Vibrational spectroscopy of resveratrol

In this article the authors deal with the experimental and theoretical interpretation of the vibrational spectra of trans-resveratrol (3,5,4'-trihydroxy-trans-stilbene) of diverse beneficial biological activity. Infrared and Raman spectra of the compound were recorded; density functional calculations were carried out resulting in the optimized geometry and several properties of the molecule. Based on the calculated force constants, a normal coordinate analysis yielded the character of the vibrational modes and the assignment of the measured spectral bands.     

A comparison of experimental and calculated spectra of HNO3 in the near-infrared using Fourier transform infrared spectroscopy and vibrational perturbation theory

This work combines new laboratory studies of the near-infrared vibrational spectra of HNO3 with theoretical predictions of these spectra as a means to understand the properties of this molecule at energies well above the fundamental region. Trends in overtone and combination band energy levels and intensities are compiled and examined. The theoretical calculations are in excellent agreement with the observed frequencies and intensities of the transitions in this spectral region. The calculations also serve as a valuable aid for assigning many of the transitions. This work validates the ab initio generated potential energy surface for HNO3 by comparing vibrational perturbation theory calculations to experimental spectra focused on combination band and overtone absorptions.     

Molecular Dynamics Simulations of Silica Nanotube: Structural and Vibrational Properties Under Different Temperatures

Four-, six-, and eight-membered ring silica nanotubes at temperatures from 300 K to 1600 Kare relaxed by classical molecular dynamics simulations with three potential models. Thesimulation results indicate that the stability of the end rings of the three silica nanotubes gradually decreases with increase in temperature. The validity of the vibrational features of silica nanotubes is shown by the vibrational density of states. Infrared spectra on the silica nanotubes under different temperatures are investigated. A detailed assignment of each spectral peak to the corresponding vibrational mode of the three nanotubes has been addressed. The results are in good agreement with the other theoretical and experimental studies.     

Eu(DBM)3纳米微晶的低频喇曼光谱

The fractal property and low frequency Raman scattering of Eu(DBM)3 nanosized microcrystals were investigated. The influence of the fractal structure of Eu(DBM)3 non-crystalline solids on low frequency scattering was studied. It was found that vibrational excitations on the fractal are localized and can be described in terms of fractions. It was shown that reduced Raman scattering intensity is of a power law dependence of the vibrational frequencies for all samples. The fractal and spectral dimension were also determined, we found that our experimental value was in fair agreement with the theoretical one.     

The umbrella motion of core-excited CH3 and CD3 methyl radicals

An accurate experimental and theoretical study of the lowest core excitation of CH(3) and CD(3) methyl radicals is presented. The complex vibrational structure of the lowest band of the x-ray absorption spectrum (XAS) is due to the large variation of the molecular geometry, which is planar in the ground state and pyramidal in the core-excited state. The XAS spectra of the two radicals were recorded at high resolution and assigned by theoretical simulations of the spectra, taking into account the coupling of symmetrical stretching and symmetrical bending (umbrellalike) deformations of the radicals. An excellent agreement between experimental and theoretical spectral profiles allowed us to accurately characterize the vibrational structure of the electronic transition. The similarities, as well as the differences, of the peculiar vibrational progression observed for the two radicals are explained by the strong anharmonicity along the umbrella coordinate and by the isotopic variation, leading to a different probing of the double-well potential energy surface of the core excited state during the nuclear motion.     

Determining the absolute configuration of two marine compounds using vibrational chiroptical spectroscopy

