Vibrational spectroscopy fingerprinting

One of the most common uses of vibrational spectroscopy is as a molecular fingerprinting tool whereby spectral features are assigned to the presence of particular fragments in molecules. These assignments are, however, only... 

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. 

Studies by Teplyakov et al. provided the experimental evidence for the formation of the Diels-Alder reaction product at the Si(100)-2 x 1 surface [239,240]. A combination of surface-sensitive techniques was applied to make the assignment, including surface infrared (vibrational) spectroscopy, thermal desorption studies, and synchrotron-based X-ray absorption spectroscopy. Vibrational spectroscopy in particular provides a molecular fingerprint and is useful in identifying bonding and structure in the adsorbed molecules. An analysis of the vibrational spectra of adsorbed butadiene on Si(100)-2 x 1 in which several isotopic forms of butadiene (i.e., some of the H atoms were substituted with D atoms) were compared showed that the majority of butadiene molecules formed the Diels-Alder reaction product at the surface. Very good agreement was also found between the experimental vibrational spectra obtained by Teplyakov et al. [239,240] and frequencies calculated for the Diels-Alder surface adduct by Konecny and Doren [237,238]. 

Vibration spectroscopy is also able to measure the concentration of ion radicals (by estimation of the band intensities). Moreover, the IR intensities of some bands in the fingerprint region for organic ion radicals may be much larger than the intensities of the bands for the neutral parent molecules. The examples are polycyclic aromatic hydrocarbons or linear polyenes and their ion radicals. The vibration patterns of the intensity-carrying modes are closely related to the electronic structure of the ion radicals (Torii et al. 1999 and references therein). 

Single-molecule vibrational spectroscopy uses a measurable change in conductance across the onset for vibrational excitation of the different modes of an adsorbed molecule to identify its vibrational fingerprint. The tip of an STM is placed on the molecule and the voltage is ramped up. When the energy given to the electrons matches a quantum of vibration, the conductance changes abruptly the STM has measured the frequency of a mode of an adsorbed molecule. 

At this point a characterization technique with a higher chemical resolution is desirable because such functionalization plus surface analytical combination experiments are extremely difficult to perform in a clean and reproducible way. Vibrational spectroscopy such as FT-1R has been developed into such a tool, after several methodical improvements concerning sample preparation and detector sensitivity. In situ oxidation experiments are still very difficult as heated black carbon is a perfect 1R emission source and interferes with any conventional detection in the spectral range of carbon-oxygen fingerprint vibrations. 

The use of infrared spectroscopy, either through fingerprint characterisation or by functional group identification, is well established. IR vibrational spectroscopy has thus been applied in spectroelectrochemistry for quite some time. ° The possibility to establish the symmetry of a molecule has made IR-SEC a most valuable tool for mixed-valence chemistry, ° allowing intramolecular electron-transfer rates in the picosecond region to be assessed and electron-transfer isomers to be established. ... 

Prior to characterization encapsulation must be ensured and clusters formed outside the cavities must be ruled out. Only then can characterization be reliably carried out. A battery of techniques is available for this purpose, such as C,Xe and metal NMR, EXAFS/XANES, XPS, IR and UV-VIS spectroscopy, electron microscopy, ESR, XRD, etc. Among these methods electronic spectroscopy plays an important role. The UV-VIS spectra reflect changes in the oxidation state of the metal as well as structural changes forced by incarceration and so serve as a valuable tool for the ascertainment of intrazeolite complexation. Although vibrational spectroscopy is most frequently applied, sometimes using the IR spectra as fingerprints for identification, it is inadequate to predict the exact structure of the clusters as these spectra maybe different from those in solution or in the sofid state due to interaction with the zeolite matrix. In any case, reliable characterization requires the combined application of complementary analytical methods. 

The search for faster screening methods capable of characterizing propolis samples of different geographic origins and composition has lead to the use of direct insertion mass sp>ectrometric fingerprinting techniques (ESf-MS and EASI-MS), which has proven to be a fast and robust method for propoHs characterization (Sawaya et al., 2011), although this analytical approach can only detect compoimds that ionize under the experimental conditions. Similarly, Fourier transform infrared vibrational spectroscopy (FITR) has also demonstrated to be valuable to chemically characterize complex matrices such as propolis (Wu et al, 2008). 

Raman spectroscopy is another form of vibrational spectroscopy that is subject to different selection rules from IR spectroscopy and therefore complementary to it. Raman spectroscopy has, for example, been used to fingerprint the framework region of zeolites (interpreting spectra in terms of characteristic building units, for example) and to investigate the incorporation of transition metals in the framework, such as titanium. Raman spectra of titanosilicates give characteristic resonances at 1125 and 960 cm, for example. 

Vibrational spectroscopy is a powerful tool for the study of molecular structure and dynamics. The typical vibrational frequency range of this spectroscopy is 100-4000 cm, which corresponds to the energy range 0.3-12 kcal/mol. Because the resolution of vibrational spectroscopy is on the order of 5 cm , the band shift on this order corresponds to a 0.02 kcal/mol. Vibrational transitions are correlated with specific vibrational motions by inspection of the transition frequencies. From identification of these fingerprint vibrational modes, conclusions can be drawn on specific structural motifs in the molecules. Vibrational transitions have bandwidths typically smaller (10-20 cm ) than those from electronic transitions (typically 200-2000 cm ), and it is thus less probable that different transition bands overlap in vibrational spectroscopy than in electronic spectroscopy. In addition, small molecular species may always be probed through their vibrations, and electronic transitions. Major disadvantages of vibrational spectroscopy, on the other hand, are the inherent lower cross sections of vibrational transitions and the frequent overlap of the absorption bands with those of the solvent [10]. 

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