Surface Hopping Molecular Dynamics Simulations for Photochemistry Involving Pyrene and CH ₃ Cl

Pure or halogenated polycyclic aromatic hydrocarbons (PAHs) and saturated halogenated hydrocarbons are both classes of harmful chemicals found in Earth's atmosphere, often involved in photochemical reactions. On a positive side, the photoreaction of PAHs with halogenated hydrocarbons serves as a model and offers routes to functionalized nanostructured carbon-based materials with tailored optoelectronic properties. Mechanistic studies on the photoreactions of these chemicals, possibly with each other, are therefore clearly of interest but still comparatively rare. In the present work, as a representative case study, the photophysics (spectra, excited-state lifetimes) and photoreaction dynamics of van der Waals or chemically bound complexes of pyrene (C₁₆H₁₀) and methyl chloride (CH₃Cl) were investigated using a combination of computational techniques, thereby delivering time- and atom-resolved information on postexcitation processes. Structural optimizations of possible reactants and products, as well as excited states and absorption spectra, were obtained by semiempirical (AM1) configuration interaction singles (CIS) and (time-dependent) density functional theory (DFT), respectively. Nonadiabatic surface hopping dynamics (NASH) based on AM1-CIS provided excited-state lifetimes and were used to explore various photochemical channels of CH₃Cl physisorbed or covalently bound to pyrene.