Ring current alkenes

It follows that aromaticity can be determined from an NMR spectrum. If the protons attached to the ring are shifted downfield from the normal alkene region, we can conclude that the molecule is diatropic, and hence aromatic. In addition, if the compound has protons above or within the ring (we shall see an example of the latter on p. 69), then if the compound is diatropic, these will be shifted upfield. One drawback to this method is that it cannot be applied to compounds that have no protons in either category, for example, the dianion of squaiic acid (p. 69). Unfortunately, NMR is of no help here, since these spectra do not show ring currents. ... 

The circulating electrons in the 7t-system of aromatic hydrocarbons and heterocycles generate a ring current and this in turn affects the chemical shifts of protons bonded to the periphery of the ring. This shift is usually greater (downfield from TMS) than that expected for the proton resonances of alkenes thus NMR spectroscopy can be used as a test for aromaticity . The chemical shift for the proton resonance of benzene is 7.2 ppm, whereas that of the C-1 proton of cyclohexene is 5.7 ppm, and the resonances of the protons of pyridine and pyrrole exhibit the chemical shifts shown in Box 1.12. 

In contrast to proton shifts, carbon-13 shifts cannot be used as criteria for aromaticity (Section 3.1.3.10). No difference exists between aromatic (128.5 ppm for benzene) and comparable alkene carbon nuclei (127.5 ppm for cyclohexene). Aromatic ring carbon nuclei are practically not influenced by the ring current (Section 3.1.3.4), which makes up a deshielding of about 2 ppm and thus is small compared with other (e.g. steric) effects on carbon-13 shifts. 

In theory then, pyridine is aromatic. But is it in real life The most important evidence comes from the proton NMR spectrum. The six protons of benzene resonate at 8h 7.27 p.p.m., some 2 p.p.m. downfield from the alkene region, clear evidence for a ring current (Chapter 11). Pyridine is not as symmetrical as benzene but the three types of proton all resonate in the same region. 

---