Aromaticity Revisited

Johansson, M.P. and Juselius, J. (2005) Arsole aromaticity revisited. Letters in Organic Chemistry, 2, 469-74. 

Watson, M., Fechtenkoetter, A., and Mullen, K. (2001) Big is beautiful - aromaticity revisited from the viewpoint of macromolecular and supramolecular benzene chemistry. Chem. Rev., 101,1267-1300. 

Watson, M. D. Eechtenkotter, A. Mullen, K. Big is Beautiful— Aromaticity Revisited from the Viewpoint of Macromolecular and Supramolecular Benzene Chemistiy. Chem. Rev. 2001, 101, 1267-1300. 

Thomae, A. V., Wunderli-Allenspach, H., Kramer, S. D. Permeation of aromatic carboxylic adds across lipid bilayers the pH-partition hypothesis revisited. Biophys. J. 2005, 89,1802-1811. 

Hiickel s rule has been abundantly verified [17] notwithstanding the fact that the SHM, when applied without regard to considerations like the Jahn-Teller effect (see above) incorrectly predicts An species like cyclobutadiene to be triplet diradicals. The Hiickel rule also applies to ions for example, the cyclopropenyl system two n electrons, the cyclopropenyl cation, corresponds to n 0. and is strongly aromatic. Other aromatic species are the cyclopentadienyl anion (six n electrons, n = 1 Hiickel predicted the enhanced acidity of cyclopentadiene) and the cyclohep-tatrienyl cation. Only reasonably planar species can be expected to provide the AO overlap need for cyclic electron delocalization and aromaticity, and care is needed in applying the rule. Electron delocalization and aromaticity within the SHM have recently been revisited [43]. 

This chapter and the next two will revisit the ring theme, but the rings will all be heterocycles rings containing not just carbon atoms, but oxygen, nitrogen, or sulfur as well. It may seem strange that this rather narrowly defined class of compounds deserves three whole chapters, but you will soon see that this is justified both by the sheer number and variety of heterocycles that exist and by their special chemical features. Chapters 43 and 44 cover heterocycles that are aromatic, and in this chapter we look at heterocycles that are saturated and flexible. Some examples, a few of which may be familiar to you, are shown below and overleaf. 

In 1974, the Tunnel Effect Theory [47] was applied to the photophysics of aromatic compounds. Subsequently, it was applied to the photodissociation of benzene [48], a field revisited in 1983 [49]. TET was also used to explore olefin isomerizations [50], quenching of aromatic molecule luminescence by paramagnetic species [51], and nonradiative rate constants of exciplexes [52]. 

Since the late 1990s nearly every major oil company announced plans to build pilot plants or conunercial plants to produce synthetically derived diesel fuel through improved processes (19). Stringent diesel exhaust emission standards and fuel specifications are compelling the petroleum industry to revisit these processes to competitively produce aromatic and sulfur level complying diesel fuel. An important part of the success of the processes is the increased reactor capacity proven within the last 10 years using slurry-bed reactors that have 100 times the capacity over some pre-1990 reactors, thus offering lower cost. 

P. Bering, Structure and vibrations of the phenol-ammonia cluster, J. Chem. Phys. 102, 9197-9204 (1995). (c) S. Tanabe, T. Ebata, M. Fujii, and N. Mikami, OH stretching vibrations of phenol-(H20) (n = 1-3) complexes observed by IR-UV double-resonance spectroscopy, Chem. Phys. Lett. 215, 347-352 (1993). (d) D. Michalska, W. Zierkiewicz, D. C. Bien ko, W. Wojciechowski, and T. Zeegers-Huyskens, Troublesome vibrations of aromatic molecules in second-order Moller-Plesset and density functional theory calculations infrared spectra of phenol and phenol-OD revisited, J. Phys. Chem. A 105, 8734-8739 (2001). 

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