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.