Selection rules organic molecules

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

Transitions between energy levels in organic molecules are subject to certain constraints, referred to as selection rules. 

Module 3, Column and Mobile Phase Design (CMP). This is the core module for ECAT. It can currently specify i) analytical column and mobile phase constituents for reverse phase chromatography of common classes of organic molecules ii) reverse phase, ion exchange phase and hydrophobic interaction chromatography of proteins and peptides iii) a limited set of specialty classes of molecules best treated by straight phase chromatography (e.g., mono- and disaccharides). The rules for selection of the HPLC detector are under development within Module 3. Some of the rules for detector mobile phase compatibility are already encoded. A set of rules for detector selection is ready but not yet encoded. 

Expansion of the data bases in Module 1 to include spectroscopic and electrochemical data to be used by the detector selection rules of Module 3. (This would include UV absorbance spectral properties of organic molecules, fluorescence quenching and activating properties of solvent environments, and electro-... 

The rules governing the water solubility of polymers are similar to those governing the water solubility of smaller organic molecules except that the extent of polymer solubility and the range of polymeric structures are more limited. Selected commercially available water-soluble polymers are shown in Figure 16.1. 

Platt has discussed the application of selection rules to the spectra of conjugated organic molecules, in particular condensed ring systems (8). He cites the various kinds of forbiddenness and then compares these with the corresponding selection rules for atomic systems. His treatment is useful and the following statements are taken directly from his article ... 

Figure 1.11. Schematic representation of the absorption spectrum of an organic molecule with some allowed transitions and some that are forbidden by spin, symmetry, or overlap selection rules (from left to right). The log e and/values of the ordinate are only meant to provide rough orientation. In particular, according to Equation (1.27), there is no simple relation between log e and/(by permission from Turro, 1978).

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