Heteronuclear Overhauser SpectroscopY HOESY

For compounds that contain a limited number of fluorine atoms, het-eronuclear 19F-4H 2D NMR experiments such as 19F-4H HETCOR and 4H-19F heteronuclear Overhauser spectroscopy (HOESY) can provide considerable assistance distinguishing structural isomers and diastere-omers as well as for conformational analysis. HOESY experiments have been frequently used for conformational analysis of biomolecules containing fluorine labels.33... 

The 2D sequence [80,81], referred to as HOESY (heteronuclear Overhauser spectroscopy. Fig. 8.52b), avoids the need for selective proton presaturation but naturally suffers from low sensitivity. The sequence parallels that of NOESY,... 

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. 

A(-Phenylpyrrole (194) is monolithiated at the 2-position of the heterocyclic ring. This monolithium compound crystallizes as the TMEDA-solvated dimer (195). This structure agrees well with the Li- H 2D heteronuclear Overhauser NMR spectroscopy (2D-HOESY). The structure serves to predict correctly that the second lithiation to a dianion occurs at the ortho position of the phenyl ring loca closest to the lithium in the monoanion. 

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%. 

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... 

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... 

NMR spectroscopy has been applied in search for molecular interactions in NADES. The existence of the hydrogen bond between the fluoride ion from choline fluoride and protons from urea was observed by using the heteronuclear Overhauser enhancement spectroscopy (HOESY) [5]. The 2D NMR spectra of 1,2-propanediol-choline chloride-water (1 1 1, molar ratio) have also revealed the strong interactions among the three components involved [35]. The formation and stability of NADES can be seriously affected by the type, the number, and the spatial orientation of the HBD/acceptor groups [18]. 

Judeinstein et al have conducted direct measurement of through-space NMR interactions that provide definitive evidence for spatial proximity of different species. Dipole-dipole interactions can be measured in principle between any NMR active nuclei with heteronuclear correlation experiments in the liquid or solid state." The dipole-dipole interactions decay quickly with the internuclear distances (r ), and are difficult to evaluate for long-range distances and even more difficult when exchange, conformation, or motion phenomena are present. However, the measurement of the nuclear Overhauser method" based on the dipole-dipole-induced crossrelaxation, was proposed to successfully measure intermolecular interactions" and the formation of ion pairs." " In agreement with recent studies, the pulsed field gradient enhanced inverse HOESY (heteronuclear Overhauser enhancement spectroscopy) sequence is usually preferred because it is more sensitive for isotope pairs H- Li and also improves the digital resolution in the H crowded spectrum." ... 

Because, in (RSn)i20i4(OH)6 X2, the anions do not interact directly with any tin atom but with the /U-2-OH that constitute the charged poles, the exchange of the anions or their ionic dissociation cause only small changes on the Sn chemical shifts (Table 2.3.1). H- H NOESY/ROESY (homonuclear Overhauser correlation spectroscopy) and HOESY (heteronuclear NOESY) have been used to... 

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