N
X
C
Hal
O
N
S
P Si
Natural
Products
Solvents
C
C
C
C C
C
253
5.14 Spectra of Solvents and References
253
5.14.2 1 H NMR Spectra of Secondary Reference Compounds
Chemical shifts in 1 H NMR spectra are usually reported relative to the peak position of tetramethylsilane (TMS) added to the sample as an internal reference. If
TMS is not sufficiently soluble, a capillary with TMS may be used as external reference. In this case, owing to the different volume susceptibilities, the local magnetic
fields in the sample and reference differ, and the peak position of the reference must
be corrected. For a D 2 O solution in a cylindrical sample and neat TMS in a capillary, the correction amounts to +0.68 and -0.34 ppm for superconducting and electromagnets, respectively. These values must be added to the chemical shifts relative
to the external TMS signal if its position is set to 0.00 ppm. Alternatively, secondary
references with (CH 3 ) 3 SiCH 2 groups may be used. The following spectra of two
such secondary reference compounds in D 2 O were measured at 500 MHz with TMS
as external reference. Chemical shifts are reported in ppm relative to TMS upon
correction for the difference in the volume susceptibilities of D 2 O and TMS. As a
result, the peak for the external TMS appears at 0.68 ppm.
Water-d 2
10
9
8
7
6
5
4
3
2
1
0
4.80
0.68 TMS
(external
reference)
3-(Trimethylsilyl)-1-propanesulfoninc acid sodium salt (sodium- 4,4-dimethyl -
4-silapentane-5-sulfonat; DSS
10
9
8
7
6
5
4
3
2
1
1.8
1.7
3.0
2.9
0.02
0.68 TMS
(external
reference)
2.93
1.78
4.80 HDO
Si
SO 3 Na
H 3 C CH 3
H 3 C
0
0
10
9
8
7
6
5
4
3
2
1
0.00
0.68 TMS
4.80 HDO
Si
COONa
H 3 C
H 3 C
H 3 C DD
D
D
2,2,3,3-D 4 -3-(Trimethylsilyl)propionic acid sodium salt
(external
reference)