Electronic energy levels of molecules

The significance of light absorption in biochemical studies lies in the great sensitivity of electronic energy levels of molecules to their immediate environment and to the fact that spectrophotometers are precise and sensitive. The related measurements of circular dichroism and fluorescence also have widespread utility for study of proteins, nucleic acids, coenzymes, and many other biochemical substances that contain intensely absorbing groups or chromophores.58... 

Visible 5,000 to 25,000 Electronic excitation accompanied by vibration-rotation changes Electronic energy levels of molecules (Section 9-9)... 

Figure 4.1 Electronic energy levels of molecules with associated vibrational and, rotational levels.

Figure 7.14. Scheme of electronic energy levels in semi-conducting solids. Ei and Eh refer to electronic energy levels of molecules in solution that may, respectively, donate electrons to or accept electrons from the solid. 

Absorption of specific frequency is due to the quantisation of electronic energy levels of molecules. Hence energy required for different transitions for different species are different. Also the portion of light used is not completely absorbed but only some fraction is utilized. These selections and utilizations are best demonstrated by Lambert-Beer s laws. 

The rotation-vibration-electronic energy levels of the PH3 molecule (neglecting nuclear spin) can be labelled with the irreducible representation labels of the group The character table of this group is given in table Al.4.10. 

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. 

Table 10.3 Electronic energy levels of some common molecules or atoms with unpaired...

The primary process consists of raising of the electronic quantum level of molecule by absorption of energy from photon. The excited molecule may then behave in different ways. The energy of the photon is transformed into heat and temperature of absorbing system is raised but the excited molecule may behave in other ways resulting in a chemical change. 

Fig. 4-22. Electron energy levels of the hydrogen electrode in electron-and-ion transfer equilibrium Hjiju) = gaseous hydrogen molecule on electrode eaj>/H2, u) = gaseous redox electron in equilibrium with the hydrogen reaction, + 2e(H-/H p,) Hp, =...

Any external electric field is minute in comparison with the internal field generated by the system of electrons and nuclei inside a molecule. The effect of the operator (8.4) is therefore always much smaller than the electronic energy of the molecule. In most cases, the effects of electric-field perturbations are also much smaller than the vibrational energy of the molecule. The interaction with an external DC field can thus be treated as a perturbation to the vibronic energy levels of molecules. 

In principle, it should be possible to obtain the electronic energy levels of the molecules as a solution of the Schrodinger equation, if inter-electronic and internuclear cross-coulombic terms are included in the potential energy for the Hamiltonian. But the equation can be solved only if it can be broken up into equations which are functions of one variable at a time. A simplifying feature is that because of the much larger mass of the nucleus the motion of the electrons can be treated as independent of that of the nucleus. This is known as the Bom-Oppen-heimer approximation. Even with this simplification, the exact solution has been possible for the simplest of molecules, that is, the hydrogen molecule ion, H + only, and with some approximations for the H2 molecule. 

As may be seen in Fig. 14 when the incident radiation, of frequency v0, falls on the molecule, the molecule is raised to a virtual state. The only requirement of this virtual state is that it does not correspond to an electronic-energy level of the molecule. From this virtual state, the molecule can either... 

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