Adsorption of Silver Clusters on Naphthalene: Theoretical Insights into Structural, Energetic, Electronic, and Infrared Properties

In the present study, we carry out a comprehensive investigation of the structural, bonding, and infrared spectral properties of the low-energy complexes of silver clusters Ag_<i>n</i> (<i>n</i> = 1-8) adsorbed on naphthalene C₁₀H₈ (Nap). The structural properties are obtained through a systematic multimethod global optimization scheme. The potential energy surfaces of the complexes are first explored at the density functional-based tight binding level of theory via extensive Monte Carlo parallel tempering simulations complemented by gradient-driven quenching, thus providing prescreened samples of low-energy structural conformations. The most stable isomers are then reoptimized at the density functional theory level using a functional that includes dispersion, namely B3LYP-D3BJ. The properties of the global minima are analyzed at the latter level. Unsurprisingly, the structures of Ag_<i>n</i> adsorbates within the most stable complexes are close to those of the bare clusters. Regarding 2D Ag_<i>n</i> structures (<i>n</i> = 2-6), those with Ag_<i>n</i> perpendicular to the Nap plane are found to be the most stable for <i>n</i> = 2-4, while parallel conformations are preferred for <i>n</i> = 5 and 6. The significant role of dispersion interactions in the stability of these complexes is shown. The study of the influence of Ag_<i>n</i> adsorption on the evolution of the HOMO-LUMO band gaps and vertical ionization potentials (VIPs) shows that the odd-even alternation of the adsorbate cluster electronic properties is maintained and that the VIP decreases with size. Finally, the influence of the adsorption of Ag_<i>n</i> on the infrared properties of Nap is analyzed: small shifts are observed as well as the appearance of new bands due to symmetry reduction. The essential novelty consists of relative intensity changes reflecting charge transfer (though not dominant) from Nap to Ag_<i>n</i>. Our study globally shows that Nap-Ag_<i>n</i> interactions lead to a modulation of the electronic and vibrational properties of Ag_<i>n</i> and Nap. This could be of interest for structural diagnosis and reactivity. Perspectives concerning the use of the present structural search methodology for more complex hybrid metal-organic systems in various application fields are suggested.