Ring Currents Aromatic and Antiaromatic

We saw in Chapter 12 that aromaticity reveals itself in various ways. Qualitatively, aromatic compounds are more stable and less reactive than alkenes. Quantitatively, their heats of hydrogenation are smaller than expected. Theory, especially Huckel s rule, furnishes a structural basis for aromaticity. Now let s examine some novel features of the NMR spectra of aromatic compounds. 

Something interesting happens when we go beyond benzene to apply the aromatic ring current test to annulenes. 

More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene. The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the outside protons. The opposite occurs in [IGlannulene, which is antiaromatic. 

An NMR spectrum also provides other useful information, including  

The number of signals, which tells us how many different kinds of protons there are. 

In addition to suggesting what structural units might be present from the chemical shifts, an NMR spectrum also provides other useful information. 

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Ring Currents Aromatic and Antiaromatic Magnetic Resonance Imaging Spectra by the Thousands Gas Chromatography GC/MS and MS/MS... 

Ring Currents Aromatic and Antiaromatic Spectra by the Thousands... 

Ring Currents Aromatic and Antiaromatic 551 Magnetic Resonance Imaging (MRI) 564 Spectra by the Thousands 575... 

The question to be answered in the first place is whether the ring current model criterion is compatible with the chief energy criterion of aromaticity and antiaromaticity. The answer will be positive if a relationship is revealed between ring currents and resonance energies. 

All the same, the quantitative determination of the aromaticity and antiaromaticity from the ring current model may be complicated by at least two problems. First, experimentally observable values of magnetic susceptibilities and their exaltations and anisotropies as well as the H-NMR chemical shifts are not necessarily determined exclusively by ring currents hence, all other effects have to be identified and removed. Naturally, for this model to work, the contribution by the ring current must be predominant. Another problem is that the calculated results on ring current intensities for molecules from the diatropic-paratropic border area may vary qualitatively depending on the method of calculation (80PAC1541). 

The question is whether or not these reliable predictions of quantitative VB theory may also arise from a qualitative VB theory. Early semiempirical HLVB calculations by Wheland (14,15) and for that matter any VB calculations with only HL structures, incorrectly predict that CBD has resonance energy larger than that of benzene. Wheland, who analyzed the CBD problem, concluded that ionic structures play an important role, and that their inclusion would probably correct the VB predictions. Indeed the above mentioned successful ab initio VB calculations implicitly include ionic structures due to the use of CF orbitals in the VB descriptions of benzene, CBD, and COT. As will be immediately seen, ionic structures are indeed essential for understanding the difference between aromatic and antiaromatic species, such as benzene, CBD, and COT. Furthermore, the inclusion of ionic structures bring in some novel insight into other features of these molecules, such as ring currents, and so on (see Exercise 5.4). 

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