The pulse sequence corresponds to that used for protein–ligand interactions by STD NMR spectroscopy. The method uses difference spectroscopy to measure transient spin saturation transfer along increasing saturation times (build-up curve), and the kinetic rate constant is easily determined from the initial slope from a single set of experiments. Here we propose a new NMR protocol for the easy determination of the kinetics of molecular systems undergoing mutual-site exchange, based on the SST experiment, but avoiding the need to assure steady state conditions and the determination of T 1 values. The latter measurement can be hampered in spectral regions with significant peak overlap. 4 SST experiments require a steady state saturation transfer between the exchanging sites (very long saturation time), and the experimental determination of the longitudinal relaxation time constant, T 1, of the monitored spin. Line-shape analysis at very high magnetic field can be experimentally of limited applicability, as the coalescence temperature exceeds the boiling points of most solvents. 3 Both of them have, however, a number of drawbacks. Kinetics of chemical exchange in small molecules have been studied by NMR using mainly two different methods: (i) line-shape analysis, 2 and (ii) spin saturation transfer experiments (SST). amide bonds), or redistribution and ligand exchange in metal complexes. butadienes), tautomerism, hindered internal rotation ( e.g. ring flip in biphenyl systems), conformational isomerism in aliphatic compounds or rings, rotation around conjugated bonds ( e.g. Examples of chemical exchanges in organic chemistry are atropoisomerism ( e.g. 1 The chemical exchange rate is as useful as any other chemical kinetic parameter, as it is related to the thermodynamics of the barrier of the exchange process. Intramolecular chemical exchange is a common phenomenon in organic molecules, and an important process when studying molecular dynamics. We report here the Initial Growth Rates SSTD NMR method, as a new powerful tool to obtain the kinetic parameters of intramolecular chemical exchange in challenging small organic and organometallic molecules.
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