Spanish term (edited): tr�nsitos electr�nicos permitidos

This is how we, chemists and physicists, say it. This the standard way textbooks say it as well.

There are rules that restrict electronic transitions to certain symmetries. Not all transitions are allowed. Those are forbidden transitions.

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Note added at 1 hr 44 mins (2006-01-12 21:25:15 GMT)
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http://www.kosi.com/raman/resources/tutorial/

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Note added at 1 hr 45 mins (2006-01-12 21:25:56 GMT)
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Metalloporphyrins, carotenoids and several other classes of biologically important molecules have strongly ** allowed electronic transitions** in the visible.

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Note added at 1 hr 47 mins (2006-01-12 21:28:19 GMT)
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Thus, an electron cannot make a transition from an "s" orbital into a "d" orbital. This will represent a change of the azimuthal quantum number (l) from 0 to 2. This is not allowed. Of course what is allowed or not allowed was decided by God (if you accept that) of by nature if you accept that explantation.

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Note added at 1 hr 53 mins (2006-01-12 21:34:16 GMT)
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In some cases, molecules can "disobey" these rules by either breaking their symmetry slightly, so that now symmetry does not have the last word, or by "borrowing" from other
types of transitions (e.g. vibrationa transitions). Then the "disallowed" transitions become a bit allowed, and one can see a weak transition signal on the spectrometer. Similar to finding a loophole in the law, I think.

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Note added at 1 hr 55 mins (2006-01-12 21:36:10 GMT)
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ZINDO semiempirical calculations on the Ni(CN)(4)(2-) ion were performed, and ground-state energies for all 41 valence-orbital-based MOs and orbital transition components of the two lowest energy fully allowed electronic transitions are reported. Gaussian 94 was used to calculate ground-state energies as a comparison.

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Note added at 1 hr 57 mins (2006-01-12 21:38:02 GMT)
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http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&d...