Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

Cryogenic ion trap vibrational spectroscopy paired with quantum chemistry currently represents the most generally applicable approach for the structural investigation of gaseous cluster ions that are not amenable to direct absorption spectroscopy. Here, we give an overview of the most popular variants of infrared action spectroscopy and describe the advantages of using cryogenic ion traps in combination with messenger tagging and vibrational predissociation spectroscopy. We then highlight a few recent studies that apply this technique to identify highly reactive ionic intermediates and to characterize their reactive sites. We conclude by commenting on future challenges and potential developments in the field.     

On the Possibility of Using UV Spectroscopy as a Measure of the Low-Barrier Hydrogen Bond

In many enzyme-catalyzed biochemical pathways, a short, strong hydrogen bond between an enzyme and substrate is an important structural feature. These bonds are termed low-barrier hydrogen bonds. In this paper, we show that UV spectra can be used as an experimental technique to determine if a system contains a low-barrier hydrogen bond (LBHB). We simulate, using the time-dependent view of UV spectroscopy, several different UV spectra: absorption, photodissociation, and emission, on systems containing a low-barrier hydrogen bond. We find several distinguishing spectral features in these UV spectra for systems that possess a LBHB.     

Diclofenac Ion Hydration: Experimental and Theoretical Search for Anion Pairs

Self-assembly of organic ions in aqueous solutions is a hot topic at the present time, and substances that are well-soluble in water are usually studied. In this work, aqueous solutions of sodium diclofenac are investigated, which, like most medicinal compounds, is poorly soluble in water. Classical MD modeling of an aqueous solution of diclofenac sodium showed equilibrium between the hydrated anion and the hydrated dimer of the diclofenac anion. The assignment and interpretation of the bands in the UV, NIR, and IR spectra are based on DFT calculations in the discrete-continuum approximation. It has been shown that the combined use of spectroscopic methods in various frequency ranges with classical MD simulations and DFT calculations provides valuable information on the association processes of medical compounds in aqueous solutions. Additionally, such a combined application of experimental and calculation methods allowed us to put forward a hypothesis about the mechanism of the effect of diclofenac sodium in high dilutions on a solution of diclofenac sodium.     

Vibrational Spectroscopy of Ion Irradiated Carbon Containing Macromolecules

A large variety of carbon rich macromolecules has been studied in our laboratory, before, during, and after ion irradiation (3-60 keV ions), by in situ IR and Raman spectroscopies. In this paper we present results obtained on: (a) Films of frozen H₂O:CH₄:N₂ mixtures that have been deposited at low temperature (12 K) on a silicon substrate, and irradiated with fast ions. Irradiation causes the formation of organic refractory residues whose spectra have been recorded and studied also after further irradiation at room temperature. (b) Pentacene (a Polyciclic Aromatic Hydrocarbon, PAH) whose Raman spectra have been studied also during ion irradiation at room temperature. (c) Thin samples of fullerene (C₆₀) films also irradiated at room temperature. Here we discuss some of the most relevant results and compare the spectra of the very different samples that after prolonged irradiation, evolve towards amorphous carbons that have common spectral characteristic. The results are discussed in terms of their astrophysical relevance. It is demonstrated that frozen hydrocarbons, PAHs and fullerenes are rapidly converted to amorphous carbon in space. Amorphous carbon has to be a widespread material in many astrophysical environments.     

拉曼光谱在锂离子电池研究中的应用

综述拉曼光谱(Raman spectroscopy)在锂离子电池碳负极材料、尖晶石LiMn2O4和LiFePO4正极材料、聚合物和室温熔盐电解质以及电极/电解质界面膜研究中的应用,分析了非原位拉曼测试手段与原位拉曼测试手段的优缺点,展望了这一领域目前有待解决的问题和可能应用的新技术.     

