Nucleus-independent chemical shift calculations

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... 

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. 

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. 

NMR shifts, nucleus-independent chemical shifts, NICS, and magnetic susceptibility exaltation, A.9 It has been found that counterions play a more important role in the behavior of dianions than dications, as shown by the improvement in the agreement of experimental and calculated shifts for dianions with the inclusion of counterions. 

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... 

A similar conclusion was drawn based on nucleus-independent chemical shift (NICS) calculations, that is, there is no significant aromatic character to a 1,2-dihydrodiazete. Natural bond orbital (NBO) analysis provides the means of investigating the extent of electron delocalization within a given structure and also indicates the bond order. In Lewis-type bonding orbitals electron density is depleted with increase in the occupancy in antibonding or non-Lewis orbitals. In the case of cis-14 or trans-14, there is less occupancy in non-Lewis orbitals and the bond order is more like localized bonds N-N (1.01), C-N (1.06) and C=C (1.76), which are similar to the nonaromatic substances. 

Calculations of the structure of the mesoionic thiopyrylium-3-olate 74 suggest that the C-S bond lengths are similar to those in the pyrylium cation at ca. 168 pm, perhaps supporting the fully charge-separated betaine structure. However, the charge at oxygen is closer to 0.5 than the 1.0 expected for such a structure. Furthermore, the nucleus-independent chemical shift value is appreciably lower than that for the thiopyrylium cation. These data point toward an ylidic structure with an acceptor moiety rather than an aromatic cation and an exocyclic oxyanion <2002IJQ(90)1055>. 

The concept of nucleus-independent chemical shifts (NICS) as a quantitative magnetic measure of aromaticity is discussed in Sections 2.2.2.2.2 and 2.3.2.2.1, where NICS values for six-membered rings and five-membered rings with one heteroatom are tabulated. Table 4 shows NICS values calculated for five-membered rings with more than one heteroatom <2002JOC1333>. Since NICS values are theoretical parameters, they depend on the computational method used. In Table 4 NICS is the value at the center of the ring and NICS(l) is the value 1 A above the center. 

---