Aromaticity nucleus independent chemical shift

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. 

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... 

The aromatic nature of the presently discussed carbocations and dications have been further established by subjecting their NMR parameters to charge density chemical shift relationship449,475 77 originally developed by Spiesecke and Schneider.478,479 Furthermore, NICS (nucleus-independent chemical shift) developed by Schleyer et al.480 offers a simple and efficient probe for aromaticity. 

Quantum-chemical calculations for pyrylium including one, two, or three water molecules using DFT and 6-31 + G(d,p) basis set revealed that the aromaticity (estimated by harmonic oscillator stabilization energy, HOSE natural resonance theory, NRT harmonic oscillator model of aromaticity, HOMA and nucleus-independent chemical shifts, NICS) is not influenced by water molecules [82],... 

The magnetic properties used to probe aromaticity arise from the presence of a diatropic ring current which tends to push an aromatic molecule out of a magnetic field (calculated property magnetic susceptibility, /). and which exerts NMR shielding on a proton at or above the ring center (calculated property nucleus-independent chemical shift, NICS). NICS values are obtained from the... 

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