Interpretation of NMR data

There are many software tools available to help with the acquisition, processing and interpretation of NMR data. Attempts have been made to automate the verification process and even perform full structural elucidations of unknown compounds. As you might guess from the complexity of the interpretation chapters, these software solutions are not foolproof It remains to be seen whether they ever will be good enough but there have certainly been some major steps forward in all of these areas. 

A fascinating insight into the impact that modelling can make in polymer science is provided in an article by Miiller-Plathe and co-workers [136]. They summarise work in two areas of experimental study, the first involves positron annihilation studies as a technique for the measurement of free volume in polymers, and the second is the use of MD as a tool for aiding the interpretation of NMR data. In the first example they show how the previous assumptions about spherical cavities representing free volume must be questioned. Indeed, they show that the assumptions of a spherical cavity lead to a systematic underestimate of the volume for a given lifetime, and that it is unable to account for the distribution of lifetimes observed for a given volume of cavity. The NMR example is a wonderful illustration of the impact of a simple model with the correct physics. 

A number of excellent publications have now appeared dealing with NMR studies of oxytocin (14,15), vasopressin (16,17), and their analogs. These follow the earlier pattern set in the gramicidin SA study and are of great value in forming ground rules for interpretation of NMR data. 

The presence of free radicals deriving from carbon black could also complicate the interpretation of NMR data in the case of filled rubbers, because radicals may cause a substantial decrease in T2. Two types of radicals have been detected in carbon-black-filled rubbers localised spins attributable to the carbon black and mobile spins deriving from rubbery chains [86]. Mobile spins are formed because of the mechanical breakdown of polymer chains when a rubber is mixed with carbon black. The concentration of mobile spins increases linearly with carbon black loading [79, 87]. 

The amount of radicals in carbon black filled rubbers decreases significantly upon extraction of free rubber with the aid of a solvent containing a free radical scavenger. The extraction nevertheless causes a substantial increase in the fraction of the T2 relaxation component with the decay time of about 0.02-0.03 ms [62], This increase is apparently caused by an increase in the total rubber-carbon black interfacial area per volume unit of the rubber due to the removal of free rubber. The T2 relaxation component with a short decay time is also observed in poly(dimethyl siloxane) (PDMS) filled with fumed silicas [88], whose particles contain a minor amount of paramagnetic impurities. Apparently, free radicals hardly influence the interpretation of NMR data obtained for carbon-black rubbers in any drastic way [62, 79]. 

This definition recognizes the fact that the electrostatic repulsion in a 4-ring is about twice as strong as in larger rings. However, the main reason of choosing this particular definition is its usefulness in the interpretation of NMR data (see below). 2q... 

An interpretation of NMR data from solubilized alkanes in aqueous lamellar phases of a nonionic surfactant is found consistent with the presence of a layer of oil located at the center of the bilayer. This supports a similar novel conclusion derived from small angle X-ray data (1). The presence of such an oil layer will have consequences, as yet not understood, on the osmotic pressures within the bilayer and the interlayer interactions responsible for the stability of the lamellar phase. 

Even if chirality is not a primary interest, the chemist should be aware of its presence, notably in the interpretation of NMR data for structure determination. In kinetically labile systems formed by self-assembly, the study of racemization kinetics offers vital information on the robustness of the system, and the possible mechanisms of decomposition or flagmentation. In short, what appears at first to be an extra complication in an already complex system does in fact offer the chemist new means of investigation and control over the properties of these compounds. 

Although the pulse sequences used to study phase transitions are usually quite simple in the examples presented in this review (one to maximum four pulses), the interpretation may be subtle. Solid-state NMR nevertheless remains a difficult technique since quantitative interpretation of the spectra rely on a profound knowledge of the chemical composition and structure of the sample analysis of NMR results also requires a model to relate the observed NMR spectral shapes or relaxation behavior to hypothesis concerning the structure and dynamics of the atoms or molecules carrying spins. That NMR motionally average the atomic and molecular displacements that occur on a time-scale faster than 10—8 10—9s is an important point that should be considered in the interpretation of data. In particular, the difference in perception between NMR and X-ray diffraction with regard to fast and slow dynamical disorder in molecular crystals undergoing phase transitions between different polymorphs was illustrated. In fact, the interpretation of NMR data almost always needs the support of other data obtained by different techniques. Therefore, we emphasized the different complementarities with X-ray (or neutron) diffraction, IQNS and other spectroscopic methods to provide, by cross-correlation of the different data, consistent picture of the phase transition. 

K. B. Wiberg and J. J. Wendoloski, Proc. Natl. Acad. Sci. U.S.A., 78, 6561 (1981). Effect of Basis Set on Electron Populations Calculated by Using Bader s Criterion for Partitioning Electron Density Between Atoms. For an interpretation of NMR data, see H. Boaz, Tetrahedron Lett., 55 (1973). Separable Contributions of Induction and Polarization to the Chemical Shift. I. Symmetrical, Saturated Hydrocarbons Having No Internal Rotation. 

NMR software Methods for fast interpretation of NMR data for protein structure analysis University of Utrechr... 

A fundamental component of the interpretation of NMR data is deciphering the NMR assignments, which correlates an observable NMR resonance with a specific atom in the molecular structure of the metabolite. This process is illustrated using the structure and H NMR spectrum of 1,3-dimethylnaphtha-lene as an example (Fig. 12.6). The two methyl groups have distinct H NMR chemical shifts because of their unique local environments. The NMR assignment process results in attributing the NMR peak at 2.57 ppm to methyl (a) and NMR peak 2.39 ppm to methyl (b). 

The conformational properties of the monosubstituted six-membered rings 1, 2 and 3 are summarized in Table 3. In contrast to previous interpretation of NMR data and MM calculations, our experimental and theoretical investigations result in a preference for the equatorial conformer in the title compound 3. Thus, the conformational properties of this compound are similar to those of 1 and 2. The A value of 3 is considerably smaller but still positive. 

R416 P. R. L. Markwick and M. Nilges, Computational Approaches to the Interpretation of NMR data for Studying Protein Dynamics , Chem. Phys., [online computer file], 2012, 396, 124. 

The current interpretation of NMR data from water biopolymer systems has been summarised in reviews and books It is not intended to re-review the subject here, merely to sununarise the conclusions. 

There are several potentially serious problems involved in the interpretation of NMR data in terms of distance constraints. First, a phenomenon called spin diffusion may result in spurious NOESY cross-peaks between protons that share a common neighboring proton, but which themselves are greater than 5 A apart. Second, biological macromolecules are always flexible to at least some degree, and NOESY cross-peak intensities are expected to reflect the inverse average sixth root of the interproton distances. As a result, it is entirely possible for a proton to appear to be adjacent to two other protons simultaneously, when the other two protons are nevertheless always greater than 10 A apart. Finally, NOESY cross-peak intensities are affected by numerous spectral artefacts and well as by other sources of relaxation, which may cause many cross-peaks to be missing. 

A conformational search carried out on BQ123 included flat-well harmonic restraints between two backbone hydrogen bonds in the potential energy function. Four of the resulting structures were found to compare favorably with one of the 20 NMR-derived structures. The LMOD method is thus proposed to be a promising technique for carrying out conformational analysis on molecules of the size described here and for aiding in the interpretation of NMR data on such molecules, where multiple conformations contribute to the observed data. 

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