NOE-enhanced proton-coupled

Figure 1.10. NMR spectra of 2,4,6-trichloropyrimidine [CeDe, 75% v/v 25 °C, 20 MHz], (a) NMR spectrum without proton decoupling (b) NOE enhanced coupled NMR spectrum (gated decoupling)...

Conditions (CD3)2CO, 25 °C, 200 MHz H), 50 MHz ( C). (a) //NMR spectrum with expanded sections (b,c) C NMR partial spectra, each with proton broadband decoupled spectrum below and NOE enhanced coupled spectrum above with expanded multiplets at 6c = 76.6 and 83.0. 

NMR provides one of the most powerful techniques for identification of unknown compounds based on high-resolution proton spectra (chemical shift type integration relative numbers) or 13C information (number of nonequivalent carbon atoms types of carbon number of protons at each C atom). Structural information may be obtained in subsequent steps from chemical shifts in single-pulse NMR experiments, homo- and heteronuclear spin-spin connectivities and corresponding coupling constants, from relaxation data such as NOEs, 7) s 7is, or from even more sophisticated 2D techniques. In most cases the presence of a NOE enhancement is all that is required to establish the stereochemistry at a particular centre [167]. For a proper description of the microstructure of a macromolecule NMR spectroscopy has now overtaken IR spectroscopy as the analytical tool in general use. 

Nuclear Overhauser effect—The nuclear Overhauser effect (NOE) occurs only between nuclei that share a dipole coupling, i.e., their nuclei are so close that their magnetic dipoles interact. Techniques that use NOE enhance spectra and allow spacial relationships of protons to be determined. 

It is possible to distinguish a 4,5-trans-disubstituted oxazolidinone or imidazolidinone from its 4,5-ds-analogue by using H NMR analysis. Indeed, both H4 and H5 protons resonate systematically at lower field for the ds-isomer than for the trans-isomer, and higher /4i5 coupling constants are observed for the ds-oxazolidinones (Table 8). In addition, nuclear Overhauser effect (NOE) enhancement experiments confirm the cis or trans relationship of the H4 and H5 protons. 

Ohrui et al. [101, 102] greatly facilitated the accessibility of stereospecifically C6-deuterated hexoses with a new short and efficient synthesis. These compounds can be utilized to study the conformer equilibrium at the C5-C6 bond with high accuracy, because of the unambiguous assignment of the coupling constants to the protons pro-S H6 and pro-R H6 and because of the smaller loss of magnetization from the enhanced proton H6 to the other in an NOE experiment (cf. below). 

---