Additional Information Obtained from Semi-empirical and Ab Initio Calculations

Additional Information Obtained from Semi-empirical and Ab Initio Calculations 

In addition to the obvious structural information, vibrational spectra can also be obtained from both semi-empirical and ab initio calculations. Computer-generated IR and Raman spectra from ab initio calculations have already proved useful in the analysis of chloroaluminate ionic liquids [19]. Other useful information derived from quantum mechanical calculations include and chemical shifts, quadru-pole coupling constants, thermochemical properties, electron densities, bond energies, ionization potentials and electron affinities. As semiempirical and ab initio methods are improved over time, it is likely that investigators will come to consider theoretical calculations to be a routine procedure. 

Jensen, Introduction to Computational Chemistry, John Wiley Sons, 1999, pp. 53-97. 

Van Duijneveldt, Molecular Interactions (S. Scheiner ed.), John Wiley Sons, 1997, pp. 81-104. 

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In addition to the chemistry that may be addressed, ab-initio calculations are needed to provide important guidance on which to develop semi-empirical and empirical schemes that use, in fact, the same formalism. There is an important interplay between all methods, and the information that we gain from ab-initio calculations is not only used to examine the chemistry of the model systems, but also used to help develop semi-empirical methods. The numbers obtained are often used to guide parameter choices where these are not available from experiment. Most importantly, discoveries in the underlying theory that we use are made in semi- empirical, density functional and ab-initio studies, and these improvements are adopted, perhaps in modified form, to affect all theories used in computational chemistry. 

The electronic structure of tetrahedral oxyanions and their derivatives has been extensively studied by many authors during the past decades. The earlier attempts were summarized by Prins [1]. Since the work of Walsh [2] and that of Wolfsberg and Helmholz [3] several semi-empirical theoretical studies have been published [4-7]. Later ab initio [8-10] and scattered wave calculations [11] have been also reported. Among the experimental investigations, Prins mentioned the electron spin resonance measurements of radicals formed by ejection or addition of an electron from or to certain oxyanions, obtaining information on just those molecular orbitals which contained unpaired electrons. [12] X-ray absorption and emission studies provided usefial information on a limited number of molecular orbitals in the valence band [13-19]... 

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