Minimizing the Risk of Reporting False Aromaticity and Antiaromaticity in Inorganic Heterocycles Following Magnetic Criteria

Abstract

Although aromaticity is a concept in chemistry, in the last years, special efforts have been carried out in order to propose theoretical strategies to quantify it as a property of molecular rings. Among them, perhaps the computation of nucleus independent chemical shifts (NICSs) is the most commonly used, since it is possible to calculate it in an easy and fast way with most used quantum chemistry software. However, contradicting assignments of aromaticity by NICS and other methods have been reported in the literature, especially in studies concerning inorganic chemistry. In this Article is proposed a new and simple strategy to use the NICS information to assess aromaticity, identifying the point along the axis perpendicular to the molecular plane where the in-plane component of NICS becomes zero; it is called free of in-plane component NICS (FiPC-NICS). This spatial point is proposed as secure to consider NICS as an aromaticity descriptor; this simple proposal is evaluated in borazine and cyclotriphosphazenes. The results are compared with carefully examined aromatic stabilization energies and magnetically induced current-density analysis.

Which NICS method is most consistent with ring current analysis? Assessment in simple monocycles

“…For example, we recently proposed the identication of the free of in-plane component NICS along the main molecular axis (FiPC-NICS), which allowed us to obtain characteristic patterns for aromatic, anti-aromatic and non-aromatic benchmark molecules. 22,34 Other more sophisticated approaches are those based on molecular dissected NICS 31,32 conjointly with their bi-or tri-dimensional plots. 34,[37][38][39][40][41] It is also important to highlight that the quantitative relationship between shielding and ring current strengths has been rigorously analyzed in the past.…”

“…22,34 Other more sophisticated approaches are those based on molecular dissected NICS 31,32 conjointly with their bi-or tri-dimensional plots. 34,[37][38][39][40][41] It is also important to highlight that the quantitative relationship between shielding and ring current strengths has been rigorously analyzed in the past. [42][43][44][45] In these works, it is assumed that the wire carrying the ring current is circular and innitely thin; thus, the induced magnetic eld is easily determined from classical electrodynamics, using the Biot-Savart's law.…”

“…In borazine, the lone pairs on the nitrogen atoms, are known to exhibits local diatropic ring currents under magnetic perturbation, which are consequently expected to contribute to the NICS locally. 34,61 In the case of the Al 4 2À cluster, Sundholm and collaborators, based on total ring current analysis, predicted a doubly (p and s) aromatic character. 62 However, Fowler and collaborators showed that the p-electron exhibits a signi-cant ring current when the molecule is perturbed by a parallel magnetic eld, which implies mobility of the p-electron cloud, but not aromaticity.…”

“…22,34,38,60,65,66 Fig. 3 depicts selected planes (perpendicular to the molecular plane) containing isolines of the s-and the p-electron contribution to NICS zz , for the set of studied molecules.…”

Testing the effectiveness of the isoelectronic substitution principle through the transformation of aromatic osmathiophene derivatives into their inorganic analogues

“…However, its chemical bonding is different: whereas benzene is aromatic, borazine has been classified as nonaromatic or marginally aromatic. [20][21][22][23][24][25][26] The aim of the present work is to evaluate, in an extreme situation, the validity of the concepts mentioned in the preceding paragraphs, in order to design new stable molecules. The chosen system to achieve this goal is [OsCl2(SC3H3)(PH3)2] + ,which presents some characteristics that make it suitable for this purpose: 1) it is a simplified model of the osmabicycles (aromatic compounds where one Os atom is present in five-membered metallacycles), studied both experimentally and theoretically by Esteruelas et al, 27 2) this system was classified as clearly aromatic, 28 with participation of Os-d-orbital (especially dxz and dyz orbitals) closing the circuit of π-orbitals and allowing electronic delocalization on the planar OsC3S fragment.…”