Evaluation of NICS changes along the main molecular axis
In this part, discussion starts analysing FiPC-NICS strategy, proposed
as a simplification of Stanger scans proposal. This strategy assumes of
that in-plane components are more sensitive to local induced fields
arising from the core and localized (lone pairs and bonding) electrons.
Local magnetic fields should be of short-range which means that they
should decay in small spatial intervals. In contrast, the out-of-plane
component is expected to highlight the induced magnetic field, as is the
case of aromatic and antiaromatic system. According to this strategy,
one can classify an aromatic (antiaromatic) system if it has a convex
(concave) slope, while a non-aromatic system behaves linearly. Fig. 2
shows that B3N3 and
B5N5 can be classified as aromatic while
B2N2,
B4N4 and
B6N6 are antiaromatic in agreement with
our RCS values. The rest of the systems (x = 7 - 11) present a linear
behaviour revealing its non-aromatic character. Additionally, is
important to identify the distance where the in-plane components of NICS
becomes zero. This distance for B2N2 and
B3N3 are 1.7 and 1.1 Å respectively. The
difference between distances are attributed to the local contributions
of the core electrons, being greater for
B2N2 due to the electronegativity of
nitrogen atoms and a smaller ring size. For the rest of the systems, the
distance increases systematically from 1.2 Å for the
B4N4 to 2.7 Å for the
B11N11. However, the increase in ring
size and the non-aromatic character of the clusters makes contributions
greater than 1.2 Å negligible (less than 1.0 ppm).