Reinvestigation of the β‑to‑α Crystal Phase Transition of Poly(butylene adipate) by the Time-Resolved X‑ray Scattering and FTIR Spectral Measurements in the Temperature-Jump Process

Poly­(butylene adipate) (PBA) exhibits the two types of crystal modification, the α and β forms, depending on the sample preparation conditions. They show the different degree of biodegradability. A majority of papers published so far reported that the phase transition from the β-form to the α-form occurs as the direct solid-to-solid process when the sample is heated up to the high temperature of around 55 °C. We have reinvestigated this β-to-α phase transition by performing the temperature-jump time-resolved measurement of the FTIR, WAXD, and SAXS measurements. This transition has been found to be not a solid-to-solid phase transition but the combined phenomena of the melting of the β-phase followed by the recrystallization to the high-temperature α-phase

Sequential Identification of Model Parameters by Derivative Double Two-Dimensional Correlation Spectroscopy and Calibration-Free Approach for Chemical Reaction Systems

A sequential identification approach by two-dimensional (2D) correlation analysis for the identification of a chemical reaction model, activation, and thermodynamic parameters is presented in this paper. The identification task is decomposed into a sequence of subproblems. The first step is the construction of a reaction model with the suggested information by model-free 2D correlation analysis using a novel technique called derivative double 2D correlation spectroscopy (DD2DCOS), which enables one to analyze intensities with nonlinear behavior and overlapped bands. The second step is a model-based 2D correlation analysis where the activation and thermodynamic parameters are estimated by an indirect implicit calibration or a calibration-free approach. In this way, a minimization process for the spectral information by sample–sample 2D correlation spectroscopy and kinetic hard modeling (using ordinary differential equations) of the chemical reaction model is carried out. The sequential identification by 2D correlation analysis is illustrated with reference to the isomeric structure of diphenylurethane synthesized from phenylisocyanate and phenol. The reaction was investigated by FT-IR spectroscopy. The activation and thermodynamic parameters of the isomeric structures of diphenylurethane linked through a hydrogen bonding equilibrium were studied by means of an integration of model-free and model-based 2D correlation analysis called a sequential identification approach. The study determined the enthalpy (Δ<i>H</i> = 15.25 kJ/mol) and entropy (<i>T</i>Δ<i>S</i> = 13.20 kJ/mol) of CO···H hydrogen bonding of diphenylurethane through direct calculation from the differences in the kinetic parameters (δΔ^⧧<i>H</i>, −<i>T</i>δΔ^⧧<i>S</i>) at equilibrium in the chemical reaction system

Solvated States of Poly‑l‑alanine α‑Helix Explored by Raman Optical Activity

Raman optical activity (ROA) reveals surprising details of the secondary structure of polypeptides and proteins in solution phase. Yet specific spectral features, such as in the extended amide III region of hydrated α-helix, did not seem explicable by the generally accepted sensitivity of ROA to the local conformation. This is reconciled in the present study by simulations of ROA spectra for model α-helical structures. Two positive ROA peaks often observed at around 1340 and 1300 cm^–1 for polypeptides and proteins have been assigned to two types of solvated α-helices; one is stable in hydrophilic environment where amide groups make hydrogen bonds to solvent molecules or polar side chains (∼1340 cm^–1), and the other is supported by a hydrophobic environment without the possibility of external hydrogen bonds (∼1300 cm^–1). For poly-l-alanine (PLA), regarded as a good model of α-helical structure, the experimentally observed relative intensity ratio of the two ROA bands has been explained by a conformational equilibrium depending on the solvent polarity. The intensities of the bands reflect solvated and unsolvated α-helical geometries, with peptide backbone torsional angles (ϕ_<i>i</i>+1, ψ_i) of (−66°, −41°) and (−59°, −44°), respectively. Quantum-mechanical simulations of the ROA spectra utilizing the normal mode optimization and Cartesian tensor transfer methods indicate, however, that the change in dielectric constant of the solvent is the main factor for the spectral intensity change, whereas the influence of the conformational change is minor

Surface-Enhanced Phosphorescence Measurement by an Optically Trapped Colloidal Ag Nanoaggregate on Anionic Thiacarbocyanine H‑Aggregate

