The Use of Thin‐Layer Electroanalysis in the Study of the Chemisorption and Anodic Oxidation of Aromatic Molecules at Smooth Polycrystalline Palladium

Thin‐layer electroanalytical chemistry has been used in the study of the chemisorption and anodic oxidation of hydroquinone and benzoquinone at smooth polycrystalline palladium electrode surfaces in aqueous sulfuric acid solutions. The results were reminiscent of those obtained previously on smooth polycrystalline platinum: i) At low aqueous‐solution concentrations, the diphenol is oxidatively chemisorbed to form surface‐coordinated benzoquinone oriented parallel to the surface. ii) At higher concentrations, the oxidative chemisorption occurs via C? H activation to yield an edge‐oriented diphenolic species. iii) Chemisorption from benzoquinone solutions leads to species identical to those from hydroquinone solutions. iv) The extent of anodic oxidation of the chemisorbed organic depends upon the initial adsorbed‐molecule orientation: the flat‐adsorbed species are oxidized completely to carbon dioxide, whereas oxidation of the edge‐chemisorbed species yields other (unidentified) products that are chemisorbed upon regeneration of the oxide‐free surface.     

Interfacial electrostatics of self-assembled monolayers of alkane thiolates on Au(111): work function modification and molecular level alignments

We have isolated at T < 150 K a weakly adsorbed dimethyl disulfide (DMDS) layer on Au(111) and studied how the vibrational states, S core hole level shifts, valence band photoemission, and work function measurements evolve upon transforming this system into chemisorbed methylthiolate (MT) self-assembled monolayers (SAM) by heating above 200 K. By combining these observations with detailed theoretical electronic structure simulations, at the density functional level, we have been able to obtain a detailed picture of the electronic interactions at the interface between Au and adsorbed thiolates and disulfides. All of our measurements may be interpreted with a simple model where MT is bound to the Au surface with negligible charge transfer. Interfacial dipoles arising from Pauli repulsion between molecule and metal surface electrons are present for the weakly adsorbed DMDS layer but not for the chemisorbed species. Instead, for the chemisorbed species, interfacial dipoles are exclusively controlled by the molecular dipole, its interaction with the dipoles on neighboring molecules, and its orientation to the surface. The ramifications of these results for alignment of molecular levels and interfacial properties of this class of materials are discussed.     

Conformationally imprinted receptors: atropisomers with "write", "save", and "erase" recognition properties

[reaction: see text] An atropisomeric receptor with "write", "save", and "erase" recognition properties is presented. The receptor adopts a complementary conformation when heating in the presence of an ethyl adenine-9-acetate guest molecule. This complementary hydrogen bonding conformation is "saved" upon cooling to room temperature due to the reestablishment of restricted rotation and is stable even upon removal of the guest. Finally, the atropisomeric receptor can be "erased" by heating in the absence of the guest.     

Infrared spectroscopy of physisorbed and chemisorbed N2 in the Pt(111)(3x3)N2 structure

Using infrared spectroscopy and low electron energy diffraction, we have investigated the adsorption of N(2), at 30 K, on the Pt(111) and the Pt(111)(1x1)H surfaces. At monolayer coverage, N(2) orders in commensurate (3x3) structures on both surfaces, and we propose that the unit cells contain four molecules in each case. The infrared spectra reveal that N(2) exclusively physisorbs on the Pt(111)(1x1)H surface, while both physisorbed and chemisorbed N(2) is detected on the Pt(111) surface. Physisorbed N(2) is the majority species in the latter case, and the two adsorption states show an almost identical uptake behavior, which indicates that they are intrinsic constituents of the growing (3x3) N(2) islands. An analysis of the infrared absorbance data, based on a simple scaling concept suggested by density functional theory calculations, supports a model in which the (3x3) unit cell contains one chemisorbed molecule in end-on atop configuration and three physisorbed molecules. We note that a classic "pinwheel" structure on a hexagonal lattice, with the end-on chemisorbed N(2) molecules acting as "pins," is compatible with this composition.     