Chiroptical techniques are increasingly employed for assigning the absolute configuration of chiral molecules through comparison of experimental spectra with theoretical predictions. For assignment of natural products, electronic chiroptical spectroscopies such as electronic circular dichroism (ECD) are routinely applied. However, the sensitivity of electronic spectral parameters to experimental conditions and the theoretical methods employed can lead to incorrect assignments. Vibrational chiroptical methods (vibrational circular dichroism, VCD, and Raman optical activity, ROA) provide more reliable assignments, although they, in particular ROA, have been little explored for assignments of natural products. In this study, the ECD, VCD, and ROA chiroptical spectroscopies are evaluated for the assignment of the absolute configuration of a highly flexible natural compound with two stereocenters and an asymmetrically substituted double bond, the marine antibiotic Synoxazolidinone A (SynOxA), recently isolated from the sub-Arctic ascidian Synoicum pulmonaria. Conformationally averaged nuclear magnetic resonance (NMR), ECD, Raman, ROA, infrared (IR) and VCD spectral parameters are computed for the eight possible stereoisomers of SynOxA and compared to experimental results. In contrast to previously reported results, the stereochemical assignment of SynOxA based on ECD spectral bands is found to be unreliable. On the other hand, ROA spectra allow for a reliable determination of the configuration at the double bond and the ring stereocenter. However, ROA is not able to resolve the chlorine-substituted stereogenic center on the guanidinium side chain of SynOxA. Application of the third chiroptical method, VCD, indicates unique spectral features for all eight SynOxA isomers in the theoretical spectra. Although the experimental VCD is weak and restricted by the limited amount of sample, it allows for a tentative assignment of the elusive chlorine-substituted stereocenter. VCD chiroptical analysis of a SynOxA derivative with three stereocenters, SynOxC, results in the same absolute configuration as for SynOxA. Despite the experimental challenges, the results convincingly prove that the assignment of absolute configuration based on vibrational chiroptical methods is more reliable than for ECD.     

Synthesis, characterization, and DFT studies of a novel azo dye derived from racemic or optically active binaphthol

We present a new azo reactive dye from racemic or optically active BINOL. This dye was characterized by UV-vis, FTIR, mass, ¹H NMR, and ¹³C NMR spectroscopic techniques and elemental analysis. The structure and spectrometry of this azo dye have been investigated theoretically by performing HF and DFT levels of theory using the standard 6-31G^∗ basis set. The optimized geometries and calculated vibrational frequencies are evaluated via comparison with experimental values. The vibrational spectral data obtained from solid phase FTIR spectra are assigned based on the results of the theoretical calculations. The theoretical electronic absorption spectra have been calculated using CIS, TD-DFT, and ZINDO methods. In addition, a good agreement between calculated and experimental NMR data is observed.     

Infrared and Raman line shapes for ice Ih. II. H2O and D2O

We present a theoretical study of infrared and Raman line shapes of polycrystalline and single crystal ice Ih, for both water and heavy water, at 1, 125, and 245 K. Our calculations involve a mixed quantum/classical approach, a new water simulation model with explicit three-body interactions, transition frequency and dipole maps, and intramolecular and intermolecular vibrational coupling maps. Our theoretical spectra are in reasonable agreement with experimental spectra (available only near the two higher temperatures). We trace the origins of the different spectral peaks to weak and strong intermolecular couplings. We also discuss the delocalization of the vibrational eigenstates in terms of the competing effects of disorder and coupling.     

A combined theoretical and experimental study of the structure and vibrational absorption, vibrational circular dichroism, Raman and Raman optical activity spectra of the L-histidine zwitterion

A combined theoretical and experimental study of the vibrational absorption (VA)/IR, vibrational circular dichroism (VCD), Raman and Raman optical activity (ROA) spectra of l-histidine in aqueous solution has been undertaken to answer the questions (i) what are the species present and (ii) which conformers of the species are present under various experimental conditions. The VA spectra of l-histidine have been measured in aqueous solution and the spectral bands which can be used to identify both species (cation, zwitterion, anion) and conformer of the species have been identified and subsequently used to identify the species (zwitterion) and conformer (gauche minus minus, gauche minus plus for the side chain dihedral angles) present in solution at pH 7.6. The VCD spectral intensities have been used subsequently in combination with further theoretical studies to confirm the conclusions that have been arrived at by only analyzing the VA/IR spectra. Finally a comparison of measured Raman and ROA spectra of l-histidine with Raman and ROA spectral simulations for the conformers and species derived from the combined VA/IR and VCD experimental and theoretical work is presented as a validation of the conclusions arrived at from VA/IR and VCD spectroscopy. The combination of VA/IR and VCD with Raman and ROA is clearly superior and both sets of experiments should be performed.     

