This is actually a misnomer because the plot is 3-dimensional with the tops of the signals cut off to yield a 2-D contour plot.
Will not focus specifically on how the technique works and limit the majority of the discussion to interpretational uses. As a brief description of what is occurring, the nuclei are deflected in 1 frame and while deflected will be deflected again ( the frequencies 1 and 2 are represented by F1 and F2).
There are a variety of techniques that may be employed to yield structural information. From this information, the number of hydrogens per carbon may be determined; the spin-spin splitting between the hydrogen signals may be seen; which hydrogen signal goes with which carbon signal; and a more detailed look at hydrogen splitting patterns may be conducted.
HOM2DJ H - H connectivity
COSY H - connectivity
APT & DEPT # of H's attached to C
HET2DJ C - H Coupling
HETCOR H to C correlation
Since the book has a rather good presentation for these techniques, the ensuing text will use geraniol as an example.
This technique is used to look at the multiplicities of protons. With many standard NMR spectra it may be difficult in many cases to see the actual splitting pattern. In order to obtain this type of spectrum, the multiplicities are plotted on one axis vs the chemical shifts on the other. This results in a contour graph showing overall multiplicities and helps to identify the splitting patterns.
The actual hydrogen-1 spectrum is provided on page 271, Figure 6.5. This is the spectrum given below. This is a 500 MHZ spectrum which is a very high resolution spectrum. For example, look at the signals they have listed as 2 and 6. On the 500 MHZ spectrum they appear as triplets.
Now, look at the same signals on the HOM2DJ spectrum, Figure 6.7 on page 273.
The groups of signals circled in red are those of the triplets that are easily seen in the main spectrum. However, notice that each group is composed of 4 smaller signals. There is an insert on Fig 6.5 for these signals that show further coupling. From the HOM2DJ spectrum we see this further splitting results in smaller quartets. This is actually a complex spectrum and is composed of triplets of quartets.
Notice that in each case the signal is due to a hydrogen-1 that is vinyl and attached to a CH2 group directly adjacent to it. In the other direction there is a vinylic carbon that has no hydrogens attached. There is at least 1 CH3 group attached to the vinylic carbon that has no hydrogen on it. This is an example of long range coupling and this does manifest itself when pi systems are involved. The signal for number 6 is further complicated by a second CH3 group exhibiting long range coupling.
Instead of the basic COSY spectrum, the double quantum filtered COSY, DFQCOSY, will be used as an example. While this technique loses some of the finer information, the result is a better resolved spectrum.
Notice on Figure 6.9, page 275, that the hydrogen-1 spectra are plotted in both the X-axis and the Y-axis. This allows us to plot the H to H connectivity. The actual spectrum appears along the diagonal.
Notice the little contours that are off the diagonal. These may be correlated to yield coupling information.
Look at the smaller red box that has been drawn. Start at the signal 2 on the diagonal and go to the right until the first off diagonal contour is contacted. Now go vertically until the diagonal is met. This shows rather strong coupling between 1 and 2 (based on the size of the contours). Notice the same result is obtained if we started at signal 1 and go to the left, then down. Basically, a box will be formed.
This also helps identify long range coupling. Look at the blue lines between signals 5 and 6. As expected they are strong signals. However, the red lines extend out to 2 very small contours. They are for signals 9 and 10. This shows a small degree of coupling between non-adjacent nuclei.
The COSY spectra may be used to map which nuclei are in close proximity to one another due to their signals.
They are used to determine the connectivities of the hydrogens and they will routinely illustrate the long range coupling
In this case the F1 and F2 are such that 2 spectra may be correlated by how the couplings change as the frequencies change
Keep in mind that the larger the contour the stronger the coupling and that long range coupling is shown by the smaller contours.
This is a technique that is used to looks at the number of protons attached to the carbons. The signals get deflected up/down based on multiplicities of the carbon-13 peaks.
