18 - TOCSY - TOCSY • We have seen through the COSY...

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1 TOCSY We have seen through the COSY experiment that we can correlate spins that are J -coupled. That is, cross peaks are present for all coupled spins. These can be observed over 2, 3, and sometimes 4 bonds (see any ring structure for the long range coupling). With TOCSY (total correlation spectroscopy), we can transfer magnetization between spins that share a common coupling partner: CH 3 CH 2 CH 2 Cl 1 2 3 1 is coupled 2 and 2 is coupled to 3 but 1 and 3 are not coupled. In a TOCSY experiment, we can correlate 1 with 3. An example: COSY map:
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2 How do we do that? This is very much like a relay race. When two spins are coupled, we can get them to transfer magnetization. If spin system 1 has transferred to 2, then 2 can transfer to 3 and so on (as long as there is coupling between each spin involved). We do this with a mixing period that we call a spin-locking period or an isotropic mixing period. What is going on during spin-locking?
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3 That is, coupling is active and chemical shift is not.
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