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Absorption spectra

During spectroscopic measurements the molecules of the investigated sample were irradiated with electromagnetic waves of ultraviolet (UV; ca. 200 - 400 nm) and visible (Vis, ca. 400 - 800 nm) light. In the process the valence electrons of the molecules were excited, i.e. they were lifted into higher energy levels. Therefore this method is a so called electron spectroscopy. With a detailed analysis of UV/Vis-spectra conclusions about the molecular bonds can be made.

Molecular orbitals and electron transfers

The electron excitation is always coupled with a whole slew of possible vibration and rotation excitations. Because the measurement is performed in solution, the single absorptions become indistinct into a mutual curve, which is detected as a broad band in the spectrum.

Radiation types and their wavelength range

The absorption spectroscopy can be established for qualitative as well as for quantitative analysis: Due to the form and the position of the absorption bands qualitative conclusions about the molecule can be made, which can e.g. be used for structure determination. A strong absorption can be traced back to the existence of specific structure elements in the molecule (so-called chromophores), which creates the condition for electron transfers. The measured absorption spectrum gives information about the existence or the absence of specific functional groups in the molecule.

The table shows some chromophoric groups and their absorption peaks:

Chromophore	Absorption peak
Carbonyl group (ketones) RR'C=O	271 nm
Carbonyl group (aldehydes) RHC=O	293 nm
Carboxyl group RCOOH	204 nm
Amido group RCONH2	208 nm
Azo group -N=N-	339 nm
Nitro group -NO2	280 nm

The exact wavelengths of the absorption bands depend in addition to the substituent of the compound on the solvent respectively of the polarity of the solvent. An increasing solvent polarity shifts the absorption bands to lower wavelengths (so-called hypsochrome shifting). A decreasing polarity causes a shifting to higher wavelengths (so-called bathochrome shifting).
The position and the intensity of the absorption peaks are however additionally dependent on other parameters such as pH-value and temperature.

Some functional groups are here listed, which can be identified due to absorption spectroscopy:

Unsaturated rings and C-C Multiple bonds
Condensed aromatic hydrocarbons and unsaturated heterocyclic and homocyclic compounds
Conjugation and cumulation
Analysis of equilibria:

Protolytic equilibria
Complex formation equilibria
Charge-Transfer-complexes
Transition metal complexes

For the quantitative analysis the very high molar extinction coefficients are responsible for the sensitive detection limit of the UV/Vis-spectroscopy for molecules. The detection limit can lie in the ppb-range. An important application is the identification and characterization of specific molecular groups.

Some possible determinations are:

Determination of elements by means of chelating agents (e.g. Dithizone method)
Determination of anions and ammonia by means of colour agents
Photometric water analysis of cations and anions by means of colour agents
Determination of organic compounds, above all conjugated systems