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26 Oct 2011 08:28:54 IST

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What is leprotes rule?explain in detail.

Chemistry

What is leprotes rule?explain in detail.

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edison

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26 Jan 2012 19:32:16 IST

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It is a spectroscopic selection rule for an allowed or forbidden electric diplole transition to take place.

Electronic spectra

The Laporte rule is a selection rule formally-stated as follows: In a centrosymmetric environment transitions between like atomic orbitals such as s-s,p-p, d-d, or f-f transitions are forbidden. The Laporte rule applies to electric dipole transitions, so the operator has u symmetry. p orbitals also have u symmetry, so the symmetry of the transition moment function is given by the triple product u×u×u, which has u symmetry. The transitions are therefore forbidden. Likewise, d orbitals have g symmetry, so the triple product g×u×g also has usymmetry and the transition is forbidden.

The wave function of a single electron is the product of a space-dependent wave function and a spin wave function. Spin is directional and can be said to have odd parity. It follows that transitions in which the spin "direction" changes are forbidden. In formal terms, only states with the same total spin quantum number are "spin-allowed". In crystal field theory, d-dtransitions that are spin-forbidden are very much weaker than spin-allowed transitions. Both can be observed, in spite of the Laporte rule, because the actual transitions are coupled to vibrations that are anti-symmetric and have the same symmetry as the dipole moment operator.

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