Specialized NMR techniques

Direct 3H observation should be much more direct and straightforward for rapid use in structure elucidation, preferably in a ID spectrum, and most subsequent attempts focussed on maintaining that speed and utility. Spin-echoes were applied first,34,45 utilizing the differences in transverse relaxation rates between the cross-linked polymer and relatively more mobile resin-bound molecule of interest. Wehler and Westman also had some success applying pre-saturation to the resin signals.34 While some attenuation is afforded, it is at the expense of some spectral intensity from the resin-bound molecule 

Lysine on Wang resin swollen in CDC13 and spun at 5 kHz. The inset shows the low artifact level of the spectrum. Reproduced with permission from Ref. 42. Copyright 1996 American Chemical Society. 

Another approach resulting in suppression of the polymer, but with the added benefit of enhanced resolution, was either E.COSY46 or 2D J-resolved 

Several groups have investigated the detailed application of TOCSY in this non-standard format - where the magnetization is subjected to modulation 

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The energy difference between 277-1,2,4,6-thiatriazines 1 and the corresponding 477-tautomers T is very small. Hence, temperature-dependent tautomeric equilibria are observed in solution. With special NMR techniques it was shown that a few percent of the tautomeric 124,2,4,6-thia-triazine 1" is present.34,54"55... 

By a special NMR technique (dipole-dipole driven NMR with field cycling), the tunnelling frequency of the methyl groups of a series on n-alkanes was determined at 4.2 K. The tunnelling frequency differs for odd- and even-numbered alkanes and can be related to the different crystalline forms of these molecules. ... 

Several detailed NMR studies with new, electron-richer ligands were undertaken around the year 2000. With these new ligands, Rh(III) solvate dihydrides, [Rh(P-P)(H)2(S)2], could be easily prepared and characterised for several diphosphines. Bargon, Brown and co-workers used chiral (but not P-stereogenic) diphosphine PHANEPHOS and special NMR techniques... 

Other specialized NMR techniques have also been applied to the problem and confirm the conclusion that the spectra observed under stable ion conditions cannot be the result of averaged spectra of two rapidly equilibrating ions. ... 

By using microwave irradiation the reaction time was shortened to 10 min and the catalyst amount was decreased to 0.1 mol%of Pd [114]. Various simple species like Pd (PPh3)4, Pd(OAc)2, or PdCl2 may, in the presence of PPhs, be utilized as catalyst precursors. To study catalytic reactions in the molten-state systems a special NMR technique was developed [116]. Using this technique intermediate dinuclear Pd complexes with the same trans-/cis- isomers ratio were detected in solvent-free/melt systems after conventional and microwave heating [116]. 

NMR spectroscopy is the premier strnctnral research tool in organic chemistry, and special NMR techniques allow the structures of small proteins to be determined with the aid of NMR. 

There are many special NMR techniques which cannot be discussed here but should be mentioned (for details see Duddeck and Dietrich 1989) (1) DEPT (Distortionless Enhancement by Polarization Transfer) in a C DEPT spectrum CH3 and CH signals are positive and CHj signals negative (2) NOE (Nuclear... 

A special NMR technique, H NOES Y, using the nuclear Overhauser effect, was applied by Bagno et al. [57] to Me NCl and Bu NI in A=W and B = MeCN mixtures, the ions of the former salt being preferentially hydrated and those of the latter one preferentially solvated by the organic component. Both ions of NaGHjCO are preferentially solvated by DMSO in its aqueous mixtures according to this technique. 

Differences in chemical shifts and dynamics of multiphase polymer systems make it possible to selectively excite or saturate one or more components of these complex materials using special NMR techniques. Such NMR experiments give information on phase separation on the nanometer (1-20 nm) scale. 

Most of the usage of CP MAS NMR for the study of organic compounds has been of a relatively routine nature, i.e., the spectra were recorded under standard conditions, with no special NMR techniques used. Frequently, the spectra are only used as fingerprints and perhaps to identify the existence of polymorphs. Generally, when there are two or more molecules in the crystallographic asymmetric unit, there will be observable splittings in the spectra. 

