Effect of Chemical Nature of the Surface on the Mechanism and Selection Rules of Charge-Transfer Surface-Enhanced Raman Scattering

The intrinsic challenge in the elucidation of charge-transfer surface-enhanced Raman scattering (SERS) has inspired the present study. It is believed that changing the surface may serve as illustrative evidence for studying the influence of the chemical nature and electronic structure of the substrate on the resonance and nonresonance chemical mechanism of SERS. With the aim of investigating the important parameters which are effective on the ground- and excited-state properties, in this work we have focused on changing the composition of the substrate. Therefore, 6- and 20-atom pure as well as bimetallic silver and gold clusters have been used as models, and the adsorption of pyridine on these clusters has been studied. It has been found that through the nonresonant chemical mechanism, the Au adsorption site becomes favorable. Therefore, the static chemical enhancement of the Au binding site is more than that of Ag, and it is related to the greater binding energy of gold. To determine the effect of the surface on the resonance chemical mechanism, we have calculated the relative intensity of the pyridine–metal cluster on the charge-transfer (CT) resonance condition. The relative intensities of simulated spectra match well with the available experimental results and suggest that changing the surface, which reveals the trend by applying negative potential on a given surface, could be explained by variation of the effective charge of the cluster. These calculations also show the importance of variation of the excited-state vector gradient and dimensionless displacement for different surfaces and their effects on the selection rules. An illuminating insight about the absolute SERS-CT intensity has been provided that shows the higher average scattering cross sections in the order of 10⁵ for binding through Ag in comparison to 10³ for binding through Au. In addition, the enhancement factor for the silver binding site has been obtained as 10⁴ in comparison to that of gold, which is 10². This factor is the intermediate value of 10³ for alloys, which confirms the idea that alloying improves the enhancement factor of gold

Investigation of the Electronic Excited States of Small Gold Clusters in Rare Gas Matrices: Spin–Orbit Time-Dependent Density Functional Theory Calculation

The effects of the weak interactions of rare gas atoms on the UV–visible absorption spectra of gold dimer and tetramer clusters are investigated. The time-dependent density functional theory based on the two-component relativistic zeroth-order regular approximation that considered spin–orbit coupling is performed to estimate the absorption spectra of Au_2,4–Rg_<i>n</i> (Rg = Ne–Xe, and <i>n</i> = 1–6) complexes. Using spin–orbit, including the appropriate functional, shows a close correlation between experiment and our calculations. It is also demonstrated that the weak interactions between rare gas atoms and gold clusters affect the UV–vis spectra of Au_2,4 clusters by shifting the electronic transition toward the blue. Moreover, we find that the order of change in peak position, Δν̃, is proportional to the strength of interactions: Δν̃_{Au_2,4–Xe} > Δν̃_{Au_2,4–Kr} > Δν̃_{Au_2,4–Ar} > Δν̃_{Au_2,4–Ne}. In addition, comparing the UV–visible spectra of Au_2,4–Rg_<i>n</i> complexes with those of isolated Au₂ and Au₄ clusters shows that for Au_2,4–Rg_2,4,6 complexes in which Rg atoms interacted symmetrically with gold clusters no additional peaks are observed compared to isolated clusters; however, for Au_2,4–Rg_1,3,5 complexes, extra peaks appear because of the decrease in symmetry

Binding of Noble Metal Clusters with Rare Gas Atoms: Theoretical Investigation

Binding of noble metal clusters (M_<i>n</i>, M = Cu, Ag, and Au; <i>n</i> = 2–4) with rare gas atoms (Rg = Kr, Xe, and Rn) has been investigated at the density functional (CAM-B3LYP) and ab initio (MP2) levels of theory. The calculation shows significant affinity of neutral metal clusters for interaction with rare gas atoms. The binding energies indicate that gold clusters have the highest and silver clusters have the lowest affinity for interaction with rare gas atoms, and for the same metal clusters, there is a continuous increase in <i>E</i>_<i>b</i> from Kr to Rn. The M–Rg bonding mechanism have been interpreted by means of the quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO), and energy decomposition analysis (EDA). According to these theories, the M–Rg bonds are found to be partially electrostatic and partially covalent. EDA results identify that these bonds have less than 40% covalent character and more than 60% electrostatic, and also NBO calculations predict the amount of charge transfer from the lone pair of rare gas to σ* and n*orbitals of metal clusters

Interactions of Glutathione Tripeptide with Gold Cluster: Influence of Intramolecular Hydrogen Bond on Complexation Behavior