A citrate-reduced Ag nanoaggregate was optically trapped on a fiber-shaped H-aggregate of an anionic thiacarbocyanine dye against Coulomb repulsion by focusing a near-infrared (NIR) laser beam. As the NIR laser power increased, namely, as the Ag nanoaggregate approaches the H-aggregate, phosphorescence from the H-aggregate with the Ag nanoaggregate excited moderately at 514 and 647 nm was strengthened, although that at 568 nm was weakened. By excitation at 568 nm, which was close to a surface plasmon resonance peak of the Ag nanoaggregate, surface-plasmon-enhanced optical trapping potential well might have deepened, and then the Ag nanoaggregate might have approached the H-aggregate too closely to enhance the phosphorescence because of energy transfer to the metal. As the excitation laser intensity increased, namely, as the surface-plasmon-enhanced optical trapping potential well was deepened, the phosphorescence enhancement factor trended upward and then downward by enhancement due to plasmon at a close distance from the Ag surface and the energy transfer at the closer distance, respectively

Effects of Hydrogen Bond Intermolecular Interactions on the Crystal Spherulite of Poly(3-hydroxybutyrate) and Cellulose Acetate Butyrate Blends: Studied by FT-IR and FT-NIR Imaging Spectroscopy

The crystal melting behaviors of poly­(3-hydroxybutyrate) (PHB) and cellulose acetate butyrate (CAB) blends were studied using infrared (IR) and near-infrared (NIR) imaging, which provided information about spherulite growth in dynamic blend systems. By analyzing the changes in the IR and NIR imaging spectra in the regions of the first and second overtones of the CO stretching vibrations of PHB and CAB, the evolution of heterogeneous spherulite during the time-resolved isothermal crystallization process was explored. Time-resolved IR and NIR imaging and polarized microscopic studies detected the PHB domains are able to separate from the PHB/CAB blends early in the process. Principal component analysis (PCA) was used to classify the distribution of the different morphologies of spherulite. The first principal component suggests that the discrimination of the imaging spectra relies largely upon the crystallinity, while the second principal component indicates the variations in the amorphous portion of PHB, the CAB contents, and the intermolecular hydrogen bonding of PHB and CAB. The PC1–PC2 scores of different parts of the spherulite suggest that the areas of low crystallinity in the blend spherulite contain both PHB and CAB

Electronic Transitions of Protonated and Deprotonated Amino Acids in Aqueous Solution in the Region 145–300 nm Studied by Attenuated Total Reflection Far-Ultraviolet Spectroscopy

The electronic transitions of 20 naturally occurring amino acids in aqueous solution were studied with attenuated total reflection far-ultraviolet (ATR-FUV) spectroscopy in the region from 145 to 300 nm. From the measured ATR spectra of sample solutions, the FUV absorption spectra attributed to the amino acids were separated from the intense solvent absorption by using a modified Kramers–Kronig transformation method. The FUV absorption spectra of the amino acids reflect the protonation states of the backbone and side-chain structures. The contributions of the side chains to the spectra were also examined from the difference spectra subtracting the corresponding Gly spectrum from each spectrum. The observed spectra were compared mostly with the electronic transition studies of the molecular fragments of the amino acids in gas phase. The FUV spectra of the amino acids exhibited the intra- and intermolecular electronic interactions of the solute–solute as well as the solute–solvent, and those are essential factors to elucidate UV photochemical processes of the amino acids in aqueous solution

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

The first electronic transition (<i>Ã</i> ← <i>X̃</i>) 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 <i>Ã</i> ← <i>X̃</i> 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

Elucidating Electronic Transitions from σ Orbitals of Liquid <i>n-</i> and Branched Alkanes by Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations

