Raman spectra of ionic liquids: a simulation study of LaCl3 and its mixtures with alkali chlorides

Theoretical Raman spectra of the elpasolite-structured crystal Cs2NaLaCl6 and of molten mixtures of LaCl6 with NaCl and CsCl have been obtained from computer simulations in order to examine how the Raman spectra reflect the coordination structure around the La3+ ions. This system is a model for many other trivalent metal halides and for examining how the network structure of the pure melts is broken down by the addition of alkali halides with different structure-breaking powers. The results suggest a way of reconciling the conclusions of Raman studies about the structures of the melts with those of neutron and x-ray-diffraction studies, which have already been examined with the same simulation methods. The Raman spectra, both polarized and depolarized, are calculated from a model for the dependence of the polarizability of the system on the ionic coordinates which was inspired by electronic structure calculations of the polarizabilities of ions in a condensed phase environment. Some results on the lifetimes of the coordination complexes responsible for the appearance of the discrete Raman bands are discussed.     

Raman spectra of ionic liquids: a simulation study of AlF3 and its mixtures with NaF

Theoretical Raman spectra of the melts of NaF/AlF3 mixtures have been obtained from computer simulations in order to examine how the Raman spectra reflect the coordination structure around the Al3+ ions. The Raman spectra, both polarized and depolarized, are calculated from a model for the dependence of the polarizability of the system on the ionic coordinates which was inspired by electronic structure calculations of the polarizabilities of ions in a condensed-phase environment. The shapes of the spectra and their evolution with composition in the mixtures conform remarkably well to those seen experimentally, and we discuss the relationship between the bands seen in the spectra and the vibrational modes of the AlFn(3-n) coordination complexes which are found in the melts. Finally, we calculate quantities which relate to the degree of cross-linking between these coordination complexes and their lifetimes.     

Lithium intercalation of phenyl-capped aniline dimers: a study by photoelectron spectroscopy and quantum chemical calculations

Structural and electronic properties of pristine and lithium-intercalated, phenyl-capped aniline dimers as a model for the lithium-polyaniline system have been studied by photoelectron spectroscopy and quantum chemical calculations. It was found that the electronic structure of reduced and oxidized forms of oligoanilines is only weakly affected by isomerism. Upon intercalation, charge transfer from the Li-atoms is remarkable and highly localized at N-atomic sites, where configurations are energetically favored in which both N atoms of the dimers are bound to Li atoms. Conversion of nitrogen sites is different for the two forms of aniline dimers and incomplete up to high intercalation levels, indicating a pronounced role of solid-state effects in the formation of such compounds.     

Vibrational spectroscopy of cyclohexylaminoglutethimide in low-temperature matrices, solutions and the solid state

The vibrational spectra of cyclohexylaminoglutethimide (CHAG) molecules were studied in low-temperature Ar and N₂ matrices and in CCl₄, CHCl₃, CS₂ and CH₃CN solutions (at different concentrations). It was found that increasing the solute concentration in CCl₄, CS₂ and CHCl₃ solutions leads to the formation of dimers. On the other hand, in CH₃CN solution only the solute–solvent complexes occur.

The molecular structure and theoretical IR and Raman spectra of CHAG were predicted with the use of ab initio RHF/6-31G^** and density functional B3LYP/6-31G^** quantum mechanical methods. A comparative analysis of theoretical and experimental spectra of monomeric (isolated) species has led to the assignment of most of the absorption bands in terms of CHAG normal modes.

The IR and Raman spectra of crystalline samples (solid film and KBr pellet) were recorded as well. A reliable assignment of the IR and Raman spectra of crystalline CHAG was obtained with the help of theoretically simulated spectra and the results of our earlier investigations of aminoglutethimide and glutethimide vibrational spectra.     

