High Resolution NMR in Solutions

The -conformation 13 is lower in energy than the Z-isomer 14. These are the smallest cyclopropyl substituted carbocations which can be investigated in solution by high resolution NMR. The corresponding primary cyclopropylmethyl cation 15 cannot directly be observed by high resolution NMR in solution because it is energetically less favorable than the bicyclobutonium ion 16 and thus only a minor isomer in the fast dynamic equilibrium of the cations 15 and 16. 13C- and H... 

High-resolution NMR in solution requires the sample to be soluble in a solvent such that the various nuclear spin interactions can be averaged or removed by molecular micro-Brownian motions. Unfortunately, elastomers used in various applications are normally crosslinked materials and therefore not soluble in any solvent. Thus, solid state NMR with magic angle-spinning technique has been used with great success in the study of cured elastomers. However, this technique demands extended instrument facilities and expertise. 

High resolution NMR in solution can be used to measure the type and number of crosslinks in polymers [13]. The C-NMR techniques possesses the required sensitivity to detect concentrations of long-chain branches (Y-type) and H-type crosslinks as low as approximately 0.5 structural units per 10,000 carbon atoms [14]. The limitation of the NMR technique is the limiting solubility arising from gel for-... 

Perhaps the widest application is that of conventional high-resolution spectroscopy in solution for the purpose of learning in detail about polymer chain structure. In this field, proton NMR, formerly dominant, has given way to carbon-13 NMR with the development of pulse Fourier transform spectrometers with spectrum accumulation. Carbon spectroscopy is capable of giving very detailed and often quite sophisticated information. For example, a very complete accounting can be provided of comonomer sequences in vinyl copolymers and branches can be identified and counted, even at very low levels, in polyethylenes. 

Lipids are commonly examined for high-resolution NMR in organic solution and are the most frequently examined nuclei. However, high-resolu-tion spectra, including spectra, of phospholipids in detergent micelles have also been recorded. 

Recent improvements in high resolution NMR of solutions have added a new dimension to the structural study of silicon compounds. Many new methods in measuring Si NMR have been reported. The low sensitivity of Si can be circumvented by isotopic enrichment and/or the use of very high fields. For aqueous Si-enriched (99%) sodium silicate solutions, experiments at high field (11.7 T) have been reported by Harris and coworkers. Furthermore, a large number of species have been identified using two-dimensional (2D) experiments such as COSY for observation of Si- Si correlations in chemical exchange studies. 

Indirect or scalar coupling of nuclear spins through covalent bonds causes the splitting of NMR signals into multiplets in high-resolution NMR spectroscopy in the solution state. The direct or... 

A high-resolution 1 1 solution NMR structure of lmPy-y9-lm-y9-lmPy-y9-Dp elucidated the role of /9-alanine in minor groove recognition (Fig. 3.7 b) [47]. The p residues allow both Im rings in the /9-lm-/9-lm subunit to adapt to the relatively large... 

A field of application of MD that is beginning to bear fruit is the refinement of data from nuclear magnetic resonance (NMR) and from diffraction experiments. High-resolution NMR at frequencies around 500 MHz is able to resolve individual proton resonances of biological macromolecules in solution with molecular weights exceeding 10,000... 

Unlike high-resolution NMR spectra of bulk solutions where NMR linewidths well below 1 Hz can be obtained routinely, NMR spectra of liquids permeating porous solids in most cases will not exhibit such a high spectral resolution. First of all, the interaction of liquid phase molecules with pore walls of the catalyst and rapid diffusion-driven intrapore transport will lead to a pronounced homogeneous broadening of the observed NMR lines. Smaller pore sizes and the presence of paramagnetic impurities in the solid material usually aggravate the situation and thus should be avoided. Another reason why NMR spectra of liquids in porous... 

Of the many areas where NMR is applied these days, two can be considered as being established. The most important is certainly its use for structure elucidation, from small molecules up to medium-sized proteins in solution no university with an analytical lab can afford to be without a liquid-state, high-resolution NMR system. Most chemistry students will come into contact with NMR at least once during their courses. Second, is diagnostic medical imaging, which many of us may have experienced personally. From the first crude and blurred NMR images that were acquired over 30 years ago, incredible developments have been achieved by the efforts of researchers and industry alike. 

With the low-frequency H 1-NMR spectrometers of the past, analysis of low-concentration additives such as antioxidants was quite problematic, in view of lack of sensitivity. Moreover, in low-frequency H NMR most polymer solvents may easily interfere with the solute resonances. In higher-field, high-resolution NMR (500 MHz), solute interferences may be overcome by means of selective signal suppression. However, this is no longer a necessary strategy for the present 900-MHz NMR generation, where both sensitivity and resolution are adequate and quantification is facilitated. 

The structure determination of organic molecules in solution—constitution, configuration, and conformation—is probably the most important application of high-resolution NMR spectroscopy. Among all existing... 

Fig High-resolution NMR spectra of biophenolA polycarbonate in deutrochloroform solution showing the effect of frequency on resolution. 

A coirplete understanding of the role of carbohydrates in biological systems requires knowledge of the distribution at equilibrium of the various conformers in aqueous solution. The conformational behavior of carbohydrates in solution can be examined from different vantage points (1,), but the most relevant approach is, no doubt, study of dilute solutions themselves. At present, high resolution NMR spectroscopy is the primary tool for determination of three-dimensional structure of oligosaccharides in solution. Optical rotation is also very sensitive to conformation (2) and there is a new, semi-enqpirical theory of optical rotation of oligosaccharides ( ). 

High Resolution Spectra of Solutions. An example of high resolution solution spectra of an elastomer system which illustrates the sensitivity of nmr to molecular structure is shown in Figure 1. Shown are spectra of ethylene propylene rubbers... 

High resolution nmr spectroscopy of elastomers is a powerful tool for identification and characterization of elastomer systems. The nmr spectra of elastomers in solution are rich in molecular structure details. The information provided... 

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