1H-1H COSY (Correlation Spectroscopy)

1H-1H COSY (Correlation Spectroscopy)

COSY

COSY (COrrelation SpectroscopY) was one of the first and most popular 2D NMR experiments to be developed. It is a homonuclear experiment that allows one to correlate different signals in the spectrum to each other. In a COSY spectrum (see Figureb), the chemical shift values of the sample’s 1D NMR spectrum are plotted along both the vertical and horizontal axes (some 2D spectra will actually reproduce the 1D spectra along the axes, along with the frequency scale in ppm, while others may simply show the scale). This allows for a collection of peaks to appear down the diagonal of the spectrum known as diagonal peaks (shown in Figureb, highlighted by the red dotted line). These diagonal peaks are simply the peaks that appear in the normal 1D spectrum, because they show nuclei that couple to themselves. The other type of peaks appears symmetric across the diagonal and is known as cross peaks. These peaks show which groups in the molecule that have different chemical shifts are coupled to each other by producing a signal at the intersection of the two frequency values.

Example of correlation spectroscopy: (a) On the left is shown a portion of a 3D or “stacked” plot of a 2D NMR COSY spectrum in which two frequency domains are plotted in two dimensions and intensity is plotted in the third. On the right is shown a contour plot, where the intensities have been depicted topographically. Spectra from Acorn NMR, Inc. (b) A spectrum of the disaccharide xylobiose (structure shown), taken from a COSY 2D NMR experiment. The red dotted line highlights the diagonal peaks. Spectrum adapted from F. Sauriol, NMR Webcourse, Department of Chemistry, Queen’s University, Ontario, www.chem.queensu.ca/facilities/nmr/nmr/webcourse/.

One can then determine the structure of a sample by examining what chemical shift values the cross peaks occur at in a spectrum. Since the cross peaks are symmetric across the diagonal peaks, one can easily identify which cross peaks are real (if a certain peak has a counterpart on the other side of the diagonal) and which are digital artifacts of the experiment. The smallest coupling that can be detected using COSY is dependent on the linewidth of the spectrum and the signal-to-noise ratio; a maximum signal-to-noise ratio and a minimum linewidth will allow for very small coupling constants to be detected.

Variations of COSY

Although COSY is very useful, it does have its disadvantages. First of all, because the anti-phase structure of the cross peaks, which causes the spectral lines to cancel one another out, and the in-phase structure of the diagonal peaks, which causes reinforcement among the peaks, there is a significant difference in intensity between the diagonal and cross peaks. This difference in intensity makes identifying small cross peaks difficult, especially if they lie near the diagonal. Another problem is that when processing the data for a COSY spectrum, the broad lineshapes associated with the experiment can make high-resolution work difficult.

In one of the more popular COSY variations known as DQF COSY (Double-Quantum Filtered COSY), the pulse sequence is altered so that all of the signals are passed through a double-quantum coherence filter, which suppresses signals with no coupling (i.e. singlets) and allows cross peaks close to the diagonal to be clearly visible by making the spectral lines much sharper. Since most singlet peaks are due to the solvent, DQF COSY is useful to suppress those unwanted peaks.

ECOSY (Exclusive COrrelation SpectroscopY) is another derivative of COSY that was made to detect small J-couplings, predominantly among multiplets, usually when J ≤ 3 Hz. Also referred to as long-range COSY, this technique involves adding a delay of about 100-400 ms to the pulse sequence. However, there is more relaxation that is occurring during this delay, which causes a loss of magnetization, and therefore a loss of signal intensity. This experiment would be advantageous for one who would like to further investigate whether or not a certain coupling exists that did not appear in the regular COSY spectrum.

GS-COSY (Gradient Selective COSY) is a very applied offshoot of COSY since it eliminates the need for what is known as phase cycling. Phase cycling is a method in which the phase of the pulses is varied in such a way to eliminate unwanted signals in the spectrum, due to the multiple ways which magnetization can be aligned or transferred, or even due to instrument hardware. In practical terms, this means that by eliminating phase cycling, GS-COSY can produce a cleaner spectrum (less digital artifacts) in much less time than can normal COSY.

Another variation of COSY is COSY-45, which administers a pulse at 45° to the sample, unlike DQF COSY which administers a pulse perpendicular to the sample. This technique is useful because one can elucidate the sign of the coupling constant by looking at the shape of the peak and in which direction it is oriented. Knowing the sign of the coupling constant can be useful in discriminating between vicinal and geminal couplings. However, COSY-45 is less sensitive than other COSY experiments that use a 90° RF pulse.

 

1H-1H COSY is used for clearly indicate correlation with coupled protons. A point of entry into a COSY spectrum is one of the keys to predict information from it successfully. 

Relation of Coupling protons is determined by cross peaks(correlation peaks) and in the COSY spectrum.

In other words, Diagonal peaks by lines are coupled to each other. Figure  indicates that there are correlation peaks between proton H1 and H2 as well as between H2 and H4. This means the H2 coupled to H1 and H4. 

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Below is a portion of a 1H-1H COSY-45 spectrum for an unknown compound. The COSY data exhibits 5 correlations: 3 diagonal and 2 off-diagonal. The 2 off-diagonal correlations at (1.45, 2.36) and (2.36, 1.45) ppm indicate 4 possible 1H-C-1H structural configurations between the protons at 1.45 and 2.36 ppm. The issue is compounded by the fact that for each off-diagonal correlation, there are 4 possibilities to consider when trying to build a fragment(s). Keep in mind that COSY data by itself may not constitute adequate information to narrow down the possibilities. However, noting the multiple possibilities will ensure that nothing is overlooked.

 

Note: the intensity of the off-diagonal correlations, judged by the number of contours in respect to the diagonal correlations, may provide a clue in eliminating some of the possibilities. However, this is dependent on how the data is collected and/or on the dihedral angles of the coupled protons.

One of the trickiest parts of interpreting a 1H-1H COSY experiment is deciding how to classify the correlations. The goal of this puzzle is to assess the COSY correlations and narrow down a set of fragments that support the data.

An expansion of the aromatic region of a 1H NMR spectrum shows four methine multiplets at 6.61, 7.15, 7.22 and 7.58 ppm with coupling constants less than 2.5 Hz. The 1H-1H COSY spectrum below points to two off-diagonal correlations: between 6.61 and 7.15 ppm and another at 7.22 and 7.58 ppm. Are the correlations on the COSY experiment relating to 3JHH or 4JHH or 5JHH coupling responses?