Special emphasis has been given to the more important techniques for solving practical problems related to the interpretation of the spectral data obtained from one- and two-dimensional NMR techniques. An introductory... 

Dupont et al. [60] studied the same reaction, but used [BMIM][PF6] and [BMIM][BF4] as ionic liquids. A special focus of their investigations was on the influence of H2-pressure on conversion. The Henry coefficient solubility constant was determined by pressure drop experiment in a reactor, which is a known procedure to measure gas solubilities [93]. The values reported by these authors were FC=3.0xl0-3 mol IT1 atm1 for [BMIM][BF4]/H2 and 8.8x10 4 mol L 1 atm-1 for [BMIM][PF6]/H2 at room temperature, which differ significantly from those determined by the 1H-NMR technique (see Table 41.2) [59]. However, their values indicated that molecular hydrogen is almost four times more soluble in [BMIM][BF4] than in [BMIM][PF6] under the same pressure. According to the authors, this is reflected by the values of conversion (ee), which were 73% (93% ee) for [BMIM][BF4] and 26% (81% ee) for [BMIM][PF6] at 50 bar H2 pressure (Table 41.9, entries 2 and 4). 

Diindeno[l,2,3,4-de l, 2, 3, 4 - tn(7p]chrysene (116a) can be considered as a symmetrical cap that was cut out of the fullerene Cgo molecule. This compound and its alkylated derivative (116b) react with Li to yield dianions and tetraanions in solution. The standard two-dimensional NMR techniques are insufficient for the structural assignment of the spectra. A special carbon edited NOESY approach was developed for total assignment of the spectra of the neutral species and the dianion. DFT calculations aided in the assignment of the tetraanion spectra ... 

Let us compare the methods applied by Pedersen for establishing the complex formation with a modern approach. Today tedious solubility studies are carried out almost exclusively with practical applications in mind, but they are not performed to prove the complex formation. For instance, one ofthe main reasons for the use of cyclodextrin complexes in the pharmaceutical industry is their solubilizing effect on drugs [8]. There, and almost only there, solubility studies are a must. As concerns spectroscopic methods, at present the NMR technique is one ofthe main tools enabling one to prove the formation of inclusion complex, carry out structural studies (for instance, making use of the NOE effect [9a]), determine the complex stability [9b, c] and mobility of its constituent parts [9d]. However, at the time when Pedersen performed his work, the NMR method was in the early stage of development, and thus inaccurate, and its results proved inconclusive. UV spectra retained their significance in supramolecular chemistry, whilst at present the IR method is used to prove the complex formation only in very special cases. 

In the synthesis of all concave acids and bases, a difunctionalized molecule A-A was cyclized with a difunctionalized bridge component B-B. Because telo- and polymerizations are the main side reactions [29] the isolated macrocycles need not be the expected [1 -I- 1] addition products, the (-A-AB-B-)i cycles, [n - - n] Telomers with the general structure (-A-AB-B-) are also possible. These molecules have identical elemental analyses and similar IR and NMR data. Therefore the mass spectral analyses of the macrocycles are very important because this is the only method which can tell [1 -t- 1] and [2 -t- 2] addition products apart. Due to the high molecular weight of the concave acids and bases, special MS techniques were necessary in some cases [30]. In the case of the macrocyclic diamine 7 [R = NEt2, X = CH2(CH20CH2)2CH2], a [2 -t- 2] addition product could be isolated and characterized besides the desired [1 + 1] product [12a]. 

Structural elucidation of natural macromolecules is an important step in understanding the relationships between the chemical properties of a biomolecule and its biological function. The techniques used in organic structure determination (NMR, IR, UV, and MS) are quite useful when applied to biomolecules, but the unique nature of natural molecules also requires the application of specialized chemical techniques. Proteins, polysaccharides, and nucleic acids are polymeric materials, each composed of hundreds or sometimes thousands of monomeric units (amino acids, monosaccharides, and nucleotides, respectively). But there is only a limited number of these types of units from which the biomolecules are synthesized. For example, only 20 different amino acids are found in proteins but these different amino acids may appear several times in the same protein molecule. Therefore, the structure of... 

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