Nucleus-independent chemical shift NICS values

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... 

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. 

Galbraith, J. M. Schreiner, P. R. Harris, N. Wei, W. Wittkopp, A. Shaik, S. A valence bond study of the Bergman cyclization geometric features, resonance energy, and nucleus-independent chemical shift (NICS) values, Chem. Eur. J. 2000, 6, 1446-1454. 

Regarding the tropicity of DBAs, Kleinpeter visualized spacial nucleus independent chemical shift (NICS) values as isochemical shielding surfaces for a number of DBAs including each constitutional isomer [24]. 

Nucleus Independent Chemical Shift (NICS) Values of Bj, B4Na2, AIJ", Al4Na2, Gai, Ga4Na2 clusters... 

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