Larger Aromatic Molecules

Studies on the 2PA properties of organic materials have been extended from the early days to a large group of aromatic compounds beyond benzene derivatives. Here, we will focus on a few examples in which studies on series of molecules under the same experimental conditions are available and were used to gain insight into the relationship between the observed properties and the molecular structure of the systems. 

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In the next part, we will briefly consider a larger aromatic molecule, viz. dimethyldibenzothiophene (DMDBT). This will give us the opportunity to describe the zeolite selectivity that is induced by the adsorption behavior of reactants to the catalytic active site on the products distribution. 

In general, however, one must be concerned with the possible dominance of chemistry by small amounts of dianions. Although not seen in electrochemistry, the naphthalene dianion has been reported in the literature ll l5°-159 167) and could dictate the results of quench reactions. In the specific case of sodium naphthalene in tetrahydro-furan, kinetic analysis of a water quench directly implicites the radical anion as the chemically dominant species 150 -158-167>. In the case of the larger aromatic molecule, perylene, however, the dianion and not the radical anion is the species quenched167a). 

The spectrum of the intramolecular vibrations in the anthracene crystal includes the region from 111 cm to 3108 cm"k For larger aromatic molecules, the lower limit shifts towards lower wavenumbers (frequencies) for smaller aromatics, it shifts towards higher wavenumbers. In naphthalene, the smallest wavenumber (frequency) of an internal mode is around 170 cm (5 THz). An analysis of the internal modes as given in Table 5.1 for anthracene has to our knowledge not been published for other molecular crystals with a similarly large number of atoms in such a complete form. Selected values of the wavenumbers of the intramolecular vibrations of other molecular crystals can, however, be found, e.g. in [3]. 

Taking advantage of its high separation strength, a combination of SEC with MS enhances the capability of MS and makes it possible for the analysis of coal liquids. SEC can be used as a coal liquid sample pretreatment method prior to GC-MS. For SEC, 1-methyl-2-pyrrolidinone (NMP) is often used as eluent. Analytical result for a coal liquefaction extract sample prepared from SEC by GC-MS showed that only the pentane-soluble components could be analyzed [16] the larger aromatic molecules identified by SEC to be present were lost in the GC column. The molecular weight of the aromatics analyzed in the coal liquids were up to 352 with very few aliphatics. 

The next few activities explore reactions involving aromatic rings. Almost all the examples in these upcoming activities will involve benzene derivatives, but the reactions also apply to larger aromatic molecules and may work with heterocyclic aromatic molecules. 

The fact that the dendritic shell can produce localized microenvironments has been used by Diederich et al. who developed water-soluble dendritic cyclophanes (dendrophanes) as models for globular proteins (Figure 16.13) [5, 170, 171], These dendrimers contain well-defined cyclophane recognition sites as initiator cores for the complexation of small aromatic guests [172-174] and steroids [174-176], Enlargement of the cyclophane core could be used as a tool to complex larger steroid molecules. 

Thyroxine is actually a simple derivative of the aromatic amino acid tyrosine (see Section 13.1), but is believed to be derived by degradation of a larger protein molecule containing tyrosine residues. One hypothesis for their formation invokes suitably placed tyrosine residues in the protein thyroglobulin being iodinated to di-iodotyrosine. These residues then react together by phenolic oxidative coupling. 

With nonpolar aromatic molecules such as benzene, naphthalene and anthracene, cohesion is almost entirely due to van der Waals interaction, but the cohesive energies are significantly larger than in aliphatic nonpolar molecules. The larger cohesive energies arise from the greater polarizability of the n-clouds. Arrangement of molecules... 

The configurations of the molecules are those expected for the resonating structures. Through resonance each bond acquires some doublebond character, which causes the adjacent bonds to strive to be co-planar. The molecules are thus brought into completely planar configurations, with 120° bond angles. This has been verified for naphthalene and anthracene and many larger aromatic hydrocarbons by careful x-ray studies. 

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