Positive NICS values

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

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... 

Nucleus-independent chemical shifts (NICS) provide a useful criterion of aromaticity/antiaromaticity. Absolute magnetic shieldings are computed at ring centers (nonweighted mean of the heavy atom coordinates) and correspond to the NMR chemical shift convention the signs are reversed, so that negative NICS values denote aromaticity and positive NICS values denote antiaromaticity. 

Note that, for simple organic molecules, a positive NICS value (paramagnetic ring current) suggests antiaromaticity, and they are expected to be distorted (nonzero BLA). However, the structures for all the Ni rings are perfectly planar polygonal. 

This interpretation is supported by model calculations for the [6,6] closed isomer of homopyracyclene, which afford large positive NICS values in the five- and six-membered rings and an even larger deshielding of the methylene hydrogens (computed 5 5 7. DFT level). 

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. 

It can be seen that the TREs calculated for polycyclic it-systems cannot always be associated with the thermodynamic or kinetic stability of the it-system. Heterobicycle 128d has large negative NICS values at all ring centers in accord with the large positive TREs. However, they must be kinetically very unstable because the nonclassical... 

Furthermore, the NICS value for cyclobutane is 2.6 ppm, with a very large, positive C-C contribution of -1-15.2 ppm. This paratropic magnetic effect is opposite to that of cyclopropane and is suggestive of perhaps a-antiaromaticity in cyclobutane. 

Nevertheless, the NICS values appear to readily classify standard molecules into three discrete categories. Aromatic molecules possess NICS values that are negative. The values at the center of the six-member rings of benzene and naphthalene and anthracene are -9.7 and -9.9, respectively. Charged aromatic molecules also have negative NICS values the values for cyclopentadienyl anion and tropy-lium cation are -14.3 and -7.6 ppm, respectively. Nonaromatic compounds like cyclohexane and adamantane have NICS values near zero. Lastly, antiaromatic molecules such as cyclopentadiene and planar Z>4 cyclooctatetrane have NICS values that are positive, 27.6 and 30.1 ppm, respectively. 

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