反丁烯二酰基桥连的带不同取代基的卟啉二联体的合成与性质

合成了反丁烯二酰基桥连的3种带不同取代基的卟啉二联体, 通过红外光谱, 紫外-可见光谱, 核磁共振波谱和质谱对化合物的结构进行了确认, 并研究了二联体的表面光电压谱, 荧光光谱和激光拉曼光谱的变化. 结果表明, 取代基的类型对卟啉二联体分子的荧光量子产率有显著影响, 带供电子基团的甲氧基增强了荧光量子产率, 而带吸电子基团的氯则降低了荧光量子产率, 并且吸电子基团的氯比供电子基团的甲氧基对荧光的影响更大. 取代基的电子效应对卟啉二联体的荧光性和激光拉曼光谱有较大影响.     

Hydrogen bonding network in a chiral alcohol: (1R,2S,5R)-(−)-menthol. Conformational preference studied by IR–Raman–VCD spectroscopies and quantum chemical calculations

A study of the molecular structure of (1R,2S,5R)-(?)-menthol and the hydrogen bond networks formed by this species in solution is carried out. Molecular structures of monomers and H-bonded dimers and trimers of the title compound are optimized using quantum chemical calculations in the isolated molecule approach. In addition, IR, Raman and VCD techniques are used to study CCl₄ solutions and thin films of the target compound. Their corresponding vibrational spectra are then analysed, both theoretically (HF and DFT) and experimentally, to characterize the different monomers (rotamers) and H-bonded oligomer species in menthol solutions as a function of the concentration.     

带不同推电子基团二聚苯撑乙烯的电子结构与发光性能

用Wittig路线合成了一系列带不同推电子取代基的二聚苯撑乙烯,用紫外光谱和循环伏安方法测定其电子结构,并用量子化学计算方法对齐聚物的电子能级进行模拟.讨论了吸收光谱、发射光谱与生色团的电子结构的关系.     

Ph[PhC(O)NH]2P=NC(O)Ph: a new mesomeric stabilized azaylide. Synthesis, crystal structure and IR spectroscopic results

The structure of Ph[PhC(O)NH]₂P=NC(O)Ph (2) in solid state and in solution is discussed on the basis of a crystal structure analysis and IR and Raman spectra. In crystalline state 2 forms dimers which are associated via two pairs of bifurcated (N–H)₂O=C hydrogen bonds. The spectroscopic data are in good agreement with the crystallographic results with respect to the hydrogen bonding and they suggest that 2 also in solution is associated.     

Molecular Recognition in Glycolaldehyde,the Simplest Sugar: Two Isolated Hydrogen Bonds Win Over One Cooperative Pair

Carbohydrates are used in nature as molecular recognition tools. Understanding their conformational behavior upon aggregation helps in rationalizing the way in which cells and bacteria use sugars to communicate. Here, the simplest α-hydroxy carbonyl compound, glycolaldehyde, was used as a model system. It was shown to form compact polar C₂-symmetric dimers with intermolecular O–H⋅⋅⋅O=C bonds, while sacrificing the corresponding intramolecular hydrogen bonds. Supersonic jet infrared (IR) and Raman spectra combined with high-level quantum chemical calculations provide a consistent picture for the preference over more typical hydrogen bond insertion and addition patterns. Experimental evidence for at least one metastable dimer is presented. A rotational spectroscopy investigation of these dimers is encouraged, also in view of astrophysical searches. The binding motif competition of aldehydic sugars might play a role in chirality recognition phenomena of more complex derivatives in the gas phase.     

Determination of hydrogen atom positions in basic lead carbonate hydrocerussite by quantum chemical methods and simulation of the vibrational spectra

Positions of hydrogen atoms in the crystal structure of basic lead carbonate hydrocerussite are determined by the PM5 quantum chemical method. Raman and infrared spectra as well as thermodynamic functions are calculated for this compound by the theory of crystal lattice dynamics.     

