和频振动光谱研究多晶金电极/溶液界面乙腈分子取向的flip-flop行为

用和频振动光谱研究乙腈/金电极界面,观测到乙腈的甲基振动峰强度随施加的电极电势而变化.当电极电势越过零电荷电势（pzc）时,甲基振动峰符号发生反转,这意味着基团取向发生反转（flip-flop）.由此推断出乙腈分子在金电极界面的吸附构型.即在零电荷电势下,电极界面吸附的乙腈分子构型为甲基靠近电极表面而腈基远离电极表面;而高于零电荷电势则电极界面吸附的乙腈分子构型发生反转,变为腈基靠近电极表面而甲基远离电极表面的构型.     

Structural Investigation of Small Nickel-Ethanol Clusters Using Vibrational Spectroscopy in a Molecular Beam

The structural identification of small nickel clusters with ethanol can help to understand fundamental steps for heterogenous catalysis. We investigate the rows [Ni_x(EtOH)₁]⁺ with x=1–4, and [Ni₂(EtOH)_y]⁺ with y=1–3 via IR photodissociation spectroscopy in a molecular beam experiment. Analyzing the CH- and OH-stretching frequencies and comparing these experimental results with density functional theory (DFT) calculations on the PW91/6-311+G(d,p) level leads to the identification of intact motifs for all clusters and hints for C−O cleavage of the ethanol in two particular cases. Furthermore, we analyze the effects of frequency shifts with the increasing clusters sizes using the results of natural bond orbitals (NBO) analyses and an energy decomposition method.     

Application of THz Vibrational Spectroscopy to Molecular Characterization and the Theoretical Fundamentals: An Illustration Using Saccharide Molecules

This work illustrates several theoretical fundamentals for the application of THz vibrational spectroscopy to molecular characterization in the solid state using two different types of saccharide systems as examples. Four subjects have been specifically addressed: (1) the qualitative differences in the molecular vibrational signatures monitored by THz and mid‐IR vibrational spectroscopy; (2) the selection rules for THz vibrational spectroscopy as applied to crystalline and amorphous systems; (3) a normal mode simulation, using α‐l ‐xylose as an example; and (4) a rigorous mode analysis to quantify the percentage contributions of the intermolecular and intramolecular vibrations to the normal mode of interest.     

Vibrational Spectroscopy As a Probe of Surface Heterogeneity: CO on NaCl(100)

The fundamental (Δv = 1 ← 0) and the first overtone (Δv = 2 ← 0) transitions of monolayer physisorbed CO on NaCl(100) single crystal surfaces at 5 K have been investigated using polarized Fourier transform infrared spectroscopy. Comparisons of the transition frequencies, bandwidths and absorbances for these two transitions together with those of dilute isotopes allow us to isolate the contribution of dynamic and static coupling effects of molecules within the monolayer. Homogeneous and heterogeneous effects can also be distinguished from band profiles of these various transitions. We conclude that the residual line broadening of 0.1 cm^?1 at 5 K for the isolated CO isotopes arises from heterogeneous surface effects. We set an upper limit of 0.07 cm^?1 for the linewidth of single CO molecules on NaCl(lOO) due to surface heterogeneity. A lower limit of × 10^?8 cm^?1 is provided by the vibrational lifetime of adsorbed CO molecules. Sharpness of the vibrational features shows that infrared spectroscopy of adsorbate is a sensitive method for probing surface and adlayer irregularities of adsorbed molecules on a single crystal.     

Cations in a Molecular Funnel: Vibrational Spectroscopy of Isolated Cyclodextrin Complexes with Alkali Metals

The benchmark inclusion complexes formed by α‐cyclodextrin (αCD) with alkali‐metal cations are investigated under isolated conditions in the gas phase. The relative αCD‐M⁺ (M=Li⁺, Na⁺, K⁺, Cs⁺) binding affinities and the structure of the complexes are determined from a combination of mass spectrometry, infrared action spectroscopy and quantum chemical computations. Solvent‐free laser desorption measurements reveal a trend of decreasing stability of the isolated complexes with increasing size of the cation guest. The experimental infrared spectra are qualitatively similar for the complexes with the four cations investigated, and are consistent with the binding of the cation within the primary face of the cyclodextrin, as predicted by the quantum computations (B3LYP/6‐31+G*). The inclusion of the quantum‐chemical cation disrupts the C₆ symmetry of the free cyclodextrin to provide the optimum coordination of the cations with the ‐CH₂OH groups in C₁, C₂ or C₃ symmetry arrangements that are determined by the size of the cation.     

