Chemical assignments resonances

A typical REMPl at 266 nm mass spectrum is shown Fig. 15.22, obtained by roasting 80 g of Arabica coffee at 225 °C. The laserpower density was adjusted to 10 -10 W/cm in order to avoid non-resonant ionisation processes. The spectrum contains predominantly molecular ions. Chemical assignment of the ion peaks was based on three distinct pieces of information the literature on coffee flavour compounds [204], the mass as observed in TOFMS and optical absorption properties. With this information, many volatiles observed in Fig. 15.22 were unambiguously identified. 

These methods work well for spectra in which there is no significant spectral overlap between compounds, or for situations where the samples to be analysed are chemically quite similar. For example, parallel process monitoring could be accomplished by using such an approach. In cases where spectra contain many resonances that overlap, it is much more difficult to assign resonances based on frequency shifts. Two other approaches have been used in this case [15,16]. One... 

This lack of confirmation of an apparent assignment points out that the method of using mutant or chemically modified Hbs to assign resonances as described above can be used correctly only if the single... 

Physiological experiments, in which resonances are manipulated by various treatments, represent a second, indirect, means of assigning resonances to particular compartments. For example, comparison of normoxic with hypoxic maize root tips [29] shows that the pH estimated from the chemical shift of the glucose 6-phosphate P resonance correlates closely with the pH estimated from the cytoplasmic P resonance, suggesting that these metabolites are in the same compartment. 

Table III. Chemical Assignments of Resonances in the CPMAS-l C-NMR-Spectra of Insect Cuticles3...

Fig. 39. ENDOR spectra of bovine SOD obtained at the five working points across the EPR spectrum. The letters indicate assigned resonances (see Table X). Reprinted with permission from Reinhard, H. Kappl, R. Hutterman, J. Viezzoli, M. S. J. Phys. Chem. 1994, 98, 8806. Copyright 1994 American Chemical Society.

It is easier to assign a structural model when the synthesized polymer has known structural or compositional features that establish chemical shift resonance-stmcture relationships. 

Nuclear magnetic resonance spectroscopy (NMR) is the determination of molecular structures by analysis of static and dynamic features of the materials [42]. In NMR experiments both a magnetic field and a radiofrequency field are applied to a solid sample or a solution resulting in an absorption of energy which is detected as a nuclear magnetic resonance. Spectrometers are also available for high resolution solid state NMR. Nuclei in different chemical environments resonate at different frequencies and thus differ in their chemical shift. Chemical shifts are used to assign these resonances to the specific structure of the sample. The nuclear environment of a nucleus results in multiple resonances that are also used to determine structural information. NMR studies are conducted to deter-... 

Phosphorus-31 NMR offers the observation of a single, readily assignable resonance with good sensitivity. In certain cases two resonances are observed (see Section III). Analysis of P relaxation must take into account the contribution from the chemical-shift anisotropy (CSA) mechanism as well as dipolar coupling to protons (see Section II, A). This additional complication can be dealt with by using chemical-shift anisotropy data provided by solid-state NMR (Terao et a/., 1977 Shindo, 1980 Opella eta/., 1981 Nall... 

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