Magnetization transfer

One can imagine two protons, and He, being part of the same molecule and undergoing chemical exchange, at a rate knu- When is irradiated, it remembers the new condition and transfers this information to Hj as a result of the chemical exchange. The newly arrived He proton does not contribute to the normal amount of signal intensity in the final NMR spectrum. If the initial intensity of Hj is 4, and the final intensity for Hg as a result of irradiation of is If, then the rate of exchange knu is defined by Eq. (7), where Tj refers to the spin-lattice relaxation time. 

A number of 2D NMR experiments have been developed which allow the study of slow exchange phenomena. The most common (Exchange Spectroscopy, EXSY), is based on the standard pulse sequence 90°x-fi-90°x-tm-90°x - FID(t2), where is an evolution delay, is the mixing time, and tj is the detection period [175, 176]. 

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Di Bari L, Kowalewski J and Bodenhausen G 1990 Magnetization transfer modes in scalar-coupled spin systems investigated by selective 2-dimensional nuclear magnetic resonance exchange experiments J. Chem. Rhys. 93 7698-705... 

MoClung RED and Aarts G FI M 1995 Multisite magnetization transfer in strongly ooupled spin systems J. Magn. Reson. A 115 145-54... 

How does magnetization transfer from H to a coupled C nucleus (polarization transfer or population transfer) affect the signal intensity of the C nucleus ... 

Two-dimensional relayed NOESY experiments (Wagner, 1984 Kessler et al., 1988) give cross-peaks that could be a superposition of nOe s resulting from different relay nuclei. This complicates the extracting of crossrelaxation rates. The 1D NOESY experiment (Kessler et al., 1989a), however, allows the path of the magnetization transfer to be clarified. 

A ID analog of the HMQC experiment with TOCSY magnetization transfer has been reported (Crouch et al, 1990). The pulse sequence is... 

The deposition chamber is a cylindrical vessel with a diameter of 50 cm and a length of 80 cm. At about 5 cm from the arc outlet, silane can be introduced via an injection ring (7.5-cm diameter) that contains eight holes of 1-mm diameter each. The distance between arc outlet and substrate is 38 cm. The substrates are heated via the substrate holder, of which the temperature can be controlled between 100 and 500°C. Samples can be loaded via a load lock equipped with a magnetic transfer arm. The substrate can be optionally RF-biased. A residual gas analyzer... 

Several variations of the ADEQUATE experiment are possible. We will begin with the 1,1-ADEQUATE experiment, which exploits 1Jcn and 1/cc couplings to allow for the identification of adjacent neighbour carbons. Other variants of the experiment are 1, n-, n, 1-, and m,n-ADEQUATE 40 The schematic representation of the magnetization transfer in the various types of ADEQUATE experiments is shown in Figure 1. 

A further detractor from the m,n-ADEQUATE experiment is the inherent ambiguity of the two long-range magnetization transfers. As in the HMBC/GHMBC experiment, the m/ci 1 transfer step may afford... 

In the case of the other correlation shown in Figure 13 as an example, the m/cH long-range magnetization transfer occurs across the anticipated three bonds from the H8 doublet to the C6 quaternary aromatic carbon. Next, the nJcc correlation, rather than occurring via two- or three bonds where n = 2 or 3, instead is via an uncanceled [ fee coupling from C6 to Cl as shown by 44. 

Aside from the initial applications of the n, 1- and m,n-ADEQUATE experiments in the elucidation of the structure of 5,6-dihydrolamellarin (3, see Sections 4.1, 5.1 and 6.5), the author is unaware of any reported applications of these experiments in the literature.38 Given the potential ambiguities associated with the two long-range magnetization transfer steps of the latter, coupled with the very low sensitivity and the fact that correlations are observed in Fi at the DQ frequency of the coupled carbons rather than at the 13C shift as in the 1,1- or 1,h-ADEQUATE experiments, it is probably quite likely that the m,n-ADEQUATE experiment will continue to be very sparingly utilized for structure elucidation. 

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