Structural preferences, argon nanocoating, and dimerization of n-alkanols as revealed by OH stretching spectroscopy in supersonic jets

n-Alkanols can occur in a multitude of energetically competitive conformational states. Using the OH stretching vibration as an infrared and Raman spectroscopic sensor in supersonic jet expansions, the torsional preferences around the Calpha-O and Cbeta-Calpha bonds are probed for n-propanol through n-hexanol. Raman detection is more powerful for isolated monomers, whereas IR spectroscopy is more sensitive for molecular complexes. The subtle IR vibrational shift induced by the nanocoating of n-alcohols with Ar atoms is shown to alternate with chain length. A large number of alcohol dimer absorptions is observed and subjected to collisional relaxation and nanocoating conditions. Essential features of the dimer spectra are modeled successfully by a simple force field approach. Exploratory quantum chemical calculations up to the MP2/aug-cc-pvqz level encourage a rigorous theoretical study of the subtle conformational aspects in monomers and possibly also in dimers of linear alcohols.     

Structure and optical properties of the 4-cyano-4′-n-pentylbiphenyl dimers and trimers

ABSTRACT

The variation of the spectral characteristics of liquid crystal molecules based on 4-cyano-4′-n-pentylbiphenyl (5CB) was analysed depending on its associates structure using the quantum–chemical density functional hybrid potential methods B3LYP/6-31G and B3LYP/6-31G**. The electronic absorption spectra and IR spectra of dimers and trimers 5CB were calculated. It was shown that spectra are sensitive to the associates’ structure. We can observe the appearance of new bands in spectra and splitting characteristic CN stretching vibrations (1–5 cm^?1) in the vibrational spectra.     

The Cluster Anion Si94−

Solely on the basis of Raman spectra and quantum chemical calculations, the previously unknown cluster anion Si₉⁴⁻ (structure shown) was characterized and its structure determined. The anion is formed as a component of solid phases by the thermal decomposition of alkali metal monosilicides.     

A DFT study on the vibrational spectroscopy of protoporphyrin IX

Infrared and Raman spectra of protoporphyrin IX were recorded. DFT quantum chemical calculations were performed. Optimised molecular geometry, electric charge distribution, vibrational force constants were computed. The normal coordinate analysis and the scaling of the force constants yielded all the necessary data for the simulation of the infrared and Raman spectra and the potential energy distribution calculations. The result was the interpretation of all vibrational modes of the molecule. Conclusions were drawn from the difficulties arisen during the assignment of the vibrational spectra of such large molecules.     

Experimental CC stretching phonon dispersion curves and electron phonon coupling in polyene derivatives

We analyze the infrared and Raman spectra (both experimentally and with the aid of quantum chemical calculations) of a series of polyenals which provide us with the fortunate case of a set of polyene chains with one of the end groups consisting of a C=O group which not only does take part in the conjugation but also pulls electrons from the chain making the whole system highly polar, thus affecting the vibrational transition moments. In the following we show, for the first time, that it is possible to derive experimental phonon dispersion curves and these prove to be different for each chain length. We support our experimental findings with Density Functional Theory quantum chemical calculations which reproduce with sufficient accuracy the IR and Raman spectral pattern and at the same time help in disentangling the assignment of the fine structure observed in the experimental spectra.     

Vibrational frequencies and structural determination of hafnium tetrahydroborate

The normal mode frequencies and corresponding vibrational assignments of of hafnium tetrahydroborate in T symmetry are examined theoretically using the Gaussian 98 set of quantum chemistry codes. All normal modes were successfully assigned to one of to one of six types of motion (B-H stretch, Hf-B stretch, B-Hf-B bend, H-B-H bend, BH4 wag, and BH4 twist) predicted by a group theoretical analysis. By comparing the vibrational frequencies with IR and Raman spectra available in the literature, a set of scaling factors is derived. Theoretical IR and Raman intensities are reported. Quantum chemical calculations predict that the molecule does not possess strict Td symmetry. The Td structure possesses one negative eigenvalue. The minimum energy structure possesses T symmetry.     