Sum frequency generation vibrational spectroscopic and high-pressure scanning tunneling microscopic studies of benzene hydrogenation on Pt(111)

Sum frequency generation (SFG) vibrational spectroscopy and high-pressure scanning tunneling microscopy (HP-STM) have been used in combination for the first time to study a catalytic reaction. These techniques have been able to identify surface intermediates in situ during benzene hydrogenation on a Pt(111) single-crystal surface at Torr pressures. In a background of 10 Torr of benzene, STM is able to image small ordered regions corresponding to the c(2 radical3 x 3)rect structure in which each molecule is chemisorbed at a bridge site. In addition, individual benzene molecules are also observed between the ordered regions. These individual molecules are assumed to be physisorbed benzene on the basis of the SFG results showing both chemisorbed and physisorbed molecules. The surface becomes too mobile to image upon addition of hydrogen but is determined to have physisorbed and chemisorbed benzene present by SFG. It was spectroscopically determined that heating the platinum surface after poisoning with CO displaces benzene molecules. The high-coverage pure CO structure of (radical19 x radical19)R23.4 degrees imaged with STM is a verification of spectroscopic measurements.     

The influence of orientation on the electrocatalytic hydrogenation of hydroquinone chemisorbed at smooth polycrystalline platinum electrodes

The influence of orientation on the electrocatalytic hydrogenation of hydroquinone (HQ) chemisorbed at smooth polycrystalline platinum electrodes in aqueous solutions has been investigated; experimental measurements, performed in the absence of bulk (unadsorbed) HQ, were based upon thin-layer electrochemical techniques. The extent of hydrogenation was characterized by (i) n_H. the average number of hydrogen atoms reacted per chemisorbed HQ molecule, and (ii) the electrolytic charge Q_ox for oxidation of chemisorbed organic which remained on the surface after the hydrogenation reaction. The measured values of n_H indicate that the extent of HQ hydrogenation is (i) dependent upon the potential E_Hyd at which hydrogenation was earned out, and (ii) a sensitive function of its initial adsorbed orientation; at a given E_Hyd, n_His larger in the flat (η⁶) than in the edge (2,3-η²) orientation. Correlation of Q_ox with n_H, indicates that an appreciable fraction of partially hydrogenated species is desorbed from the surface; this fraction, which is a function of E_Hyd, is larger in the 2.3-η² than in the η⁶ orientation.     

Photoionization dynamics of glycine adsorbed on a silicon cluster: "on-the-fly" simulations

Dynamics of glycine chemisorbed on the surface of a silicon cluster is studied for a process that involves single-photon ionization, followed by recombination with the electron after a selected time delay. The process is studied by "on-the-fly" molecular dynamics simulations, using the semiempirical parametric method number 3 (PM3) potential energy surface. The system is taken to be in the ground state prior to photoionization, and time delays from 5 to 50 fs before the recombination are considered. The time evolution is computed over 10 ps. The main findings are (1) the positive charge after ionization is initially mostly distributed on the silicon cluster. (2) After ionization the major structural changes are on the silicon cluster. These include Si-Si bond breaking and formation and hydrogen transfer between different silicon atoms. (3) The transient ionization event gives rise to dynamical behavior that depends sensitively on the ion state lifetime. Subsequent to 45 fs evolution in the charged state, the glycine molecule starts to rotate on the silicon cluster. Implications of the results to various processes that are induced by transient transition to a charged state are discussed. These include inelastic tunneling in molecular devices, photochemistry on conducting surfaces, and electron-molecule scattering.     

In situ raman spectroelectrochemistry of electron transfer between glassy carbon and a chemisorbed nitroazobenzene monolayer