Spectroscopic characterization of peroxynitrous acid in cis-perp configurations

This paper presents experimental evidence, supported by two-dimensional theoretical calculations, that HOONO can be observed in cis-perp (cp) configurations in a pulsed supersonic expansion. The spectral properties (transition frequency, rotational constants, and transition type) of OH overtone transitions originating from a state with predominately cp character are predicted theoretically and compared with those associated with a weak feature at 6996.2 cm(-1) observed experimentally using infrared action spectroscopy. This spectral feature is attributed to HOONO in cp configurations based on its vibrational frequency, rotational band contour, and resultant OH product state distribution.     

Vibrational echo spectroscopy of aqueous sodium bromide solutions from first principles simulations

A theoretical study of the time-dependent vibrational echo spectroscopy of sodium bromide solutions in deuterated water at two different concentrations of 0.5 and 5.0 M and at temperatures of 300 and 350 K is presented using the method of ab initio molecular dynamics simulations. The instantaneous fluctuations in frequencies of local OD stretch modes are calculated using time-series analysis of the simulated trajectories. The third-order polarization and intensities of three pulse photon-echo are calculated from ab initio simulations. The timescales of vibrational spectral diffusion are determined from the frequency time correlation functions (FTCF) and short-time slope of three pulse photon echo (S3PE) calculated within the second-order cumulant and Condon approximations. It is found that under ambient conditions, the rate of vibrational spectral diffusion becomes slower with increase in ionic concentration. Decay of S3PE calculated for different systems give timescales, which are in close agreement with those of FTCF and also with the results of experimental time-dependent vibrational spectroscopic experiments. © 2019 Wiley Periodicals, Inc.     

Unraveling the Electronic Structure,Spin States,Optical and Vibrational Spectra of Malaria Pigment

A detailed knowledge of the electronic structure and magnetic and optical properties of hemozoin, the malaria pigment, is essential for the design of effective antimalarial drugs and malarial diagnosis. By employing state‐of‐the‐art electronic structure calculations, we have performed an in‐depth investigation of the malaria pigment. Specifically, molecular bond lengths and spin states of the two Fe^III heme centers and their exchange interaction, the UV/Vis absorption spectrum, and the IR vibrational spectra were calculated and compared with available experimental data. Our density functional theory (DFT)‐based calculations predict a singlet ground spin state that stems from an S=5/2 spin state on each of the Fe heme centers with a very weak antiferromagnetic exchange interaction between them. Our theoretical UV/Vis and IR spectra provide explanations for various spectroscopic studies of hemozoin and β‐hematin (a synthetic analogue of hemozoin). A good comparison of calculated and measured properties demonstrates the convincing unveiling of the electronic structure of the malaria pigment. Based on the predicted vibrational spectra, we propose a unique spectral band from the nuclear resonance vibrational spectroscopy (NRVS) results that could be used as a key fingerprint for malarial detection.     

DFT and ab initio calculations of the vibrational frequencies and visible spectra of triazenes derived from cyclic amines

We present a detailed analysis of the structural, infrared spectra and visible spectra of the 4-substituted aminoazo-benzenesulfonyl azides. The preparation of 4-sulfonyl azide benzenediazonium chloride with cyclic amines of various ring sizes (pyrrolidine, piperidine, 4-methylpiperidine, N-methylpiperazine, morpholine and hexamethyleneimine) have been investigated theoretically by performing HF and DFT levels of theory using the standard 6-31G* basis set. The optimized geometries and calculated vibrational frequencies are evaluated via comparison with experimental values. The vibrational spectral data obtained from solid phase FT-IR spectra are assigned modes based on the results of the theoretical calculations. The observed spectra are found to be in good agreement with the calculations.     

