Infrared spectroscopy relation, vibrational frequencies

In general, though, Raman spectroscopy is concerned with vibrational transitions (in a manner akin to infrared spectroscopy), since shifts of these Raman bands can be related to molecular structure and geometry. Because the energies of Raman frequency shifts are associated with transitions between different rotational and vibrational quantum states, Raman frequencies are equivalent to infrared frequencies within the molecule causing the scattering. 

Infrared spectroscopy is a powerful method of studying molecules. For small molecules of known structure, it gives information on the force constants of chemical bonds, which can be related to their strengths. Larger molecules have more complicated spectra, arising from the existence of several different types of vibrational motion, with different frequencies and quantized energy levels. Studying these can help identify molecules and determine their structures. 

It should be noted that, in all the examples discussed so far, infrared spectroscopy produces information relating to the catalyst in an indirect manner, via hydroxyl groups on the support, or via the adsorption of probe molecules such as CO and NO. The reason why it is often difficult to measure the metal-oxide or metal-sulfide vibrations of the catalytically active phase in transmission infrared spectroscopy is that the frequencies are well below 1000 cm-1, where measurements are difficult due to absorption by the support. In this respect, infrared emission and Raman spectroscopy (see below) offer better opportunities. 

Vibrational spectroscopy has been used in the past as an indicator of protein structural motifs. Most of the work utilized IR spectroscopy (see, for example, Refs. 118-128), but Raman spectroscopy has also been demonstrated to be extremely useful (129,130). Amide modes are vibrational eigenmodes localized on the peptide backbone, whose frequencies and intensities are related to the structure of the protein. The protein secondary structures must be the main factors determining the force fields and hence the spectra of the amide bands. In particular the amide I band (1600-1700 cm-1), which mainly involves the C=0-stretching motion of the peptide backbone, is ideal for infrared spectroscopy since it has an large transition dipole moment and is spectrally isolated... 

Up to now, infrared spectroscopy has been used mainly to determine the types of hydroxyl groups and the acidity of zeolites (39). The frequencies of the vertical and horizontal vibrations (with respect to the cavity wall) of H2O molecules adsorbed in zeolite A were determined by measurements in the far infrared ( 220 and —75 cm" ) (37). These values are in agreement with a simple theoretical model. A number of ultraviolet and ESR studies are reviewed (33). The difference has been established between the specific molecular interaction of aromatic molecules on zeolites cationized with alkali cations and the more complex interactions involving charge transfer in CaX and deca-tionized X and Y zeolites. These more complex interactions with CaX zeolites containing protonized vacancies and with decationized zeolites are similar. These phenomena are related to the interactions of molecules with acidic centers in zeolites which are stronger, as compared with the molecular adsorption. 

S. Gnanakaran and R. M. Hochstrasser (2001) Conformational preferences and vibrational frequency distributions of short peptides in relation to multidimensional infrared spectroscopy. J. Am. Chem. Soc. 123, p. 12886... 

A related method that has been applied to Langmuir monolayers is sum-frequency generation (SFG) spectroscopy.The signal is generated by mixing an infrared wave of frequency (u,r with a visible wave of frequency CO to yield an output cosp. When co,r matches a vibrational frequency, there is a resonant enhancement of the output. The nonlinear polarization that governs the SFG process can be written... 

Thus, if a transition exists which is related to the frequency of the incident radiation by Planck s constant (h = 6.626-1 O 34), then the radiation can be absorbed. Conversely, if the frequency (v) does not satisfy Planck s expression, then the radiation will be transmitted. A plot of the frequency of the incident radiation against some measure of the percent radiation absorbed by the sample provides the absorption spectrum of the compound or component. The absorption spectrum is characteristic for the compound and this spectrum is often called the fingerprint of the compound. Infrared spectroscopy is based on the measurement of the absorption of electromagnetic radiation that arises from the altering of the vibration level of the component s molecule. An example of the adsorption and transmission of the infrared radiation is shown in Figure 2.30. 

