Molecular electronic spectra

In an analysis of a molecular spectrum, the primary task is, for purpose of characterisation, to assign each narrow spectral feature to a transition between two molecular states specified with rotational, vibrational and electronic quantum numbers or other indices. Nearly as important as the former, another task is to... 

Figure 2.11 Potential energy diagrams for molecular oxygen electronic energy states and the absorption spectrum of oxygen molecule.

Electronic Transitions In an electronic transition an electron is excited from an occupied to an empty molecular orbital (M.O.). The energy of such transitions normally corresponds to photons in the near IR, visible or UV region of the electromagnetic spectrum. Electronic absorption bands give rise to the colours of compounds, including ones without transition metals. 

Density functional techniques are available for the calculation of the molecular and electronic structures of ground state systems. Techniques of this kind are also applied extensively in the study of classical liquids, where applications cover a broad spectrum ranging from fluids at interfaces to theories of freezing and nucleation. The area of nuclear physics is still in a very early stage of development in the use of DFT, mainly because there is not yet a complete theory, as there is in the molecular and atomic cases. This chapter has focused on the theoretical aspects of other applications not related to the use of the Kohn-Sham procedure for electronic systems, with the hope that a better understanding of the problems and successes in the respective areas would help in the development of improved functionals. 

The electronic spectra of the five-membered ring compounds have been intensively studied by the experimental and theoretical works. These molecules are fundamental units in many important biological systems. Furthermore, their excitation spectra are benchmark examples for theoretical studies of molecular excited states [51,55-58]. For furan and thiophene, various types of excitation spectra were measured the vacuum ultraviolet (VUV) spectrum, electron energy-loss (EEL) spectrum and magnetic circular dichroism (MCD) spectrum. The SAC-Cl method offered consistent interpretations of these electronic spectra [51-53]. 

The isoindole solution emits a greenish blue fluorescence in ultraviolet light (medium pressure Hg). The mass spectrum (electron impact) shows a strong molecular ion, with important fragments at m/e 90 (M — HCN)+, 89 (M - HCN - H)+, and 63 (C5H3)+.6 22... 

From the spectrum of a molecule we can obtain experimental information about the geometry of the molecule (bond lengths), and the energy states from which bond strengths are ultimately obtained. The molecular spectrum depends on the characteristics of the nuclear motions as well as on the electronic motions. In Section 23.1, by invoking the Born-Oppenheimer approximation, we discussed the electronic motion that produces the bonding between the atoms as a problem separate from that of the nuclear motions. We begin the discussion of molecular spectroscopy with a brief recapitulation of the description of the nuclear motions. 

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