Isolated conjugated molecule

Stabilities of cyclic conjugated molecules encountered in the laboratory seem to owe much to kinetic rather than thermodynamic properties. Many unstable conjugated molecules including anti-aromatic molecules are now known to be isolated as stable ones when they are protected by bulky substituents from intermolecular reactions. 

Here Ha and Hb are the Hamiltonians of the isolated reactant molecules, Hso is the Hamiltonian of the pure solvent, and Vmt is the interaction energy between reactants and between reactant and solvent molecules, i.e., it contains the solute-solute as well as the solute-solvent interactions, qa and reactant molecules A and B, respectively, and pa and pb are the conjugated momenta. If there are na atoms in molecule A and tib atoms in molecule B, then there will be, respectively, 3ua coordinates c/a and 3rt j coordinates c/b Similarly, R are the coordinates for the solvent molecules and P are the conjugated momenta. In the second line of the equation, we have partitioned the Hamiltonians Hi into a kinetic energy part T) and a potential energy part V). 

The VEH energies correspond to isolated chains in vacuo. It is therefore no surprise that the ionization potentials turn out to be too large, since in the solid state a chain is surrounded by a polarizable medium of dielectric constant 2 to 4. Analysis of a very large amount of data on molecular solids has shown that the difference between the ionization potentials of a conjugated molecule and the corresponding solid, that is, the polarization... 

In contrast to the acid/base behaviour of polymeric bulk water, monomeric water is a relatively weak acid and base in the gas phase compared to its substituted derivatives (R—OH, R—O—R, etc.), whose conjugated base or acid ions are stabilized by polarization of the alkyl groups. The gas-phase basicity of water is 138 kJ/mol (33 kcal/mol) below that of ammonia. Its gas-phase acidity is comparable to that of propene and it is less acidic than phenol by about 167 kJ/mol (40 kcal/mol). With respect to the well-known acid/base properties of water, ammonia, and phenol in aqueous solution, one has to conclude that enormous solvation energies must contribute to the difference from the behaviour of isolated water molecules. See Section 4.2.2 for further discussions and references. 

During the past eight years, we have studied the reactions of thermally generated silicon atoms with a variety of low-molecular-weight reactants in an argon matrix. The reaction products were identified by means of IR and UV/Vis spectroscopy aided by comparison with calculated spectra. The method turned out to be very versatile and successful [1]. The selected substrates were mainly molecules with isolated, conjugated or aromatic n bonds, and compounds containing a bonds and at the same time free electron pairs. 

This section deals with analytical properties of the Hiickel molecular orbital theory and the associated isolated molecule method of predicting the active positions in a conjugated molecule. We shall deal with polarizability coefficients defined as certain partial derivatives with respect to the coulomb and resonance integrals described in Section III. The important derivatives are those relating to the total tt electron energy and to the charges q, fi ee valences and bond orders... 

Figure 10-1. Energy scheme of the relevant electronic levels that are affecting stimulated emission in an isolated dye molecule in solution (a) and in a conjugated polymer (b). In both systems absorption of a photon induces a transition from the singlet ground state Sq to a vibronically excited state within the Si-manifold. After vibronic relaxation, a red-shifted emission can be observed. This transition can occur in a spontaneous as well as in a stimulated manner. A photoinduced absorption may occur as competing processes in both systems (PIA) due to the population of the triplet state T. A pair of oppositely charged carriers (GP) can be generated in the conjugated polymer due to dissociation of the 5i-state. This can lead to additional photoinduced absorption bands that compete with the stimulated emission.

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