ATR-far-ultraviolet spectroscopy: a challenge to new σ chemistry

This review reports the recent progress on ATR-far ultraviolet (FUV) spectroscopy in the condensed phase. ATR-FUV spectroscopy for liquids and solids enables one to explore various topics in physical chemistry, analytical chemistry, nanoscience and technology, materials science, electrochemistry, and organic chemistry. In this review, we put particular emphasis on the three major topics: (1) studies on electronic transitions and structures of various molecules, which one cannot investigate via ordinary UV spectroscopy. The combined use of ATR-FUV spectroscopy and quantum chemical calculations allows for the investigation of various electronic transitions, including σ, n-Rydberg transitions. ATR-FUV spectroscopy may open a new avenue for σ-chemistry. (2) ATR-FUV spectroscopy enables one to measure the first electronic transition of water at approximately 160 nm without peak saturation. Using this band, one can study the electronic structure of water, aqueous solutions, and adsorbed water. (3) ATR-FUV spectroscopy has its own advantages of the ATR method as a surface analysis method. ATR-FUV spectroscopy is a powerful technique for exploring a variety of top surface phenomena (∼50 nm) in adsorbed water, polymers, graphene, organic materials, ionic liquids, and so on. This review briefly describes the principles, characteristics, and instrumentation of ATR-FUV spectroscopy. Next, a detailed description about quantum chemical calculation methods for FUV and UV regions is given. The recent application of ATR-FUV-UV spectroscopy studies on electronic transitions from σ orbitals in various saturated molecules is introduced first, followed by a discussion on the applications of ATR-FUV spectroscopy to studies on water, aqueous solutions, and adsorbed water. Applications of ATR-FUV spectroscopy in the analysis of other materials such as polymers, ionic liquids, inorganic semiconductors, graphene, and carbon nanocomposites are elucidated. In addition, ATR-FUV-UV-vis spectroscopy focusing on electrochemical interfaces is outlined. Finally, FUV-UV-surface plasmon resonance studies are discussed.

Observation of Solid-Liquid Phase Transitions of Brine Using the Far-Ultraviolet Charge-Transfer-to-Solvent Band

Although determining the chemical states of salts and ions is critical in numerous fields, such as elucidating biological functions and maintaining food quality, the current direct observation methods are insufficient. We propose a spectral analysis method of directly observing the phase transitions of NaCl solutions using the changes in the charge-transfer-to-solvent band and the absorption band representing the first electron transition (Ã ← X̃) of H₂O. The intensities of these bands may be observed using attenuated total reflection far-ultraviolet spectroscopy. According to the well-known phase diagram of aqueous NaCl, we observe spectral changes during freezing-thawing and may spectroscopically detect the phase transitions from liquid to mixed liquid-solid and solid phases, including eutectic crystals, in addition to their coexistence curves.

Attenuated Total Reflection Far-Ultraviolet (ATR-FUV) Spectroscopy is a Sensitive Tool for Investigation of Protein Adsorption

Attenuated total reflection far-ultraviolet (ATR-FUV) spectra in the 145-250 nm region were studied for four kinds of proteins (two α-helix-rich proteins: bovine serum albumin (BSA) and lysozyme and two β-sheet rich proteins: concanavalin A and γ-globulin) in different solutions (pure water and phosphate buffered saline, or PBS) with different concentrations. All the spectra show a band at 191 nm due to the π-π^* transition of amide bonds of the proteins. The wavelength of the band does not change with their second structures, suggesting that the corresponding electronic transition mode is localized and polarized in the direction that is not affected by the difference in the peptide folding. The intensity of the 191 nm band differs with the concentration of salt in the solution, suggesting that the band intensity reflects the adsorption density of a protein on the internal reflection element (IRE) made of a sapphire glass prism. According to the intensity changes of the band at 191 nm, it is revealed that the properties in adsorption are different from one protein to another. It is assumed that there are two types of forces on the protein adsorption: one is that among the molecules and the other is that between a molecule and a substrate. The origin of force includes localized electrostatic polarity and affinity to water. The ions in the solvent give a marked effect on these forces, resulting in the difference in the response to adsorption density against the salt concentration in the solvent.

Local structures and dynamics of interfacial imidazolium-based ionic liquid depending on the electrode potential using electrochemical attenuated total reflectance ultraviolet spectroscopy

Recently, ionic liquids (ILs) have attracted attention as prospective electrolytes for Li-ion batteries, with safe performance. Herein, the dynamics of the IL at the electrochemical interface, which is the key to the electrochemical reaction, was monitored using attenuated total reflectance far- and deep-ultraviolet (ATR-FUV-DUV) spectroscopy. An original measurement system, which combined an ATR-FUV-DUV spectrometer with a Kretschmann type (fully metal-coated prism) electrochemical setup, was assembled. Spectral measurements and assignments were performed for the 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([EMIM][TFSI])/Pt electrode (∼7 nm) interface. The incident light in the FUV and DUV regions entered a measurement system comprising an [EMIM][TFSI]/Pt electrode/ATR sapphire prism, and the potential-dependent absorption spectra were measured in the 180-450 nm range. This in-situ spectroscopic technique is unique in that the electronic transition spectra of the interfacial IL can be obtained. By switching the applied potentials, temporal spectral changes (i.e. relaxation signals) were tracked at wavelengths of 450 nm and 221 nm, where the direct electronic absorption of the IL was active and inactive, respectively. Comparing these relaxation times, it was revealed that the absorption signal at 221 nm changed more slowly than that at 450 nm. This indicated that the molecular conformations that affected the electronic absorption of the interfacial ILs changed slowly. Considering the surface-normal dipole selection rule for molecules on a metal surface, it is suggested that the slow changes in the molecular conformations can be ascribed to the potential-dependent interfacial orientations of [EMIM]⁺.