Polymer Structure. In addition to visualization, profiling, thickness measurements and chemistry of polymer wear it is frequently desirable to know whether the polymer is in the amorphous or crystalline state because other properties relate to state. Raman spectroscopy is very useful in studying very low frequency modes associated with vibrations of polymer chain backbones and the lattice modes of polymer crystals. It complements infrared spectroscopy. 

In infrared spectroscopy, a beam of infrared light is passed through a sample, and light is absorbed if the frequency of the light is the same as the frequency of a normal mode of vibration. This method is sensitive to molecular bonding between atoms. Beer s law relates the absorbance (A) of a vibration to its absolute concentration ... 

Note that deuteration will decrease enormously the amplitude of peaks related to hydrogen, whereas in infrared (IR) spectroscopy there would only be a change in the vibrational frequency. That is, the information on molecular-scale dynamics can be multiplied if IR, Raman, and neutron spectroscopies are applied together. 

As mentioned above, different types of bonds absorb infrared radiation at different frequencies related to the local molecular environment, allowing the use of IR spectroscopy in the identification of compounds. Because of the complexity of molecular vibrations, no two compounds will have exactly the same IR spectrum. By matching the IR spectrum of an unknown compound to that of a compound in a database, a process known as fingerprinting, is a definitive method of identification. The figure shows the IR spectrum of a relatively simple molecule (CH2=CHC=N or 2-propenenitrile) and an assignment of the major peaks to different molecular vibrations. 

Quantum-chemical calculations [21,22] have shown that the ground state of CO4 is planar, with the C atom coordinated to three O atoms. The C2V cluster was detected [23] by infrared spectroscopy via its Vj vibrational frequency of about 1941 cm . Elliott and Boldyrev [22] also found a low-lying isomer of >2 symmetry, with the C atom coordinated to four O atoms. The calculations of Cabria and March [19] agree with those results. The energy of the >24 structure is 1.61 eV above the planar state (1.43 eV in the calculations of Elliott and Boldyrev [22]). Cabria and March [19] also found an isomer related to the structure, but with lower symmetry. In... 

More recently, Dong and Hill [49] used FT infrared (FT-IR) spectroscopy to study copolymer composition and monomer sequence distribution in styrene-methacrylonitrile copolymers. They determined the dependence of the frequencies of the individual spectral peaks on the copolymer composition, in particular, the vibration frequencies for the nitrile group is discussed. Correlations were established to relate changes in the peak positions to changes in the copolymer composition and monomer sequence distribution. Vibration band frequencies for blends of poly(methacrylonitrile) and polystyrene were examined to compare the effects of inter- and intra-chain interactions in these bands. 

Raman spectroscopy uses an incident laser beam that is focused on the sample. The intensity of the scattered light is measured as a function of its frequency. Bands appear in the spectrum that are shifted in frequency from the frequency of the incident light. Each shift in frequency corresponds to one of the vibrational frequencies of the molecule. Infrared spectroscopy measures the absorption of infrared light as a function of its frequency. Absorption bands appear in the spectrum that correspond to vibrational frequencies in the DNA. Infrared spectroscopy is hindered by the strong almost continuous absorption band of water while Raman spectroscopy benefits from the fact that water is a weak Raman scatterer. Infrared spectroscopy is of greater usefulness in the studies of films and fibers while Raman is of use in obtaining the vibrational frequencies of DNA in crystals, films, fibers, and aqueous solutions. A large amount of evidence, both theoretical and experimental, now exists which shows that there is a close relation between the conformation of a DNA and the frequencies and the intensities of certain bands in the Raman spectra. The theory for the relation between the frequencies and intensities and the DNA conformation is outlined in the next two sections. 

The vibrational and rotational frequencies of molecules can be studied by Raman spectroscopy as well as by infrared spectroscopy. While they are related to each other, the two types of spectra are not exact duplicates and each has its individual strong points. In Raman spectroscopy, only the wave-number is used. 

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