A 2D homonuclear correlation experiment used to analyse scalar (J) coupling networks between protons. It has a similar appearance to the 2D COSY spectrum. However, COSY crosspeaks are limited to identifying directly coupled spins, that is, those spins that share a mutual J-coupling, A-B. TOCSY is able to “relay” magnetisation between spins, A-B-C-D.., and can therefore show correlations amongst spins that are not directly coupled (eg A-C and A-D) but exist within the same spin system. This proves useful in the analysis of crowded spectra where correlations from a single resolved proton may be used to trace the coupling network. Popular for the analysis of peptides and oligosaccharides where molecules are typically composed of discrete subunits (spin systems) ie. amino-acids or saccharide units.
