TOCSY is the abbreviation for TOtal Correlation SpectroscopY. Also called HOHAHA (HOmonuclear HArtmann HAhn). During this pulse sequence, after the evolution period t1, the magnetization is spin-locked (for example by a series of 180` pulses). During this mixing time the magnetization exchange through scalar coupling. During this spin-lock period, the magnetization behaves as a strongly coupled spin system and evolve under the influence of a "collective spin-mode". In that collective mode, coherence transfer is possible between all coupled nuclei in a spin system, (even if the are not directly coupled). One of the most popular mixing scheme in TOCSY is the MLEV-17.
For small mixing period, (e.g. 20-30 msec) Relay1-COSY type of data can be observed. As the mixing period gets longer, correlation with more distant protons can be observed (e.g. mix=80-100 msec can correlate H1 to H6 in carbohydrate). The extent of correlation depends mainly on the length of the mixing period.
This sequence is much more useful than the Relay-COSY as it very efficiently transfer magnetization before relaxation takes place.
If you want to see how to analyze a 2D spectra like above, check this animation . (this animation shows COSY, TOCSY, NOESY, Homonuclear 2DJ, HMQC and HMBC analysis)
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