pi-pi Stacking between Polyaromatic Hydrocarbon Sheets beyond Dispersion Interactions

TL;DR

This study uses high-level ab initio calculations to analyze pi-pi stacking interactions in polyaromatic hydrocarbons, revealing significant deviations from planarity and providing binding energies that align with experimental data.

Contribution

It offers detailed computational insights into the geometry and energetics of pi-pi stacking beyond dispersion interactions, comparing various theoretical methods.

Findings

Computed geometries show significant non-planarity in sheets.

Binding energies agree with experimental defoliation energies.

DFT with empirical dispersion underestimates binding strength.

Abstract

High level ab initio calculations ranging from coupled cluster methods including explicitly correlated approaches to standard second order M{\o}ller-Plesset theory using spin scaling (SOS-MP2) have been performed on sandwich and slipped parallel dimer structures of a series of quasi one-dimensional acenes and on two-dimensional sheets containing the series pyrene to coronene encircled with two layers of benzene rings. Sandwich (graphitic AA type) and slipped parallel (AB type) structures were considered and, within given symmetry restrictions, full geometry optimizations were performed. Basis set superposition effects have been considered. The computed geometries show a significant biconcave deviation of the twodimensional sheets from planarity with the central intersheet C...C distances considerably smaller that van der Waals distances. The computed intersheet binding energy per carbon atom extrapolated for N ->$\infty$ of -74.3 meV (1.713 kcal/mol)/atom agrees quite well with the experimental defoliation energy of -52 meV (1.199 kcal/mol)/atom (-67 meV (1.545 kcal/mol)/carbon atom without corrections for H binding contributions) for polyaromatic hydrocarbons (PAHs) from graphite. A limited investigation of density functional theory (DFT) calculations using empirical dispersion contributions has been performed also showing a significant underbinding character of the D3 method. For most of the DFT variants investigated the graphene sheet models retain a quasi-planar structure in strong contrast to the aforementioned SOS-MP2 results.