Quantum chemical calculations consequence

The first-principles calculation of NIS spectra has several important aspects. First of all, they greatly assist the assignment of NIS spectra. Secondly, the elucidation of the vibrational frequencies and normal mode compositions by means of quantum chemical calculations allows for the interpretation of the observed NIS patterns in terms of geometric and electronic structure and consequently provide a means of critically testing proposals for species of unknown structure. The first-principles calculation also provides an unambiguous way to perform consistent quantitative parameterization of experimental NIS data. Finally, there is another methodological aspect concerning the accuracy of the quantum chemically calculated force fields. Such calculations typically use only the experimental frequencies as reference values. However, apart from the frequencies, NIS probes the shapes of the normal modes for which the iron composition factors are a direct quantitative measure. Thus, by comparison with experimental data, one can assess the quality of the calculated normal mode compositions. 

The solid-state structures of orthorhombic S864 and monoclinic Se866 are shown in Figure 15. As may be seen from the average E-E-E bond and EE EE torsion angles, this structure is almost completely relaxed and consequently lone pair repulsion and ring strain are lowest in this arrangement. Other S8 structures were assessed by quantum chemical calculations.69... 

The black box situation of SCF applications has not yet been reached for the multiconfigurational SCF theory. This constitutes a major problem, since MCSCF is a much better starting point for quantum chemical calculations on many interesting chemical problems (a good example is studies of transition states for chemical reactions). A development towards more automatized procedures can consequently be expectedto take place in MCSCF theory too. 

The quantum chemical calculations on Fe(CO)5 indicate that the lowest-energy accessible excited state is MC in character. The oscillator strength for the ground state to MC transition is small and most ultrafast experiments use either single or multiphoton excitation to MLCT states. The difference in product distribution depending on the excitation pulse duration points to an enhanced absorption cross section for the Fe(CO)5 excited state over the ground-state species to both pump and probe pulses. This tends to complicate the apparent photochemistry. Consequently the use of short-pulse single-photon excitation provides a better picture of the excited-state dynamics. 

X-ray structure determination is used as a constant in the quantum chemical calculation (and all other parameters fully optimized), the energy is only 1.5 kcal/mol higher than that of the free optimized molecule as a consequence of the flat potential energy surface for the B-N bond. 

This review will focus on theoretical calculations to advance understanding of gas phase oxidafion of gaseous elemental mercury (GEM) by halogen species. Computational and experimental studies to help parameterize models have been performed to make a more reliable description of the dynamics of mercury in the atmosphere so that the consequences of abatement strategies can be assessed. Quantum chemical calculations are the only way to viably investigate the mechanisms and advance what is observed in field and laboratory studies. 

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