Current Organic Chemistry - Volume 5, Issue 10, 2001

The characteristic features of ¹³C solid state NMR applications are discussed. The spectra are routinely measured with cross-polarisation (CP) pulse sequence and fast spinning under magic angle (MAS). Additional techniques (dipolar dephase, variation of cross-polarisation contact time) influence the signal intensities and enable selective observation of nonprotonated or protonated carbons. Solid state NMR is complementary to X-ray diffraction and provides structural information about solids when the attempts to grow suitable single crystals have failed. However the combination of both methods is more successful in characterisation of solid state conformations and dynamic processes. Many medium sized molecules such as: saccharides, amino acids, peptides, steroids, polyphenols (flavonoids), and tocopherol derivatives have been studied. Samples that are insoluble in commonly used solvents or easily decompose in solution should be investigated by solid state NMR. Since the method can be applied to crystalline as well as to amorphous powders it is especially interesting tool in the studies of macromolecules. Numerous results concerning peptides, proteins and polysaccharides are reported.

Most recent research works on the hydrogen-bonded structure of solid ¹⁷O-labeled peptides and polypeptides by means of solid -state 17O NMR are presented. It has been demonstrated that ¹⁷O-labeling andsolid -state ¹⁷O NMR provide useful informations about the hydrogen-bonded structure between the amide oxygen and proton of peptides and polypeptides in the solid state, in addition to ¹H, ¹³C and ¹⁵N techniques.

In situ MAS NMR has been demonstrated to be a superior technique for analyzing desorbed reactants, reacting adsorbates, intermediates and products. The results are useful for getting deeper insights into catalytic processes and reaction kinetics, and for re vealing the mechanisms of organic reactions. The utility of in situ NMR spectroscopy has demonstrated that this technique is an important means for the studies of catalytic organic reactions. The objective of this paper is to give a review on the generatio n of apparatus for in situ MAS NMR experiments, and to illustrate the applications of the in situ MAS NMR techniques in various important organic reactions catalyzed by solid catalysts. The typical reactions studied by in situ MAS NMR techniques include conversion of methanol to hydrocarbons (MTG process), aromatization of methane, ethylbenzene reaction over zeolite HMOR, observation of carbenium intermediates formation, and photocatalytic organic reaction.

Nuclear magnetic resonance (NMR) has become established as a powerful tool for qualitative and semi-quantitative analysis of complex mixtures of body fluids, intact cells, tissues and their extracts. Such studies are finding increasing applications in clinical and medical sciences. ¹H and ³¹P NMR provide a noninvasive probe of high-energy compounds, amino acids, and compounds of phospholipid metabolism in cells and tissues. ¹³C NMR has been used to measure and characterize high concentration of molecules. The introduction of ¹³C-labeled substrates allows one to follow metabolism by the ¹³C NMR method. In this review, we have described one dimensional inverse detection techniques and various two dimensional homonuclear and heteronuclear methods with special emphasis on methods for studies of cells. Application of NMR to detect me tabolites in various cells and tissues has been discussed with illustrative examples, to highlight the type of information NMR can provide.

The small-sized cyclic imides and amides have been for a long time known as convenient models for study of numerous biochemical processes. In addition, owing to the structural resemblance with their naturally occurring analogues, biological activity has been documented for a number of these compounds. Recent development of the experimental and especially high-level theoretical structural methods has provided new evidences for their structure and properties. The present review aims to summarize the vibrational, structural and combined data published during the last decade on selected five- and six-membered ring imides and derivatives thereof, in the attempt to update the existing knowledge for this class of compounds. As an exceptionally suitable and thoroughly studied model system that has also been a subject for research to the authors for quite some time, the vibrational-structural results for sulfobenzimide (saccharin) and its compounds are also reviewed. The inferences are designed to be applied to other, less-studied imide systems. In spite of the recent vast progress of the analytical methods, simultaneous use of vibrational and diffraction experimental/theoretical results has proven to be far more reliable for better structural interpretation than any of these alone.