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[عاطف خليفة]

Electronic Configurations



High Spin/Low Spin

There is no evidence that high-spin ( electron configuration) to low-spin () transitions are geologically important. Such transitions are therefore not important in

considering the interiors of planets
..


Inert


A substance that does not undergo chemical reactions (such as the noble gases) is said to be inert. A complex whose ligands are slowly replaced (low-spin , low-spin , low-spin , ) is also called






Jahn-Teller Effect

The spontaneous Jahn-Teller effect is the spontaneous distortion of geometry in an electronically excited state which results when levels are split to reduce the energy of the overall system.
The static Jahn-Teller effect occurs if the lowest energy level of a molecule is degenerate, in which case it will distort spontaneously so as to remove the degeneracy and make one energy level more stable. The proof is technical and difficult, and requires a rather sophisticated application of group theory to quantum mechanics. However, without a complete calculation, the geometric nature and amplitude of the effect cannot be computed.
For octahedral coordination, susceptible species are , , and low spin in which 1 or 3 electrons occupy . The effect is small when the degeneracy is in the group.

For tetrahedral coordination, susceptible species are high spin , , , and low spin .


Laporte Rule

For a free ion, transitions which involve a redistribution of electrons in a single quantum shell are forbidden. However, if the environment about the ion lacks a center of symmetry (as in tetrahedral coordination), mixing of the and orbitals may occur and Laporte transitions may occur. In addition, vibronic (vibrational and electronic) coupling between wavefunctions with opposite parities may occur.

Period

Elements with the same outermost shell are said to belong to the same period.



Selection Rules


Describe types of electronic transitions which are permitted. Forbidden transitions are forbidden "to first order," which means they may occur in practice, but with low probabilities. These rules include

Spin Multiplicity Rule

In a free ion, electronic transitions involving a change in the number of unpaired electrons are forbidden (they do not conserve spin angular momentum). However, spin-orbit coupling makes such transitions possible, although with low probability.



Tanabe-Sugano Diagram


A diagram which plots the (normalized) energy difference of an electronic transition (in units of ) on the vertical axis and the (normalized) crystal field splitting energy (in units of , where is the crystal field splitting energy and B is a normalization constant) on the horizontal axis for each possible electronic state of the system. The lowest energy state is usually placed along the x-axis, and all other states plotted relative to it. The number of curves intersected by a vertical line for a given gives the number of possible transitions and therefore the number of expected spectral absorption features. For more than 1 and less than 9 d electrons , there are more than 2 ways of arranging electrons in orbitals leading to different configurations, called Russell-saunders (or spectroscopic ) states.

[usama_mustafa]

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