DIMENSIONS IN NMR SPECTROSCOPY

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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
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.

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

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

2. Caryophyllene oxide 2.1 COSY spectrum of caryophyllene oxide 2.2 Expanded view of COSY spectrum of caryophyllene oxide        chemical shift (ppm) indicated protons correlations 4.99 olefinic coupled to each other 4.81 olefinic 2.86 methine coupled to 2 resonances at 1.28 and 2.23 ppm 2.60 allylic methine coupled to 3 resonances (methine 1.76 ppm and methylene 1.43, 1.47 ppm) 2.37, 2.11 methylene coupled to 2 resonances at 2.23 and 1.28 ppm 2.23, 1.28 methylene coupled to protons at 2.11, 2.37 and 2.86 ppm 2.06, 0.95 methylene coupled to 2 resonances at 1.45 and 1.63 ppm 1.76 methine coupled to proton at 1.45 ppm 1.63, 1.45 methylene coupled to 2 resonances at 0.95 and 2.06 ppm 1.47, 1.43 methylene coupled to allylic methine at 2.60 ppm 1.19 methyl - 1.01 methyl coupled to each other 0.98 methyl 2.3 HETCOR spectrum of caryophyllene oxide        13C chemical shift (ppm) 1H chemical shift (ppm) indicated part of structure 153 - quarternary C-8 113 4.81, 4.99 methylene C-12 63.6 2.86 methine C-5 59.7 - quarternary C-4 50.9 1.76 methine C-1 48.7 2.60 methine C-9 39.8 1.43, 1.47 methylene C-10 39.2 0.95, 2.06 methylene C-3 34.0 - quarternary C-11 30.1 1.28, 2.23 methylene C-6 30.0 not available methyl C-13 29.9 2.11, 2.37 methylene C-7 27.2 1.45, 1.63 methylene C-2 22.6 not available methyl C-14 or C-15 16.9 not available methyl C-14 or C-15

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