Chemistry Reference
In-Depth Information
Table 7.1
Typical values of proton NMR chemical shifts
Type of proton
Approximate
d
value
Alkane
a
CH
3
0.9
Alkane
a
CH
2
a
CH
3
1.3
Alkane
a
CH
a
1.4
O
2.1
C
CH
3
CCH
2.5
R
a
CH
2
a
X
(X
halogen,
a
O
a
3.4
5-6
CCH
1.7
CC
3
Ph
a
H
7.2
Ph
a
CH
3
2.3
R
a
CHO
9-10
R
a
COOH
10-12
R
a
OH
Variable, about 2-5
Ar
a
OH
Variable, about 4-7
R
a
NH
2
Variable, about 1.5-4
and arises due to the phenomenon of spin-spin coupling. When a sample is
placed into a magnetic field, the NMR-active nuclei align themselves either
with the external field (i.e. in the same direction) or opposed to the external
field (in the opposite direction). This means that the field experienced by a
nucleus depends on whether neighbouring nuclei align to augment the
applied field or to decrease it. The result is that the original signal now
arises from nuclei experiencing two different values of chemical shift and
appears to split into two distinct signals. In proton NMR, when there is one
neighbouring proton, the signal is split into two peaks of equal intensity (a
'doublet'); when there are two neighbouring protons, the signal is split into
three signals (a 'triplet') with intensities 1:2:1; and if there are three
neighbouring identical protons, the signal is split into four peaks (a
'quartet') with intensities 1:3:3:1, and so on. Figure 7.23 illustrates this
effect for two neighbouring hydrogen atoms.
