Saturday, January 20, 2007

Comparison between valence bond theory and molecular orbital theory

In some areas, valence bond theory is better than the molecular orbital theory. When applied to the two-electron molecule, H2, valence bond theory, even with Heitler-London approach to the most simple, to provide energy approach closer ties and a more accurate representation of the behavior of electrons when chemical bonds are formed and broken. In contrast, molecular orbital theory predicts that the hydrogen molecules will dissociate into a linear superposition of the hydrogen atom and hydrogen ions positive and negative. This prediction does not match the physical description. This in part explains why the total energy curve of the distance between atoms on the valence bond method is above the curve using the molecular orbital method. This situation occurs in all diatomic molecules homonuklir and appears clearly in F2 when the minimum energy curve using molecular orbital theory is still higher than the two atomic energy F.

The concept of hybridization is very useful and variability in bonding in most organic compounds are very low, leading to this theory is still an integral part of organic chemistry. However, the work of Friedrich Hund, Robert Mulliken, and Gerhard Herzbergmenunjukkan that molecular orbital theory provides a more precise description of spektrokopi, ionization, and magnetic properties of molecules. Lack of valence bond theory become clearer in the berhipervalensi molecules (eg, PF5) when the molecule is described without the use of d orbitals is very crucial in the bonding hybridization proposed by Pauling. Metal complexes and compounds yangkurang electrons (such as diborana) is described very well by molecular orbital theory, although the explanation that using valence bond theory has also been made.

In 1930, two competing methods to realize that both are only an approximation to the theory better. If we take the simple valence bond structure and incorporate all covalent and ionic structures are possible on a group of atomic orbitals, we get what is called full configuration interaction wave function. If we take the simple molecular orbital description of the ground state and to combine these functions with the functions that describe the whole possibility of excited states using unfilled orbitals from the same set of atomic orbitals, we also get a full configuration interaction wave functions. Seen that the simple molecular orbital approach is too focused on the structure of the ion, whereas the valence bond theory is simple too little emphasis on ion structure. Can we say that the molecular orbital approach too delocalized, whereas the valence bond approach in too much of localization.

Now the two approaches considered to meet each other, each giving his own views on issues on chemical bonds. Modern calculations in quantum chemistry usually starts from (but in the end away) molecular orbital approach than the valence bond approach. This is not because the molecular orbital approach is more accurate than the approach of valence bond theory, but because the molecular orbital approach makes it easier to convert into numerical calculations. But program-program a better bond valence is also available.

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