Theoretical Investigation on the Substituent Effect of Halogen Atoms at the C₈ Position of Adenine: Relative Stability, Vibrational Frequencies, and Raman Spectra of Tautomers

We have theoretically investigated the substituent effect of adenine at the C₈ position with a substituent X = H, F, Cl, and Br by using the density functional theory (DFT) at the B3LYP/6-311+G­(d, p) level. The aim is to study the substituent effect of halogen atoms on the relative stability, vibrational frequencies, and solvation effect of tautomers. Our calculated results show that for substituted adenine molecules the N9H8X tautomer to be the most stable structure in gas phase at the present theoretical level. Here N9H8X denotes the hydrogen atom binds to the N₉ position of imidazole ring and X denotes H, F, Cl, and Br atoms. The influence of the induced attraction of the fluorine substituent is significantly larger than chlorine and bromine ones. The halogen substituent effect has a significant influence on changes of vibrational frequencies and Raman intensities

Theoretical Study on Thermodynamic and Spectroscopic Properties of Electro-Oxidation of <i>p</i>‑Aminothiophenol on Gold Electrode Surfaces

The electro-oxidation of <i>p</i>-aminothiophenol (PATP) on gold electrodes has been investigated by means of density functional theory. A combination of thermodynamic calculations and surface Raman and infrared (IR) spectral simulations has allowed us to reveal the electro-oxidation mechanism and reaction products of PATP on gold electrodes in acidic, neutral, and basic solutions. PATP can be first oxidized to the radical cation PATP­(NH₂^•+) or the neutral radical PATP­(NH^•) depending on the pH of aqueous solutions, and this is the rate-determining step. The radical cation or neutral radical can then transform to the dimerized products through a radical coupling reaction. In the acidic medium, the radical cation reacts with its resonance hybrid through a N–C4 coupling to form 4′-mercapto-<i>N</i>-phenyl-1,4-quinone diimine (D1), which can further undergo hydrolysis to yield 4′-mercapto-<i>N</i>-phenyl-1,4-quinone monoimine (D2). In the neutral medium, the neutral radical reacts with its resonance hybrid through the N–C2(6) coupling to form 4,4′-dimercapto-<i>N</i>-phenyl-1,2-quinone diimine (D3). In the basic medium, the neutral radical reacts with its resonance structure through the N–N coupling to form 4,4′-di­mercapto­azo­benzene (D4). The adsorbed dimer products exhibit reversible redox properties. The calculated standard electrode potentials of the above four species decrease in the order D3, D1, D2, and D4. Finally, the characteristic bands for the surface Raman and IR spectra of D1 to D4 redox pairs are clearly assigned. This study provides mechanistic insight into the electrochemical reaction properties of PATP on metal electrodes

DFT Study of Hydrogen-Bonding Interaction, Solvation Effect, and Electric-Field Effect on Raman Spectra of Hydrated Proton

Strong hydrogen-bonding interaction and Raman spectra of hydrated proton have been investigated using hybrid density functional theory method B3LYP. The solvation model of density (SMD) approach is employed in the present calculation to simulate hydrated protons in aqueous solution. Focusing on the different hydrogen-bonded Eigen-water and Zundel-water interactions, we present a better assignment of Raman signals of hydrated proton on the basis of vibrational analysis in different environments. Our results showed that B3LYP calculations could give a good prediction for characteristic vibrational frequencies of Eigen and Zundel isomers in liquid phase. The O–H stretching vibrational frequencies from Eigen and Zundel units are very sensitive to hydrogen-bonding interaction with solvent water molecules. Moreover, the solvation effect and the external electric-field effect lead to the proton deviating from the central position of Zundel structure and finally resulting in a transition to Eigen one in aqueous solution. Furthermore, by combining theoretical prediction and Raman scattering theory, we calculate absolute Raman intensities of characteristic signals based on the polarizability tensor derivatives of hydrated proton clusters. This is very helpful to infer the microstructure of hydrated protons in aqueous solution by using Raman measurements

Theoretical Study on Electroreduction of <i>p</i>‑Nitrothiophenol on Silver and Gold Electrode Surfaces

The electroreduction of <i>p</i>-nitrothiophenol (PNTP) on gold and silver electrodes has been investigated by means of density functional theory. A combination of thermodynamic calculations and surface Raman/IR spectral simulations has allowed us to reveal the reaction mechanism and reaction products of electroreduction of PNTP on metal electrodes. First, thermodynamic calculations were carried out to calculate the standard electrode potentials of PNTP and its possible intermediates. The potential energy curves of PNTP reduction as a function of the applied potential are obtained on the basis of the calculated standard electrode potentials of the elementary electrochemical reactions. Second, surface vibrational spectral simulation was performed to provide theoretical assignments of reaction products for the in situ Raman/IR experimental studies of electroreduction of PNTP. The most interesting finding in the reaction product identified by IR spectroscopy is PATP; however, Raman spectroscopy shows that the main product is <i>p</i>,<i>p</i>′-dimercaptoazobenzene (DMAB). The difference between IR and Raman measurements arises from the fact that the incident laser used in Raman measurement can induce the formation of DMAB by photoreduction of PNTP or photo-oxidation of PATP. Finally, the reaction mechanism of electroreduction of PNTP was compared with its photoreduction mechanism

