Ultraviolet-visible spectroscopy electronic transitions

Ultraviolet-visible spectroscopy (UV = 200 - 400 nm, visible = 400 - 800 nm) corresponds to electronic excitations between the energy levels that correspond to the molecular orbital of the systems. In particular, transitions involving n orbital and ion pairs (n = non-bonding) are important and so UV/VIS spectroscopy is of most use for identifying conjugated systems which tend to have stronger absorptions... 

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. 

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. 

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

Atomic spectroscopy in the ultraviolet-visible region involves transitions of valence shell electrons and the spectra are thus sensitive to the chemical and physical form of the element of interest. For sensitive quantitative work the sample is normally converted to free atoms in the gas phase. This can be achieved by vaporization from a furnace, by... 

The ultraviolet-visible method is useful for the study of electronic transitions in molecules and atoms. Although various forms of ultraviolet-visible spectroscopy can be used to study a myriad of important chemical and physical properties, we will be most concerned with its use in quantitative analysis. It is probably the single most frequently used analytical method, with the possible exception of the analytical balance. For example, a single clinical analysis laboratory in a major hospital may perform a million chemical analyses a year, primarily on serum and urine, and about 707o of these tests are done by ultraviolet-visible absorption spectroscopy. Atomic absorption and emission spectroscopy (Chaps. 10 and 11) is used primarily to analyze for metallic elements in a variety of matrices—serum, natural waters, tissues, and so forth. 

To put these values for nuclear spin energy levels in perspective, energies for transitions between vibrational energy levels observed in infrared (IR) spectroscopy are 8 to 63 kj (2 to 15 kcal)/mol. Those between electronic energy levels in ultraviolet-visible spectroscopy (see Section 20.3) are 167 to 585 kJ (40 to 140 kcal)/mol. Nuclear transitions involve only small energies, on the order of a few hundredths of a calorie. 

Ultraviolet visible (UV VIS) spectroscopy (Section 13 21) An alytical method based on transitions between electronic en ergy states in molecules Useful in studying conjugated systems such as polyenes... 

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. 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

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

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