磁场作用下葡萄糖近红外光谱检测技术研究

采用傅立叶变换红外光谱仪(FTIR)研究了磁场强度对葡萄糖溶液近红外光谱的影响,发现磁场作用下葡萄糖的近红外光谱吸收强度和部分峰位发生显著变化。分析了磁场对葡萄糖溶液近红外光谱吸收的影响机理。采用偏最小二乘回归法(PLS)建立了磁场作用下葡萄糖溶液的定量分析模型,使用验证集进行验证。研究结果表明,磁场对葡萄糖分子基团偶极矩产生诱导作用,使偶极矩增大,吸收增强;同时磁场作用下,葡萄糖分子趋于沿平行于磁场的方向排列,其基团振动频率(特征吸收峰)吸光度与浓度变化的线性关系得到极大的改善。该研究有助于提高葡萄糖分子吸收强度及其测量精度,为进一步提高血糖检测精度提供技术支持。     

Study of Molecular Orientation by Vibrational Spectroscopy: From Polymers to Silk

Infrared and Raman spectroscopies are very efficient techniques to characterize molecular orientation in macromolecular systems. In the present paper, two examples of the application of vibrational spectroscopy to the study of molecular orientation in synthetic and natural macromolecules will be presented. In the first example, the dynamics of orientation and relaxation of stretched films of bimodal blends of polystyrene (PS) and deuterated polystyrene (dPS) has been studied in situ by polarization modulation infrared linear dichroism while, in the second one, polarized Raman microspectroscopy has been used to determined quantitatively the orientation of β-sheet domains in single filaments of Bombyx mori silk.     

Structure and Fluxionality of B13+ Probed by Infrared Photodissociation Spectroscopy

We use cryogenic ion vibrational spectroscopy to characterize the structure and fluxionality of the magic number boron cluster B₁₃⁺. The infrared photodissociation (IRPD) spectrum of the D₂‐tagged all‐¹¹B isotopologue of B₁₃⁺ is reported in the spectral range from 435 to 1790 cm⁻¹ and unambiguously assigned to a planar boron double wheel structure based on a comparison to simulated IR spectra of low energy isomers from density‐functional‐theory (DFT) computations. Born–Oppenheimer DFT molecular dynamics simulations show that B₁₃⁺ exhibits internal quasi‐rotation already at 100 K. Vibrational spectra derived from these simulations allow extracting the first spectroscopic evidence from the IRPD spectrum for the exceptional fluxionality of B₁₃⁺.     

A Vibrational Spectroscopic Study of Ion Solvation in Lithium Perchlorate/Propylene Carbonate Electrolyte

Abstract

The infrared (IR) and Raman spectra of propylene carbonate (PC) containing various concentrations of LiClO₄ have been measured and analyzed. The difference in spectra of PC with and without LiClO₄ was attributed to the interaction of the PC molecules and lithium ions. This interaction occurs mainly on the carbonyl oxygen atom of the PC molecule. The ring deformation, symmetric ring deformation, carbonyl stretching and stretching of ring oxygens for PC are sensitive to this interaction. The solvation number of Li⁺ is also calculated. On the other hand, the structure of the ClO^? ₄ is also affected by PC molecule, forming the solvent separated ion pairs.     

Vibrational spectroscopic study of hydrated uranyl oxide: Curite

Raman and infrared spectra of the uranyl oxyhydroxide hydrate: curite is reported. Observed bands are attributed to the (UO₂)²⁺ stretching and bending vibrations, U–OH bending vibrations, H₂O and (OH)⁻ stretching, bending and librational modes. U–O bond lengths in uranyls and O–H…O bond lengths are calculated from the wavenumbers assigned to the stretching vibrations. These bond lengths are close to the values inferred and/or predicted from the X-ray single crystal structure. The complex hydrogen-bonding network arrangement was proved in the structures of the curite minerals. This hydrogen bonding contributes to the stability of these uranyl minerals.     

