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.