Overhauser effect parameters

See also Chemical Exchange Effects in NMR Fourier Transformation and Sampling Theory NMR Relaxation Rates NMR Spectrometers Nuclear Overhauser Effect Parameters in NMR Spectroscopy, Theory of. 

Summary The utility of the NMR parameters longitudinal relaxation time (Tj) and nuclear Overhauser effect (NOE) for a deeper insight into the molecular structure and motion of polymer siloxanes was tested. A few characteristic examples of siloxanes investigated were presented to show problems and results. 

Since the discovery of the nuclear Overhauser effect (NOE, see previous section) [4, 5] and scalar coupling constants [36, 37] decades ago, NMR-derived structure calculations of biomolecules largely depended on the measurement of these two parameters [38]. Recently it became possible to use cross-correlated relaxation (CCR) to directly measure angles between bond vectors [39] (see also Chapt 7). In addition, residual dipolar couplings of weakly aligned molecules were discovered to measure the orientation of bond vectors relative to the alignment tensor (see Sect 16.5). Measurement of cross-correlated relaxation was described experimentally earlier for homonuclear cases [40, 41] and is widely used in solid-state NMR [42 14]. 

The compound 35 possesses exo- and air-stereochemistry of the methyl and the benzoate groups based on the nuclear Overhauser effect (NOE) data. Based on the C-N distance (r), the Woodward parameter (h) and the sum at the N-atom (SN) value from X-ray data, N-l is in a pyramidal environment. N-3 is on maximum resonance with the Jt-framework of the adjacent C=0 group. This is reflected in a shorter C(2)-N(3) bond (1.375(2) A, compared to the C(2)-N(l) bond distance [1.438(2)A]. JZN values are as follows N-3 = 359.99°, and ]N-1 =311.74° and, = 0,601 A. The X-ray crystallographic data support some of the conclusions derived from AMI calculations, viz. N-l and N-3 atoms are in chemically distinct environments and form C-N bonds of different strengths. This would have implications on the... 

The chemical shift is an extremely important NMR parameter but there are many other parameters that can be discerned from NMR spectra. Indeed, NMR is unique among many forms of spectroscopy in that there are so many parameters associated with a spectrum other than just peak intensity and frequency. These include coupling constants, which provide information on local conformations and also on molecular connectivities nuclear Overhauser effects (NOEs), which provide information on internuclear distances and relaxation parameters, which provide information on molecular dynamics. Table 12.1 summarizes the main NMR parameters that may be measured and highlights their applications in the drug discovery process. 

Several different analytical methods are needed to determine the structure and dynamics and to map the intermolecular interactions that prevail in supramolecular systems in solution [1]. NMR is one of the most powerful of these methods [Id-e]. The conventional NMR parameters that are used to characterize the structure and dynamics of supramolecular systems in solution are chemical shifts, spin-spin coupling, relaxation times, NOEs (Nuclear Overhauser Effect) and the correlation thereof. We shall demonstrate that the diffusion coefficient, which is currently underused, should be added to this arsenal of NMR parameters when characterizing supramolecular systems in solution. 

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