Advances, challenges and perspectives of quantum chemical approaches in molecular spectroscopy of the condensed phase

The purpose of this review is to demonstrate advances, challenges and perspectives of quantum chemical approaches in molecular spectroscopy of the condensed phase. Molecular spectroscopy, particularly vibrational spectroscopy and electronic spectroscopy, has been used extensively for a wide range of areas of chemical sciences and materials science as well as nano- and biosciences because it provides valuable information about structure, functions, and reactions of molecules. In the meantime, quantum chemical approaches play crucial roles in the spectral analysis. They also yield important knowledge about molecular and electronic structures as well as electronic transitions. The combination of spectroscopic approaches and quantum chemical calculations is a powerful tool for science, in general. Thus, our article, which treats various spectroscopy and quantum chemical approaches, should have strong implications in the wider scientific community. This review covers a wide area of molecular spectroscopy from far-ultraviolet (FUV, 120-200 nm) to far-infrared (FIR, 400-10 cm-1)/terahertz and Raman spectroscopy. As quantum chemical approaches, we introduce several anharmonic approaches such as vibrational self-consistent field (VSCF) and the combination of periodic harmonic calculations with anharmonic corrections based on finite models, grid-based techniques like the Numerov approach, the Cartesian coordinate tensor transfer (CCT) method, Symmetry-Adapted Cluster Configuration-Interaction (SAC-CI), and the ZINDO (Semi-empirical calculations at Zerner's Intermediate Neglect of Differential Overlap). One can use anharmonic approaches and grid-based approaches for both infrared (IR) and near-infrared (NIR) spectroscopy, while CCT methods are employed for Raman, Raman optical activity (ROA), FIR/terahertz and low-frequency Raman spectroscopy. Therefore, this review overviews cross relations between molecular spectroscopy and quantum chemical approaches, and provides various kinds of close-reality advanced spectral simulation for condensed phases.

ATR-far-ultraviolet spectroscopy in the condensed phase-The present status and future perspectives

Far-ultraviolet (FUV) spectroscopy in the region of 140-200 nm of condensed-phase has received keen interest as a new electronic spectroscopy. The introduction of the attenuated total reflection (ATR) technique to the FUV region has opened a new avenue for FUV spectroscopy of liquids and solids. ATR-FUV spectroscopy enables the study of electronic structures and transitions of most types of molecules. It also holds great promise for a variety of applications, i.e., from the application to basic sciences to practical applications. In this review, the characteristics and advantages of ATR-FUV spectroscopy in the condensed phase are described first; then, a brief historical overview is provided. Next, the ATR-FUV spectroscopy instrumentation is outlined. After these introductory parts, a variety of AFT-FUV spectroscopy applications are introduced, starting from applications to investigations of electronic structure and transitions of alkanes, graphenes, and polymers. Then, time-resolved ATR-FUV spectroscopy is discussed. The applications to materials research, such as the research on inorganic semiconductors and ionic liquids, follow. In the last part, the FUV spectroscopy perspective is emphasized.

Attenuated total reflectance far-ultraviolet and deep-ultraviolet spectroscopy analysis of the electronic structure of a dicyanamide-based ionic liquid with Li+

Elucidating the unique electronic structure of ionic liquid molecules around Li⁺ using electronic absorption spectroscopy, theoretical calculations, and chemometric analyses.

The Intrinsic Stability of Metal Ion Complexes with Nanoparticulate Fulvic Acids

The electrostatic contributions to metal ion binding by fulvic acids (FAs) are characterized in light of recent theoretical developments on description of the net charge density of soft nanoparticles. Under practical electrolyte concentrations, the radius of the small, highly charged soft nanoparticulate FAs is comparable to the electrostatic screening length and their electric potential profile has a bell shape that extends into the surrounding aqueous medium. Consequently, accumulation of counterions in the extraparticulate zone can be significant. By comparison of experimentally derived Boltzmann partitioning coefficients with those computed on the basis of (i) the structural FA particle charge and (ii) the potential profile for a nanoparticulate FA entity equilibrated with indifferent electrolyte, we identify the thickness of the extraparticulate counter charge accumulation shell in 1-1 and 2-1 electrolytes. The results point to the involvement of counterion condensation phenomena and call into question the approaches for modeling electrostatic contributions to ion binding that are invoked by popular equilibrium speciation codes. Overall, the electrostatic contributions to Cd_aq²⁺ and Cu_aq²⁺ association with FA are weaker than those previously found for much larger humic acids (HA). The intrinsic chemical binding strength of CdFA is comparable to that of CdHA, whereas CuFA complexes are weaker than CuHA ones.

