Computational Approaches to the Electronic Properties of Noble Metal Nanoclusters Protected by Organic Ligands

Organometallic nanoparticles composed by metal cores with sizes under two nanometers covered with organic capping ligands exhibit intermediate properties between those of atoms and molecules on one side, and those of larger metal nanoparticles on the other. In fact, these particles do not show a peculiar metallic behavior, characterized by plasmon resonances, but instead they have nonvanishing band-gaps, more along molecular optical properties. As a consequence, they are suitable to be described and investigated by computational approaches such as those used in quantum chemistry, for instance those based on the time-dependent density functional theory (TD-DFT). Here, I present a short review of the research performed from 2014 onward at the University of Modena and Reggio Emilia (Italy) on the TD-DFT interpretation of the electronic spectra of different organic-protected gold and/or silver nanoclusters

Modelling of spectroscopic and structural properties using molecular dynamics

The work described here was carried out at the European Lab. for Non-Linear Spectroscopy (LENS) to achieve a better understanding of molecular vibrations employing computer simulations. H-bonds are the main intermolecular interactions affecting vibrational spectra and here it’s shown how they usually induce a (red or blue) shift on the vibrational frequencies of the groups engaged in them, and how this shift nicely correlates with structural properties. H-bonds can be present also in a bifurcated arrangement. In systems such as confined water, this bifurcated configuration has long lifetimes, allowing it to be studied by both spectroscopic and computational means. The computational protocols implemented and adopted here allow for a direct comparison between structural features and vibrational spectra

Raman and DFT study of methimazole chemisorbed on gold colloidal nanoparticles

The adsorption of methimazole on gold colloidal nanoparticles was investigated using a combination of surface-enhanced Raman scattering and density functional theory calculations, which allowed identifying the thiolate anion as the molecular species chemically interacting with the active sites of the gold surface, modeled as zero-charge metal adatoms, only through the sulfur atom. This result can be important for the use of these ligand/metal nanohybrids in the process of drug delivery. Moreover, functionalized gold nanoparticles are able to promote the Raman enhancement in the red-light region as well as in the near-infrared, where generally no fluorescence emission occurs. This paves the way for the use of these nanosystems in a biological environment, even in vivo experiments

Can DFT Calculations Provide Useful Information for SERS Applications?

: Density functional theory (DFT) calculations allow us to reproduce the SERS (surface-enhanced Raman scattering) spectra of molecules adsorbed on nanostructured metal surfaces and extract the most information this spectroscopy is potentially able to provide. The latter point mainly concerns the anchoring mechanism and the bond strength between molecule and metal as well as the structural and electronic modifications of the adsorbed molecule. These findings are of fundamental importance for the application of this spectroscopic technique. This review presents and discusses some SERS-DFT studies carried out in Italy as a collaboration between the universities of Modena and Reggio-Emilia and of Florence, giving an overview of the information that we can extract with a combination of experimental SERS spectra and DFT modeling. In addition, a selection of the most recent studies and advancements on the DFT approach to SERS spectroscopy is reported with commentary

DFT and TD-DFT Study of the Chemical Effect in the SERS Spectra of Piperidine Adsorbed on Silver Colloidal Nanoparticles

The surface-enhanced Raman scattering (SERS) spectra of piperidine adsorbed on silver/chloride colloids were studied by a combined density functional theory (DFT)/time dependent DFT (TD-DFT) approach. The mechanism of chemical enhancement on the Raman signals is due to at least two contributions: the first comes from the changes in the molecular force constants and the dynamic polarizabilities of the normal modes, when the molecule is chemisorbed. DFT calculations satisfactorily reproduce the SERS spectra of piperidine adsorbed on silver, showing that the species formed on the silver particle is a complex formed by a deprotonated piperidine linked to a silver cation. A second contribution to the SERS chemical enhancement is due to a resonance Raman effect occurring when the wavelength of the Raman excitation falls within the electronic excitation band of the molecule/metal complex. Actually, the SERS spectra of piperidine show a significant dependence on the wavelength of the laser excitation, with a marked enhancement in the green-light region. TD-DFT calculations on the most-probable complex explain this behavior, because a strong excitation band of the complex is calculated in the green spectral region. This pinpoints that a resonance between the exciting radiation and the absorption band of this complex is responsible for this enhancement effect

