2D EXSY spectrum

Fig. 16. NMR 2D-EXSY spectrum of an acidic aqueous solution containing the thallium species Tl(OH2)g, TKCN), and T1(CN)2, showing the dominating cyanide...

Figure 8.44. The 128 MHz boron-11 2D EXSY spectrum of a 1 1 mix of BCI3 and BBr3 at 400 K. The spectrum was recorded with a mixing time of 50 ms (reproduced with permission from [51].

Fig. 12. 2D EXSY spectrum of HP adsorbed in ITQ-6 under MAS conditions showing exchange between chaimels, cavities, inter-lamellar spaces, and the gas phase. (Reprinted from ref. 237 with permission. Copyright 2003, The Owners Society.)...

Von Philipsbom et al. exempfify the use of EXSY NMR for the interconversion of diastereomeric complexes for Rh(I)-catalyzed asymmetric hydrogenation [13] (Scheme 13.1). The solvent complex dimer [Ph(S,S-chiraphos)(CDjOD)2] (S,S-chiraphos = (2S,3S)-bis(diphenylphosphino)butane) was found to react with mac((Z)-a-N-acetyncinnamate) to two diastereomeric complexes in approximately 10 1 ratio. Ina( P, P)- H) 2D EXSY spectrum of the mixture, each diagonal peak has an exchange cross peak for the other diastereomer, but these cross peaks have unequal intensities, which is contradictory to a pure intermolecular exchange mechanism. This clearly demonstrates that both intra- and intermolecular diastereomer... 

Figure 10.3. The 2D EXSY spectrum of W(SjCjH(p-CHjOPh)), at -40 C in the dithiolenic proton region witht =2 s.

Fig. 5.3.9 Hp-129Xe 2D EXSY NMR spectra frequency discrimination (states) with 64 scans recorded during combustion for various ex- per spectrum. The average experimental time change times. Adapted from Ref. [2], The per EXSY with 0.5-s recycle delay was 40—50...

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)... 

Figure 11 shows the contour plots of the 2D 51V EXSY spectrum of a two-site system (40 mM total vanadate at pH 10.9) in which V exchanges with V2. Quantification of the EXSY spectrum and calculation of the error propagated to the rate constants from the integration precision gives a 25% error on the rate constant. The results (both the rate constants and the errors) correspond nicely to the results obtained from a ID magnetization transfer experiment on the same sample (27). The EXSY spectrum of a sample containing 12.5 mM total vanadate at 1.0... 

Fig. 17. Two-dimensional exchange spectroscopy (2D-EXSY) spectra of an aqueous solution containing TKCNls, T1(CN)4 , and HCN in a concentration ratio 0.021 0.018 0.166 M, enriched to 95% in C. Temperature = 25°C. Reprinted from Batta et al. (168). Copyright 1993 American Chemical Society, (a) C NMR spectrum, recorded at 100 MHz, mixing time = 0.035 s. The spin-spin coupling between C and H is observed because the proton exchange between HCN and bulk water is slow (309a). (b) T1 NMR spectrum, recorded at 231 MHz, mixing time = 0.020 s.

Fig. 26. HP 2D EXSY spectra acquired during combustion showing exchange as a function of the mixing time. Experimental time was 40-50min per spectrum. (Courtesy of Thomas Meersmann. Reprinted from ref. 334 with permission. Copyright 2003, American Chemical Society.)...

Another important NMR experiment for monitoring catalyst performance is the 2D EXSY pulse sequence, which is equal to the 2D NOESY sequence (nuclear Over-hauser effect spectroscopy). The diagonal on these two-dimensional homonuclear spectra represents the ID spectrum. One wiU find two kinds of cross peaks in the... 

There is a growing realization of the power of the technique in the catalysis area, especially when combined with the use of modern multipulse procedures such as distortionless enhancement by polarization transfer (DEPT), 2D tritium/proton correlation, 2D /-resolved NOE difference spectroscopy (NOESY), and 2D exchange spectrum (EXSY). Thus, in the... 

Figure 29 2D EXSY NMR spectrum of PIO-O-PIMB in the B4 phase. For clarity, the aliphatic region is omitted. Reprinted with permission from J. Phys. Chem. B 2013, 117, 6830 [67], Copyright 2013 American Chemical Society.

Figure 2.6 H 2D EXSY NMR spectrum of 19a (300 MHz, CDCI3, 313 K, mixing time 0.5 s) asterisks indicate the signals of 19b.

The mechanism of this process was determined by 2D EXSY NMR spectroscopy. The cross-peaks between and and between and O observed in the spectrum testify that a reversible bonds shifting characteristic for all cyclooctatetraenes is observed in the borane 39 (Scheme 2.25). 

Figure 2.12 2D H- H EXSY spectrum (400 MHz, benzene-dg) of phenalenyl(di- -propyl)borane 24 at 298 K (mixing time 0.5 s). O. L. Tok et al., J Chem. Soc. Chem. Commun. 311-312 (2000). Reproduced by permission of The Royal Society of Chemistry.

The effect of exchange processes can be observed in two-dimensional spectrum and be analyzed in a very similar way. The 2D exchange spectroscopy (EXSY) is in principle, identical to the NOESY experiment. Cross peaks in 2D EXSY experiments arise from noncoherent magnetization transfer between sites with different resonances. Noncoherent magnetization transfer takes place either by exchange of nuclei between different sites or by cross-relaxation (NOE). However, the mixing time in EXSY is usually chosen to be shorter,... 

The speciation of vanadate in aqueous systems is found to be both concentration and pH dependent. The NMR spectrum of the vanadate solution contains four main peaks at —542, —564, —574 and —582 ppm, which are attributed to vanadate monomer, dimer, tetramer and pentamer, respectively [166,167]. A dynamic analysis combined with exchange rates determined by using the 2D EXSY NMR spectrum, allowed quantification of exchange pathways, and has yielded information about the kinetic stability of the vanadate oligomers (Figure 5.9) [167]. The major pathway for monomer formation is unimolecular decomposition of the dimer conversely, the major pathway for... 

Figure 5.9 (A) Different vanadate species are observed in a 10 mM vanadate solntion (pH 8.6), and varions temperature V NMR spectra show the exchange between them. (B) 2D EXSY NMR spectrum shows the entire exchange map of the vanadates. VI, V2, V4 and V5 represent vanadate monomer, dimer, tetramer and pentamer, respectively. Adapted with permission from D. C. Crans et al., J Am. Chem. Soc. 112, 2901 (1990) [167]. Copyright (1990) American Chemical Society.

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