Negative NICS values

The computational results point to an aromatic character of the silanaphthalenes 18-20. The chemical shifts which were observed for 1 are similar to those calculated for 19a-c (Table 3). The observed (in 1) and calculated (for 19a-c) <5(29Si) and 5(13C) chemical shifts are similar to those previously reported for other sp2-hybridized Si and C atoms32,8. All the NMR signals are, as expected, in the aromatic region (Table 3). The highly negative NICS values calculated for 18,19a and 20 (Figure 3), which are very similar to those of benzene, also indicate that all the calculated silanaphthalenes (18-20) are aromatic. 

It has been postulated that perfluorinated 1,3-dioxa systems 1 are unlikely to fragment easily and may be therefore thermodynamically stable, because perfluorination induces a negative NICS value (cf. Section 13.11.2) corresponding... 

The NICS values calculated at the ring centres for the compounds 1-7 are depicted in Fig. 13. Large negative NICS values are found for the top and bottom six-membered rings. The NICS values for these rings are shifted 10 ppm upfield with respect to the NICS values of the central six-membered rings, which is in line with the resonance criterion, derived above. 

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... 

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. 

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... 

In their study on the aromatic character of electrocyclic and pseudopericyclic reactions de Lera et examined the thermal cyclization of (2Z)-hexa-2,4-5-trienals and their Schiff bases. They used the NICS at the RCP to detect aromaticity, since a large negative NICS value is associated with aromatic character. 

More recently, a new and widely used index of aromaticity, the nucleus-independent chemical shift (NICS), has originally been proposed by Schleyer and coworkers [44,45]. It is defined as the negative value of the absolute shielding computed at a ring center or at some other interesting points of the system. Negative NICS values correspond to aromaticity, while positive values are associated with antiaromaticity. There is a whole family of NICS methods, which include NICS computations at ring centers [43], NICS(O) and above [46], NICS(I) and NICS(2), dissected NICS values, i.e., the total NICS at a particular point in space may be dissected into paratropic and diatropic components, which mainly arise from the C-C cr and jt multiple bonds, respectively [46,47] and MO contributions to NICS,... 

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. 

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