Ring current of aromatic compounds

In toluene, the methyl protons resonate at 2.34 8 where as a methyl group attached to an acyclic alkane appears at 1.958. This is due to the greater deshielding influence of the ring current in aromatic compound as compared to the other. 

The stability of isothiazole derives from the fact that it has an aromatic delocalized ir-electron system. The NMR chemical shifts, which depend, inter alia, on ring currents, and the high stability of the molecular ions in mass spectrometry, are typical of aromatic compounds, and X-ray measurements confirm the partial double bond character of all the bonds of the ring. 

The NMR spectrum of this compound shows a diamagnetic ring current of the type expected in an aromatic system. X-ray crystal structures of 1 and its carboxylic acid derivative 2 are shown in Fig. 9.2. Both reveal a pattern of bond lengths very similar to that in naphthalene (see p. 534). ... 

A novel method for estimating ring current in aromatic molecules has recently been provided (79TL437). This involves measurement of the H NMR a- and y-methyl chemical shifts in compounds (258), readily prepared from ArNth and 2,4,6-trimethyIpyrylium perchlorate. Quantity D defined by equation (18) is a qualitative measure of ring current... 

A special magnetic behaviour of aromatic compounds is believed to result from the occurrence of aromatic ring currents. The following magnetic criteria can be used to determine if a molecule is aromatic. 

This ring current in benzene is termed diamagnetic and is characteristic of aromatic compounds in general. The presence of a diamagnetic ring current provides a useful experimental criterion for the presence of aromaticity in a compound. Other examples of the use of this method are provided in Section 16.10. 

These various structures raise the question of the aromaticity of the l,6,6a 4-trithiapen-talene system. From a qualitative viewpoint, thermal stability and electrophilic attack on atoms 3 and 4 may be considered as consistent with an aromatic character. Furthermore, NMR deshielding of ring protons <71AHC(13)161, p. 230) suggests a strong ring current similar to those of aromatic compounds. 

Interestingly, the bond distances and angles of l,l-dimethyl-2,4,6-triphenyl-/l -phosphorin determined by X-ray diffraction are similar to those of /( -derivatives (Figure 18), yet the physical and chemical properties show that the 2 -phosphorins with electron-donor substituents (H, Me, SiHs) at the P heteroatom are not aromatic, and that the tetracoordinated phosphorus atom interrupts the ring current in such compounds. Flowever, when the P-substituents of /l -phosphorins have higher electronegativities (F, OFl, Cl, Br) the NlCS(l) values indicate aromaticity, but somewhat less than phosphabenzene (01HCA1578). 

But the most precise attack was probably mounted by J. Musher who argued that the N.M.R. behaviour of aromatic compounds might be described as the sum of contributions from localized electrons of both a and it charac-ter besides, he emphasizes the fact that the so-called ring current is a phenomenon which depends essentially on the group symmetry of the molecules and that this restriction greatly weakens the generality of this concept... 

The simplest aromatic compound is benzene. It has a ring current of tt electrons, which shows strong ti -> ti absorptions at 184 nm 60,000), and at 204 nm 7900). (This is called a primary band.) Benzene exhibits a low intensity band at 256 nm 200) (Known as a secondary or fine-structure band), with a series of fine-structue bands between 230 and 270 nm). Any substitution n the benzene ring, irrespective of its electronic character... 

The simplest aromatic compound is benzene. It has a ring current of tt electrons, which shows strong absorptions at 184 nm 60,000),... 

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