The average local ionization energy as a tool for identifying reactive sites on defect-containing model graphene systems

Abstract

In a continuing effort to further explore the use of the average local ionization energy $\overline{\mathrm{I}}\left( \mathbf{r} \right)$ as a computational tool, we have investigated how well $\overline{\mathrm{I}}\left( \mathbf{r} \right)$ computed on molecular surfaces serves as a predictive tool for identifying the sites of the more reactive electrons in several nonplanar defect-containing model graphene systems, each containing one or more pentagons. They include corannulene (C₂₀H₁₀), two inverse Stone-Thrower-Wales defect-containing structures C₂₆H₁₂ and C₄₂H₁₆, and a nanotube cap model C₂₂H₆, whose end is formed by three fused pentagons. Coronene (C₂₄H₁₂) has been included as a reference planar defect-free graphene model. We have optimized the structures of these systems as well as several monohydrogenated derivatives at the B3PW91/6-31G* level, and have computed their $\overline{\mathrm{I}}\left( \mathbf{r} \right)$ on molecular surfaces corresponding to the 0.001 au, 0.003 au and 0.005 au contours of the electronic density. We find that (1) the convex sides of the interior carbons of the nonplanar models are more reactive than the concave sides, and (2) the magnitudes of the lowest $\overline{\mathrm{I}}\left( \mathbf{r} \right)$ surface minima (the ${{\overline{\mathrm{I}}}_{{\mathrm{S}\text{,}\min }}}$) correlate well with the interaction energies for hydrogenation at these sites. These ${{\overline{\mathrm{I}}}_{{\mathrm{S}\text{,}\min }}}$ values decrease in magnitude as the nonplanarity of the site increases, consistent with earlier studies. A practical benefit of the use of $\overline{\mathrm{I}}\left( \mathbf{r} \right)$ is that a single calculation suffices to characterize the numerous sites on a large molecular system, such as graphene and defect-containing graphene models.