Cyclic ring systems proton chemical shifts

Up to this time there has been no report of the experimental determination of the structure of the parent homotropenylium ion. The three simplest systems that have been studied are 18, 19 and the iron complex 20. Cations 18 and 19 each have an oxygen-containing electron-donor substituent and, as such, appear to have smaller induced ring currents than the parent ion. In fact 18 and 19 have almost identical chemical shift differences (A<5 = 3.10 ppm) between the two C(8) protons. In the case of 20, A<5 is very small and it was considered to be a non-cyclically delocalized model for the bicyclo[5.l.OJheptadienyl cation69. 

Benzene is aromatic because it has six electrons in a cyclic conjugated system. We know it is aromatic because it is exceptionally stable and it has a ring current and hence large chemical shifts in the proton NMR spectrum as well as a special chemistry involving substitution rather than addition with electrophiles. This chapter and the next are about the very large number of other aromatic systems in which one or more atoms in the benzene ring are replaced by heteroatoms such as N, O, and S. There are thousands of these systems with five- and six-membered rings, and we will examine just a few. 

Aromatic compounds are defined as cyclic or polycyclic systems which sustain a diamagnetic ring current and consequently exhibit a total diatropic lowfield 1H NMR chemical shift relative to that of vinylic protons. Paramagnetic ring current is expected and was shown to be induced in antiaromatic species and to result in a total highfield H NMR band displacement, while non aromatic compounds give rise to characteristic vinylic XH patterns. 

Due to their cyclic structure, the aliphatic cyclic esters often display non-equivalence in chemical shift for the two protons of the methylene group in the ring system. 

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