Intheoretical chemistry,molecular electronic transitions take place whenelectrons in amolecule areexcited from oneenergy level to a higher energy level. The energy change associated with this transition provides information on thestructure of the molecule and determines many of its properties, such ascolour. The relationship between the energy involved in the electronic transition and thefrequency ofradiation is given byPlanck's relation.
The electronic transitions inorganic compounds and some other compounds can be determined byultraviolet–visible spectroscopy, provided that transitions in theultraviolet (UV) orvisible range of theelectromagnetic spectrum exist for the compound.[1][2] Electrons occupying aHOMO (highest-occupied molecular orbital) of asigma bond (σ) can get excited to theLUMO (lowest-unoccupied molecular orbital) of that bond. This process is denoted as aσ → σ* transition. Likewise, promotion of an electron from api-bonding orbital (π) to anantibonding pi orbital (π*) is denoted as aπ → π* transition.Auxochromes withfree electron pairs (denoted as "n") have their own transitions, as doaromatic pi bond transitions. Sections of molecules which can undergo such detectable electron transitions can be referred to aschromophores, since such transitions absorbelectromagnetic radiation (light), which may be hypothetically perceived as color somewhere in the electromagnetic spectrum. The following molecular electronic transitions exist:
In addition to these assignments, electronic transitions also have so-called bands associated with them. The following bands are defined (by A. Burawoy in 1930):[3]
For example, theabsorption spectrum forethane shows aσ → σ* transition at 135 nm and that ofwater an → σ* transition at 167 nm with anextinction coefficient of 7,000.Benzene has threearomatic π → π* transitions; two E-bands at 180 and 200 nm and one B-band at 255 nm with extinction coefficients respectively 60,000, 8,000 and 215. These absorptions are not narrow bands but are generally broad because the electronic transitions are superimposed on the othermolecular energy states.
The electronic transitions of molecules insolution can depend strongly on the type ofsolvent with additionalbathochromic shifts orhypsochromic shifts.
Spectral lines are associated with atomic electronic transitions and polyatomicgases have their ownabsorption band system.[4]
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