Biography of Zhao-Wu Tian

Biography of Zhao-Wu Tia

Mobility and Reactivity of Oxygen Adspecies on Platinum Surface

The adsorption and mobility of oxygen adspecies on platinum (Pt) surface are crucial for the oxidation of surface-absorbed carbon monoxide (CO), which causes the deactivation of Pt catalyst in fuel cells. By employing nanoelectrode and ultramicroelectrode techniques, we have observed the surface mobility of oxygen adspecies produced by the dissociative adsorption of H₂O and the surface reaction between the oxygen adspecies and the preadsorbed CO on the Pt surface. The desorption charge of oxygen adspecies on a Pt nanoelectrode has been found to be in proportion to the reciprocal of the square root of scan rate. Using this information, the apparent surface diffusion coefficient of oxygen adspecies has been determined to be (5.61 ± 0.84) × 10^–10 cm²/s at 25 °C. During the surface oxidation of CO, two current peaks are observed, which are attributed to CO oxidation at the Pt/electrolyte interface and the surface mobility of the oxygen adspecies on the adjacent Pt surface, respectively. These results demonstrate that the surface mobility of oxygen adspecies plays an important role in the antipoisoning and reactivation of Pt catalyst

Biography of Zhao-Wu Tian

Biography of Zhao-Wu Tia

Structural and Charge Sensitivity of Surface-Enhanced Raman Spectroscopy of Adenine on Silver Surface: A Quantum Chemical Study

The interaction of adenine with silver surfaces has been investigated using density functional method. Two isomers of N9H and N7H were included to model surface species. Considering the complexity of silver surfaces in surface-enhanced Raman spectroscopy, neutral and positive silver clusters were used to mimic the substrate. Following the bonding principle, we consider adenine-approached silver clusters in different configurations and their relation to the Raman spectra. For neutral adenine Ag_<i>n</i> (<i>n</i> = 4, 7, and 9) complexes, N9H–Ag_<i>n</i> complexes are more stable than N7H–Ag_<i>n</i> ones. The corresponding Raman spectra strongly depended on the structure of adenine and the adsorption sites. Moreover, we find N7H interacts with one positively charged silver cluster via N3 and N9 at the same time as the most stable surface complex, which can reproduce the experimental surface Raman spectra of adenine well on silver surfaces

Solvent Effect and Hydrogen Bond Interaction on Tautomerism, Vibrational Frequencies, and Raman Spectra of Guanine: A Density Functional Theoretical Study

Stable structures and Raman spectra of guanine have been investigated by density functional theory (DFT). Focusing on solvent effect and hydrogen bonding interaction, we have calculated the two keto-amino tautomers G17K and G19K as well as their guanine–water complexes and tetramers. The results show G17K is more stable than G19K in the gas phase, whereas in polar solvents G19K dominates. The vibrational fundamentals of G17K have been reassigned based on normal-mode analysis, since the previous assignment was limited to the G19K only. In the Raman spectra, the modes of the ring breathing vibration and those in the fingerprint region (from 1000 to 1600 cm^–1) affected by the solvent effect and the hydrogen bonding interaction dramatically. The band at 1163 cm^–1 of G17K in gas has a large blue shift when water molecule forms hydrogen bonds with N₇–H₁₆ and C₆O₁₃ sites. The blue shift can be explained by the influence of hydrogen bonding interaction along with shortening the N₁–C₆ bond distance. In addition, the dominant existing tautomer in polycrystalline and powder guanine is proposed to be G17K, whose calculated vibrational frequencies agree with the experimental Raman spectra reported before

Plasmonic Smart Dust for Probing Local Chemical Reactions

Locally probing chemical reactions or catalytic processes on surfaces under realistic reaction conditions has remained one of the main challenges in materials science and heterogeneous catalysis. Where conventional surface interrogation techniques usually require high-vacuum conditions or ensemble average measurements, plasmonic nanoparticles excel in extreme light focusing and can produce highly confined electromagnetic fields in subwavelength volumes without the need for complex near-field microscopes. Here, we demonstrate an all-optical probing technique based on plasmonic smart dust for monitoring local chemical reactions in real time. The silica shell-isolated gold nanoparticles that form the smart dust can work as strong light concentrators and optically report subtle environmental changes at their pinning sites on the probed surface during reaction processes. As a model system, we investigate the hydrogen dissociation and subsequent uptake trajectory in palladium with both “dust-on-film” and “film-on-dust” platforms. Using time-resolved single particle measurements, we demonstrate that our technique can in situ encode chemical reaction information as optical signals for a variety of surface morphologies. The presented technique offers a unique scheme for real-time, label-free, and high-resolution probing of local reaction kinetics in a plethora of important chemical reactions on surfaces, paving the way toward the development of inexpensive and high-output reaction sensors for real-world applications