Comparison of spectra

One of the most positive identification methods of an organic compound is to find a reference IR spectrum that matches that of the unknown compound. This approach is facilitated using spectral libraries as compiled by various societies or 

The algorithm using the absolute difference is the simplest for each of the i spectra in the library the sum Sj, for the n defined points along the abscissa, is calculated as the absolute difference between the corresponding absorbance of the unknown and that of spectrum j in the library (expression 10.10). Then the i summations Sj are classified in increasing order. The results are presented along with an index of correlation. 

Other expressions can be chosen replacing the differences above by their squares or by the differences of these squares or taking the incremental difference between two consecutive points. Each algorithm may minimize or increase the value of particular factors (differences of intensity, signal-to-background improvement, difference in gradient.. . ). 

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This broad band at 1500 cm was ascribed by Kaufman. Metin, and Saper-stein [10], to an IR observation of the amorphous carbon Raman D and G bands. This is forbidden by the selection rules, and has been attributed to the symmetry breaking introduced by the presence of CN bonds in the amorphous network. As carbon and nitrogen have different electronegativities, the formation of CN bonds gives the necessary charge polarity to allow the IR observation of the collective C=C vibrations in the IR spectrum. This conclusion was stated by the comparison of spectra taken from films deposited from N2 and N2. In the N2-film spectrum, no shift was observed for the 1500-cm band, whereas all other bands shifted as expected from the mass difference of the isotopes. Figure 25 compares... 

Figure 7.37 Comparison of spectra of different TLC plates (a) silica gel (b) cellulose and (c) polyamide 11. After Everall et al. [779]. From N.J. Everall et al., Applied Spectroscopy, 46, 597-601 (1992). Reproduced by permission of the Society for Applied Spectroscopy...

In principle, all of the methods for selective oxidations of di- and polyenes28 can be employed for analytical derivatization. However, the complexity of products obtained rules out some of these. Despite this, epoxidation of selected double bonds is used for comparison of spectra (see previous discussion on MS). Epoxidation of isolated non-conjugated... 

This measure is equivalent to the correlation coefficient between two sets of mean-centered data—corresponding here to the vector components of xA and xB. It is frequently used for the comparison of spectra in IR and MS. 

This assures better reproducibility of spectra, and therefore allows comparison of spectra obtained from different mass spectrometers or from mass spectral databases (Chap 5.7). 

Fig. 10.8. Comparison of spectra of the organic dye pigment red 144 as obtained by (a) LDI, (b) solvent-based and (c) solvent-free MALDI sample preparation. Adapted from Ref. [109] by permission. John WUey Sons, 2001. 

Figure 2.26 Comparison of spectra of cyclohexane extracts of floated pyrrhotite with that of sulphur dissolved in cyclohexane (Heyes and Trahar, 1984)...

First introduced to polymer chemistry by Schaefer and collaborators, CP-MAS spectroscopy has already yielded interesting results in both stractural and dynamic studies. The comparison of spectra in solution and in bulk permits identification of frozen conformations, distinction between spectra of crystalline and amorphous phases and measurement of the rate of several eonformational transitions. For example, the C spectrum of the poly(phenylene oxide), 74, in solution consists of five signals while the CP-MAS spectrum displays six. In the solid state the resonance of the aromatic CH appears split into two components. The phenomenon is attributed to the forbidden rotation of the benzene ring around the O. .. O axis, which makes the two carbon atoms indicated with an asterisk no longer equivalent. 

A precise treatment of the vibrations of a complex molecule is not feasible thus, the IR spectrum must be interpreted from empirical comparison of spectra and extrapolation of studies of simpler molecules. Many questions arising in the interpretation of an IR spectrum can be answered by data obtained from the mass and NMR spectra. 

Finally, in a fingerprint comparison of spectra, or any other situation in which the shapes of peaks are... 

Figure 20-16 Comparison of spectra recorded in 5 min by a photomultiplier tube and a charge coupled device. [From P. M. Epperson. J. V. Sweedler, R. B. Bilhom, G. R. Sims, and M. B. Denton, Applications of Charge Transfer Devices In Spectroscopy." Anal. Chem. 1988,60,327A.]... 

Considerable effort has been made to explain the IR spectra of M2(OEt)10 compounds. By using 180 labelling and by making careful comparisons of spectra, both the bridging and terminal v(M—O) vibrations were identified.165... 

An H-NMR study of thalsimine (Section 1I,B,13) showed that the alkaloid exists at room temperature as a mixture of two conformers. The spectrum in perdeuteriopyridine shows 10 OMe and 2 NMe peaks, but on heating the sample to 95°C, the spectrum collapses to the expected pattern (13). The NMR spectra of bisbenzylisoquinolines are often very sensitive to temperature or variations in acidity or basicity, making exact comparisons of spectra with literature values difficult (53). 

Although only one peak is indicated by the GSR, a comparison of spectra obtained from the front of the peak with those which elute in the later half of the peak can be made, when this technique was applied to these data, it was found that certain changes in the spectra were observed. Figure 3 shows the first 6 spectra in the C-H stretching region obtained over the major peak in the GSR. Each of these spectra represent 8 coadded files, resulting in a time resolution of about 4 seconds per file. Infrared spectra taken from the front of the peak show the lowest methyl to methylene ratio. As each subsequent file is ratioed to keep the methylene peak constant, the absorbances due to methyls increase over the width of the peak. Subsequent files show a leveling of this ratio. 

The first application we would like to discuss involves the analysis of thiofanox formulated on a clay carrier. Two specific questions for which FTIR provided answers were (1) are there interactions between the compound and the carrier material and (2) is the active ingredient completely removed by solvent extraction The answer to these questions required a comparison of spectra from the blank carrier and the formulated material before and after extraction. These spectra were obtained by thoroughly grinding each sample, mixing with KBr, and pressing... 

Finally, in a fingerprint comparison of spectra, or any other situation in which the shapes of peaks are important, we should be aware of the substantial differences in the appearance of the spectrum in changing from a spectrum that is linear in wavenumber to one that is linear in wavelength (Figure 2.6). 

The main purpose of FFT or FHT of discrete spectra is not the reduction of storage space because a tabular representation of peaks would be usually short enough (see Chapter 4). Nevertheless, an efficient (w ith a respect to the computation time) manipulation and comparison of spectra calls for short and uniform representation and the use of reduced transforms is justified and helpful, especially w hen searching spectral collections resident personal computers. 

Compound 18 is a mono-antennary, lower-branch, asialo oligosaccharide that has been isolated from the urine of patients suffering from GM,-gangliosidosis52,64 643 or from64,65 Morquio syndrome type B. The 500-MHz, H-n.m.r. spectrum of the relatively pure pentasaccharide is presented in Fig. 18b. The signals corresponding to 18 in Fig. 18a can readily be traced by comparison of spectra 18a and 18b. The chemical shifts of its structural-reporter groups are summarized in Table VIII. 

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