Nucleus-Independent Chemical Shift NICS

NMR chemical shifts for aromatic systems generally appear in the region from 7 to 9 ppm, a downfield shift of about 2 ppm from typical alkenes. These are 

It must first be remarked that there is no way to compare these computed NICS values with an experimental measurement, since there is no nucleus (typically) at the center of aromatic rings. The values in Table 3.24 demonstrate some small basis set dependence. Schleyer recommended the use of diffuse functions for the evaluation of NICS. NICS is a local measure, a magnetic property at a single point. There are concerns over using such a local property to evaluate the global nature of 

Nevertheless, the NICS values appear to readily classify standard molecules into three discrete categories. Aromatic molecules possess NICS values that are negative. The values at the center of the six-member rings of benzene and naphthalene and anthracene are -9.7 and -9.9, respectively. Charged aromatic molecules also have negative NICS values the values for cyclopentadienyl anion and tropy-lium cation are -14.3 and -7.6 ppm, respectively. Nonaromatic compounds like cyclohexane and adamantane have NICS values near zero. Lastly, antiaromatic molecules such as cyclopentadiene and planar Z 4 cyclooctatetrane have NICS values that are positive, 27.6 and 30.1 ppm, respectively. 

Schleyer demonstrated the utility of NICS as an aromaticity metric by comparing NICS values with the values of ASE for a series of five-member ring systems. The correlation is quite acceptable, suggesting that NICS might be useful for rank-ordering relative aromatic character. An example of NICS used in this way was in the study of a series of dimethyldihydropyrene derivatives. NICS and other aromaticity measures were found to correlate.  

NICS methods (NICS(O) and NICS(l)) have found widespread use in evaluating relative aromatic and antiaromatic characters for a variety of systems—aromatic transition states, ° antiaromatic dications,and aromatic bowls  

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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... 

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. 

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

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

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. 

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