In situ Raman spectroscopy was used to monitor 4-nitroazobenzene (NAB) in an electrochemical cell, both as a free molecule and as a chemisorbed monolayer on a glassy carbon (GC) electrode surface. Reduction of free NAB exhibited two well-defined voltammetric couples in acetonitrile, and the accompanying spectral changes supported a mechanism involving two successive 1-e(-) transfers. Raman spectra of NAB chemisorbed to GC via diazonium ion reduction were obtained in acetonitrile with a high-sensitivity, line-focused CCD spectrometer. The chemisorbed NAB spectra were quite different from the free NAB spectra, and were sufficiently strong to monitor as a function of applied potential. In the potential range of +400 to -800 mV vs Ag/Ag(+), the intensity of the Raman bands associated with the phenyl-NO(2) moiety varied, implying an electronic interaction between the pi system of the graphitic substrate and the chemisorbed NAB molecules. Negative of -800 mV, a 1-e(-) voltammetric reduction peak was observed, which was reversible on the positive voltage scan. This peak was accompanied by significant spectral changes, particularly the loss of the N=N and NO(2) stretches. The spectra are consistent with formation of a quinoid structure containing a C=C double bond between the NAB and the graphitic surface. The electron transfer and spectral changes occurred over a wider potential range than expected for a conventional Nernstian equilibrium, but did not appear to be broadened by slow electron-transfer kinetics. The results imply a significant perturbation of electron transfer between the GC and the monolayer, caused by strong electronic coupling between the graphitic pi system and the NAB orbitals. Rather than a discrete electron transfer to a free molecule, the electron transfer to chemisorbed NAB is more gradual, and is presumably driven by the electric field at the electrode/solution interface.     

Frontispiz: Biomass Oxidation: Formyl C?H Bond Activation by the Surface Lattice Oxygen of Regenerative CuO Nanoleaves

An integrated experimental and computational investigation reveals that surface lattice oxygen of copper oxide (CuO) nanoleaves activates the formyl C H bond in glucose and incorporates itself into the glucose molecule to oxidize it to gluconic acid. The reduced CuO catalyst regains its structure, morphology, and activity upon reoxidation. The activity of lattice oxygen is shown to be superior to that of the chemisorbed oxygen on the metal surface and the hydrogen abstraction ability of the catalyst is correlated with the adsorption energy. Based on the present investigation, it is suggested that surface lattice oxygen is critical for the oxidation of glucose to gluconic acid, without further breaking down the glucose molecule into smaller fragments, because of C C cleavage. Using CuO nanoleaves as catalyst, an excellent yield of gluconic acid is also obtained for the direct oxidation of cellobiose and polymeric cellulose, as biomass substrates.     

Biomass Oxidation: Formyl CH Bond Activation by the Surface Lattice Oxygen of Regenerative CuO Nanoleaves

An integrated experimental and computational investigation reveals that surface lattice oxygen of copper oxide (CuO) nanoleaves activates the formyl C? H bond in glucose and incorporates itself into the glucose molecule to oxidize it to gluconic acid. The reduced CuO catalyst regains its structure, morphology, and activity upon reoxidation. The activity of lattice oxygen is shown to be superior to that of the chemisorbed oxygen on the metal surface and the hydrogen abstraction ability of the catalyst is correlated with the adsorption energy. Based on the present investigation, it is suggested that surface lattice oxygen is critical for the oxidation of glucose to gluconic acid, without further breaking down the glucose molecule into smaller fragments, because of C? C cleavage. Using CuO nanoleaves as catalyst, an excellent yield of gluconic acid is also obtained for the direct oxidation of cellobiose and polymeric cellulose, as biomass substrates.     

Methanethiolate adsorption site on Au(111): a combined STM/DFT study at the single-molecule level

The chemisorptive bonding of methanethiolate (CH(3)S) on the Au(111) surface has been investigated at a single-molecule level using low-temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT). The CH(3)S species were produced by STM-tip-induced dissociation of methanethiol (CH(3)SH) or dimethyl disulfide (CH(3)SSCH(3)) at 5 K. The adsorption site of an isolated CH(3)S species was assigned by comparing the experimental and calculated STM images. We conclude that the S-headgroup of chemisorbed CH(3)S adsorbs on the 2-fold coordinated bridge site between two Au atoms, consistent with theoretical predictions for CH(3)S on the nondefective Au(111) surface. Our assignment is also supported by the freezing of the tip-induced rotational dynamics of a single CH(3)SH molecule upon conversion to CH(3)S via deprotonation.     

