Two Dimensional Experiments

There are two types of two dimensional experiments: through-bond and through-space.

Through-Bond Experiments:

Through-bond experiments correlate a spin 1/2 nucleus (virtually always proton) with another spin 1/2 nucleus that are coupled to each other directly or indirectly through another nucleus. There are two types of through-bond experiments: homonuclear and heteronuclear.

Homonuclear Through-Bond Experiments:

Homonuclear through-bond experiments correlate 2 protons with each other.

COSY experiments correlate protons that are directly couple to each other (the protons are separated by less than ~3 bonds or 4 bonds in some cases). In COSY experiments there will be a crosspeak in the 2D that corresponds to the two coupled protons. Coupling constants can be measured in some types of COSY experiments, particularly DQF-COSY or E-COSY, but not in the default COSY experiment.

TOCSY experiments correlate protons that are directly AND indirectly coupled to each other, meaning if proton A is coupled to B and B is coupled to C, but A is not couple to C, then in a COSY there will a crosspeak from A to B and B to C but not A to C, whereas in a TOCSY, there will crosspeaks from A to B and C, from B to A and C, and from C to A and B.

Heteronuclear Through-Bond Experiments:

Heteronuclear through-bond experiments correlate a proton and an X nucleus, usually ¹³C, ¹⁵N, or ³¹P.

HSQC and HMQC experiments correlate a proton and the X nucleus that it is covalently bound to. There is no diagonal in this experiment, simply one peak for every proton in the molecule that is bound to the X nucleus that you are indirectly detecting. For example, in a ¹³C/¹H HSQC, nitrogen bound protons will not be in the spectrum.

HMBC and Heteronuclear COSY experiments correlate protons with X nuclei that are within 3-4 bonds. The HMBC filters out the X nuclei that are directly bound to the proton, the Heteronuclear COSY does not.

Through-Space Experiments:

Through-space experiments correlate protons with each other through-space, so protons that are far apart in the molecule through-bond but are close in 3 dimensional space (~< 0.5 Angstroms) will have an NOE to each other. There are two types of through-space 2D experiments- NOESY and ROESY. NOESY is the choice for molecules > ~2000 MW, ROESY is the better choice for molecules < ~2000 MW. Crosspeaks between two protons indicate that they are close in space. The intensity of the crosspeak is ~ inversely proportional to the distance between the protons.