Hidden in Plain Sight: Aromaticity of Hexagonal Boron Nitride

Abstract

Hexagonal boron nitride (hBN) and graphene are similar in many ways - they are isoelectronic, have the same structure, are chemically inert and show persistence. All of these properties are indicators of a deeper connection that has, thus far, been overlooked. Unlike graphene, which has been shown to be aromatic, it is not known if hBN is aromatic. In this density functional theory-based work, we investigate the aromaticity (or lack thereof ) of hBN. By employing the magnetic criterion, supported by group theoretic and energetic considerations, we show that hexagonal boron nitride is indeed aromatic, even if weakly so, as compared to graphene. Since aromaticity is used to understand physical and chemical properties of planar compounds, the picture developed in this work is important to bridging the gap between the physical and chemical understanding of hBN's properties.

Figures (8)

• Figure 1: Flakes with the general chemical formula of B$_{3n^{2}}$N$_{3n^{2}}$H$_{6n}$, $n=1, 2, 3,...$ (shown on the right) created from a bulk hBN monolayer (on the left). The smallest possible flake, borazine ($n=1$), is obtained from the hydrogenated pink hexagonal region of 2D hBN. Larger flakes are obtained by including additional hBN rings, which are indicated by the different colored regions of the 2D hBN on the left.
• Figure 2: Magnetic criterion for aromaticity: (a) Magnetically-induced aromatic ring current (in blue) and the resulting shielding (deshielding) effect of the current-induced magnetic field (in red) in the endocyclic (exocyclic) region. (b) The BQ-probe (ghost atom), shown in purple, used to obtain the NICS-index at a distance $r$ above the ring center. (c) A series of BQ-probes placed along the X-direction to survey the NICS-indices across the molecule. (d) The $\sigma$-Only model for borazine obtained by placing hydrogen atoms below the B- and N-atoms.
• Figure 3: (a) The lateral scan direction (X-axis) is shown by the gray arrows along which NICS values are calculated by placing multiple ghost atoms on benzene. NICS-X-scans yielding (b) $\textrm{NICS}_{zz}$ and (c) $\textrm{NICS}_{\pi zz}$ values at different heights (1.0 - 3.0 Å in 0.1 Å increments) for benzene. In (b) and (c), the distance along the x-axis is given relative to the geometrical center of the ring. (d) X-scan path (indicated by the gray arrow), (e) $\textrm{NICS}_{zz}$ X-scan, and (f) $\textrm{NICS}_{\pi zz}$ scan values for borazine. Note that the y-axis scales used to plot NICS-scans of borazine are different from those for benzene.
• Figure 4: $\textrm{NICS}_{zz}$ values at 2.0 Å above the ring centers for (a) B$_{3}$N$_{3}$H$_{6}$, (b) B$_{12}$N$_{12}$H$_{12}$, (c) B$_{27}$N$_{27}$H$_{18}$, and (d) B$_{48}$N$_{48}$H$_{24}$. $\textrm{NICS}_{zz}$ scans along: (e, f, g, h) X-direction and (i, j, k, l) Y-direction for the hBN flakes of different sizes. The insets in (e) and (i) show the scan directions for borazine.
• Figure 5: (a) ACID isosurface plot for benzene, showing the $\pi$-contribution. Current density vectors (green arrows) plotted onto the ACID isosurface (drawn in yellow) show a strong diatropic ring current from $\pi$-delocalization. Also drawn is a schematic illustration of the benzenic ring current (solid red line). (b) ACID plot for borazine showing the $\pi$-contribution. A close-up of different regions shows a weak global diatropic circulation (in dashed red line), along with stronger local ring current (in solid red) around the nitrogen atoms. The schematic diagram shows global and local rings currents to highlight the differences in magnetic response for benzene and borazine. (c) ACID and schematic plots for C$_{24}$H$_{12}$, showing a strong global ring current (diatropic), along with a weak local paratropic current (in blue) around the central ring. (d) ACID and schematic plots for B$_{12}$N$_{12}$H$_{12}$, showing a global, diatropic ring current in the outer rim, along with local diatropic currents around nitrogen atoms. All ACID plots are for the isovalue of 0.03.