Nucleus independent chemical shift aromaticity/antiaromaticity

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic. 

Several aspects of aromaticity have been studied <2002JOC1333> using statistical analyses of quantitative definitions of aromaticity. ASEs, REs, magnetic susceptibility exaltation (A), nucleus-independent chemical shift (NIGS), the harmonic oscillator model of aromaticity (HOMA), (/j) and (Aj), evaluated for a set of 75 five-membered 7t-electron systems and a set of 30 ring-monosubstituted compounds (aromatic, nonaromatic, and antiaromatic systems) revealed statistically significant correlations between the various aromaticity criteria, provided the whole set of compounds is used. The data in Table 9 have been found for arsole (AsH) 1 (E = As, R = H), its anion (As ), and protonated species (AsH2 ). 

Stanger, A. Nucleus-independent chemical shifts (NICS) distance dependence and revised criteria for aromaticity and antiaromaticity, J. Org. Chem. 2006, 71,883-893. 

An alternative approach to characterizing the aromaticity of planar compounds is the calculation of a nucleus-independent chemical shift (NICS), which is a computed value of the magnetic shielding a virtual (ghost) nucleus would experience at specific locations near a n system. A NICS(0) value refers to shielding in the center of the n system, while a NICS(l) value reports the shielding expected at a point 1A above the center of the molecular plane. Aromatic compounds have negative NICS(O) values, and antiaromatic structures have positive values. NICS values provide a quantitative measure of aromatic character and have been shown to correlate well with other measures of aromaticity. 

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... 

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,... 

Schleyer PvR, Manoharan M, Wang Z-X, Kiran B, Jiao H, Puchta R, Hommes NJRVE (2001) Dissected nucleus-independent chemical shift analysis of n-aromaticity and antiaromaticity. Org Lett 3 2465-2468... 

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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