EXSY method

Modern NMR techniques such as quantitative analysis of multisite exchange using either ID magnetization transfer experiments (JOS) or the 2D exchange spectroscopy (EXSY) method (104,105) promise to be of great help in unraveling the complex stereochemical exchange networks involved in cluster fluxionality. The usefulness of EXSY in the context of this article is illustrated by the phase-sensitive 13C 1H EXSY spectrum (255 K, tm = 0.5 sec)... 

Two-dimensional exchange spectroscopy (2D EXSY) offers an enormous extension of accessible rates towards slow and very slow exchange processes. Extending the timescale is not the only merit of 2D EXSY methods before discussing some practical examples, there should be a few general remarks. The basic pulse sequence used is shown in scheme 4. 

Me2CgH3 ligands were found to have approximately equal amounts of syn- and anti-alkylidene isomers in solution, which allowed for a study of the interconversion of the two by employing EXSY methods. The values... 

The 2D EXSY method has been applied to the rearrangements of tris(dithiolene) complexes of the general type M(S2C2R]R2)3, where M is W or Mo, R( is phenyl or substituted phenyl and Rj is H or phenyl. The structures in solution are believed to be trigonal prismatic, based on the crystal structures of analogous complexes, and the asymmetric substitution gives the possibility of cis and trans isomers. However, there is the possibility that these are fac and mer isomers. The authors structural assignments are shown by the left-hand structures in Scheme 10.6, where the R, substituents have been omitted for clarity and Rj = H. 

Structural characterization of the dihydrides of Rh and Ir phosphine complexes with properly modified 2D NMR methods (such as COSY, HSQC, HMQC, and NOESY) was reported, which, in particular, can address heteronuclear correlations involving such nuclei as and ° Rh [27,63]. Dynamic transformations can be addressed with ID and 2D NOESY/EXSY methods. The exchange between the dihydrides and pH2 can be studied efficiently as it leads to the permanent replenishment of hyperpolarization of the dihydride complex. Ligand (e.g., CO, PPhj) exchange has been addressed as well. 

Comment Whether it is used for simple monitoring, or to prove the existence of traces of a key intermediate (EXSY or PHIP), NMR spectroscopy has clearly developed into one of the catalytic community s most valuable analytical tools. NOE data will never afford a molecular picture which is quite as structurally exact as that from X-ray crystallography however, this latter method cannot determine kinetic parameters, recognize equilibria, mimic catalytic conditions (HP) or recognize when ion-pairing is important (PGSE). NMR is not a very sensitive method indeed, most methods are far more amenable to quantitative results. Nevertheless, its proven flexibility makes it indispensable. 

For exchange in multisite systems, both line-shape analysis and ID magnetization transfer methods become tedious and time-consuming. Because 2D magnetization transfer methods (2D EXSY) provide simultaneous quantitative information about every exchange process taking place in a system, they present an attractive alternative to studying the dynamics of complex systems. 

Quantification of EXSY data produces a map of individual site-to-site rate constants. Because all of the data are derived from a single experiment, this method is preferable to analogous ID methods whenever the number of exchanging sites in the system exceeds two. 

Pyridine complexes can show analogous hindered rotation to that observed with metal-aryl complexes. This has been observed for over SOyears, but recent interest in supramolecular chemistry, particularly by Fujita and Stang, wherein bipyridyls have been used as linkers has spurred renewed interest in the phenomenon. Several articles on dynamics provide leading references for applications of EXSY and other DNMR methods in these systems. BINAP complexes with picolines and methyl-substituted isoquinolines, which show syn and anti isomerism, have been investigated, for example, (48), (49), and (50). ... 

Exchange spectroscopy (EXSY) is a two-dimensional method that can detect chemical exchange and fluxional processes before line broadeiting occurs (Figure 22)." ... 

Many other pulse-sequence techniques besides COSY can be used to produce multidimensional NMR spectra. It will suffice here to simply list the acronyms of some of the better known methods EXSY (exchange spectroscopy), NOESY (nuclear Overhauser effect spectroscopy), TOCS Y (total correlation spectroscopy), ROESY (rotational nuclear Overhauser effect spectroscopy). The nuclear Overhauser effect (NOE) refers to a change in intensity of one NMR peak when another peak is irradiated. 

The standard techniques available for DNMR investigations [166,167,172] are today supplemented by 2D methods, where the 2D EXSY experiment [173-175] (pulse sequence (x)) can be applied to systems that are in slow exchange ... 

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