with an asterisk (*) in the Table of Increments, Z i ).
Conformation
K
synperiplanar (eclipsed)
C X
-4.0
synclinal (gauche)
X
C
-1.0
anticlinal
C
X
0.0
antiperiplanar (anti)
X
C
2.0
not fixed
0.0