Quantifying aromaticity according to ring current strength (RCS)
and NICSzz based descriptors
Systems studied in this work are BxNxclusters (x = 2 – 11) depicted in Fig. S1 in the supporting
information. This section is aimed to the quantification of the
(anti)aromatic character of the mentioned clusters. For this purpose,
ring current strengths (RCS) were computed and compared with different
quantification strategies based on NICSzz. RCS values
were obtained by numerical integration of the current flow through
interatomic surfaces by means of quantum theory of atoms in molecules
(QTAIM) proposed by Bader. RCS values have been widely used in
aromaticity studies and are known to be a reliable indicator of the
(anti)aromatic character of both organic and inorganic systems.
According to table 1, B2N2, present a
strong antiaromatic character with a RCS value of -13.0
nA.T-1 in clear disagreement with Matxain et
al . results but consistent with electron counting rules.
B4N4 and
B6N6, present low negative values (-2.7
and -1.5 nA.T-1 respectively) revealing weak
antiaromaticity. On the other hand, borazine-like
B3N3 present a RCS value of 3.8
nA.T-1, in agreement with recent reported
results55, whereas
B5N5 have a RCS value 2.2
nA.T-1 revealing its weak aromatic character. The rest
of the systems (x = 7 – 11) show values close to zero, therefore, they
are non-aromatic. Additionally, NICSzz was measurement
at 0.0 Å, 1.0 Å, 2.0 Å and FiPC (Free in plane component, where the
in-plane contributions are zero) can be seen in Fig. 1, these values
correlate quite well with the RCS, however it is necessary to be very
cautious with the interpretation. As an example, let us take the
NICSzz(1) value of B5N5,
which is -0.44 ppm, this result can be interpreted as non-aromatic and
not (weak) aromatic, as indicated by its RCS value. The same behaviour
is observed for the NICSzz(2) of
B4N4 (0.32 ppm) which could be
interpreted as non-aromatic, however the RCS values showed that this
system is (weak) antiaromatic.
Table 1. RCS (in nA.T-1) values and
NICSzz (in ppm?) computations for the
BxNx (x = 2 – 11) planar clusters. RCS
positive (negative values denote aromaticity (antiaromaticity), values
close to zero, nor aromatic character. Negative (Positive) values of
NICSzz descriptors states an aromatic (Antiaromatica) character, whereas
values close to zero are for non-aromatic compounds.