Predicting IR and Raman Spectra

Vibrational frequencies, used to predict IR and Raman spectra, are computed from the Hessian matrix, assuming a harmonic oscillator approximation. Errors in the... 

The predicted IR and Raman spectra, based on the data printed out from the vibrational analysis in O Fig. 10-6 are shown in O Fig. 10-7. These are direct screen captures from PQSView, which is the job output and visualization component of the PQSMol GUI (PQS 2010). For each spectrum, the frequency range (horizontal axis), the intensity range (vertical axis), and the band half-width (fitted using a Lorentzian band profile) can be varied it is also possible to zoom in on selected regions of the spectrum. 

The final SQM-predicted IR and Raman Spectra for 1,2-dichloroethane are shown in O Fig. 10-9 and O Fig. 10-10, respectively. In each case individual spectra for the trans and gauche conformers are given followed by a combined spectrum assuming a room-temperature distribution of 85% trans and 15% gauche. 

To predict the IR and Raman spectra of molecules (frequencies and intensities). 

We ve included several papers in the References section which perform theoretical and experimental studies of the IR and Raman spectra for these compounds. These compounds were among the earliest ab initio frequency studies of such systems. In addition, in the case of propellane, theoretical predictions of its energy and structure preceded its synthesis. 

Vibrational spectra are very sensitive to symmetry changes in a molecule. The splitting of bands in infrared (IR) and Raman spectra correlates with the point group of the molecule which changes when distortions appear. The nature of the splitting, i.e. the number of resulting bands in the distorted state, can be predicted from simple group theory considerations. 

Bands are observed in the IR and Raman spectra of a molecule that correspond to normal modes of vibration of that particular structure. These normal modes can be calculated from knowledge of the three-dimensional structure of the molecule and of its vibrational force field. If a reliable force field is available, it is therefore possible to predict the vibrational frequencies of a known or hypothesized structure, and to draw significant structural conclusions on the basis of comparisons with observed IR and Raman bands. 

The wideline NMR, IR, and Raman spectra of maleic hydrazide were determined and discussed in connection with its tautomeric forms. NMR techniques have been used to predict the structure of cyclization products from 3-carboxyacryloylhydrazines. The NMR spectra of JV-methyl-pyridazinium iodides and pyridazine iV-oxide were investigated, and the... 

Figure 2.31 shows the IR and Raman spectra of these stereoisomers. In the Os—Cl stretching region (350-300 cm ), the trans isomer (2b, exhibits one IR E and two Raman (A and bands, as expected from the selection rules. For the cis isomer (2a, C21 ), the selection rules predict four (2A, Bi, andB bands both in IR and Raman spectra. However, only three IR and two Raman bands are apparent in Fig. 2.24. Such... 

Also, the IR and Raman spectra of Al-free faujasite structures were compared. The spectra show bands in the asynunetric (1200-1000 cm ) and symmetric (850-700 cm ) stretching regions of the framework vibrations however, they contain a lower number of bands than predicted as IR- and Raman-active. Strong Raman bands around 500 cm were ascribed by Geidel... 

Theoretical calculation of the frequencies of the normal modes of vibration is possible and has been demonstrated for moderately complex molecules such as acetazolamide [34]. This facilitates assignment of the observed spectral bands beyond what may be possible by comparison with correlation tables and manual interpretation. The ability to predict the visual appearance of IR and Raman spectra is, however, more challenging since the inclusion of selection rules to ascertain which bands will be IR or Raman active and the calculation of their intensity must be considered. 

IR and Raman may be used to confirm results obtained from structure prediction software. In a study on eniluracil [35], the crystal structure was predicted from the X-ray powder diffraction pattern. Two feasible crystal sttuctures were identified in which the hydrogen bonding pattern was very similar with only the orientation of the non-bonded carbonyl differing between the two. The IR and Raman spectra of eniluracil support the presence of such a... 

Since the molecular symmetry is important in vibrational spectroscopy few explanations will be given shortly with regards to comments within the tables. For more details the reader is referred to basic literatures on the theory of molecular symmetry. Vibrations of functional groups of a polymer chain can be described by the so-called point groups. The point group theory means that during the vibration at least one point in the molecule remains unaffected. Furthermore, for molecular units with some symmetry, point group theory can be used to predict how many vibrational modes should appear in the IR and Raman spectra. As different molecular symmetries lead to different numbers of IR and Raman active bands this is a... 

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