Overhauser effect spectroscopy HOESY

Bauer, W. (1995). NMR of organolithium compounds general aspects and application of two-dimensional heteronuclear Overhauser effect spectroscopy (HOESY). In Lithium Chemistry, ed. Sapse. A.-M., and Schleyer, P. V. R., Wiley-lnterscience, New York, 125-172. 

The HOESY (Heteronuclear Overhauser Effect Spectroscopy) experiment... 

With the aid of 13C NMR, 6Li NMR and XH HOESY (heteronuclear Overhauser effect spectroscopy) NMR of a-lithiomethoxyallene (106) and l-lithio-l-ethoxy-3-J-butylallene (107) as well as by ab initio model calculations on monomeric and dimeric a-lithiohy-droxyallene, Schleyer and coworkers64 proved that 106 and 107 are dimeric in THF (106 forms a tetramer in diethyl ether) with a nonclassical 1,3-bridged structure. The 13C NMR spectrum of allenyllithium in THF is also in agreement with the allenic-type structure the chemical shift of C2 (196.4 ppm) resembles that of neutral allene (212.6 ppm), rather than C2 of propyne (82.4 ppm). 

Coordination of [6Li]-a-(phenylthio)benzyllithium with 9 was studied by H Li-HOESY NMR technique (HOESY = heteronuclear Overhauser effect spectroscopy) <1998JOM(550)359>. This interaction results in the formation of contact ion pair and ligand and tetrahydrofuran (THF) solvent molecules compete for three coordination sites. The fourth site is occupied by the anionic benzylic carbon atom in an qMike manner. 

The heteronuclear variant of NOESY is HOESY (Heteronuclear Overhauser Effect SpectroscopY). Figure 4.60 shows a HOESY spectmm for the tetramethylethylenediamine (tmeda) adduct of 2-lithio-l-phenylpyrrole, whose dimeric structure is also shown in the figure. The normal H and Li NMR spectra are shown along the axes of the 2D contour plot, which contains just three peaks. The lithium atom is therefore close (i.e. less than about 3.5 A) to three different sets of three protons, which can be readily identified as H(7) and H(ll), equivalent by virtue of fast rotation about the N(l)-C(6) bond in solution, H(3), and the methyl protons of the tmeda ligand. Note that the hydrogen atoms are numbered according to the numbers of the carbon atoms to which they are attached. The close contact between Li and H(11) seen in the crystal structure is thus maintained in solution, and it is of chemical significance, as it leads to... 

A different approach to locate the metal in a lithium organyl, which exploits dipolar rather than scalar interactions, was presented by the group of Berger who appUed both 2D- C, Li HOESY and lD- C Li difference spectroscopy to measure C, Li heteronuclear Overhauser effects and demonstrated that the resulting data can be used for C-Li distance calculation. Ihe major drawback of this technique is that the use of doubly [ Li, C]-labelled samples is mandatory to ensure reliable measurement of very small NOE effects of some 1%. 

Chiappe et al. reported the use of rotating-frame Overhauser spectroscopy (ROESY) and HOESY to investigate two pyrazolium-based ILs with different anions [83]. Using homo- and heteronuclear NOE, they showed the presence of aggregation, how sensitive the aggregation is to steric hindrance, and the nature of anions. Specifically they found that aggregation motives are in a head-to-tail and head-to-head manner. From the HOESY experiments, the researchers probed the formation of loose ion pairs, which are also sensitive to steric effects. Castner s group has utilized H— F HOESY to probe-specific cation—anion interactions for isoelectronic... 

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