The Electrostatic Interaction of a Charged Particle with a Surface: The Effect of Surface Charge Rearrangement

In this paper we investigate the electric interaction between a charged particle and a surface in which the charged ions are capable of moving in response to the electric potential disturbance caused by the approach of the charged particle. Such surfaces include ionic surfactants distributed in air-water interface and charged lipids in bilayer membranes. On the basis of the mean field theory, the free energy of the system, which includes the electrostatic internal energy and the entropy of the mobile ions and surface ions, can be written down. The surface charge-potential relation is then derived by the calculus of variation. When the potential disturbance is small enough, a linear charge regulation model is obtained. The interaction energy associated with a long rod parallel to the interface is studied and an analytical expression is obtained. When a rod approaches an oppositely charged surface, the interaction can change from attraction to repulsion, depending on the ratio of the characteristic regulation length to the Debye length. At low surface charge density, the surface behaves as under the condition of constant charge density and acts as that of constant potential for high enough charge density. Copyright 2001 Academic Press.     

Transport of Propylene Carbonate-LiTFSI Electrolytes in P(VDF-HFP) Using Time-resolved ATR-FTIR Spectroscopy: Diffusion Coefficients and Molecular Interactions

The time-resolved attenuated total reflectance-Fourier transform infrared(ATR-FTIR) spectroscopy is employed to investigate the transport mechanism of gel electrolytes by monitoring the diffusion behavior of propylene carbonate-lithium bis(trifluoromethylsulfonyl)imide(PC-LiTFSI) solution through poly(vinylidene fluoride-co-hexafluoropropylene)(P(VDF-HFP)) films. Fickian behavior has been observed for both TFSI-and PC. Higher temperature leads to faster diffusion of TFSI~-and PC, which could be related to the increased free volume in P(VDF-HFP)matrix and rapid molecular movements upon heating. Various molecular interactions among LiTFSI, PC and P(VDF-HFP) have been recognized.During the diffusion process, PC molecules, in the form of small clusters, can firstly diffuse through the P(VDF-HFP) film and interact with P(VDFHFP) by dipole-dipole interaction, acting as the plasticizer. Then, Li~+ diffuses into P(VDF-HFP) with the help of ion-dipole interactions between Li~+and C=O of PC. Meanwhile, TFSI-diffuses through the polymer matrix in solvation states. In addition, slight ion-dipole interactions between Li~+and P(VDF-HFP) have been observed as well. Results in this work contribute to a better understanding of transport process in gel polymer electrolytes for lithium-ion batteries and support the development of improved gel polymer electrolytes by rationally regulating molecular interactions.     

Interfacial Behavior of beta-Lactoglobulin at a Stainless Steel Surface: An Electrochemical Impedance Spectroscopy Study

The electrochemical impedance spectroscopy technique was used to investigate the interfacial behavior of beta-lactoglobulin at an austenitic stainless steel surface over the temperature range 299 to 343 K at an open circuit potential. The electrode/electrolyte interface and corresponding surface processes were successfully modeled by applying an equivalent-electrical-circuit approach. A charge-transfer resistance value was found to be very sensitive to the amount of adsorbed protein (surface concentration), thus indicating that the adsorption of the protein (i) was accompanied by the transfer of the charge, via chemisorption, and (ii) influenced the mechanism and kinetics of the corrosion reaction. This was also apparent from the large decrease in the corrosion activation energy (16 kJ mol(-1)) caused by the adsorption of the protein. Adsorption of beta-lactoglobulin onto the stainless steel surface at an open circuit potential resulted in a unimodal isotherm at all the temperatures studied and the adsorption process was described with a Langmuir adsorption isotherm. From the calculated Gibbs free energies of adsorption it was confirmed that beta-lactoglobulin molecules adsorb strongly onto the stainless steel surface. The enthalpy and entropy values indicated that the molecule partially unfolds at the surface upon adsorption. The adsorption process was found to be entirely governed by the change in entropy. Copyright 2000 Academic Press.     

New Insights into the Nature of Observable Reaction Intermediates in Cytochrome P450 NO Reductase by Using a Combination of Spectroscopy and Quantum Mechanics/Molecular Mechanics Calculations

Cytochrome P450 NO reductase is an unusual member of the cytochrome P450 superfamily. It catalyzes the reduction of nitric oxide to nitrous oxide. The reaction intermediates were studied in detail by a combination of experimental and computational methods. They have been characterized experimentally by UV/Vis, EPR, Mössbauer, and MCD spectroscopy. In conjunction with quantum mechanics/molecular mechanics (QM/MM) calculations, we sought to characterize the resting state and the two detectable intermediates in detail and to elucidate the nature of the key intermediate I of the reaction. Six possible candidates were taken into account for the unknown key intermediate in the computational study, differing in protonation state and electronic structure. Two out of the six candidates could be identified as putative intermediates I with the help of the spectroscopic data: singlet diradicals Fe^III‐NHO ^{. ?} and Fe^III‐NHOH ^. . In a companion publication (C. Riplinger, F. Neese, ChemPhysChem­ 2011, 12, 3192 ) we have used QM/MM models based on these structures and performed a kinetic simulation. The combination of these two studies shows the nature of the key intermediate to be the singlet diradical, Fe^III‐NHOH ^. .     

