Vibrational spectra predictions

The computational prediction of vibrational spectra is among the important areas of application for modem quantum chemical methods because it allows the interpretation of experimental spectra and can be very instrumental for the identification of unknown species. A vibrational spectrum consists of two characteristics, the frequency of the incident light at which the absorption occurs and how much of the radiation is absorbed. The first quantity can be obtained computationally by calculating the harmonic vibrational frequencies of a molecule. As outlined in Chapter 8 density functional methods do a rather good job in that area. To complete the picture, one must also consider the second quantity, i. e., accurate computational predictions of the corresponding intensities have to be provided. 

Fig. 18. The hindered translational region of the vibrational spectrum as predicted by the Wores Rice model (from Ref. 64>)...

In many applied areas, calculations can be used to screen a variety of materials to select materials with useful properties. In this case, if DFT calculations (or any other kind of theoretical method) can reliably predict that material A is significantly better than material B, C, D, or E for some numerical figure of merit, then they can be extremely useful. That is, the accuracy with which trends in a property among various materials can be predicted may be more important than the absolute numerical values of the property for a specific material. In other venues, however, the precision of a prediction for a specific property of a single material may be more important. If calculations are being used to aid in assigning the modes observed in the vibrational spectrum of a complex material, for example, it is important to understand whether the calculated results should be expected to lie within 0.01, 1, or 100 cm-1 of the true modes. 

An impressive application of infrared and Raman spectroscopy was demonstrated in studies of superelectrophilic diprotonated thiourea, [H3NGSH jNfE 2+ 2AsFf,. The Raman spectrum (taken at — 110°C) corresponded reasonably well with calculated vibrational bands predicted by density functional theoiy.38 Coupled with computational methods for predicting vibrational frequencies, it is expected that vibrational spectroscopic techniques will be useful for the observations of these and other superelectrophiles. 

Experimental information on the molecular vibrations of the dimers is restricted to a few, mostly, tentative assignments in matrix-IR spectra (Wesley and DeKock, 1971 Hastie et al., 1975 Feltrin and Cesaro, 1996). On the other hand, the complete vibrational spectrum has been computed for La22f6, l-)y2AV Ce2Br6 and Ce2l6 (Kovacs, 1999, 2000 Kapala et al., 2002) and the data are summarised in table D.6. However, due to the failure of the computations to accurately predict the bending frequencies of the monomers at low wavenumbers, the reliability of these frequencies of the dimers may be limited also. 

Ab initio SCF-MO calculations327 and Hartree-Fock-Slater studies328 predict that the polarization of coordinated N2 will be Ru—N—N. Bands in the vibrational spectrum have been assigned329,330 to the N=N and Ru—N(N2) stretching modes. The frequency and intensity of rN=N are found to be dependent upon counterion and solvent, due to electrostatic interactions in solution.188,331 Mossbauer and 15N NMR studies on [Ru(NH3) N2]2+ have been reported.270 120,332,333 Linkage isomerism of coordinated N2 has been detected in the solid state (t1/2 = 2 1 days at 22°C in the dark) and solution (t1/2 = 2h at 25°C),318 a weak 7i-bonded intermediate being postulated318 (equation 34). 

VO(acac)2. - The predicted vibrational spectrum of this molecule is shown in Table 1 along with the observed spectrum ... 

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