عاطف خليفة
17-10-2008, 01:56 AM
Electronic Configurations
High Spin/Low Spin
There is no evidence that high-spin (http://scienceworld.wolfram.com/chemistry/himg40.gif electron configuration) to low-spin (http://scienceworld.wolfram.com/chemistry/himg41.gif) 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 (http://scienceworld.wolfram.com/chemistry/Ligand.html) are slowly replaced (low-spin http://scienceworld.wolfram.com/chemistry/iimg9.gif, low-spin http://scienceworld.wolfram.com/chemistry/iimg10.gif, low-spin http://scienceworld.wolfram.com/chemistry/iimg11.gif, http://scienceworld.wolfram.com/chemistry/iimg12.gif) 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 (http://mathworld.wolfram.com/GroupTheory.html) http://scienceworld.wolfram.com/images/crossrefs/math.gif to quantum mechanics (http://scienceworld.wolfram.com/physics/QuantumMechanics.html). http://scienceworld.wolfram.com/images/crossrefs/physics.gif However, without a complete calculation, the geometric nature and amplitude of the effect cannot be computed.
For octahedral coordination, susceptible species are http://scienceworld.wolfram.com/chemistry/jimg1.gif, http://scienceworld.wolfram.com/chemistry/jimg2.gif, and low spin http://scienceworld.wolfram.com/chemistry/jimg3.gif in which 1 or 3 electrons occupy http://scienceworld.wolfram.com/chemistry/jimg4.gif. The effect is small when the degeneracy is in the http://scienceworld.wolfram.com/chemistry/jimg5.gif group.
http://scienceworld.wolfram.com/chemistry/jimg6.gif
For tetrahedral coordination, susceptible species are high spin http://scienceworld.wolfram.com/chemistry/jimg7.gif, http://scienceworld.wolfram.com/chemistry/jimg8.gif, http://scienceworld.wolfram.com/chemistry/jimg3.gif, and low spin http://scienceworld.wolfram.com/chemistry/jimg1.gif.
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 http://scienceworld.wolfram.com/chemistry/limg13.gif and http://scienceworld.wolfram.com/chemistry/limg14.gif 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 http://scienceworld.wolfram.com/chemistry/simg2.gif
http://scienceworld.wolfram.com/chemistry/simg3.gif
http://scienceworld.wolfram.com/chemistry/simg4.gif
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 http://scienceworld.wolfram.com/chemistry/timg1.gif) on the vertical axis and the (normalized) crystal field splitting energy (http://scienceworld.wolfram.com/chemistry/CrystalFieldSplittingEnergy.html) (in units of http://scienceworld.wolfram.com/chemistry/timg2.gif, where http://scienceworld.wolfram.com/chemistry/timg3.gif is the crystal field splitting energy (http://scienceworld.wolfram.com/chemistry/CrystalFieldSplittingEnergy.html) 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 http://scienceworld.wolfram.com/chemistry/timg3.gif 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 (http://scienceworld.wolfram.com/physics/Electron.html) http://scienceworld.wolfram.com/images/crossrefs/physics.gif, there are more than 2 ways of arranging electrons (http://scienceworld.wolfram.com/physics/Electron.html) http://scienceworld.wolfram.com/images/crossrefs/physics.gif in orbitals leading to different configurations, called Russell-saunders (http://scienceworld.wolfram.com/physics/Russell-SaundersStates.html) http://scienceworld.wolfram.com/images/crossrefs/physics.gif (or spectroscopic (http://scienceworld.wolfram.com/physics/SpectroscopicState.html) http://scienceworld.wolfram.com/images/crossrefs/physics.gif) states.
High Spin/Low Spin
There is no evidence that high-spin (http://scienceworld.wolfram.com/chemistry/himg40.gif electron configuration) to low-spin (http://scienceworld.wolfram.com/chemistry/himg41.gif) 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 (http://scienceworld.wolfram.com/chemistry/Ligand.html) are slowly replaced (low-spin http://scienceworld.wolfram.com/chemistry/iimg9.gif, low-spin http://scienceworld.wolfram.com/chemistry/iimg10.gif, low-spin http://scienceworld.wolfram.com/chemistry/iimg11.gif, http://scienceworld.wolfram.com/chemistry/iimg12.gif) 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 (http://mathworld.wolfram.com/GroupTheory.html) http://scienceworld.wolfram.com/images/crossrefs/math.gif to quantum mechanics (http://scienceworld.wolfram.com/physics/QuantumMechanics.html). http://scienceworld.wolfram.com/images/crossrefs/physics.gif However, without a complete calculation, the geometric nature and amplitude of the effect cannot be computed.
For octahedral coordination, susceptible species are http://scienceworld.wolfram.com/chemistry/jimg1.gif, http://scienceworld.wolfram.com/chemistry/jimg2.gif, and low spin http://scienceworld.wolfram.com/chemistry/jimg3.gif in which 1 or 3 electrons occupy http://scienceworld.wolfram.com/chemistry/jimg4.gif. The effect is small when the degeneracy is in the http://scienceworld.wolfram.com/chemistry/jimg5.gif group.
http://scienceworld.wolfram.com/chemistry/jimg6.gif
For tetrahedral coordination, susceptible species are high spin http://scienceworld.wolfram.com/chemistry/jimg7.gif, http://scienceworld.wolfram.com/chemistry/jimg8.gif, http://scienceworld.wolfram.com/chemistry/jimg3.gif, and low spin http://scienceworld.wolfram.com/chemistry/jimg1.gif.
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 http://scienceworld.wolfram.com/chemistry/limg13.gif and http://scienceworld.wolfram.com/chemistry/limg14.gif 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 http://scienceworld.wolfram.com/chemistry/simg2.gif
http://scienceworld.wolfram.com/chemistry/simg3.gif
http://scienceworld.wolfram.com/chemistry/simg4.gif
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 http://scienceworld.wolfram.com/chemistry/timg1.gif) on the vertical axis and the (normalized) crystal field splitting energy (http://scienceworld.wolfram.com/chemistry/CrystalFieldSplittingEnergy.html) (in units of http://scienceworld.wolfram.com/chemistry/timg2.gif, where http://scienceworld.wolfram.com/chemistry/timg3.gif is the crystal field splitting energy (http://scienceworld.wolfram.com/chemistry/CrystalFieldSplittingEnergy.html) 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 http://scienceworld.wolfram.com/chemistry/timg3.gif 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 (http://scienceworld.wolfram.com/physics/Electron.html) http://scienceworld.wolfram.com/images/crossrefs/physics.gif, there are more than 2 ways of arranging electrons (http://scienceworld.wolfram.com/physics/Electron.html) http://scienceworld.wolfram.com/images/crossrefs/physics.gif in orbitals leading to different configurations, called Russell-saunders (http://scienceworld.wolfram.com/physics/Russell-SaundersStates.html) http://scienceworld.wolfram.com/images/crossrefs/physics.gif (or spectroscopic (http://scienceworld.wolfram.com/physics/SpectroscopicState.html) http://scienceworld.wolfram.com/images/crossrefs/physics.gif) states.