The 2D COSY (COrrelation SpectroscopY) experiment is the most simple and widely used 2D experiment. It is an homonuclear chemical shift correlation experiment based on the transfer polarization by a mixing pulse between directly J-coupled spins. Thus, homonuclear through-bond interactions can be trace out by simple analysis of the 2D map providing a more general and more useful alternative to classical 1D homodecoupling experiments.

Easy implementation on any AVANCE spectrometer.

The 2D COSY experiment ( 71JEENER , 76JCP2229 , and 81JMR542-44 ) is a simple two-pulse sequence in which a variable evolution period is inserted between these two pulses (see COSY block). During this period, both J-coupling and chemical shift evolutions take place (after fourier transformation will give the F1 dimension). The signal is detected as usual (F2 dimension).

The basic COSY experiment can be performed in magnitude or phase-sensitive modes by selecting the appropriate phase programs and transform algorithm. In addition, a lot of versions of COSY experiment exist as, for instance:

• Delayed or Long-Range COSY experiment (or COSYLR): Allows to detect small or long-range H-H couplings with reasonable cross-peak intensity by  inserting fixed evolution delays into the variable evolution period and just prior to detection of the conventional 2D COSY experiment  ( 81JMR542-44 ).
• Multiple-quantum filtered COSY experiment (or COSYMQF).
• Exclusive and Purgued COSY experiments:
• P.COSY ( 88JMR528-80 ).
• P.E.COSY ( 87JMR191-72 and 88JMR429-79 ): It's based in the COSY-b experiment and affords a COSY spectrum with simplified multiplet structure, allowing for the accurate measurements of homonuclear coupling constants. Thus a COSY-45 experiment gives good suppression of remote connectivities but undesired strong and dispersive diagonal peaks are still present and cross-peaks are weak. If a COSY-0 reference spectrum is acquired, only diagonal peaks are present. When this FID is substracted from the original, the dispersive character of the diagonal is largely canceled, but cross-peaks signals remain. These E.COSY-like spectra permits to measure accurate proton-proton coupling constants. However, the long rotational correlation times in large molecules render the linewidths comparable to the J-splittings and the validity of the coupling constant measurements has been questioned ( 93JMRA106-104 , 93JACS3026 and 94JMRA195-111 ). A recent work dealing with these errors has been published ( 93JMRA106-104 ).
• z-filtered COSY experiment (or z-COSY) ( 86JMR559-69 , 87JMR475-72 , and 92JMR191-96 ). A z-filter (delay-90º(1H) block) is inserted prior to acquisition in order to remove dispersive signal components for both diagonal and cross peaks.
•  Constant-time COSY experiment (also COSYDEC) ( 81JMR542-44 , 84JMR513-57 , 94JMRA99-108 , and 01JMR242-151 ). By inserting a movable 180º pulse in the variable evolution t1 period it is possible to completely suppress couplings in the F1 dimension or to downscale them.
•  SUPERCOSY ( 84JMR143-60 , 85JMR123-62 , 85JMR107-63 , 85MRC470 , and 87JMR417-73 ) and  ISECR COSY ( 90JMR1-86 and 91JMR244-95 ) experiments. Removing of antiphase character from the off-diagonal peaks, by transfer it into the diagonal peak.
• Incorporation of solvent presaturation.
•  Gradient-enhanced versions of COSY experiments: ge-2D COSY and ge-2D COSY-MQF experiments.
• COSY experiments can also be applied to other nuclei than 1H as, for instance, 11B, 13C ...

The COSY experiment is usually recorded in routine and automation modes. For optimal sensitivity, a mixing pulse of 60 degree is recommended, while of 45 degree improves the intensity ratios and diagonal streamlining effect with only a moderate loss in signal-to-noise ratio. Magnitude or phase-sensitive data will be chosen according to the desired resolution. In spectrometers equipped with gradient technology, gradient-based COSY versions are highly recommended.
Details on practical implementation of several variants of the 2D COSY experiment on AVANCE spectrometers can be found in the corresponding tutorials:

Tutorials: 2D homonuclear experiments

Tutorials: 2D gradient-based homonuclear experiments

Tutorials: 2D gradient-based homonuclear experiments in H₂O

A COSY spectrum correlates chemical shifts of the same nucleus in both dimensions. Three main peaks can be present:

• Diagonal peaks (showing in-phase negative pure dispersion pattern in phase-sensitive spectra).
• Off-diagonal peaks (showing anti-phase pure absorption pattern with respect to the active J-coupling, and in-phase with all passive J-couplings in phase-sensitive spectra). These peaks reveal J-coupling connectivity.
• Axial peaks (appearing a F1=0) that are efficiently removed by a proper phase cycle.

Quantitation of scalar coupling constants can be performed from COSY spectra by analyzing the fine structure of cross peaks. However, when the multiplet structure is not well resolved due to low spectral resolution or partial cancelation of multiplet lines, the constant values are more conveniently measured from E.COSY-type spectra.

The theoretical background of the COSY experiment has been extensively described and discussed in many books and review articles. Extensive descriptions can be found in 88BOOK1 (pages 58-94), 94BOOK1 (pages 295-315), 96ENC1455 and 96ENC1462 .

Related experiments:

2D homonuclear experiments

2D gradient-based homonuclear experiments