A Quantum Chemical Study of Bonding Interaction, Vibrational Frequencies, Force Constants, and Vibrational Coupling of Pyridine−Mn (M = Cu, Ag, Au; n = 2−4)

To dimerize or not: para-aminothiophenol on a bismuth heterostructure

Surface enhanced Raman spectroscopy (SERS) of p-aminothiophenol (PATP) was investigated on β-Bi₂O₃/Bi₂O₂CO₃ nanoparticles, a novel bismuth based metal substrate with the lowest limit of detection of 1 mM. Unlike on noble metal surfaces where PATP gets converted to p,p'-dimercaptoazobenzene (DMAB) due to photocatalytic coupling, no such transformation of PATP was observed on β-Bi₂O₃/Bi₂O₂CO₃ nanoparticles. Density functional theory (DFT) calculations at the PW91PW91/LANL2DZ/6-311+G(d,p) level of theory supported the experimental results exceedingly well. Also, the charge transfer direction from PATP to β-Bi₂O₃/Bi₂O₂CO₃ nanoparticles was revealed by the projected density of states calculation.

A Density Functional Theoretical Study on the Charge-Transfer Enhancement in Surface-Enhanced Raman Scattering

The chemical enhancement due to ground-state charge transfer (GSCT) and photon-driven charge transfer (PDCT) in surface-enhanced Raman scattering (SERS) has been investigated by density functional theory. Para-substituted thiophenol derivatives adsorbed on silver and gold surfaces are selected as model systems to evaluate the chemical enhancement factor. By changing the functional groups on thiophenol, we are allowed to modulate the chemical interactions between the thiophenol and the metal cluster in both ground state and charge transfer excited state. Both off-resonance and pre-resonance SERS spectra are simulated to calculate the chemical enhancement factors. The GSCT enhancement factor, EF_GSCT , shows a roughly linear relationship to (ω_TP /ω_M-TP )⁴ , where ω_TP denotes the HOMO-LUMO gap of free molecule, and ω_M-TP denotes the energy difference between the HOMO of the molecule and the LUMO of the metal. The PDCT enhancement factor, EF_PDCT , is governed by the energy difference between the incident light energy and the excitation energy to the CT excited state. EF_PDCT first increases and then decreases with the increase of incident light energy.

Conformations of 2-thiouracil in the aqueous solution and its adsorption behavior on the gold substrates explored by DFT calculations and experimental methods

2-Thiouracil, a thio-derivative of uracil, may appear in various tautomeric forms due to the different positions of protons. In this paper, the adsorption behavior and conformations of 2-thiouracil on the gold substrates are inspected by means of surface-enhanced Raman scattering (SERS) and density functional theory (DFT) calculations. The results indicate that all the enhanced bands are assigned to in-plane vibration modes. Besides, most of the bands related to N and S atoms are significantly enhanced and have obvious shifts in the SERS spectra. Furthermore, the CO stretching band at 1695 cm(-1) also appears in the SERS spectra. The theoretical SERS spectra of 2TU-Au₄ and 2TU2-Au₄ complexes agree well with the experimental SERS spectra of 2-thiouracil at 0.04 mM. Meanwhile, we calculate that the binding energies for 2TU2-Au₄ and 2TU-Au₄ are ca. 70 and 50 kcal/mol, respectively. Those results above imply that the 2TU and 2TU2 conformers can exist stably in the aqueous solution and both of them are vertically chemisorbed on the gold surfaces. For the 2TU, it is adsorbed on the gold surfaces through N1H position and the sulfur atom. While the 2TU2 adsorbed on the gold substrates through the N1 site and its deprotonated sulfur atom.

The nature of interfacial binding of imidazole and carbene ligands with M20 nanoclusters (M = Au, Ag and Cu) – a theoretical study

To inspire more exciting developments in the design and advances of self-assembled monolayers (SAMs), the fundamental understanding of the nature of interaction between metal nanoparticles and certain functional groups is very crucial.

Magic Numbers in DNA-Stabilized Fluorescent Silver Clusters Lead to Magic Colors

DNA-stabilized silver clusters are remarkable for the selection of fluorescence color by the sequence of the stabilizing DNA oligomer. Yet despite a growing number of applications that exploit this property, no large-scale studies have probed origins of cluster color or whether certain colors occur more frequently than others. Here we employ a set of 684 randomly chosen 10-base oligomers to address these questions. Rather than a flat distribution, we find that specific color bands dominate. Cluster size data indicate that these "magic colors" originate from the existence of magic numbers for DNA-stabilized silver clusters, which differ from those of spheroidal gold clusters stabilized by small-molecule ligands. Elongated cluster structures, enforced by multiple base ligands along the DNA, can account for both magic number sizes and color variation around peak wavelength populations.

