Spectrum prediction semiempirical methods

The MO-type calculation has been employed by Bramanti et al. (4) to explain the absorption spectrum of Tl+ in KC1. The calculated positions of the TT energy levels in KC1 explained changes in the spectrum on going from free ions to the solid state. Similarly, in the case of hydrogen impurity in LiF, Hayns (5) has shown that the excitation energy predicted by calculation is in accord with experimental results. These results have inspired confidence in the semiempirical method as a means for providing qualitative explanations for several electronic phenomena. 

Thus, among of the used semiempirical methods only PM6 provides a mathematical model for predicting of the normal vibrations frequencies position of the carbonyl group in the IR spectrum of the benzoyl peroxide symmetric derivatives. 

Many attempts have been made in the recent years to calculate IR spectra on the basis of semiempirical or ab initio methods. Since the computational times required are rather high, the practical use of these techniques is limited. In both NMR and IR spectroscopy, spectrum prediction techniques based on structure-oriented libraries are the methods of choice. 

The most common approaches to predicting spectra are based on empirical modeling, linear additivity, database retrieval, rule sets, and semiempirical methods. The availability of large spectral libraries has proved to be a valuable resource in these studies. Although ab initio theory relating to spectrum prediction is well advanced, the theoretical equations necessary for application to real-world structures are large and complex... 

Several sets of theoretical calculations have been performed on the parent ring system. HMO calculations of total rr-electron densities and frontier electron densities successfully predicted that the nucleus would undergo electrophilic substitution at the 6- and 8-positions. Two groups, " have compared the electronic structure of indolizine and various aza derivatives using the SCF or semiempirical antisymmetric configuration interaction method. The results allowed interpretations of the electronic spectrum to be made which were in good agreement with experiment. 

A positive sign of the spin-spin coupling constant J(P-H) (see Table 13) was established by the NMR spectrum of PH3 taken in a liquid-crystal solvent (A/-(p-ethoxybenzylldene)-p -n-butylaniline, EBBA) [3], supporting the prediction J(P-H)>0 from semiempirical calculations [20]. Ab initio [21,22, 23] and semiempirical [24, 25,26] calculations of J(P-H) were reported. A qualitative perturbation model [27] and a nonempirical double perturbation method [28] were applied to calculate J(P-H) in PH3. 

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