.
Now many NMR techniques are now available for identifying molecular structures. Chemists can now clearly understand information about spin - spin coupling and the exact conectivity of atoms in molecules with techniques called multidimensional NMR spectroscopy. The most common techniques are two-dimensional NMR or 2D NMR such as COSY, HETCOR, and others.
In a COSY spectrum, a H 1 spectrum is shown along both horizontal and vertical axes, and the intensity of correlation peaks is shown as mountains.
Example : COSY spectrum of geraniol
1. Basic COSY spectrum of geraniol, in CDCl3 at 500 MHz
From the basic COSY spectrum, we can see that H-5 and H-6 are coupled by each other. However, the
signals for 3 methyl groups at C-8, C-9, and C-10 are severely
overlaped, as are those for the 2 methylene groups at C-4 and C-5.
Moreover, it lacks of the H-1----H-4
and H-6----H-8 couplings, and the differentiation between H-8 and H-9 is uncertain.
These problems can be less by using a double quantum filtered COSY (DQFCOSY); the intense singlets of noncoupled methyl groups are greatly reduced.
2. The DQFCOSY spectrum of geraniol, in CDCl3 at 500 MHz
In
the DQFCOSY spectrum, we can see that the H-8 and H-9 methyl proton
signals are clearly separated. The long-range coupling of H-8 and H-9
methyl groups with one another and the H-1----H-4 and H-6----H-8
couplings are present. However, the differentiation between H-8 and H-9
is still uncertain.
The HETCOR experiment correlates 13C nuclei with attached 1H nuclei; these are one-bond couplings.
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1D - NMR Spectrum
When
we talk about an 1D-NMR spectrum, we mean a spectrum (2-dimensional
graph) in which an abcissa axis shows the frequency (chemical shift) and
an ordinate axis shows the intensity of peaks.
2D - NMR Spectrum
When
we talk about a two-dimensional spectrum, we mean a spectrum
(3-dimensional graph) in which both abcissa and ordinate axes show
chemical shifts and the third dimension shows the intensity of the
peaks.
2-D J-resolved spectrum
F1 scalar coupling F2 chemical shift |
2D-correlated spectrum
F1 & F2 chemical shift withn scalar spin-spin or dipolar copling a) 1H vs 1H b) 1H vs 13C |
Now many NMR techniques are now available for identifying molecular structures. Chemists can now clearly understand information about spin - spin coupling and the exact conectivity of atoms in molecules with techniques called multidimensional NMR spectroscopy. The most common techniques are two-dimensional NMR or 2D NMR such as COSY, HETCOR, and others.
When 2D NMR is applied to 1H-NMR, it is called 1H-1H COrrelation SpectroscopY or COSY. COSY spectra are useful for deducing proton-proton coupling relationships.
2D spectra can be obtained to indicate coupling between hydrogens and the carbons to which they attached. In this case it is called 1H-13C HETeronuclear CORrelation spectroscopy or HETCOR.
In a COSY spectrum, a H 1 spectrum is shown along both horizontal and vertical axes, and the intensity of correlation peaks is shown as mountains.
The
important information from the COSY spectrum comes from the correlation
peaks (mountains) that appear off the diagonal (cross peaks). If
we start at a given cross peak and imagine that two perpendicular lines
lead back to the diagonal, these lines are coupled to each other. The
intersepted peaks indicate that they are coupled to each other. It
is found that the cross peaks above the diagonal are found
symmetrically so only cross peaks on one side of the diagonal need to be
interpreted.
Example : COSY spectrum of geraniol
1. Basic COSY spectrum of geraniol, in CDCl3 at 500 MHz
and H-6----H-8 couplings, and the differentiation between H-8 and H-9 is uncertain.
These problems can be less by using a double quantum filtered COSY (DQFCOSY); the intense singlets of noncoupled methyl groups are greatly reduced.
2. The DQFCOSY spectrum of geraniol, in CDCl3 at 500 MHz
The HETCOR experiment correlates 13C nuclei with attached 1H nuclei; these are one-bond couplings.
In a HETCOR spectrum, a 13C spectrum is shown along one axist and a 1H
spectrum is shown along the other axist. The cross peaks that relate
the two types of spectra to each other are found in the third dimension.
The cross peak indicates that the hydrogen giving rise to the 1H NMR signal on one axist is coupled (and attached) to the carbon that giving rise to the corresponding 13C NMR signal on the other axist. Therefore, we can indicate which hydrogens are attached to which carbon in a molecule.
Example : HETCOR spectrum of geraniol
The HETCOR spectrum of geraniol, in CDCl3 at 500 MHz for 1H and 125.7 for 13C
We
have the proton assigned from the COCY spectrum, so we can now
correlate them with the carbon atoms in the HETCOR spectrum of geraniol.
From high to low field in the 13C axis, we assign
- the methyl groups 10, 8, and 9
- the methylene 5, 4, and 1
- the alkene methines 6 and 2
- the quarternary carbon atoms 3 and 7 are not correlated with protons
- the methyl groups 10, 8, and 9
- the methylene 5, 4, and 1
- the alkene methines 6 and 2
- the quarternary carbon atoms 3 and 7 are not correlated with protons
1. Ipsenol
1.1 COSY spectrum of ipsenol
chemical shift (ppm)
|
indicated protons
|
correlations
|
6.35
|
olefinic
|
coupled to olefinic protons at d 5.08 ppm
|
5.08 (group)
|
olefinic
|
coupled to olefinic protons at d 5.35 ppm and methylene protons at d 2.22 and 2.48 ppm
|
3.83
|
carbinol methine
|
coupled to 4 protons corresponding to 2 adjacent methylene groups
|
2.48
|
methylene
|
coupled to carbinol methine proton and each other
|
2.22
|
methylene
|
|
1.82
|
isopropyl methine
|
coupled to 3 protons corresponding to 2 adjacent metnylene groups
|
1.80
|
hydroxylic
|
-
|
1.49
|
methylene
|
coupled to carbinol methine proton and each other
|
1.26
|
methylene
|
|
0.93
|
2 overlaping methyl doublets
|
coupled to isopropyl methine proton
|
1.2 HETCOR spectrum of ipsenol
13C chemical shift (ppm)
|
1H chemical shift (ppm)
|
indicated part of structure
|
143
|
-
|
olefinic (quarternary C)
|
138
|
6.35
|
olefinic
|
117, 119
|
5.08, 5.15
|
olefinic
|
117
|
5.24, 5.26
|
olefinic
|
69
|
3.83
|
carbinol methine
|
41
|
2.48, 2.22
|
methylene
|
25
|
1.82
|
isopropyl methine
|
-
|
1.80
|
hydroxylic
|
47
|
1.26, 1.49
|
methylene
|
22, 24
|
0.93
|
2 overlaping methyl doublets
|
|
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