Heteronuclear Overhauser Effects

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

Just as in the COSY type of experiments this cross-relaxation effect is not restricted to protons, but can also involve heteronuclei the acronym HOESY (heteronuclear Overhauser effect) is used in these cases. This can be used, for example, to show that an anion such as IT1., is in close proximity to the ligands of the organometallic compound, as was carried out by Macchioni et al. with a 19F-xH HOESY experiment [24]. 

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 tlirough-space nuclear overhauser effect (NOE) can provide information on sites where metals interact even if the metals do not form stable bonds through which spin coupling can be transferred. For instance, metal-HS interactions can be studied by NOE spectroscopy (NOESY) by measuring interactions between the protons within the HS molecules before and after the addition of metals to understand the conformational changes that occur within the molecules (Kingery et al., 2001). An alternative approach is to measure the heteronuclear overhauser effect (HOE) directly between the metal ion and the HS proton in close proximity by HOE spectroscopy (HOES), as has been demonstrated for organo-Li complexes (Bauer, 1995). 

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. 

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%. 

D. Canet, "Systematic errors due to improper waiting times in heteronuclear Overhauser effect measurements by the gated decoupling technique," J. Magn. Resonance 23, 361-364 (1976). 

W. Bauer, NMR of Organolithium Compounds General Aspects and Application of Two-Dimensional Heteronuclear Overhauser Effect Spectroscopy. In Lithium Chemistry (Eds. A.-M. Sapse and P.V.R. Schleyer), Wiley, New York, 1995, pp. 125-172. 

For an accurate quantification of the content of substructures, it is essential that the NMR spectra are recorded under appropriate experimental conditions. In general, two effects can falsify the signal intensities (i) an incomplete spin-lattice (Ti) relaxation and (ii) a heteronuclear Overhauser effect, resulting from H decoupling. Typically, the NMR signal quantification is less crucial from the proton spectrum than from spectra of other nuclei, which require the correct choice of experimental conditions. To date, very few relaxation studies have been reported comparing data for terminal, linear, and dendritic units. Longer Tm values were... 

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... 

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