Fingerprinting spectra, fingerprint region

Fingerprint region (Section 13 20) The region 1400-625 cm of an infrared spectrum This region is less character istic of functional groups than others but varies so much from one molecule to another that it can be used to deter mine whether two substances are identical or not Fischer esterification (Sections 15 8 and 19 14) Acid cat alyzed ester formation between an alcohol and a carboxylic acid... 

Fingerprint region (Section 12.7) The complex region of the infrared spectrum from 1500 to 400 cm-1. 

A further aid to identification comes from the fact that a molecule commonly has complex series of absorptions spanning a range of wavelengths. This fingerprint region of the spectrum may be too difficult to analyze in detail, but its presence enables us to recognize the substance by comparing the spectrum to an atlas of spectra. 

Figure 3. Fingerprint region of the COSY spectrum of RpII in H O showing the amide to a proton connectivities. Cross peaks are labeled with the identity of the amino acid in the sequence from which they arise.

The next most useful is vibrational spectroscopy but identification of large molecules is still uncertain. In the laboratory, vibrational spectroscopy in the infrared (IR) is used routinely to identify the functional groups in organic molecules but although this is important information it is not sufficient to identify the molecule. Even in the fingerprint region where the low wavenumber floppy vibrational modes of big molecules are observed, this is hardly diagnostic of structure. On occasion, however, when the vibrational transition can be resolved rotationally then the analysis of the spectrum becomes more certain. 

It is interesting to observe here that this particular region of the spectrum is densely populated with bands. As we know that no two fingerprints could be identical in human beings, exactly in a similar manner no two compounds may have the same fingerprint region . Thus, each and every molecule essentially gives rise to a unique spectrum which offers a characteristic feature of the same. 

The fingerprint region lies between 1300-400 cm-1 which is considered to be the most valuable component of the spectra and mainly comprises of a specifically large number of unassigned vibrations. Therefore, IR-spectroscopy aids in the identification of unknown compound by comparing its spectrum with a standard spectra recorded under exactly similar experimental parameters. Thus, pharmaceutical substances that exhibit the same infrared spectra may be inferred as identical. 

Albertidine, isolated from Leontice Albertii Rgl. (196,197), is a crystalline, optically active tribase. There are IR absorption bands for a trani-quinolizidine system at 2750 and 2793 cm and a six-membered lactam carbonyl at 1640 cm The absorption in the fingerprint region is similar to that of matrine. The mass spectrum of albertidine is characterized by ion peaks at miz 247 (M -1), 219, 205, 192, 177, 150, 137, 98, and 96 which are typical for matrine alkaloids (209). On the basis of spectroscopic data and taking into account the tranj-quinolizidine band in the IR spectrum, the probable structure 182, with rings A/B-trans, was proposed. 

After the on-resonant imaging, the frequency of t 2 was changed snch that the frequency difference corresponds to none of the Raman-active vibrations ( off-resonant ). Figure 10.11 shows a normal Raman spectrum of the DNA in a part of the fingerprint region. The solid arrows on the spectrum denote the frequencies adopted for the on-resonant and off-resonant conditions in CARS imaging. 

The picosecond IR absorption spectrum of the tS state in the fingerprint region is different in w-heptane and in acetonitrile. The spectrum recorded for Si tS in the nonpolar solvent w-heptane is consistent with a species that has a center of symmetry. In acetonitrile, the spectrum exhibits additional weak bands near 1570, 1250, and 1180 cm which are approximately at the same frequencies as strong Raman bands assigned to in-plane vinylic vibrational modes in 5i. This result was taken to suggest a molecular structure for 5i that lacks a center of symmetry in acetonitrile. However, because the intensities of these three bands are weak, it was concluded that either the polarization of 5i or the contribution from polarized S structures to all of the S structures in acetonitrile may be small. 

Between 42 and 24 THz (1400 and 800cm ) there are many peaks which are difficult to interpret. However, this range, called the fingerprint region, is useful for determining whether compounds are identical. (It is virtually impossible for two different organic compounds to have the same ir spectrum, because of the large number of peaks in the spectrum.)... 

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