Molecular orbital calculations thermochemistry

Even though the problem of the hydrogen molecule H2 is mathematically more difficult than, it was the first molecular orbital calculation to appear in the literature (Heitler and London, 1927). In contrast to Hj, we no longer have an exact result to refer to, nor shall we have an exact energy for any problem to be encountered from this point on. We do, however, have many reliable results from experimental thermochemistry and spectroscopy. 

There have been a number of theoretical studies describing the thermochemistry and structural properties of SiC Hm+ cations. Ketvirtis and coworkers investigated several small organosilicon cations by means of ab initio molecular orbital calculations, and found that, in general, the isomers which contain Si atoms that have no hydrogens bonded to them are considerably more stable than other isomers. This certainly is a consequence of the significantly weaker Si—H bond strength as compared to that of C—H bonds. 

The continuous development and implementation of molecular orbital theory ab initio methods have enlarged the applications to this area too. Indeed, the impact of theoretical calculations in thermochemistry is substantial. Experimental groups often use calculations as a supplement to the interpretation of their results. In this section we will mention a few recent and representative studies that are directly associated with the bond dissociation energies of silanes. Early theoretical investigations of the Si—H bond strength in silanes have been summarized [13]. 

The use of computational chemistry to address issues relative to process design was discussed in an article. The need for efficient software for massively parallel architectures was described. Methods to predict the electronic structure of molecules are described for the molecular orbital and density functional theory approaches. Two examples of electronic stracture calculations are given. The first shows that one can now make extremely accurate predictions of the thermochemistry of small molecules if one carefully considers all of the details such as zero-point energies, core-valence corrections, and relativistic corrections. The second example shows how more approximate computational methods, still based on high level electronic structure calculations, can be used to address a complex waste processing problem at a nuclear production facility (Dixon and Feller, 1999). 

Developments in theoretical calculations have provided much information on the structure and thermochemistry of relatively complicated heterocyclic compounds. An ab initio self-consistent-field molecular orbital analysis of structural features of pyrazine has been carried out by 4-3IG <85JOC4894>, 3-21G/STO-5G <89JST(187)95> and 6-31G /STO-5G levels <91JST(236)283>. The optimized structure of pyrazine, computed at the 6-3IG level, is represented by formula (7) which is... 

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