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Tuesday, July 22, 2008

1D Selective Gradient TOCSY as a Function of Mixing Time

Total Correlation SpectroscopY , TOCSY (see links here, here and here) is a technique that employs a spin lock during the mixing time of the sequence for which chemical shifts are invariant but J couplings evolve. The technique is used to correlate spins in the same J coupled spin system. The 1D selective TOCSY uses a shaped pulses to select a single spin and one gets a convenient 1D spectrum with all spins in the same spin system as the selected spin. The intensity of the coupled resonances depends on the duration of the mixing time as well as the magnitude of the coupling between spins. For short mixing times, one can often trace out the spin system by running a series of spectra as a function of the mixing time. The figure below shows a series of 1D selective gradient TOCSY spectra for 3-heptanone (selectively irradiated at the 6 position) collected as a function of mixing time. This molecule has two spin systems, separated by the carbonyl group, namely the protons in positions 6, 5, 4 and 3 and those in positions 1 and 2. With a short 10 msec mixing time, one observes the nearest protons at position 5. For 20 msec, one observes the protons at positions 5 and 4. For 30 msec and 50 msec mixing times, one observes all of the protons in the same spin system as the protons in the 6 position. Furthermore, one is able to observe the protons in the 3 position independent of the overlapping protons in position 2 which are not in the same spin system.

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