Electronic transitions, ultraviolet-visible

In the electronic transitions in visible and ultraviolet region with liquids or solutions, we do not get vibrational bands along with rotational fine structure, but we get a continuous broad electronic band and hence such curves do not give much valuable information. This is because the vibrational fine structure gets suppressed due to overlapping of vibrational spacings. 

FIGURE 7.11 An energy level diagram for a molecule or complex ion expanded to show an electronic energy transition and a vibrational energy transition. The electronic transition involves visible or ultraviolet light while the... 

Reaction selectivity is observed on laser-induced desorption and dissociation of NO and CO chemisorbed on Ni, Pd, and Pt surfaces via the electronic transition using visible and ultraviolet nanosecond-pulsed lasers, as listed in Table 6. The open circle shows that desorption and dissociation have been observed, while the cross mark means that they have not been observed. These metals are isoelectronic and the band structure is very similar, but the activity on laser-induced desorption and dissociation is remarkably different. The origin of the different desorption activity between Pt and other transition metals of Ni and Pd may be closely related with the nature of the antibonding 2-ira state in adsorbed NO and CO [11]. 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

The electronic transitions which produce spectra in the visible and ultraviolet are accompanied by vibrational and rotational transitions. In the condensed state, however, rotation is hindered by solvent molecules, and stray electrical fields affect the vibrational frequencies. For these reasons, electronic bands are very broad. An electronic band is characterised by the wave length and moleculai extinction coefficient at the position of maximum intensity (Xma,. and emai.). 

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... 

As discussed earlier in Section lOC.l, ultraviolet, visible and infrared absorption bands result from the absorption of electromagnetic radiation by specific valence electrons or bonds. The energy at which the absorption occurs, as well as the intensity of the absorption, is determined by the chemical environment of the absorbing moiety. Eor example, benzene has several ultraviolet absorption bands due to 7t —> 71 transitions. The position and intensity of two of these bands, 203.5 nm (8 = 7400) and 254 nm (8 = 204), are very sensitive to substitution. Eor benzoic acid, in which a carboxylic acid group replaces one of the aromatic hydrogens, the... 

The concept of a chromophore is analogous to that of a group vibration, discussed in Section 6.2.1. Just as the wavenumber of a group vibration is treated as transferable from one molecule to another so is the wavenumber, or wavelength, at which an electronic transition occurs in a particular group. Such a group is called a chromophore since it results in a characteristic colour of the compound due to absorption of visible or, broadening the use of the word colour , ultraviolet radiation. 

Absorption and Fluorescence Spectra. The absorption spectra of actinide and lanthanide ions in aqueous solution and in crystalline form contain narrow bands in the visible, near-ultraviolet, and near-infrared regions of the spectmm (13,14,17,24). Much evidence indicates that these bands arise from electronic transitions within the and bf shells in which the Af and hf configurations are preserved in the upper and lower states for a particular ion. 

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. 

Addition of water across a C=N bond in a conjugated system breaks the conjugation and alters the electronic transitions. The ultraviolet and visible spectra of anhydrous and hydrated species are therefore usually dissimilar, and such differences have been used as the basis for much of the qualitative and quantitative work done on covalent hydration. 

Only electron transitions down to the second orbit cause emission of visible light. Other transitions may involve infrared or ultraviolet light. 

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... 

Absorption of ultraviolet and visible radiation in organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy (Figure 4). The spectrum of a molecule containing these chromophores is complex. This is because the superposition of rotational and vibrational transitions on the electronic transitions gives a combination of overlapping lines. This appears as a continuous absorption band. 

Chemiluminescence (CL) is the emission of the electromagnetic (ultraviolet, visible, or near infrared) radiation by molecules or atoms resulting from a transition from an electronically excited state to a lower state (usually the ground state) in which the excited state is produced in a chemical reaction. The CL phenomenon is relatively uncommon because, in most chemical reactions, excited molecules... 

The ultraviolet and visible spectra are usually comprised of a few broad peaks at most. This is due to the fact that electronic transitions... 

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. 

Transitions between different electronic states result in absorption of energy in the ultraviolet, visible and, for many transition metal complexes, the near infrared region of the electromagnetic spectrum. Spectroscopic methods that probe these electronic transitions can, in favourable conditions, provide detailed information on the electronic and magnetic properties of both the metal ion and its ligands. 

---