Quantifying the Impact of Halogenation on Intermolecular Interactions and Binding Modes of Aromatic Molecules

Halogenation
of aromatic molecules is frequently used to modulate
intermolecular interactions with ramifications for optoelectronic
and mechanical properties. In this work, we accurately quantify and
understand the nature of intermolecular interactions in perhalogenated
benzene (PHB) clusters. Using benchmark binding energies from the
fixed-node diffusion Monte Carlo (FN-DMC) method, we show that generalized
Kohn–Sham semicanonical projected random phase approximation
(GKS-spRPA) plus approximate exchange kernel (AKX) provides reliable
interaction energies with mean absolute error (MAE) of 0.23 kcal/mol.
Using the GKS-spRPA+AXK method, we quantify the interaction energies
of several binding modes of PHB clusters ((C6X6)n; X = F, Cl, Br, I; n = 2, 3). For a given binding mode, the interaction energies increase
3–4 times from X = F to X = I; the X–X binding modes
have energies in the range of 2–4 kcal/mol, while the π–π
binding mode has interaction energies in the range of 4–12
kcal/mol. SAPT-DFT-based energy decomposition analysis is then used
to show that the equilibrium geometries are dictated primarily by
the dispersion and exchange interactions. Finally, we test the accuracy
of several dispersion-corrected density functional approximations
and show that only the r2SCAN-D4 method has a low MAE and correct
long-range behavior, which makes it suitable for large-scale simulations
and for developing structure–function relationships of halogenated
aromatic systems.