Since there are cleaner methods of obtaining this information, this technique will not be covered in deta
This technique is similar to HOM2DJ spectrum except it show the multiplicities of the carbon-13 signals. Overall, it shows how many hydrogens are attached to each carbon. The same general approach as used with the HOM2DJ spectra is used.
Using the spectrum in Figure 6.12 on page 279 as the example, a variety of signals are highlighted in red. Starting on the left, the first 2 are single signals which is the same as an off-resonance singlet (meaning no hydrogens attached). The third signal is a doublet which indicates a carbon with 1 hydrogen. Look very closely at this set of signals and you will see that it actually accounts for 2 different carbons with very similar chemical properties. The remaining (circled) signals illustrate a CH2 group and 2 different CH3 groups. Handle this type of spectrum like you would an off-resonance spectrum.
This technique correlates the carbon signals with the signals of the hydrogens (& vice-versa). This allows us to properly identify each nucleus.
Using the data from Figure 6.13 on page 279, a number of interesting observations may be made. In this case, the carbon-13 spectrum is the X-axis and the hydrogen-1 spectrum is the Y-axis.
In order to correlate the signals, simply draw a line straight down from a C-13 or across from a H-1 signal. The contour at which they connect shows which C-13 signal correlates to which H-1 signal.
For example, look at the C-13 signal at 58 ppm (the dotted blue line) and draw a line to the contour. At this point, draw a horizontal line to the H-spectrum. This connects with the signal at 4.2 ppm on the H-1 spectrum.
However, something strange happens when the 2 signals between 135-140 ppm are evaluated. Drawing a line straight down no contour is encountered. Also, the result occurs when the signal on the H-1 spectrum at 2.8 ppm is evaluated. When this happens there is no correlation between the two spectra.
The reason signals at 135-140 ppm have no contour is that there are no hydrogens attached to them (the vinylic carbons with no hydrogens). The reason the H-1 signal at 2.8 ppm has no contour is that it has no carbon attached (the O-H group itself)
Before you start working on the multidimensional aspects of the spectrum, be certain that you don't lose the basic information that is provided. Notice the molecular formula is provided, as well as, the hydrogen count under each signal. Remember that multidimensional spectra provide ADDITIONAL information to a typical NMR spectrum (and not different information).
The calculation for sites of unsaturation yields 1 site. This could be a double bond (C=C or C=O) or a ring structure.
We can rule out a C=C due to the chemical shifts of the signals. There are no signals in the 5-6 ppm range. However, there are two CH2 groups in the 2-2.3 ppm range which is typical for hydrogens on carbons that are alpha to C=O groups. This suggests we have a ketone.
We can rule out the aldehyde because there is no signal in the 9.7-10 ppm range which is due to the hydrogen on the aldehyde group.
A quick inventory shows he have four CH2's and 2 CH3's. Adding the units that are present we find that only a C and an O are missing (further proof of an C=O).
Keep in mind that CH3 groups are capping groups because they end chains or branches. Also notice that we haven't considered the contours of the 2-D spectrum ... yet!
The only structural that is easily gleaned from NMR spectrum is that each of the CH3 groups appear as a triplet which means they are both connected to a CH2 group (this rules out a methyl ketone)
Now we can look at the COSY to see what signals are connected. Look at the boxes that are drawn on the spectrum.
Start with the second triplet from the right. This signal is coupled to the CH2 group at 1.2 ppm. The CH2 at 1.2 ppm is coupled with the CH2 at 1.4 ppm. This signal is in turn coupled to the CH2 at 2.4 ppm. This means we have a fragment that looks like this CH3-CH2-CH2-CH2-.
Go to the first CH3 group (at 0.8 ppm). This signal is connected to the CH2 at 2.3 ppm to yield a CH3-CH2- fragment.
Since we still have a C=O to consider, all we have to do is connect these fragments to the C=O to yield 3-heptanone as the identity of the compound.