Structure determination chemical shift

However, in molecules where groups are constrained for whatever steric reasons, molecular anisotropy can play a large part in determining chemical shifts. Take for example, the molecule in Structure 6.7. 

Structure elucidation of three related derivatives of ring system 2 ( temozolomide 9a, mitozolomide 9b, and the related acid derivative 9c) has been carried out by 13C and 15N NMR spectroscopy <1995J(P1)249> (Scheme 2). The 1SN NMR chemical shifts measured in dimethyl sulfoxide (DMSO) solutions are listed in Table 1. For compound 9a, Lowdin charges of the nitrogen atoms have also been calculated and found to have a linear relationship with the experimentally determined chemical shifts of these atoms. The NMR data of 9a have been correlated with those of a series of heterocycles of related structure by the same team <2002MRC300>. 

The chemical structures of five dextrans were partially determined by methylation, and found to be branched molecules having the following types of substitution (a) 6-0 and 3,6-di-O, (b) 6-0, 3-0, and 3,6-di-O, (c) 6-0,3,6-di-O, and 2,3-di-O, (d) 6-0, 4-0, and 3,4-di-O, and (e) 6-0 and 2,3-di-O. At 27° and pH 7 (external, Me4Si standard), the 13C shifts ofO-substituted, non-anomeric carbon atoms were C-2 (76.5), C-3 (81.6), and C-4 (79.5). The C-l resonances were also recorded, and may be used for reference purposes. Some variation of chemical shifts, relative to each other, was observed with changing temperature. (The work serves to emphasize the importance of accurately measuring the temperature of the solution when determining chemical shifts.102)... 

Once the sequential assignment is complete, the secondary structure of the protein can be determined from the secondary-structure specific chemical shifts of the 13Ca nuclei. Other nuclei, including the 13C/9 and the 13CO, also show characteristic chemical shifts that can be used to identify the secondary structure [55-60]. In addition, the secondary structure can also be assigned with the help of the identified secondary structure-specific sequential NOEs and coupling constants obtained from the HNHA experiment... 

Empirically determined chemical shift additivity parameters have been determined- for diene-type polymers. The shift contribution of a quaternary carbon which is fllto the carbon in question was not determined by those authors. However, using their additivity parameters and the shift positions of the ( carbons in Figure 8, a value of +15.4 ppm can be estimated for the contribution of a neighboring (0() quaternary carbon. Using this value, the shift positions of the carbons in structures VII and VIII are calculated as shown. If the first 1,2 unit were on the chain in a 2,1 manner, the methylene carbon resonance would be at a considerably higher field, but it would be difficult to estimate its position with any certainty because the quaternary effect... 

The C chemical shifts are in ppm relative to tetramethylsilane, determined with solutions containing 5 mL/100 mL tetramethylsilane as the internal standard. The multiplicity of the peaks is given in parentheses br indicates a broad peak without resolvable fine structure. The chemical shifts can be dependent on solute, concentration, and temperature. 

The spectrum and expansions provided in this problem are for one of the compounds shown below. The broad peak at 5.25 ppm is solvent dependent. Using calculations of the approximate chemical shifts and the appearance and position of the peaks (singlet and doublets), determine the correct structure. The chemical shifts may be calculated from the information provided in Appendix 6. The calculated values will be only approximate, but should allow you to determine the correct structure. 

After assignment of the resonances of the protein, secondary-structure information can be retrieved from chemical shifts quite directly. For H , C , Cp, and carbonyl resonances, one observes conformation induced changes of the chemical shifts that can be referenced to chemical shifts measured from the same amino acid in a peptide. As a rule, these structure-induced chemical shifts are low field for H and Cp, and high field for C and carbonyl in a )8-strand. For a-helices, the structure-induced chemical shift is opposite [66] [67]. Thus, the secondary-structure elucidation is possible based on the assignment of the resonances and the determination of the chemical shifts. The result from this analysis for the CaM/C20W complex is shown in Fig. 30... 

Graphite surface (in particular, EG surface) contains a lot of structural imperfections, such as oxidized groups and residual molecules of oxidizers (Morimoto and Miura 1985). They can be involved in formation of H-bonded complexes with water-type molecules. Active protons in such associations should be subjected to a deshielding effect of electron-donating atoms, and, besides, in this case, one could expect the appearance of H NMR signals with down-field shift relative to the corresponding resonance lines for a condensed phase. As no such peaks were found in the spectra, and, in addition, experimentally determined chemical shifts for water are close to calculated values, it may be concluded that the concentration of these sites in EG is low in comparison with hydrophobic adsorption sites on basal graphite planes of a carbon surface. 

Determining molecular structure from chemical shift measurements is the first step in NMR characterization of ILs. Although H and are routinely used, multinuclear NMR provides a wealth of information that is helpful to determine molecular structures for ILs that contain a variety of heteroatoms. F, and have been used for the characterization of novel ILs containing these heteroatoms [1—4]. Figure 1 shows H, Li, and B NMR spectra of a new class of mesogenic IL electrolytes made up of long-chain imidazolium cations mixed with Hthium salts [5]. 

NMR is a powerful tool for the determination of structures from first principles and the chemical shift is the most important NMR parameter in structural analysis. For estimating the relationship between chemical structures and chemical shifts three possibilities exist the calculation of the chemical shift values by empirical methods [137], the computation by quantum chemical procedures, e.g., with the IGLO-method (Individual Gauge for Localized Orbitals [ 129]), or the use of large compilations of NMR spectra and the associated chemical structures. The access to relevant reference data for identical or similar compounds can facilitate the assignment process enormously. Reference data may assist by reducing the amount of experimental and/or interpretive effort required or increase confidence in the suggested structure. 

Structural parameters for the modelling of complete C-njn.r. spectra of disaccharides have been derived from forty experimental spectra 1 use of molecular mechanics techniques. The average difference between simulated and experimentally determined chemical shift values was 0.45 ppra. ... 

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