Slow chemical exchange

T2 relaxation rate, however, will continue to get shorter and shorter as Tc increases. 

In practice, relaxation times are rarely used to elucidate the structure of smaller molecules. Relaxation studies involving macromolecules (polymers) and other large molecules, however, are well known to yield important structural information. 

Rapid chemical exchange is often observed in H spectra when our sample contains labile protons. Labile protons are most often those found on heteroatoms in hydroxyl, carboxyl, and amino groups. In special cases, other H s may be observed to undergo rapid chemical exchange if there is a combination of several conditions that each contribute toward making a particular especially labile, e.g., if the is alpha to several carbonyls or if there is a strong propensity for the molecule to tautomerize. 

Rapid chemical exchange means that the exchange takes place on a time scale faster than any that can be resolved by using the instrument. As an aside, the time scale that can be observed with an NMR spectrometer is referred to as the NMR time scale in fact, the NMR time scale may vary over many orders of magnitude, with the specific time scale depending on what experiment is being conducted. 

Rapid chemical exchange A chemical exchange process that occurs so rapidly that two or more resonances coalesce into a single resonance. 

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In a coupled spin system, the number of observed lines in a spectrum does not match the number of independent z magnetizations and, fiirthennore, the spectra depend on the flip angle of the pulse used to observe them. Because of the complicated spectroscopy of homonuclear coupled spins, it is only recently that selective inversions in simple coupled spin systems [23] have been studied. This means that slow chemical exchange can be studied using proton spectra without the requirement of single characteristic peaks, such as methyl groups. 

The selective inversion experiment for slow chemical exchange... 

Bain. A. and Cramer, J.A. (1993) A method for optimizing the study of slow chemical exchange by NMR spin-relaxation measurements. Application to tripodal carbonyl rotation in a metal complex. 

Slow chemical exchange in an eight-coordinated bicentered ruthenium complex studied by one-dimensional methods. Data fitting and error analysis. J. Magn. Reson. Ser. A, 118 (1), 21-27. 

A and B are coincident Q and R cannot be reliably distinguished as we are not sure of the preferred conformation of the iso-propyl group. Note the small but characteristic COSY cross peak between the angular methyl P and la. The 13 - and 14 -hydroxyl protons arc in slow chemical exchange, but are exchanging rapidly enough with water in the solvent to show saturation transfer. 

Poppe et determined the conformation of the sLe tetrasaccharide in the free state and bound to E-, P-, and L-selectin-lg fusion proteins, from NMR measurements of interglycosidic coupling constants and nOe involving hydroxyl protons in slow chemical exchange at low temperatures." " From their data. 

It has been demonstrated that cross-polarization/magic-angle spinning (CP/MAS) NMR spectroscopy is the most powerful means available to characterize the structure and dynamics of inorganic polymers in the solid state [1], The most recent development of a variable-temperature (VT) spinner provided an indispensable means to study slow chemical exchange, molecular motion and phase transitions of inorganic polymers. 

Slow chemical exchange can be more difficult to observe by NMR. For example, a may slowly exchange over time with deuterons in the solvent. Immediately after the solute is dissolved in the solvent, it maybe possible to observe a resonance due to this slowly exchanging... 

Li Y, Palmer AG (2009) TROSY-selected ZZ-exchange experiment for characterizing slow chemical exchange in large proteins. J Biomol NMR 45 357-360... 

The time scale is probably not longer than a few microseconds for if it were, the motions could be detected by well known methods for characterizing slow chemical exchange motions which range between 100 microsec... 

Spectroscopic studies on metal carbonyl complexes were relatively abundant in 1993. They include Iridium carbonyl complexes investigated via NMR O NMR studies on (mesitylene)M CO)3 complexes (M = Cr, Mo, W) an interesting NMR method for optimizing the study of slow chemical exchange has been announced natural abundance 0 NMR spectra of metal carbonyl clusters of the iron triad . 

The CEST effect" " is observed for compounds containing protons in slow chemical exchange with bulk water (medium). On selective saturation of the NMR signal of these slowly exchanging protons, the chemical exchange also transfers the saturation to the bulk water protons resulting in a decreased intensity of their proton signal. Slowly... 

In the limit of slow chemical exchange, the NMR spectrum of a proton in I would consist of two resonances one at Vj for free I and another at Vei for bound I. When chemical exchange is fast, the NMR spectrum of the same proton in I consists of a single peak with a resonance frequency v given by... 

There are two main applications of slow chemical exchange one is to determine the qualitative mechanism, and the other is to measure the rates of the processes as accurately as possible. For the first case, in which we have a spectrum in slow exchange, we need to establish the mechanism which site is... 

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