INTRODUCTION
Mixed boron-nitrogen (BN) materials have been subject of both
experimental and theoretical studies in order to understand their
properties1, such as their thermal stability for
example, hexagonal boron nitride (h-BN) are analogous to graphene and
has been used as thermally stable engineering ceramic and most recently
in optoelectronic devices2. Additionally, BN clusters
are interesting due to their participation in formation of thin solid
films of β-BN, that possess the particularity of scratching
diamond3. Recently, the application of atomic clusters
as assembly blocks for the design of new materials has grown in interest
and the term “Cluster Assembled Materials” (CAM) has been
coined4, so knowing the structures and electronic
properties of these clusters is of great importance5.
BxNy clusters have been characterized
through the interaction between pulsed laser evaporation followed by
matrix infrared spectroscopy and theoretical
predictions6,7. On the other hand,
BxNx clusters offer three-dimensional
structures that are similar to fullerene.
B12N12,
B76N76,
B208N208, among others, these have been
obtained through intense irradiation of boron nitride
samples8,9. Furthermore, small clusters offer planar
structures that resemble carbon clusters10.
Some years ago, Matxain et al .11 carried out an
exhaustive analysis of the potential energy surface (PES) of
BxNx clusters (x = 2 – 15) in singlet
state through Quantum Monte-Carlo calculations in order to know the
geometries of these clusters, obtaining as global minima planar
structures for x = 2 – 11 and three-dimensional for x = 12 - 15.
Additionally, authors concluded that planar structures with 2, 3, 5, 7
BN moieties are π-aromatic, structures with 4, 6 moieties are
antiaromatic and 8 – 11 BN moieties are non-aromatic according to their
Nucleus Independent Chemical Shift (NICS) calculations. Results for
B2N2 are counter intuitive according to
traditional electron counting rules such as Hückel’s
rule12, which states that cyclic systems with 4n+2
(4n) electrons have an aromatic (antiaromatic) character. Along with
this, the difference in electronegativities between the B and N atoms
leads to a very different electronic distribution than typical organic
aromatic systems, leading to a reduction in aromaticity. This
characteristic has been studied in various systems such as
cyclotriphosphazenes, metallabenzenes, among
others13-16.
The concept of aromaticity17, despite not being an
experimental observable, has been of great utility to rationalize
stability, electronic structure and chemical bond among other properties
in organic and inorganic chemistry18. There are
several criteria to rationalize aromaticity, such as the
geometric19, electronic
delocalization20 and the magnetic
criteria21, being the latter the latter being very
popular in recent years. In 1996, Schleyer et al . proposed the
Nucleus independent chemical shift (NICS)22, defined
as the negative of the isotropic magnetic shielding evaluated at the
centre of the ring of an (anti)aromatic system. This index quickly
became popular due to its ease calculation in many standard quantum
chemical software. However, its indiscriminate use has been criticized
by several authors, indicating that the validity of the NICS is limited
due to spurious contributions from the in-plane tensor components which
are not related with aromaticity23-31.
To avoid these problems, different ways to analyse NICS have been
suggested. For instance: to compute the property 1.0 angstrom (Å) above
the molecular plane NICS(1)32, and to evaluate the
out-of-plane component of the NICS tensor, the so called
NICSzz which has been proven to be particularly
sensitive to π-electron delocalization33,34. Other
strategies proposed are related to the analysis of NICS in the axis
perpendicular to the molecular plane. Stanger et
al .35 and Solá et al. 36,
independently, proposed NICS scans and more recently Torres-Vegaet al .13 proposed the FiPC-NICS strategy which
allows to obtain characteristic profiles for aromatic, antiaromatic, and
non-aromatic molecules. Another more sophisticated approaches are
related to dissected NICS approaches, bi-dimensional maps,
three-dimensional grids and quantification related to its
topology37-41.
In this article the aromaticity of BxNxclusters (x = 2 – 11) has been revisited by means of magnetic criteria
of aromaticity, and chemical bond has been analysed through Adaptative
Natural Density Partitioning (AdNDP) method42,43. A
complete analysis considering different NICS-based strategies proposed
in literature were carried out. Additionally, ring current strengths
(RCS) were computed measuring the flow of ring current passing through
interatomic surfaces according to quantum theory of atoms in molecules
(QTAIM) proposed by Bader44-47.