Vibrational spectroscopy and molecular dynamics of water monomers and dimers adsorbed on polycyclic aromatic hydrocarbons

Abstract

This paper reports structures, energetics, dynamics and spectroscopy of H₂O and (H₂O)₂ systems adsorbed on coronene (C₂₄H₁₂), a compact polycyclic aromatic hydrocarbon (PAH). On-the-fly Born–Oppenheimer molecular dynamics simulations are performed for temperatures T varying from 10 to 300 K, on a potential energy surface obtained within the self-consistent-charge density-functional based tight-binding (SCC-DFTB) approach. Anharmonic infrared (IR) spectra are extracted from these simulations. We first benchmark the SCC-DFTB semi-empirical hamiltonian vs. DFT (Density Functional Theory) calculations that include dispersion, on (C₆H₆)(H₂O)_1,2 small complexes. We find that charge corrections and inclusion of dispersion contributions in DFTB are necessary to obtain consistent structures, energetics and IR spectra. Using this Hamiltonian, the structures, energetics and IR features of the low-energy isomers of (C₂₄H₁₂)(H₂O)_1,2 are found to be similar to the DFT ones, with evidence for a stabilizing edge-coordination. The temperature dependence of the motions of H₂O and (H₂O)₂ on the surface of C₂₄H₁₂ is analysed, revealing ultra-fast periodic motion. The water dimer starts diffusing at a higher temperature than the water monomer (150 K vs. 10 K respectively), which appears to be consistent with the binding energies. Qualitative and quantitative analyses of the effects of T on the IR spectra are performed. Anharmonic factors in particular are derived and it is shown that they can be used as signatures for the presence of PAH–water complexes. Finally, this paper lays the foundations for the studies of larger (PAH)_m(H₂O)_n clusters, that can be treated with the efficient computational approach benchmarked in this paper.