TOCSY mixing

Figure 6.12 2D TOCSY spectrum with 31P decoupling acquired at row 11 (stop at 3.3 min) spectral size, 4k x 160 data points 8 scans per increment 65 ms TOCSY mixing time...

Figure 6.18 Stop-flow 2D WET-TOCSY spectrum of 3,5-dichloro-2-hydroxybiphenyl. Protons (H4.H6) of the chlorinated ring can be identified due to the magnetization transfer between the signals at 7.23 (H4) and 7.41 (H6) ppm. Conditions spectrometer, Bruker DRX 600 probe head, 4mm z-gradient LC probe. 13C decoupling was applied during the WET-pulse train and acquisition. The spectral size was 8k x 512 data points, with 16 scans per increment, and a 64 ms TOCSY mixing time...

If we used a continuous-wave spin lock (ROESY mixing scheme) for TOCSY mixing, the only way efficient transfer could occur via 7 couplings within a spin system would be if the two protons have opposite resonance offsets (v0 - vr), where vr is the carrier or reference frequency. This is a special case of the Hartmann-Hahn match ... 

In both ROESY mixing and TOCSY mixing, the transfer is from in-phase net magnetization on the spin-lock axis to in-phase net magnetization on the spin-lock axis for example, ly ->1 (TOCSY) or ly - - Iy (ROESY). This is analogous to the NOESY transfer, It -> — for small molecules and in contrast to the INEPT transfer 21 -> 21 1 , which is antiphase to antiphase. 

Now consider what happens during the ideal TOCSY mixing scheme. There are no chemical shift differences but the / couplings are still active, so all protons in a spin system are extremely strongly coupled, like the CH2 groups in a long, straight-chain hydrocarbon. 

A selective TOCSY experiment starts with putting the net magnetization of just one resonance in the x -y plane and locking it with the TOCSY mixing spin lock. After an appropriate mixing time, the spin lock field is turned off and we simply start acquiring the FID. These steps can be summarized as follows ... 

Mixing Apply the TOCSY mixing sequence (spin lock) on the y axis for a period of time between 30 and 85 ms. 

Factors That Affect TOCSY Mixing Efficiency... 

Exercise Estimate the Varian power level settings for TOCSY mixing (8333 Hz), for ROESY mixing (3333 Hz), and for H decoupling (2778 Hz) if the 90° pulse is 21.3 jjis at a power setting of 59 dB. 

If we apply the TOCSY spin lock at this point on the x axis, we will destroy the first and fourth terms (Bi field inhomogeneity) and lock the second and third terms. Because the TOCSY mixing sequence transfers coherence from in-phase to in-phase, only the third... 

NMR pulse sequence without getting tied up in the details of pulse phases and a mountain of sine and cosine terms only the essential elements of the sample net magnetization will be described at each point. Finally, the formal Hamiltonian description of solution-state NMR will be described and applied to explain two related phenomena strong coupling ( leaning of multiplets) and TOCSY mixing (the isotropic mixing sequence). 

THE HAMILTONIAN MATRIX STRONG COUPLING AND IDEAL ISOTROPIC (TOCSY) MIXING... 

Now let s look at something we do not know the answer to the ideal isotropic mixing Hamiltonian. This is the ideal TOCSY mixing sequence that leads to in-phase to in-phase coherence transfer. The ideal sequence of pulses creates this average environment expressed by the Hamiltonian. The Zeeman Hamiltonian that represents the chemical shifts goes away and we have only the isotropic (i.e., same in all directions) /-coupling Hamiltonian ... 

TOCSY (total correlation spectroscopy) is an extension of the COSY experiment, in which the coherence transfer is not limited to a single jump from one proton to another via a J coupling. Instead, coherence is spread out over an entire spin system of coupled protons via multiple /-coupled jumps. For example, in a string of carbons CHa-CHb-CHc-CHd, coherence can be transferred by the TOCSY mixing sequence from Ha to Hc or from Ha to Hj. Thus, crosspeaks will be observed at F% = va and I = i b, vc or (Fig. B.5). 

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