Summation of Nucleus Independent Chemical Shifts as a Measure of Aromaticity

Abstract

When the nucleus independent chemical shifts, NICS(1)zz, for a set of aromatic and antiaromatic hydrocarbons are summed, they show an excellent linear relationship with the magnetic susceptibility exaltation, Lambda, for neutral, cationic, and monoanionic species. Aromatic and antiaromatic dianions show a similar relationship but with a different slope. However, when both Lambda and the summation of NICS(1)zz are divided by the area of the ring squared, the vast majority of the aromatic and antiaromatic species fall on the same line, indicating that both NICS and Lambda are affected by the size of the ring. The species that deviate slightly from the line are a few of the anionic compounds, which may be a result of the difficulties of calculating magnetic properties of anions. This is the first report of the relationship of NICS to ring area. In addition, the excellent correlation between Lambda and the summation of NICS(1)zz demonstrates that summation of NICS(1)zz values for individual ring systems of polycyclic ring systems to give a measure of the aromaticity of the entire system is justified. By extension, the excellent correlation also serves to demonstrate that the NICS(1)zz values for individual ring systems are reliable measures of local aromaticity/antiaromaticity. Finally, the excellent correlation between experimental shifts and the 13C NMR shifts calculated with density functional theory, B3LYP/6-311+g(d,p), serves as an indirect validation of the accuracy of the NICS(1)zz calculated by the same method and basis set.

Dications of Benzylidenefluorene and Diphenylmethylidene Fluorene: The Relationship between Magnetic and Energetic Measures of Antiaromaticity

“…23 The good linearity also allowed us to demonstrate that the sum of the NICS values for each individual ring in a polycyclic system is an effective measure of the aromaticity of the system as a whole. This study also showed the value of using a combination of aromatic and antiaromatic species because the greater range of values obtained with the inclusion of antiaromatic species gave a more convincing demonstration of linearity.…”

“…22 The NICS(1) zz values are reported for the individual ring systems as well as for the summation of the three rings in the fluorenyl system (Tables 3 and 4). 23 The dihedral angles of the phenyl rings of 1 2þ and 2 2þ are approximately 50°; those for the phenyl rings of 3 2þ and 4 2þ are approximately 60°( see the Supporting Information). Because the top and bottom sides of the fluorenyl systems are not exactly the same, there is a slight difference in the magnitude of the NICS values.…”

“…We have averaged the NICS values for each ring of the fluorenyl system for Tables 3 and 4, but the unaveraged NICS values are available in the Supporting Information. We have determined that NICS values are inversely related to the square of the area of the ring, 23 and even though the ring systems examined via NICS are very similar, the correction for ring area has an effect on the comparison of NICS between systems (see the Supporting Information). The values for 1 2þ and 3 2þ are given in Table 3 and the values for 2 2þ and 4 2þ in Table 4.…”

“…of the relationship between P NICS and magnetic susceptibility exaltation, 23 the inclusion of data from both aromatic and antiaromatic species expanded the range sufficiently so that the linearity of the relationship was more apparent.…”

Reduction of Benzylidene Dibenzo[a,d]cycloheptenes: Over-Reduction of Antiaromatic Dianions to Aromatic Tetraanions

“…However, to place these values in a larger context by comparison with other aromatic and antiaromatic species, each value needs to be normalized by the square area of the ring system. We have demonstrated a linear relationship between the ΣNICS(1) zz /sq area and Λ/sq area, 22 and that relationship is shown in Figure 1, 23 The plot also includes planar (D 4h ) cyclooctatetraene, which is presumed to be the transition state for inversion of the tub-shaped geometry of cyclooctatetraene. 24 Attempted Preparation of 1a 2À À1f 2À through Reduction with Lithium or Potassium.…”