Assessment of the basis set effect on the structural and electronic properties of organic-protected gold nanoclusters

We have investigated the structural and optical properties of five monolayer-protected gold nanoclusters with a combination of exchange–correlation functionals, namely B-PBE for the geometry relaxation and CAM-B3LYP for the time-dependent calculations. We have tested the accuracy of five different basis sets in reproducing the experimental structures of these nanoclusters, and we have found that even a rather small basis set (single zeta) can outperform a significantly larger one (double zeta) if some selected atoms are treated with polarization functions. Namely, the sulfur and phosphorous atoms of the capping thiols and phosphines usually are hypervalent when bonded to the gold inner core; therefore, polarization functions allow them significantly more structural flexibility. With the two best performing basis sets, we carried out optical calculations and found that the resulting UV–Vis profiles are largely similar, in particular the energy and orbital contributions of the optical gaps are very close. The results support the use of the small basis set proposed here to investigate larger nanoclusters with general hybrid and range-corrected hybrid functionals

Quantum-Chemistry Study of the Hydrolysis Reaction Profile in Borate Networks: A Benchmark

This investigation involved an ab initio and Density Functional Theory (DFT) analysis of the hydrolysis mechanism and energetics in a borate network. The focus was on understanding how water molecules interact with and disrupt the borate network, an area where the experimental data are scarce and unreliable. The modeled system consisted of two boron atoms, bridging oxygen atoms, and varying numbers of water molecules. This setup allows for an exploration of hydrolysis under different environmental conditions, including the presence of OH− or H+ ions to simulate basic or acidic environments, respectively. Our investigation utilized both ab initio calculations at the MP2 and CCSD(T) levels and DFT with a range of exchange–correlation functionals. The findings indicate that the borate network is significantly more susceptible to hydrolysis in a basic environment, with respect to an acidic or to a neutral pH setting. The inclusion of explicit water molecules in the calculations can significantly affect the results, depending on the nature of the transition state. In fact, some transition states exhibited closed-ring configurations involving water and the boron–oxygen–boron network; in these cases, there were indeed more water molecules corresponding to lower energy barriers for the reaction, suggesting a crucial role of water in stabilizing the transition states. This study provides valuable insights into the hydrolysis process of borate networks, offering a detailed comparison between different computational approaches. The results demonstrate that the functionals B3LYP, PBE0, and wB97Xd closely approximated the reference MP2 and CCSD(T) calculated reaction pathways, both qualitatively in terms of the mechanism, and quantitatively in terms of the differences in the reaction barriers within the 0.1–0.2 eV interval for the most plausible reaction pathways. In addition, CAM-B3LYP also yielded acceptable results in all cases except for the most complicated pathway. These findings are useful for guiding further computational studies, including those employing machine learning approaches, and experimental investigations requiring accurate reference data for hydrolysis reactions in borate networks

The antioxidant properties of Ce-containing bioactive glass nanoparticles explained by Molecular Dynamics simulations

Molecular dynamics simulations of two glass nanoparticles with composition 25Na2O\ub725CaO 50SiO2 mol% (Ce-K NP) and 46.1SiO2\ub724.4Na2O\ub726.9CaO\ub7 2.6P2O5 mol.% (Ce-BG NP) doped with 3.6 mol% of CeO2 have been carried out in order to explain the enhanced antioxidant properties of the former glass with respect to the latter. The present models show that the different catalase mimetic activity of the two NPs is related to the Ce3+/Ce4+ ratio exposed at their surface. In fact, this ratio is about 3.5 and 13 in the bulk and at the surface of the Ce-BG NP, and 1.0 and 2.1 in the bulk and at the surface of the Ce-K NPs, respectively. Since both oxidation states are necessary for the catalysis of the dismutation reaction of hydrogen peroxides, NPs with a very high Ce3+/Ce4+ ratio possess poorer antioxidant properties. Moreover, our simulations reveal that the already low silicate connectivity found in the bulk glasses examined here is further reduced on the nanoparticle surface, whereas the Na+/Ca2+ ratio rapidly increases. Sodium, calcium and cerium sites in proximity of the surface are found to be under-coordinated, prone to quickly react with water present in physiological environments, thus accelerating the glass biodegradatio