DFT study of interaction of Palladium Pdn (n = 1–6) nanoparticles with deep eutectic solvents

In this study, we use density functional theory (DFT) calculations to investigate the stability, reactivity and interactions of Palladium Pdn (n = 1–6) nanoparticles with ChCl:U and ChCl:EG based deep eutectic solvents (DESs). We find that the DES … Pdn complexes are stabilized by two types of binding; Pdn-X anchoring bonds (X = N atom of –NH2 group in urea and [Cl]- anion) and Pdn…H-X (X = C, N and O) unconventional H-bonds. Analyses based on AIM, NBO, NCI, and EDA suggest that the anchoring bonds, which are electrostatic in nature are stronger than the unconventional H-bonds, which are van der Waals in nature. The Energy Decomposition Analysis reveals that the charge transfer plays an important role in the stability of DES…Pdn complexes. Thermochemical calculations, including enthalpy (ΔH) and free energy (ΔG), indicate that the formation of the DES…Pdn complexes is exothermic and occurs spontaneously. The binding energy (ΔEb) calculations show that the ChCl:U DES has a stronger interaction with the Pdn nanoparticles than their ChCl:EG DES counterparts. On the other hand, a similar trend for the ΔEb, ΔH and ΔG values of the complexes is observed with increasing nanoparticle size of Pdn (DES…Pd5> DES…Pd6> DES…Pd4> DES…Pd3> DES…Pd2> DES…Pd1). Our results show that the magnitude of charge transfer (ΔQ) value in the complexes follow the order observed for the ΔEb values. It is also observed that increasing the energy gap Eg values of the complexes decreases the ΔEb and ΔQ values of the complexes. The reactivity parameter calculations of the complexes show that the Eg and chemical hardness (η) values of ChCl:U…Pdn and ChCl:EG…Pdn complexes decrease with an increase in the nanoparticle size. Additionally, the global electrophilicity index (ω) values of the DES…Pdn complexes increase with an increase in the Pdn nanoparticle size, while no clear trend is seen for the chemical potential (μ) values of the complexes. The urea-based DES shows better suitability towards Pdn nanoparticles than the ethylene glycol-based DES. Overall, such DESs are potentially promising green solvents for nanoparticle synthesis and activity.