Theoretical infrared line shapes of H‐bonds within the strong anharmonic coupling theory and Fermi resonances effects

In this article, we extend a previous work toward presenting a theoretical study of the effects of Fermi resonances and the fundamental anharmonic coupling parameter α between the high‐frequency mode and the H‐bond bridge. The model incorporates (i) both intrinsic anharmonicities of the fast mode (double well potential) and the H‐bond Bridge (Morse potential), (ii) strong anharmonic coupling theory, (iii) Fermi resonances by the aid of an anharmonic coupling between the fast mode and one or several harmonic bending modes, (iv) quadratic modulation of both the angular frequency and the equilibrium position of the X? …Y stretching mode on the intermonomer ? H… motions, and (v) the quantum direct (fast and bending modes) and indirect dampings (slow mode). The IR spectral density is obtained by Fourier transform of the autocorrelation function of the transition dipole moment operator of the X? H bond. The numerical calculation shows that Fermi resonances generate very complicated profiles with multisubstructure and also provide a direct evidence of Fermi resonances which were predicted to be a major feature of H‐bonds. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Self-Metalation of Anchored Porphyrins on Atomically Defined Cobalt Oxide Surfaces: In situ Studies by Surface Vibrational Spectroscopy

Metalation of anchored porphyrins is essential for their functionality at hybrid interfaces. In this work, we have studied the anchoring and metalation of a functionalized porphyrin derivative, 5-(4-carboxyphenyl)-10,15,20-triphenylporphyrin (MCTPP), on an atomically-defined CoO(100) film under ultrahigh vacuum (UHV) conditions. We follow both the anchoring to the oxide surface and the self-metalation by surface Co²⁺ ions via infrared reflection absorption spectroscopy (IRAS). At 150 K, MCTPP multilayer films adsorb molecularly on CoO(100) without anchoring to the surface. Upon heating to 195 K, the first layer of porphyrin molecules anchors via formation of a bridging surface carboxylate. Above 460 K, the MCTPP multilayer desorbs and only the anchored monolayer resides on the surface up to temperatures of 600 K approximately. The orientation of anchored MCTPP depends on the surface coverage. At low coverage, the MCTPP adopts a nearly flat-lying geometry, whereas an upright standing film is formed near the multilayer coverage. Self-metalation of MCTPP depends critically on the surface temperature, the coverage and on the molecular orientation. At 150 K, metalation is largely suppressed, while the degree of metalation increases with increasing temperature and reaches a value of around 60 % in the first monolayer at 450 K. At lower coverage higher metalation fractions (85 % and above) are observed, similar as for increasing temperature.     

Vibrational Spectroscopy of the Dehydrogenated Uracil Radical by Autodetachment of Dipole‐Bound Excited States of Cold Anions

Molecules with large enough dipole moments can bind an electron by the dipole field, which has little effect on the molecular core. A molecular anion can be excited to a dipole‐bound state, which can autodetach by vibronic coupling. Autodetachment spectroscopy of a complex anion cooled in a cryogenic ion trap is reported. Vibrational spectroscopy of the dehydrogenated uracil radical is obtained by a dipole‐bound state with partial rotational resolution. Fundamental frequencies for 21 vibrational modes of the uracil radical are reported. The electron affinity of the uracil radical is measured accurately to be 3.4810±0.0006 eV and the binding energy of the dipole‐bound state is measured to be 146±5 cm^?1. The rotational temperature of the trapped uracil anion is evaluated to be 35 K.     

Vibrational Circular Dichroism Spectroscopy of Three Multidentate Nitrogen Donor Ligands: Conformational Flexibility and Solvent Effects

A series of multidentate nitrogen donor ligands have been synthesized and characterized and their conformational distributions in solution have been investigated. Vibrational absorption (VA) and vibrational circular dichroism (VCD) spectroscopy, complemented with DFT calculations, have been used to probe the conformations of these important ligands in solution directly. These three ligands demonstrate very different conformational flexibility; the pyridine subunits and amine groups may adopt a number of different conformations. Experimental VA and VCD data measured in CDCl₃ have been compared to the theoretical spectra of all possible most stable conformers. Solvent effects have been taken into account by using the implicit polarizable continuum model and explicit solvation model. The explicit hydrogen‐bonding solvation model is important for explaining the VCD sign‐reverse phenomenon in the amide I region. Good agreement has been achieved between experimental and predicted spectra for all three ligands; thus allowing detailed examination of the related conformational structures and distributions in solution.