Direct comparison of spectra

Barlin and Batterham223 have studied the effects of solvent on chemical shifts of the anionic and cationic species of imidazoles. Protonation shifts, obtained by direct comparison of spectra in deuteriochloroform and trifluoroacetic acid, observed for 1-methyl-imidazoles are consistent with stabilization of the resulting cations by an amidinium-type resonance (47). Thus, for 1-methylimidazole, which... 

In addition, grease, oil, waxes and paraffins can be detected by IR spectroscopy, either by a direct comparison of spectra from the stain and from unstained areas, with the possibility of subsequent subtraction of spectra, or after extractions and concentration by spectroscopy of the extraction residue. 

This commonality in the radiolysis was established not only for cooked meats irradiated to high doses at -40°C, but also for raw meats irradiated to lower doses at 77 K [29]. The ESR spectra for raw pork, beef sirloin, and chicken breast (Figure 14) show the singlets associated with radicals formed by electron addition to the carbonyl groups, the yields of which linearly increased with dose. After annealing at -78°C, the spectral features changed to the predominant asymmetric doublet associated with the peptide backbone radical. Moreover, a direct comparison of spectra at -78°C for raw and roasted turkey breasts irradiated to 3.8 kGy showed no differences, indicating that native and denatured conformers of the protein respond radiolytically in similar ways [29]. 

On direct comparison of stephanaberrine (26) and metaphanine (2) (75), close similarity of the H-NMR and mass spectra was observed. The stereochemical problem of the ethylamine linkage was resolved by the fact that the optical activity of 26, [a]D —47.5°, agrees with that of 2, [a]D —41.4° (72). Thus, structures 18 and 26 were proposed for prostephanaberrine and stephanaberrine, respectively (72). 

Variations on the spectral peaks from different species of the same genus were also observed. Three species of Pseudomonas produced the spectra shown in Figure 14.2. These spectra are clearly unique and were used to correctly identify unknown samples. Because of peak ratio reproducibility issues in bacterial protein profiles obtained by MALDI MS,11 a fingerprint approach that had been used for other mass spectrometry approaches has not been used. The profile reproducibility problem was first recognized by Reilly et al.12,13 and later researched by others in the field.14,15 As a later alternative, a direct comparison of the mass-to-charge ratio (m/z) of the unknown mass spectral peaks with a database of known protein masses has been used to identify unknown samples.14... 

In a attempt to compensate for poor long-term reproducibility in a longterm identification study, Chun et al.128 applied ANNs to PyMS spectra collected from strains of Streptomyces six times over a 20-month period. Direct comparison of the six data sets, by the conventional approach of HCA, was unsuccessful for strain identification, but a neural network trained on spectra from each of the first three data sets was able to identify isolates in those three datasets and in the three subsequent datasets. 

Secondly, the five additional catheter samples (2 through 6) were analyzed using H NMR spectroscopy to provide a direct comparison of these samples to the results for catheter sample 1. Approximately 30 mg of each additional sample was dissolved in 1 mL of deuterated 2,2,2-trifluoroacetic acid, and these solutions were analyzed using 1H NMR analysis. The 1H NMR spectra of all five of these... 

The detection of a chromophore permits us to deduce the presence of a structural fragment or a structural element in the molecule. The fact that it is the chromophores and not the molecules as a whole that give rise to spectral features is fortunate, otherwise spectroscopy would only permit us to identify known compounds by direct comparison of their spectra with authentic samples. This "fingerprint" technique is often useful for establishing the identity of known compounds, but the direct determination of molecular structure building up from the molecular fragments is far more powerful. 

Molecules generally have a large number of bonds and each bond may have several IR-active vibrational modes. IR spectra are complex and have many overlapping absorption bands. IR spectra are sufficiently complex that the spectmm for each compound is unique and this makes IR spectra very useful for identifying compounds by direct comparison with spectra from authentic samples "fingerprinting"). 

Once an identification has been made, the name and registry number of the data base compound are reported to the user. If necessary, the data base spectrum can be listed or, if a CRT terminal is being used, plotted, to facilitate direct comparison of the unknown and standard spectra. 

Later in 1978, Masamune reported the first matrix Fourier transform IR spectrum of cyclobutadiene that now allowed a direct comparison of the observed spectrum of 1 with the calculated spectra. In Figure 6, Masamune s experimental spectrum is compared with the computed IR spectra (STO-4G and 4-3IG basis sets). It is seen that both calculated spectra, which were performed with relatively small basis sets, are in good qualitative agreement with the observed spectrum of 1. One could therefore conclude that the ground state structure of cyclobutadiene is indeed rectangular. 

One has to be very cautious in interpreting, e.g., valence band spectra of actinides generally a direct comparison of valence band spectra with the density of states as derived from band ground state calculations is not appropriate. 

The analytical chemist can use Raman and infrared spectra in two ways. At tile purely empirical level, they provide fingerprints of the molecular structure and, as such, permit the qualitative analysis of individual compounds, either by direct comparison of the spectra of the known and unknown materials run consecutively, or by comparison of the spectrum of the unknown compound with catalogs of reference spectra. 

Gibberellin A5 and bean factor II, isolated by West and Phinney (18) from the seed of French bean, are identical. West has kindly made a direct comparison of the infrared spectra, which are identical. 

The present study was initiated to provide a direct comparison of IETS and IR spectra for an identical molecule adsorbed on an aluminum oxide covered, evaporated aluminum substrate. Further, it was of interest to see if a weakly acidic C-H bond, such as that present in 1,3-dialkanediones, would show dissociative chemisorption similar to the well-known chemisorptions of Bronsted acids containing acidic O-H bonds (see above). The molecules chosen for this study were acetic acid and 2,4-pentanedione. Both oxide covered copper and aluminum were used as substrates in order to see the effects of substrate oxide on the chemisorption spectra. 

Fig. 13 Direct comparison of the experimental Sc-L23 x-ray absorption spectra of Sc2 C84 (line+symbols) with a simulation (grey solid line) based upon an atomic multiplet model with an initial state composed of 61% d° and 39% d L configurations. For details,see Ref. [34]...

Zero-order first and second derivative UV absorption spectra of 18 purines and pyrimidines were determined in aqueous solution at 298 K. The procedure, based on the direct comparison of the derived maximum and minimum wavelengths and of the sequences of relative amplitudes, can be used for concentrations of purines as low as 5 x 10-6 M <85MI 7li-0l>. A similar study has shown that second derivative spectra can be used for the identification of eight mixtures of purines and pyrimidines <88TAL513>. The structure and equilibrium of purine in ethanol has been studied by UV spectroscopy which indicates the existence of annular automery in purine and the presence of associated species of only one of its tautomers <88JST(174)83>. Both polarized UV and Raman... 

FIGURE 16 Re L3-edge XANES of rhenium metal (Re0)—5d5 (solid) Re02 (Re4+)—5d (dashed) and NH4Re04 (Re7+)—5d° (dash-dot). The XANES spectra have been aligned in energy to allow a more direct comparison of the white line intensities. 

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