Nucleus-independent chemical shifts

The observed planarity and bond length equalization in 1,3,2-diazaphospholenium cations likewise suggest that these compounds have substantial n-electron delocalization and possess possibly aromatic character. Several studies were undertaken to quantify the degree of n-delocalization by computational calculations using the interpretation of population analyses, ELF calculations, evaluation of magnetic criteria [nucleus independent chemical shift (NICS) values], and the... 

The aromaticity of indolizine was examined using the nucleus-independent chemical shift values at several different levels of theory confirming the trend indole > isoindole > indolizine <2003JMT157>. 

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. 

This provides support for the reliability of the nucleus independent chemical shift values reported in Table I, which are calculated by the same method. These values show that the fluorenyl systems are antiaromatic in 9 and 10. In addition, the antiaromaticity can be varied by the nature of the substituent and their placement. 

Table I. Nucleus independent chemical shift (NICS(l)) of the fluorenyl... 

Table II. Redox potentials0 and nucleus independent chemical shifts for...

Aromaticity Evaluations of Three-dimensional Structures Nucleus-Independent Chemical Shift (NICS)... 

Density functional theory and MC-SCF calculations have been applied to a number of pericyclic reactions including cycloadditions and electrocyclizations. It has been established that the transition states of thermally allowed electrocyclic reactions are aromatic. Apparently they not only have highly delocalized structures and large resonance stabilizations, but also strongly enhanced magnetic susceptibilities and show appreciable nucleus-independent chemical-shift values. 

Schieyer PV, Maerker C, Dransfeld A et al (1996) Nucleus-independent chemical shifts a simple and efficient aromaticity probe. J Am Chem Soc 118 6317-6318... 

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