Attenuated total reflection far-ultraviolet (ATR-FUV) spectra containing Rydberg states of <i>n-</i>alkanes (C_<i>m</i>H_2<i>m</i>+2; <i>m</i> varies in the range 5–9) and branched alkanes observed in the liquid phase were investigated by quantum chemical calculations with the aim of elucidating electronic transitions from σ orbitals of liquid <i>n-</i> and branched alkanes. New assignments are proposed based on the time-dependent density functional theory (TD-DFT) and symmetry-adapted cluster configuration interaction (SAC-CI) calculations, and the differences in these spectra are analyzed in detail. The FUV spectra of <i>n-</i>alkanes show a broad asymmetric feature near 8.3 eV. The strong band at ∼8.3 eV shows a red shift with a significant increase in intensity as the carbon chain length increases, which is attributed to the overlapping transitions from the third (or fourth) highest occupied molecular orbitals HOMO–2 (or HOMO–3) and HOMO–1 to Rydberg 3p_<i>y</i> by the TD-DFT and SAC-CI calculations. This band was previously assigned to the overlap of two peaks arising from the transition from the HOMO to 3p and from the HOMO–1 to 3s based on their term values. Although the most intense transition, T1, is from HOMO-2 for <i>m</i> = 5 and 6 and HOMO–3 for <i>m</i> varying in the range of 7–9, the shape of Kohn–Sham molecular orbital for T1 is similar among the all-alkanes investigated. The theoretical result also has demonstrated that the red shift originates in both stabilization of the Rydberg 3p_<i>y</i> and destabilization of the occupied orbitals. The intensity of the shoulder at 7.7 eV drastically increases in the spectra of the branched alkanes, especially for those with quaternary carbon atoms such as 2,2-dimethyl butane. This increase in intensity is caused by a reduction in symmetry in the branched alkanes, which leads the forbidden transitions to Rydberg 3s to allowed transitions. In this way, the present study has provided new insight into the existence of their Rydberg transitions and the shape of the relevant MOs of the transitions

Generation of Pronounced Resonance Profile of Charge-Transfer Contributions to Surface-Enhanced Raman Scattering

A chemically enhanced mechanism of surface-enhanced Raman scattering (SERS) was investigated using a series of metal-charge-transfer (CT) complex systems fabricated by a self-assembly method. The developed Ag/4-mercaptophenols (MPH)/<i>n</i>-TiO₂ system presented layer number-dependent SERS spectra. By using the electron density values of the Ag₁₃/MPH and Ag₁₃/MPH/TiO₂ system calculated using the density functional theory (DFT) and by using these values in combination with the results of our previous investigations on the mechanism of the Ag/MPH/TiO₂ system, the absorption threshold of the CT complexes was clearly defined. The degree of CT was selected to study the layer number-dependent SERS spectra. Based on the layer number-dependent SERS data, it has been inferred that the degree of CT represents a resonance phenomenon. In addition, the CT resonance occurs at higher energy in the Ag/MPH/<i>n</i>-TiO₂ system than in the monolayer TiO₂ system owing to the blue-shift of CT states with the continuous introduction of TiO₂. Thus, we provide a good example of the use of a CT complex system to investigate the chemical mechanism of SERS

Temperature Drift of Conformational Equilibria of Butyl Alcohols Studied by Near-Infrared Spectroscopy and Fully Anharmonic DFT

Conformational isomerism of aliphatic alcohols with respect to the internal rotation of C–O­(H) group and its impact on near-infrared (NIR) spectra has been known in the literature. However, no attempt has ever been made to investigate systematically whether and how the conformational flexibility of the aliphatic chain determines the observed NIR data of aliphatic alcohols. In the present study NIR spectra of four kinds of butyl alcohols, 1-butanol, 2-butanol, isobutanol, and <i>tert</i>-butyl alcohol, were investigated in diluted (0.1 M) CCl₄ solutions. The experimental NIR spectra of butyl alcohols were accurately reproduced and explained in a fully anharmonic DFT study by means of generalized second-order vibrational perturbation theory (GVPT2). Entire conformational populations were taken into account in each case. On the basis of the theoretical study, influences of conformational flexibility with respect to internal rotations not only about the C–O bond, but also about the C–C bonds have been well evidenced in the experimental spectra. The conformational isomerism affects significantly the shape of NIR spectra. The temperature-dependent NIR spectra of butyl alcohols show changes in the band shape and a blue-shift of the overtone band due to the stretching mode of free OH group, and its intensity decreases with increasing temperature. These effects can be closely monitored by two-dimensional correlation spectroscopy (2D-COS). In this work, the experimental 2D-COS patterns have been successfully reproduced, based on DFT calculated NIR spectra of conformational isomers of the studied molecules and their Boltzmann coefficients over the corresponding temperature range. Thus, the experimentally observed effects are fully reflected in the DFT study, which leads to the conclusion that the main factor in the temperature-dependent spectral changes of 2ν_OH band of aliphatic alcohols in the diluted phase, where no self-association occurs, is played by the changes in the relative population of their conformational isomers