N-Aromatics

As in the case of density or specific gravity, the refractive index, n, for hydrocarbons varies in relation to their chemical structures. The value of n follows the order n paraffins < n naphthenes < n aromatics and it increases with molecular weight. 

Scheme 3.2-44. From two-dimensional n aromatics (planar 2, C2BH3) to three-dimensional o aromatics (polyhedral 15, 39 and higher members of the series) via folded aromatics (3, 2 ) connecting both classes.

Gopishetty, S.R., Heinemann, J., Deshpande, M. and Rosazza, J.P.N., Aromatic oxidations by Streptomyces griseus biotransformations of naphthalene to 4-hydroxy-1-tetralone. Enzyme Microbiol TechnoL, 2007, 40, 1622. 

Attack on the aromatic ring and formation a n-complex or electron donor-acceptor complex N02 + ArH —> ArH N02. This complex involves high electrostatic and charge-transfer interactions between the n-aromatics and nitroninm ion. 

Stepnowski, R, Nichthauser, Mrozik, W., and Buszewski, B., Usefulness of jt- -n aromatic interactions in the selective separation and analysis of imidazolium and pyridinium ionic liquid cations. Anal. Bioanal. Chem., 385, 1483-1491,2006. 

Figure 10. The specific structure of flux-promoting compound 6. S3 anchors to the membrane surface through N -aromatic interaction and the hydrophobic Si locates in the interior membrane, thereby spanning the oxyethylene part S2. ...

An interesting comparison can be made on the basis of the hydrogen distribution of the three macerals. The aromatic hydrogen content (%H.r) has been calculated from the total area of the infrared absorption between 11 and 14 n (aromatic HCC rocking vibrations) (6). The aliphatic hydrogen content (% H.i) has been derived from the absorption of the aliphatic H-C stretching vibration between 3.3 and 3.6 m (7). The values found for the three macerals under investigation are reported in Table II, columns 3 and 4. 

These pentapyrrolic macrocycles contain a 22-jr-electron ring system, and are expected to be the largest members of the [An + 2] n aromatics. In fact, their spectroscopic characteristics, i.e. the intense absorption band in visible region, the large diamagnetic shift of peripheral H and inner NH NMR signals, and relatively strong parent (M+) and dicationic (M2+) mass peaks, indicate the existence of a delocalized cyclic aromatic n system (Table 11). 

Because of the forbiddeness of the transition, Tx —> S0 + ho, the natural phosphorescent lifetime, t°, of a triplet state is long—from approximately 10"3 sec for an n,it triplet to 30 sec for a rr,n aromatic triplet. At room temperature in solution, phosphorescence is often not observed because ISC of Tx to S0 and quenching of Tx by impurities and molecular 02 (see below) competes effectively with phosphorescence. Therefore most phosphorescence studies must be carried out at low temperatures in carefully purified, outgassed, rigid media. Under these conditions the quantum yield of phosphorescence, 9P, defined by Equation 13.10, is often high and approaches 1.0 for some aromatic carbonyls. 

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