Raman spectra of ionic liquids: interpretation via computer simulation

Theoretical Raman spectra of the complex-forming ionic liquids LaCl3 and ScCl3, derived from molecular dynamics computer simulations, are presented. These simulations, which use polarizable ion interaction models, have previously been shown to predict structural properties in excellent agreement with diffraction experiments. The dependence of the polarizability of the melt on the ionic positions, which determines the Raman spectrum through the time dependence of the polarizability correlation function, is modeled on the basis of ab initio electronic structure calculations carried out on alkali chlorides. New simulation techniques are introduced in order to allow the spectrum to be calculated with acceptable statistics. The calculated spectra are in semiquantitative agreement with experimental data. The distinctive bands which appear in the spectra of such complex melts are linked to the vibrations of the transient coordination complexes which form in these melts and new interpretations for the origin of several well-known features are proposed. The simulations thus enable a link between the structure of a melt as perceived through Raman spectroscopy and through diffraction experiments to be made.     

Molecular geometry and vibrational spectroscopy of potassium O,O-diethyldithiophosphate

The aim of this work was to build a good theoretical and experimental basis for the further study of changes in structure and spectra of the O,O-diethyldithiophosphate anion upon its adsorption on the surfaces of transition metal sulfides.Infrared and Raman spectra of potassium O,O-diethyldithiophosphate were recorded. High level quantum chemical calculations were carried out to optimize the molecular geometry of both the potassium salt and its anion. Vibrational force constants were calculated from the second derivative of the molecular energy function with respect to the Cartesian coordinates of the atoms. With the aid of the optimized geometry and the calculated vibrational force constants a normal coordinate analysis was carried out to characterize the molecular vibrational modes and to assign the vibrational frequencies.     

Structure and Raman spectrum of clavulanic acid in aqueous solution

The calculation of the vibrational Raman spectrum of enzyme-bound beta-lactamase inhibitors may be of help to understand the mechanisms responsible for bacterial drug resistance. Here, we present a study of the solvation structure and the vibrational properties of clavulanate, an important beta-lactamase inhibitor, in aqueous solution as obtained from full quantum and hybrid empirical/quantum molecular dynamics simulations at ambient conditions. The analysis of the vibrational density of states indicates that hybrid empirical/quantum mechanical simulations are able to properly describe the vibrational levels of clavulanate in solution. In addition, we propose a computationally efficient protocol to calculate the vibrational Raman effect for large solute molecules in water, which is able to faithfully reproduce the experimentally recorded clavulanate Raman spectrum and discloses the possibility to employ hybrid simulations to assign the experimental Raman spectra of inhibitors bound to beta-lactamases.     

The effect of intermolecular dipole-dipole interaction on Raman spectra of polyconjugated molecules: density functional theory simulations and mathematical models

In this work, we analyze the effect of intermolecular dipole-dipole interactions on Raman spectra of polyconjugated molecules. In particular, the behavior of push-pull polyenes has been studied. By means of density functional theory (DFT) calculations on isolated molecules and dimers, we have found that both the frequencies and intensities of the strongest Raman lines (R mode) are strongly influenced by intermolecular interactions. The results have been rationalized within the effective conjugation coordinate (ECC) theory developed in the past. The calculations for different configurations have also shown that the Raman spectra are sensible to different intermolecular geometries, thus implying a possible application of vibrational spectroscopy to the study of supramolecular properties of polyconjugated systems. The comparison with the available experimental spectra confirms the results obtained with the DFT computations. Finally, a very simple mathematical model is proposed for the prediction of the Raman frequencies of interacting systems. From the knowledge of just a few quantities for the isolated molecule and of some geometrical parameters, an estimate of the frequency of the dimers can be obtained. Despite its simplicity, this model gives results in very good agreement with DFT calculations carried out explicitly on dimers in several different arrangements.