Borazine: to be or not to be aromatic

Abstract

Aromaticity of borazine, which has been subject of controversial discussions, is addressed. Beside a short review on aromaticity of borazine we report a detailed analysis of two molecular fields, the induced magnetic field (B ind ) and the electron localization function (ELF). The induced magnetic field of borazine shows a long-range shielding cone perpendicular to the molecular plane, as in benzene, but lower in magnitude. Contrary to benzene, borazine shows two weakly paratropic regions, one of them inside the ring, and the second one enveloping the boron atoms. It is necessary to separate r and p contributions to identify whether borazine exhibits p-aromatic character comparable to benzene. Nucleus-independent chemical shift (NICS) isolines show that the r electrons are much stronger localized than p electrons, their local paramagnetic contributions generate a short-range response and a paratropic (deshielding) region in the ring center (similar to an anti-aromatic response). Three regions can be identified as chemically meaningful domains exhibiting an internally strong electron delocalization (ELF = 0.823). Borazine may be described as a p aromatic compound, but it is not a globally aromatic species, as the electronic system is not as delocalized as in benzene.

Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules

“…Very recently Islas et al [94] in their publication entitled Borazine: to be or not to be aromatic based on a detailed analysis of two molecular fields, the induced magnetic field (B ind ) and the electron localization function (ELF) indicated that borazine can be described as a π-aromatic compound but it is not a globally aromatic species, since the delocalized system is not as homogeneous as that of benzene. The induced magnetic field in borazine is weaker than that of benzene but nevertheless pronounced and long-ranged.…”

The aromaticity and electronic properties of monosubstituted benzene, borazine and diazadiborine rings: an ab initio MP2 study

“…However, some investigations have shown that borazine may undergo electrophilic substitution in the gas phase under basic conditions much like benzene [1,2]. On the other hand, the aromaticity of borazine has been a controversial matter [3][4][5][6] and most reliable aromaticity criteria advocate non-aromaticity of borazine [7,8].…”

A Hybrid Organic/Inorganic Benzene

“…The isoelectronic and isostructural relationship between the BÀN and C=C bond and its consequence on the aromaticity of borazine has been a topic of discussion. [5] From a more applied perspective, borazine serves as a precursor to BN-based ceramic materials.…”