Raman spectral study of metal-cytosine complexes: a density functional theoretical (DFT) approach

The fluctuation of surface-enhanced Raman scattering (SERS) spectra has been an obstacle to the analysis of the adsorbate on the metal surface. In this paper, we aim at using the density functional theory (DFT) to study the fluctuant Raman spectra of the cytosine molecule which interacts with a coinage metal atom or cation via N1 and N3 sites. The results show that the adsorption site strongly influences the Raman spectral property of cytosine molecule, especially the relative intensity of some bands. In addition, the SERS spectra of cytosine which is adsorbed on the gold, silver, and copper electrodes are measured, and the possible orientation and adsorption site of the cytosine molecule adsorbed on metal electrodes surface are proposed with the help of DFT simulations.

Predicting the binding capability of benzothiazoline-2-thione and its derivatives with gold: a DFT and FT-Raman combined studies

The activities of chemical systems can be evaluated successfully by combining vibrational spectroscopic analysis and quantum chemical calculation based on density functional theory (DFT). Two tautomers of 5-fluorobenzo[d]thiazole-2(3H)-thione (FBTT), 7H-[1,3]dioxolo[4',5',4,5]benzo[1,2-d]thiazole-6-thione (DBTT) and 5-chloro-6-methylbenzo[d]thiazole-2(3H)-thione (CMBTT) were investigated by FT-Raman spectroscopy and DFT calculations at B3LYP/6-311+G** level. The molecular properties and activity relationships were determined by the HOMO energies, Mulliken charges and the binding energies with metal. It is concluded that three derivatives exhibited stable conformation of the thione form both in the isolated powder monomers and in their complexes with gold. The binding capability with gold was in the order of DBTT>BTT approximately CMBTT>FBTT. The derivatives with the electron-donor substitutes in benzene ring were favorable to metal for the p-pi conjugate effect.

Pulsed-field ionization electron spectroscopy and molecular structures of copper-(pyridine)n (n = 1, 2) complexes

Cu-(pyridine)n (n = 1, 2) complexes are prepared in a pulsed laser ablation cluster source and identified using laser photoionization time-of-flight mass spectrometry. High-resolution electron spectra of these complexes are obtained using pulsed-field ionization zero electron kinetic energy (ZEKE) photoelectron spectroscopy. Metal-pyridine and pyridine-based vibrational modes are identified by comparing the ZEKE spectra with previous spectroscopic studies of isolated pyridine, pyridine adsorbed on metal surfaces, and other Cu complexes. Ground electronic states and molecular structures are determined by comparing the ZEKE spectra with ab initio and multidimensional Franck-Condon factor calculations. Metal-pyridine bond energies of the neutral complexes are derived from the measured ionization energies and thermochemical relations. The mono-ligand complex has C2v symmetry in both the neutral and ionized forms, whereas the di-ligand complex has an eclipsed pyridine configuration with D2h and C2 symmetries for the ion and neutral species, respectively. Although both the mono- and di-pyridine Cu complexes are formed by Cu binding to nitrogen atoms, important binding differences are found between these two complexes.Key words: pulsed-field ionization, ZEKE, photoelectron, ab initio, copper-pyridine complexes.[Traduit par la Rédaction]

Solvation Effects on the Stability of Silver(I) Complexes with Pyridine-Containing Ligands Studied by Thermodynamic and DFT Methods

The formation of Ag(I) complexes with 2,2'-bipyridine (bipy), 2,2'6',2' '-terpyridine (terpy), 2-(aminomethyl)pyridine (amp), and bis((2-pyridyl)methyl)amine (dpa) is studied in dimethyl sulfoxide (dmso) by means of potentiometric and calorimetric measurements. Enthalpy-stabilized mononuclear MLj complexes are formed, whereas entropy changes counteract complex formation. Additionally, a comparison with analog Ag-polyamine species is made to evidence the significant different coordination behavior of these classes of ligands. The results are discussed in terms of different basicity and steric requirements of the ligands and solvation effects. The dpa ligand, with an unprecedented coordination pattern, forms also a bimetallic complex [Ag2(dpa)2]2+ that has been structurally characterized in the solid state by X-ray diffraction. The influence of solvent, water and dmso, on the binding energy of the monodentate pyridine to Ag(I) has also been assessed by means of density functional theory (DFT) calculations. This study has been extended also in vacuum to the reaction of Ag(I) with the simple monoamine methylamine (mea). These results are correlated with the experimental evidence and used to interpret the different affinities of pyridine for the Ag(I) ion in the two media.