Understanding the nature of the interaction between metal nanoparticles and biomolecules has been important in the development and design of sensors. In this paper, structural, electronic, and bonding properties of the neutral and anionic forms of glutathione tripeptide (GSH) complexes with a Au₃ cluster were studied using the DFT-B3LYP with 6-31+G**-LANL2DZ mixed basis set. Binding of glutathione with the gold cluster is governed by two different kinds of interactions: Au–X (X = N, O, and S) anchoring bond and Au···H–X nonconventional hydrogen bonding. The influence of the intramolecular hydrogen bonding of glutathione on the interaction of this peptide with the gold cluster has been investigated. To gain insight on the role of intramolecular hydrogen bonding on Au–GSH interaction, we compared interaction energies of Au–GSH complexes with those of cystein and glycine components. Our results demonstrated that, in spite of the ability of cystein to form highly stable metal–sulfide interaction, complexation behavior of glutathione is governed by its intramolecular backbone hydrogen bonding. The quantum theory of atom in molecule (QTAIM) and natural bond orbital analysis (NBO) have also been applied to interpret the nature of interactions in Au–GSH complexes. Finally, conformational flexibility of glutathione during complexation with the Au₃ cluster was investigated by means of monitoring Ramachandran angles

Meta-Hybrid Density Functional Theory Study of Adsorption of Imidazolium- and Ammonium-Based Ionic Liquids on Graphene Sheet

In this study, two types of ionic liquids (ILs) based on 1-butyl-3-methylimidazolium [Bmim]⁺ and butyltrimethylammonium [Btma]⁺ cations, paired to tetrafluoroborate [BF₄]⁻, hexafluorophosphate [PF₆]⁻, dicyanamide [DCA]⁻, and bis­(trifluoromethylsilfonyl)­imide [Tf₂N]⁻ anions, were chosen as adsorbates to investigate the influence of cation and anion type on the adsorption of ILs on the graphene surface. The adsorption process on the graphene surface (circumcoronene) was studied using M06-2X/cc-pVDZ level of theory. Empirical dispersion correction (D3) was also added to the M06-2X functional to investigate the effects of dispersion on the binding energy values. The graphene···IL configurations, binding energies, and thermochemistry of IL adsorption on the graphene surface were investigated. Orbital energies, charge transfer behavior, the influence of adsorption on the hydrogen bond strength between cation and anion of ILs, and the significance of noncovalent interactions on the adsorption of ILs on the graphene surface were also considered. ChelpG analysis indicated that upon adsorption of ILs on the graphene surface the overall charge on the cation, anion, and graphene surface changes, enabled by the charge transfer that occurs between ILs and graphene surface. Orbital energy and density of states calculations also show that the HOMO–LUMO energy gap of ILs decreases upon adsorption on the graphene surface. Quantum theory of atoms in molecules analysis indicates that the hydrogen-bond strength between cation and anion in ILs decreases upon adsorption on the graphene surface. Plotting the noncovalent interactions between ILs and graphene surface shows the role and significance of cooperative π···π, C–H···π, and X···π (X = N, O, F atoms from anions) interactions in the adsorption of ILs on the graphene surface. The thermochemical analysis also indicates that the free energy of adsorption (Δ<i>G</i>_ads) of ILs on the graphene surface is negative, and thus the adsorption occurs spontaneously

How is the Observation of High-Order Overtones and Combinations Elucidated by the Charge-Transfer Mechanism in SERS?

The charge-transfer chemical mechanism is responsible for altering the molecular spectral pattern and providing valuable insights into the properties of adsorbates. The impact of charge transfer becomes more pronounced in SERS spectra when CT states can gain intensity through vibronic coupling with high-intensity excitations. Experimental SERS spectra of diamino molecules, such as 4,4′-diaminostilbene (DAS) and 4,4′-diaminotolane (DAT), featuring bright CT transitions, have been compared to dipyridyl compounds, such as 1,2-bis(4-pyridyl) ethylene (BPE) and 1,2-di(4-pyridyl) acetylene (DPA), characterized by nearly dark CT excitations. This comparison aims to elucidate the effect of CT transitions on the presence of overtones and combination bands. We explain this distinction using Albrecht’s formalism for resonance Raman spectroscopy within the framework of path integral time-dependent density functional theory considering the Herzberg–Teller corrections. It is worth noting that the energy gap between the highest occupied metallic orbital and the lowest unoccupied molecular orbital in diamino derivatives is noticeably smaller than in compounds featuring two pyridyl rings. The high-intensity SERS-CT spectra for diamino derivatives, primarily driven by the Albrecht A term, were acquired and used to elucidate the experimental observation of high-order modes with a significant Huang–Rhys factor. Conversely, the absolute intensity of SERS-CT for dipyridyl compounds is at least 106 times smaller than that for diamines, and the C term makes a significant contribution, explaining the silent overtones

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<p>General characteristics, mean daily intake and percentile distribution of energy, some food groups and food items</p

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<p>Characteristics, dietary and cardiometabolic variables in tertiles of energy-adjusted total bread intake</p

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<p>Characteristics, dietary and cardiometabolic variables in tertiles of energy-adjusted total bread-rice intake</p