Study of first electronic transition and hydrogen bonding state of ultra-thin water layer of nanometer thickness on an α-alumina surface by far-ultraviolet spectroscopy

The first electronic transition (A˜←X˜) and the hydrogen bonding state of an ultra-thin water layer of nanometer thickness between two α-alumina surfaces (0.5-20nm) were studied using far-ultraviolet (FUV) spectroscopy in the wavelength range 140-180nm. The ultra-thin water layer of nanometer thickness was prepared by squeezing a water droplet (~1μL) between a highly polished α-alumina prism and an α-alumina plate using a high pressure clamp (~4.7MPa), and the FUV spectra of the water layer at different thicknesses were measured using the attenuated total reflection method. As the water layer became thinner, the A˜←X˜ bands were gradually shifted to higher or lower energy relative to that of bulk water; at thicknesses smaller than 4nm, these shifts were substantial (0.1-0.2eV) in either case. The FUV spectra of the water layer with thickness <4nm indicate the formation of structured ice-like hydrogen bond (H-bond) layers for the higher energy shifts or the formation of slightly weaker H-bond layers as compared to those in the bulk liquid state for lower energy shifts. In either case, the H-bond structure of bulk liquid water is nearly lost at thicknesses below 4nm, because of steric hydration forces between the α-alumina surfaces.

Systematic analysis of various ionic liquids by attenuated total reflectance spectroscopy (145–450 nm) and quantum chemical calculations

Electronic absorption spectra in 140–450 nm were investigated by attenuated total reflectance spectroscopy and quantum chemical calculations.

Changes in the Electronic States of Low-Temperature Solid n-Tetradecane: Decrease in the HOMO-LUMO Gap

Intermolecular interactions between alkyl chains such as CH···HC should be reflected in the phase behavior of organic compounds. We measured the attenuated total reflectance spectra in the far-UV region (145-300 nm) of n-tetradecane (T _m = 5.9 °C), through both cooling and heating, from 15 to -38 °C, to determine its temperature dependency. The chosen temperature range, with the minimum temperature much lower than the melting point of n-tetradecane, allowed for a detailed examination of the liquid-solid phase transition. With decreasing temperature, the absorption band at 153 nm in the liquid phase becomes weaker, and new bands appear at around 200 and 230 nm. Because the observed variations in the band intensities are reversible during both cooling and heating processes and the conversion from the 153 nm band to 200 and 230 nm bands takes place at the melting temperature, we concluded that these spectral changes are caused by changes in the electronic states because of the phase change. To understand the origin of this drastic change observed experimentally, we carried out time-dependent density functional theory calculations using a model dimer structure of n-pentane and periodic density functional theory calculations of the polyethylene crystal structure. Although the simulated absorption spectra of both the optimized and crystal structures did not explain the changes observed experimentally in the absorption spectra, compressed structures were found to be a potential explanation for the experimental findings. These results suggest that an unusually compressed structure might be generated on the surface at low temperatures, and this phase change, which is reversible, is responsible for the unusual absorption observed in the attenuated total reflection-far ultraviolet spectra.

Interpretation of the à ← X̃ transition of hydrated protons in aqueous solutions observed in the far-UV region with quantum chemical calculations

A substantial blue-shift of the first electronic transition band of liquid water with a H₂SO₄ concentration (0–14.4 M) observed in the far-UV region.

Electronic absorption spectra of imidazolium-based ionic liquids studied by far-ultraviolet spectroscopy and quantum chemical calculations

Electronic absorption spectra of imidazolium-based ionic liquids were studied by far- and deep-ultraviolet spectroscopy and quantum chemical calculations.

Far-ultraviolet spectroscopy of solid and liquid states: characteristics, instrumentation, and applications

Far-ultraviolet spectroscopy (≥200 nm) can greatly contribute to the basic science of electronic structures for almost all materials and their applications.

Surface Effect of Alumina on the First Electronic Transition of Liquid Water Studied by Far-Ultraviolet Spectroscopy

The first electronic transition (à ← X̃) of liquid water (H2O and D2O) on an α-alumina substrate was studied using variable angle attenuated total reflection far-ultraviolet (VA-ATR-FUV) spectroscopy in the wavelength region 140-180 nm (8.86-6.89 eV). A variation in the penetration depth of the evanescent wave of a probe light (25-19 nm) directly determined individual FUV spectra associated with bulk water (distance from the alumina surface >2 nm) and interfacial water (<2 nm). We found that the à ← X̃ band of the interfacial water was markedly blue-shifted and red-tailed relative to the bulk water. The electronic state difference of the interfacial water from the bulk water mainly arose from the hydrogen-bond structure and energy affected by the alumina surface.

Significant enhancement of photocatalytic activity of rutile TiO2compared with anatase TiO2upon Pt nanoparticle deposition studied by far-ultraviolet spectroscopy

The charge-separation efficiency of rutile TiO₂is considerably enhanced compared to that of anatase TiO₂upon the Pt nanoparticle deposition.