FT-IR, FT-Raman and SERS spectra of pyridine-3-sulfonic acid

FT-IR and FT-Raman spectra of pyridine-3-sulfonic acid are recorded and analysed. Surface enhanced Raman scattering (SERS) spectrum is recorded in a silver colloid. The bands due to upsilonCH, upsilonSO are enhanced in the SERS spectrum. A likely 'perpendicular orientation' of the molecule on the silver surface is suggested.

Quantification of Intramolecular Nonbonding Interactions in Organochalcogens

Intramolecular nonbonding interactions between chalcogen atoms in a series of ortho substituted arylselenides (S/O...Se-Y, with Y = -Me, -CN, -Cl, and -F) are quantified using the coupled cluster CCSD(T)/cc-pVDZ level of theory. A homodesmic reaction method as well as an ortho-para approach are employed in evaluating the strength of intramolecular interactions. Comparison of the results obtained using the ab initio MP2 method and pure and hybrid density functional theories are performed with that of the coupled cluster values to assess the quality of different density functionals in evaluating the strength of nonbonding interactions. The interaction energies are found to be higher when the thioformyl group acts as the donor and the Se-F bond acts as the acceptor. In a given series with the same donor atom, the strength of the interaction follows the order Me < CN < Cl < F, exhibiting fairly high sensitivity to the group attached to selenium (Se-Y). Analysis of electron density at the S/O...Se bond critical point within the Atoms in Molecule framework shows a very good correlation with the computed intramolecular interaction energies.

Binding Interactions and Raman Spectral Properties of Pyridine Interacting with Bimetallic Silver-Gold Clusters

The binding interactions between pyridine and bimetallic silver-gold clusters are investigated using density functional theory (DFT). The binding energies of pyridine-bimetallic cluster complexes indicate that the bonding depends strongly on the binding site (Au or Ag atom) and bonding molecular orbitals in a given configuration. The donation of the lone-pair electrons of the nitrogen of pyridine to an appropriate unoccupied orbital of each metal cluster plays an important role. The low-lying excited states and charge-transfer states of four stable complexes of interest are calculated on the basis of a time-dependent DFT method. In nonresonance Raman scattering processes, the influence of binding interactions on the relative Raman intensity of totally symmetric pyridine vibrational modes is discussed. These calculated relative Raman intensities are compared with observed surface-enhanced Raman spectra of pyridine adsorbed on silver-gold alloy surfaces.

A combined Raman, DFT and MD study of the solvation dynamics and the adsorption process of pyridine in silver hydrosols

The adsorption of pyridine onto silver colloids has been investigated by Raman spectroscopy experiments and by ab initio DFT and MP2 calculations. The solvation dynamics of the pyridine in water has been studied by a molecular dynamics simulation. The results are compared with the latest available experimental and theoretical data. It is found that the pyridine is essentially hydrogen bonded to one solvent molecule. Calculations based on pyridine-water and pyridine-Ag(+) complexes allow the reproduction of the experimentally observed Raman features and explain the adsorption process of the ligand in silver hydrosols.

Effect of displacement and distortion of potential energy surfaces and overlapping resonances of electronic transitions on surface-enhanced Raman scattering: Models and ab initio theoretical calculation

A previously developed theory for the temperature-dependent resonance Raman scattering is used to study the surface-enhanced Raman scattering. Two models, the displaced oscillator model and the displaced-distorted oscillator model, based on the harmonic potential energy surfaces are carried out to calculate the surface-enhanced Raman scattering excitation profiles of the pyridine molecule adsorbed on a silver electrode, for which the density functional theory method is applied to evaluate the potential energy surfaces of the adsorption structure. In this framework, the distortion effect on the surface-enhanced Raman scattering will be discussed by comparing both models. The overlapping resonance of multiexcited electronic transitions is also studied, in which the interference between electronic transitions has been taken into account. It will be used to study the abnormal band at 1005.6 cm(-1) with the exciting radiation 457.9 nm.