Fig. 2. Plots of the NICSin-plane vs. NICSout-of-plane for BxNx clusters (x = 3 – 11).
Complementary, NICSzz-scans are showed in Fig. 3, measurements were carried out at center of the ring and above up to 5 Å. Through these plots it is possible to see the long-range effects characteristic of (anti)aromatic systems. B3N3 is the only system which presents long-range negative values characteristic for aromatic systems.
For B5N5, no long-range effects are observed, in disagreement with the RCS values which classify it as weakly aromatic, but in agreement with the NICSzzvalues. Recently, Cina pointed out that when NICS values moves from negative to positive regions are good candidates for NICS failures which is the case of this cluster56. Additionally, this system presents two minimum values of -7.4 ppm and -6.8 ppm at 0.0 and 1.4 Å respectively, this may be due to local contributions from core electrons, these local contributions have been studied and are important in similar systems such as borazine57.
On the other hand, B2N2, B4N4 and B6N6 present positive long-range values which is consistent with their antiaromatic character, while aromatic (According RCS calculations) B5N5 and B7N7 present small short-range. The rest systems have very low values due to the contributions of the core electrons.