Aromaticity magnetic measures

A simple consequence of the above is the large vertical resonance energy of the distorted benzene in Figure 6 and its aromatic magnetic behavior both computed and measured.32 169-171 In fact, as emphasized by Choi and Kertesz,87 even when the aromatic [nj-annulenes undergo bond alternating distortion, their magnetic properties are still those of an aro-... 

Nucleus-independent chemical shift (NICS) values for a number of five-membered heterocycles have been reported and are used as a quantitative magnetic measure of aromaticity. They are a measure of diamagnetic ring current and are not a measure of thermodynamic stability. NICS values are theoretical parameters and the values depend on the computational method used and the position above the ring. Table 2 shows NICS values for a selection of heterocyclic systems where NICS is the value at the centre of the ring. The aromaticity of five-membered heterocycles is discussed in Sections 2.3.4.2 and 2.4.4.2. 

The concept of nucleus-independent chemical shifts (NICS) as a quantitative magnetic measure of aromaticity is discussed in Sections 2.2.2.2.2 and 2.3.2.2.1, where NICS values for six-membered rings and five-membered rings with one heteroatom are tabulated. Table 4 shows NICS values calculated for five-membered rings with more than one heteroatom <2002JOC1333>. Since NICS values are theoretical parameters, they depend on the computational method used. In Table 4 NICS is the value at the center of the ring and NICS(l) is the value 1 A above the center. 

Magnetic susceptibilities of 2-aminothiazole have been measured (139) in order to evaluate the aromatic character of the compound. 

Nuclear magnetic resonance measurements have led to the conclu-sion that 2-pyridones have about 35% of the aromaticity of benzene and that the formally related l,2-dihydro-2-methylenepy-ridine is not aromatic. A substantial contribution by such resonance is indicated by the electronic spectrum of 2-quinolone, which is... 

The numerous aromaticity measures proposed in recent decades have widened the number of descriptors in the literature considerably [2,8,9,44,45], Aromaticity descriptors can be classified as magnetic, energetic, electronic, and structural [46]. 

Aromaticity, defined as a structural feature, was used as a predictive tool for compounds that had not been prepared previously, whereas reactivity, bond length, or magnetic criteria had to await the isolation of a compound and its experimental investigation. Only recently has the development of quantum-chemical methods reached the point where one can predict with sufficient accuracy the magnetic properties, the bond lengths, and the reactivity patterns of aromatics. The multidimensional character and the definition and measurement of aromaticity generated confusion and conflicts.43 A recent review discussed the multidimensional character of aromaticity and theoretical and experimental approaches to aromatic structures and their predictions, and references are indicated extensively.66... 

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