Electronic Transitions in Molecules

The SHG/SFG technique is not restricted to interface spectroscopy of the delocalized electronic states of solids. It is also a powerful tool for spectroscopy of electronic transitions in molecules. Figure Bl.5.13 presents such an example for a monolayer of the R-enantiomer of the molecule 2,2 -dihydroxyl-l,l -binaphthyl, (R)-BN, at the air/water interface [ ]. The spectra reveal two-photon resonance features near wavelengths of 332 and 340 mu that are assigned to the two lowest exciton-split transitions in the naphtli-2-ol... 

Electronic transitions in molecules in supersonic jets may be investigated by intersecting the jet with a tunable dye laser in the region of molecular flow and observing the total fluorescence intensity. As the laser is tuned across the absorption band system a fluorescence excitation spectrum results which strongly resembles the absorption spectrum. The spectrum... 

Shorter-wavelength radiation promotes transitions between electronic orbitals in atoms and molecules. Valence electrons are excited in the near-uv or visible. At higher energies, in the vacuum uv (vuv), inner-shell transitions begin to occur. Both regions are important to laboratory spectroscopy, but strong absorption by make the vuv unsuitable for atmospheric monitoring. Electronic transitions in molecules are accompanied by stmcture... 

Af,Af-dimethylaniline, which becomes yellow in air or oxygen but colorless again when the oxygen is removed. Such weak charge-transfer complexes make certain electronic transitions in molecules more intense they are also a plausible first stage in photooxidations. 

The majority of electronic transitions in molecules occur in the UV region of the spectrum because the energy separations of electron states typically correspond to the energies of photons in the UV region. However, some molecules have electronic energy separations that correspond to radiation in the visible region (see Fig. 12.3). One such class of compounds includes the coordination compounds that contain transition metal ions (see Section 20.6). 

Similar considerations apply to electronic transitions in molecules. Thus, a 7T 7t transition in a carbonyl group may be accompanied by a displacement of electron charge from the O atom to the C atom, and hence provides an interaction mechanism with the electromagnetic radiation. On the other hand, an n(cr) n transition is essentially a rotational migration of charge, with no dipolar effect. 

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. 

The term transient absorption is commonly used for techniques that detect relatively fast (< 1 sec) changes in absorption arising from electronic transitions in molecules in the wavelength region between 200 and 1200 nm, that is, from the far-UV to the near-IR. Transient IR absorption spectra (vibrational transitions) can be studied, but the required techniques are not discussed here. A variety of different processes can be investigated... 

Three distinct types of electrons are involved in valence electron transitions in molecules. First are the electrons involved in single bonds, such as those between carbon and hydrogen in alkanes. These bonds are called sigma a) bonds. The amount of energy required to excite electrons in cr bonds is usually more than UV photons of wavelengths >200 nm possess. For this reason, alkanes and other saturated compounds (compounds with only single bonds) do not absorb UV radiation and are therefore frequently very useful as transparent solvents for the smdy of other molecules. An example of such a nonabsorbing compound is the alkane hexane, CgH. ... 

So far nothing has been said about the nature of the electronic transitions in molecules. These, of course, determine the fabric on which the vibrational and rotational fine structure of the visible spectra is embroidered. 

The majority of electronic transitions in molecules occurs in the UV region of the spectrum because the energy separations of electron states typically correspond to the energies of photons in the UV region. However, some molecules... 

Radiation in the ultraviolet and visible regions gives rise to electronic transitions in molecules, often involving pi electrons, and hence this type of spectroscopy gives information about the energies of the molecular orbitals in a molecule or atomic orbitals in an atom or ion. 

Determine selection rules for electronic transitions in molecules. 

Raman scatter, and excitation emission fluorescence spectroscopy (EEFS). They use interaction with radiation from different regions of the electromagnetic spectrum to identify the chemical nature of molecules. For example, absorption of UV and VIS radiation causes valence electron transitions in molecules which can be used to measure species down to parts per million concentrations for fluorophores (i.e., EEFS) determination can even go down to parts per billion levels. Whereas UV, VIS, and EEFS are limited to a smaller, select group of molecules, the NIR, IR, and Raman scatter spectroscopy techniques are probing molecular vibrations present in almost any species their quantification limits are somewhat higher but can still be impressive. The reader is referred to textbooks for further details on basic principles of these spectroscopic techniques [3]. 

Electronic transitions in molecules need energies correlated to wavelengths that are in the range of the ultraviolet (UV) and visible spectral regions. The absorption of a UV or visible photon by a molecule induces changes in the distribution of the electrons surrounding the nuclei, and in the forces between the atomic nuclei of a molecule. As a result, molecules in electronically excited states often have very different chemical and physical properties than their electronic ground states. 

As it will be shown in the next paragraph, the value of ellipticity is related to the absorption difference for the two linearly polarized components, and depends on the wavelength of the radiation. The aim of CD spectroscopy is the measurement of the ellipticity as a function of wavelength. Although this kind of spectroscopy can be carried out in several spectral regions, the most popular case is that of CD in the UV-visible region, which arises from electronic transitions in molecules. Infrared CD spectroscopy, involving vibrational transitions, is less common yet used in several research laboratories. 

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