Magnetic criteria of aromaticity

Magnetic criteria of aromaticity are based on the model of interatomic teratomic ring currents induced in conjugated cyclic molecules by external magnetic fields. 

One of the well-accepted magnetic criteria of aromaticity is the nucleus independent chemical shift (NICS) which is defined as the negative value of the absolute magnetic shielding calculated at some selective points. The NICS(O) and NICS(l) values are computed at the centre and lA above the aromatic ring respectively. Aromatic systems possess negative NICS values as that indicates the presence of an induced diatropic ring current. Positive NICS values refer to paratropicity and are present in antiaromatic... 

Zhou and Navangul calculated MNDO hardnesses for 14 benzenoid hydrocarbons and showed it to be a good indicator for their aromaticity.Using the hardness criterion, kekulene, coronene, and the corannulene tetraanion were all predicted to be superaromatic, also confirmed by calculations of Babic and Trinajstic and Cioslowski, O Connor, and Fleischmann in the case of kekulene. " Jiao and Schleyer, however, concluded, based on geometrical, energetic, and magnetic criteria of aromaticity, that kekulene is not superaromatic. ... 

Aihara J (2006) Circuit resonance energy a key quantity that links energetic and magnetic criteria of aromaticity. J Am Chem Soc 128 2873-2879... 

Bird, C. W. 1996. The relationship of classical and magnetic criteria of aromaticity. Tetrahedron 52 9945-9952. 

Though it is usually not difficult to classify a given compound as aromatic, nonaromatic or antiaromatic from a qualitative point of view, much more complex problems arise in attempting to describe the aromaticity in quantitative terms. Until now, three main groups of quantitative criteria of aromaticity have been elaborated energetical, structural and magnetic. 

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. 

We have used a different approach to compare the aromaticities of phosphole (8) and pyrrole (10) [23, 24], From literature data on derivatives of 8 and 9 it is known that the inversion barrier of phosphole is about 67 kJ mol-1 (70.2 kJ mol-1 at the B3LYP/aug-cc-pVTZ level) [25] while that of tetrahydrophosphole amounts to 163 kJ mol-1. This is explained by the fact that the planar transition state of 8 is highly aromatic. Pyrrole (10) is planar and pyrrolidine has a calculated inversion barrier of 15-17 kJ mol-1. Several aromaticity indices were used in this study, based on different criteria of aromaticity energetic (aromatic stabilization energy, ASE), geometric (harmonic oscillator model of aromaticity, HOMA, and /5), and magnetic (NICS). 

Electronic, geometric, energetic, and magnetic conventional criteria of aromaticity when applied to cobalta- and iridacyclopentadiene showed that these heterocycles are firt-electronic and aromatic <2004JOM(689)1050>. However, there is controversy in the literature concerning the existence and extent of Jt-delocalization in metalla-cyclopentadienes, -trienes, and metallacyclopentynes, and more systematic theoretical computations are needed. 

We will not concern ourselves here with the attempt to discern a true definition of aromaticity, nor with the largely unproductive " desire to rank-order relative aromaticity. Rather, we will focus on how computational chemistry has been used to help define two of the criteria of aromaticity. We will discuss how to compute the stabilization energy of an aromatic species and then how we can compute magnetic properties that can be related to aromaticity. We will then conclude with a discussion of how these measures have been employed to investigate the nature of annulenes and the MiUs-Nixon effect. 

The ASE values correlate with magnetic susceptibility for the five-membered heteroaromatic compounds. Magnetic and polarizability criteria put the order of aromaticity as thiophene > pyrrole > furan. The other criteria of aromaticity discussed in Section 8.2 are also applicable to heterocyclic compounds. HOMO-LUMO gaps and Fukui functions (see Topic 1.5) have been calculated for compounds such as indole, benzofuran, and benzothiophene and are in accord with the known reactivity patterns of these heterocycles. 

An MO multicenter bond index involving a- and 7t-electron population is related to both energetic and magnetic criteria. Since aromaticity is certainly related to the mutual simultaneous interaction of all bonds of an aromatic ring, 7,ing is defined as a measure of aromaticity <2000PCP3381>. 7, ng values for 1, 1,2,4- and 1,3,5-triazine are 0.087 5,... 

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