CONCLUSIONS
A complete analysis of the (anti)aromatic character of
BxNx clusters (x = 2 – 11) has been
performed according to magnetic criteria of aromaticity through ring
current strength (RCS) analysis, 1D, 2D, 3D NICSzzdescriptors and AdNDP analysis. The RCS values, FiPC strategy and
NICSzz-3D-grids patterns show that the
B2N2 cluster is anti-aromatic.
Additionally, the isolines of dissected NICSzz and the
bond pattern according to AdNDP reveal that this cluster is doubly
anti-aromatic, showing paratropic long-range regions in σ- and
π-contributions and delocalized σ- and π-bonds complying with the
electron counting rules. For the case of
B3N3 and
B5N5 clusters, the
NICSzz-based strategies show patterns that suggest an
aromatic character, in line with this, the AdNDP results show three
delocalised 3c-2e bonds and five delocalised 5c-2e bonds for the
B3N3 and
B5N5 respectively. For the latter, the
NICSzz 3D grids show paratropic contributions in the
center of the ring that can be attributed to the contributions of the
core electrons of the N and B atoms, for this reason, isolines of
dissected NICSzz were obtained which show that
B5N5 is non-aromatic σ- and weakly
π-aromatic, this is confirmed with RCS values revealing that both
clusters are weakly aromatic. The rest of the studied systems (X = 7 –
11) are non-aromatic, FiPC, NICSzz 3D-grids and RCS
calculations confirm these results. The AdNDP pattern shows that these
clusters have 2X localised σ- and π-bonds which explains the bond length
equalisation. Description of aromaticity using
NICSzz-based descriptors confirms its limitations even
though it has good correlations with RCS values, For example, the
results of NICSzz(0) present non-negligible positive
values for the clusters x = 7 - 11 which could be interpreted as having
a certain degree of anti-aromaticity, or the
NICSzz-scans which present patterns that could lead to
incorrect conclusions as mentioned by Cina. Finally, the use of
NICSzz descriptor to diagnose (anti)aromaticity must be
done carefully. Thus, the use of 1D, 2D or 3D tools based on
NICSzz are necessary to have a complete and appropriate
description of the aromaticity.