Chemical Enhancement and Quenching in Single-Molecule Tip-Enhanced Raman Spectroscopy

Despite intensive research in surface enhanced Raman spectroscopy (SERS), the influence mechanism of chemical effects on Raman signals remains elusive. Here, we investigate such chemical effects through tip-enhanced Raman spectroscopy (TERS) of a single planar ZnPc molecule with varying but controlled contact environments. TERS signals are found dramatically enhanced upon making a tip–molecule point contact. A combined physico-chemical mechanism is proposed to explain such an enhancement via the generation of a ground-state charge-transfer induced vertical Raman polarizability that is further enhanced by the strong vertical plasmonic field in the nanocavity. In contrast, TERS signals from ZnPc chemisorbed flatly on substrates are found strongly quenched, which is rationalized by the Raman polarizability screening effect induced by interfacial dynamic charge transfer. Our results provide deep insights into the understanding of the chemical effects in TERS/SERS enhancement and quenching.     

Catalytic Activity of Polyaniline in the Molecular Oxygen Reduction: Its Nature and Mechanism

By comparing experiments in air and an inert atmosphere, additional evidence for the polyaniline catalytic action on the air oxygen electroreduction is obtained. The action is compared with that of graphite substrates under the same conditions. The effect of the oxygen supply method and the polyaniline film thickness (weight) on the oxygen reduction rate is evaluated. To elucidate the nature of the polyaniline catalytic activity, quantum-chemical modeling of polyaniline and its adsorption complexes with oxygen is carried out. According to calculations, molecular oxygen can be reversibly chemisorbed at the polyaniline surface that serves as a donor of electron density. The bonding order for the O₂*molecules chemisorbed at polyaniline decreases by 30% and the bond length increases by 24%. Thus, oxygen molecules acquire higher activity in a chemisorbed state and can then participate in the reduction more actively.     

The chemisorption of spin polarised NO on Ag{111}

Spin-density functional theory calculations are presented for NO adsorbed on Ag{111}. The ground state for the monomeric species is chemisorbed in an upright configuration, but retains 90% of the spin-density of the free molecule, in the molecular 2π^* orbital. In constrast, two NO molecules in upright configuration chemisorbed at neighbouring fcc and hcp sites have zero spin-density, and charge density difference plots demonstrate π bonding as in the free dimer, (NO)₂.     

汞离子的高灵敏度裸眼识别和荧光传感探针

设计合成了一种以耐尔蓝为母体的Hg2+光学探针分子1-苯甲酰-3-{2-[9-(乙氨基)-10-甲基-9H-苯并[α]苯酚-5-胺基]乙基}硫脲盐酸盐(NBET). 在pH=7.4的Tris-HCl缓冲液中, 探针分子最大吸收波长为640 nm, 此时溶液为淡蓝色; 加入汞离子可以诱导探针分子在640 nm处的吸收降低, 并在556 nm处产生新的吸收峰, 溶液变为浅紫色, 而其它金属离子的加入未引起显著变化, 基于此可对水溶液中的痕量Hg2+进行裸眼识别. 荧光光谱显示, 汞离子可以特异性地猝灭探针分子在660 nm处的荧光发射. 该探针分子的灵敏度、选择性及荧光量子产率高, 激发/发射波长长, 可以实现水溶液中0.005 μmol/L Hg2+的荧光检测.     

Electrocatalytic mediation of oxidation of H2 at gold by chemisorbed states of anions

On Au, free surface atoms are not active for dissociative oxidation of the H₂ molecule but at potentials where anions are chemisorbed or complexes of partially discharged OH^(1−γ− with the chemisorbed anions arise, the H₂ molecule can be dissociated and consequently the H-atom electrochemically oxidized.This phenomenon is observed on Au (110), (100) single-crystal surfaces, and to a lesser degree on the (111) plane, as well as on polycrystalline Au. The rate of the reaction is proportional to the quantity of discharged species and is higher when the oxidation occurs by mediation by the complexes. The reaction is inhibited, however, by fully discharged OH. A microscopic surface-chemical model of the reaction is proposed and examined in relation to the experimental behaviour at the (110) plane in 0.01 M HClO₄.     