Vibrational quenching of CO2(010) by collisions with O(3P) at thermal energies: a quantum-mechanical study

The CO(2)(010)-O((3)P) vibrational energy transfer (VET) efficiency is a key input to aeronomical models of the energy budget of the upper atmospheres of Earth, Venus, and Mars. This work addresses the physical mechanisms responsible for the high efficiency of the VET process at the thermal energies existing in the terrestrial upper atmosphere (150 K相似文献   

Heteronuclear NMR Spectroscopy as a Surface‐Selective Technique: A Unique Look at the Hydroxyl Groups of γ‐Alumina.

The surface hydroxyl groups of γ‐alumina dehydroxylated at 500 °C were studied by a combination of one‐ and two‐dimensional homo‐ and heteronuclear ¹H and ²⁷Al NMR spectroscopy at high magnetic field. In particular, by harnessing ¹H–²⁷Al dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the ¹H magic‐angle spinning (MAS) NMR spectrum was demonstrated thanks to ¹H–²⁷Al RESPDOR (resonance‐echo saturation‐pulse double‐resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {¹H}‐²⁷Al dipolar heteronuclear multiple quantum correlation (D‐HMQC), which was used to establish a first coordination map. Then, in combination with ¹H–¹H double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO₂ reactivity with alumina.     

Front Cover: Investigation of the Second Harmonic Generation at the Water–Vacuum Interface by Using Multi‐Scale Modeling Methods (ChemistryOpen 1/2023)

The Front Cover shows the interfacial selectivity of second harmonic generation at the water‐vacuum interface, which is targeted in this work. In more details, the molecular first hyperpolarizability responses have been calculated by combining classical molecular dynamics and quantum chemistry simulations, and our model was able to distinguish between the bulk and the interfacial contributions. More information can be found in the Research Article by Tárcius N. Ramos et al.     

Heme-Peptide Models for Hemoproteins. 2. N-Acetylmicroperoxidase-8: Study of the pi-pi Dimers Formed at High Ionic Strength Using a Modified Version of Molecular Exciton Theory

AcMP8 is the Cys-14-acetylated water-soluble heme-octapeptide fragment obtained proteolytically from cytochrome c. Two successive dimerization equilibria are observed with increasing ionic strength in aqueous solution at neutral pH (part 1, preceding article). The electronic spectra of the two pi-pi dimers were extracted from the absorption envelopes at 2.01 and 4.02 M ionic strength and resolved by Gaussian analysis. The principal transitions were assigned using a tailored version of molecular exciton theory based on coupling of the main x- and y-polarized transition dipole moments of the interacting heme groups. The spectra of both pi-pi dimers indicate that the y-polarized exciton states are blue-shifted relative to the excited states of the monomer, while the x-polarized exciton states exhibit a red shift. These shifts were correctly predicted by a simple dipole-dipole coupling model. From an analysis of the resultant transition dipole moments to the exciton states with B(x)()(0,0) and B(y)()(0,0) character and the magnitudes of their red and blue exciton shifts, respectively, we have determined the dipole-dipole interaction geometries for both dimers. The principal difference between the interaction geometry in the first dimer and that in the second is a stronger interaction for the y-polarized transition dipoles and somewhat weakened interaction for the x-polarized transition dipoles. From an analysis of available crystallographic data for porphyrin and metalloporphyrin pi-pi dimers (Scheidt, W. R.; Lee, Y. J. Struct. Bonding 1987, 64, 1) and the results of our exciton model, we conclude that the origin of the coordinate system for the Soret transition dipole moments of AcMP8 is not metal-centered. Furthermore, since the true directions of the x- and y-axes of the low-symmetry heme chromophore in AcMP8 are unknown, we have not been able to determine the structures of the pi-pi dimers from a knowledge of their transition dipole-dipole interaction geometries. This study therefore highlights one of the shortfalls of molecular exciton theory.     

Effect of S arrangement on Fe(110) properties at 1/3 monolayer coverage: a DFT study

Using density functional theory calculations, we compare the relative stabilities and properties of different arrangements of S on Fe(110) at a 1/3 monolayer coverage, including two observed experimentally. For all studied arrangements, S is adsorbed in the three high-symmetry adsorption sites: 4-fold hollow, 3-fold hollow, bridge, and atop sites. The binding energy, work function change, adsorption geometry, charge density distribution, magnetic properties, and density of states are determined and compared. The most stable overlayer arrangement corresponds to the overlayer seen by experiment after dissociative adsorption of H2S and has S adsorbed in 4-fold hollow sites. In the other arrangements, the S atoms are located closer to each other on the surface reducing the stability of the overlayer. S causes a minor adsorbate-induced reconstruction of the Fe surface and quenches the magnetic moment of the Fe atoms it bonds to directly. It adsorbs as an electropositive species, causing a positive work function change and forms polar covalent bonds to the surface.