Ultraviolet spectroscopy electronic transitions

The preceding empirical measures have taken chemical reactions as model processes. Now we consider a different class of model process, namely, a transition from one energy level to another within a molecule. The various forms of spectroscopy allow us to observe these transitions thus, electronic transitions give rise to ultraviolet—visible absorption spectra and fluorescence spectra. Because of solute-solvent interactions, the electronic energy levels of a solute are influenced by the solvent in which it is dissolved therefore, the absorption and fluorescence spectra contain information about the solute-solvent interactions. A change in electronic absorption spectrum caused by a change in the solvent is called solvatochromism. 

In ultraviolet and visible region, electronic transition of atoms and molecules are observed. This is why it is called electronic spectroscopy. In infrared region the absorption of radiation by an organic compound causes molecular vibrations and so it is called vibrational spectroscopy. 

The use of infra-red or ultraviolet spectroscopy to examine the molecular groups present in a chemical compound is familiar to any chemist. One of the main uses of this technique is to apply a range of electromagnetic frequencies to a sample and thus identify the frequency at which a process occurs. This can be characteristic of, say, the stretch of a carbonyl group or an electronic transition in a metal complex. The frequency, wavelength or wavenumber at which an absorption occurs is of most interest to an analytical chemist. In order to use this information quantitatively, for example to establish the concentration of a molecule present in a sample, the Beer-Lambert law is used ... 

In connection with visible-ultraviolet spectroscopy, know the meaning of nanometer, electronic transition, Beer s law, molar absorptivity or extinction coefficient. 

Ultraviolet-visible (UV) spectroscopy (Section 15.1) A type of spectroscopy that employs ultraviolet or visible light UV-visible spectroscopy uses transitions between electronic energy levels 10 provide information about the conjugated part of a compound. 

Ultraviolet (UV) spectroscopy, covered in Chapter 15, observes electronic transitions and provides information on the electronic bonding in the sample. 

We now study ultraviolet (UV) spectroscopy, which detects the electronic transitions of conjugated systems and provides information about the length and structure of the conjugated part of a molecule. UV spectroscopy gives more specialized information than does IR or NMR, and it is less commonly used than the other techniques. 

The ultraviolet spectroscopy of formaldehyde has been studied almost exhaustively, and there is an excellent review on this subject (171). A majority of the bands in the electric-dipole-forbidden vibronically allowed A 2 +X Aj transition have been assigned mostly due to the work of Brand (37), Robinson and DiGiorgio (196), Callomon and Innes (44), and Job, et al. (124). As briefly mentioned earlier, the ground electronic state (X) is planar and the first excited singlet state (A) is pyramidal. It is valid to use the C2V point group symmetry for both electronic states, rather than the C2 point group symmetry (see ref. 171), although the emission could certainly be treated as a -A" - 1A transition. 

Which types of electronic transitions are associated with ultraviolet/visible spectroscopy ... 

U V- Vis. Spectroscopic measurements in the ultraviolet and visible range of the electronic spectrum (UV-Vis) can be used to probe electronic transitions in certain metal atoms and ion complexes. The energy of an electronic transition can depend upon the symmetry of the metal ion being different for transitions in a metal complex displaying tetrahedral (Td) symmetry from the same metal showing an octahedral (Oh) symmetry. Thus, it is possible to use UV-Vis spectroscopy to interrogate the symmetry of certain metal ions bound to oxide surfaces. We show here a few examples of the use of UV-Vis spectroscopy to characterized supported metal oxides. 

Ultraviolet and Visible Spectroscopy Spectroscopic examination of the electronic transitions of H bonded substances offers the novel prospect of revealing the H bonding properties of excited states. This is, in a sense, a means of determining chemical properties of excited molecules. Of course, UV and visible spectra have already shown usefulness in determining thermodynamics of H bond equilibria. 

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