Raman spectra from ethylene and deuterated ethylene chemisorbed on a silica-supported nickel catalyst

The Raman spectra of ethylene and deuterated ethylene chemisorbed on silica-supported nickel have been measured in the frequency range 50–3400 cm^?1. At room temperature, a Raman spectrum is observed which corresponds to ethylene chemisorbed under dehydrogenation and it is rather similar to the spectrum of chemisorbed acetylene. For a comparison therefore, the Raman spectra of acetylene and deuterated acetylene were also measured. In addition, the vibrational spectrum of chemisorbed benzene was recorded. At temperatures T ? 200 K, ethylene is found to be associatively chemisorbed without dehydrogenation.The vibrations observed are described in the approximation of a surface molecule with covalent bonding to two or three surface nickel atoms. The symmetry seems to be slightly distorted C_2v or C_s. The vibrational spectrum is discussed with respect to a metal- surface selection rule. In order to improve the reliability of the assignments for localized vibrational modes, a normal coordinate analysis and a force constant calculation have been done for chemisorbed acetylene.     

D-penicillamine adsorption on gold: an in situ ATR-IR spectroscopic and QCM study

The adsorption of penicillamine from ethanol on gold was studied in situ by attenuated total reflection infrared (ATR-IR) and quartz crystal microbalance (QCM) experiments. Both ATR-IR and QCM reveal a fast mass uptake. In ethanol, the molecule adopts a zwitterionic form. Upon adsorption, part of the molecules deprotonate at the amine group, which is a relatively slow process that goes along with a strong shift of the nu(as)(COO(-)) mode. Both ATR-IR and QCM confirm a physisorbed layer. ATR-IR furthermore shows that the latter consists of zwitterionic molecules only, whereas both zwitterionic and anionic species are found in the chemisorbed layer. The infrared spectra of the physisorbed and chemisorbed layers are rather different, and the molecules within both layers seem to be oriented with respect to the surface. The ATR-IR spectra furthermore indicate that all three functional groups of penicillamine (i.e., thiol, carboxylate, and amine) interact with the surface, and density functional theory calculations support this finding. QCM also shows that the molecule uses considerably more space on the surface than molecules of similar size, which supports a three-point interaction. The latter leads to a strong anchoring of the molecule to the metal, which may explain the exceptional capability of penicillamine to bind metals.     

Evaluation of "credit card" libraries for inhibition of HIV-1 gp41 fusogenic core formation

Protein-protein interactions are of critical importance in biological systems, and small molecule modulators of such protein recognition and intervention processes are of particular interest. To investigate this area of research, we have synthesized small-molecule libraries that can disrupt a number of biologically relevant protein-protein interactions. These library members are designed upon planar motif, appended with a variety of chemical functions, which we have termed "credit-card" structures. From two of our "credit-card" libraries, a series of molecules were uncovered which act as inhibitors against the HIV-1 gp41 fusogenic 6-helix bundle core formation, viral antigen p24 formation, and cell-cell fusion at low micromolar concentrations. From the high-throughput screening assays we utilized, a selective index (SI) value of 4.2 was uncovered for compound 2261, which bodes well for future structure activity investigations and the design of more potent gp41 inhibitors.     

Biatomic substrates for bulk-molecule interfaces: the PtCo-oxygen interface

We develop an ab initio procedure for materials participating in electron transfer reactions, to consider biatomic backgrounds (e.g., substrates, catalysts, electrodes), thus the precise effects of the continuum and long-range interactions as well as the effects of the discrete and local nature of the chemistry can be combined to study molecules under such biatomic backgrounds. We test this new procedure by studying the reactivity of molecular oxygen on bimetallic clusters of platinum and cobalt. The reaction of molecular oxygen on the surface of the L1(2) phase of Co(3)Pt yields three distinct chemisorbed precursors, two of which are energetically almost degenerate. One precursor is formed across a bridge site with the molecule parallel to the surface, the second one is formed at the bridge with the molecule again parallel to the surface, and the third is formed in the threefold fcc hollow site, with the molecule slightly tilted in a top-hollow-bridge geometry.