Broadband pump-probe spectroscopy with sub-10-fs resolution for probing ultrafast internal conversion and coherent phonons in carotenoids

We use pump-probe spectroscopy with broadband detection to study electronic energy relaxation and coherent vibrational dynamics in carotenoids. A fast optical multichannel analyzer combined with a non-collinear optical parametric amplifier allows simultaneous acquisition of the differential transmission dynamics on the 500–700 nm wavelength range with sub-10-fs temporal resolution. The broad spectral coverage enables on the one hand a detailed study of the ultrafast bright-to-dark state internal conversion process; on the other hand, the tracking of the motion of the vibrational wavepacket launched on the ground state multidimensional potential energy surface. We present results on all-trans β-carotene and on a long-chain polyene in solution. The developed experimental setup enables the straightforward acquisition and analysis of coherent vibrational dynamics, highlighting time–frequency domain features with extreme resolution.     

Vibrational state dependence of beta and D asymmetry parameters: the case of the highest occupied molecular orbital photoelectron spectrum of methyl-oxirane

The beta angular asymmetry and D dichroic asymmetry parameters of the methyl-oxirane highest occupied molecular orbital (HOMO) band have been experimentally investigated with vibrational resolution using synchrotron radiation. A theoretical calculation of the Franck-Condon factors between vibrational ground state and different ionic vibrational states, in the Born-Oppenheimer harmonic approximation, has been performed in order to gain information on the vibrational states mainly involved in the HOMO photoelectron band. The general good agreement between theoretical and experimental results allows a reliable assignment of the major features. The experimental determination of beta and D shows their dependence on the different final vibrational states. This paper reports, for the first time, experimental evidence of the dependence of the dichroic D parameter on the vibrational excitation of the ion.     

石墨烯分子振动模式因子群分析与密度泛函计算

运用因子群分析法对石墨烯的分子振动模式进行了理论分析,得到石墨烯的分子振动模式,计算出各振动模式的光谱特性.对所建立的石墨烯晶体的布拉维单胞模型采用基于密度泛函理论的第一性原理进行分子振动频率与模式的计算,所得的振动模式数目以及各振动频率的光谱特性同因子群分析方法所得结论一致.结合上述计算结果,通过系统比较石墨与石墨烯之间的红外光谱和拉曼光谱的差别,从理论上解释了具有D6h对称的石墨烯的A2u、E1u红外活性特征振动模式没有在红外光谱中出现的原因.     

Temperature-dependent vibrational spectroscopic studies of pure and gold nanoparticles dispersed 4-n-Hexyloxy-4’-cyanobiphenyls

Raman spectra of pure and 2 wt.% gold nanoparticles (GNPs) dispersed liquid crystalline compound 4-n-Hexyloxy-4?- cyanobiphenyls (6OCB) has been recorded as a function of temperature from room temperature (solid crystal) to 80°C (isotropic liquid) in the spectral region of 500–2500 cm^?1. The variation of Raman spectral parameters (peak positions and line width) with temperature is used to explain the changes in molecular alignment and its effect on inter-/intra-molecular interactions at crystal-Nematic (K-N) transition. To understand the change in molecular structure during phase transition and on account of dispersion of gold nanoparticles in pure liquid crystal more precisely, two spectral regions 1000–1500 cm^?1 and 1500–2400 cm^?1 have been selected separately. From the detailed study, it is concluded that increased orientational/vibrational freedom of the molecules as well as delocalisation of electron clouds results in the spectral anomalies at K-N transition. The geometrical structure of 6OCB was optimised using density functional theory (DFT) and theoretical Raman spectra have been obtained for comparison with experimental spectra. The tentative assignment of vibrational modes observed in our region of study was calculated based on potential energy distribution (PED) using vibrational energy distribution analysis (VEDA) calculation.     

DFT, ab initio and FT-IR studies of the structure of sulfonamide triazenes

We present an in-depth investigation of the structural, infrared spectra and visible spectra of various triazenes with a sulfonamide moiety. The preparation of 4-sulfonyl amide benzenediazonium chloride with cyclic amines of various ring sizes (pyrrolidine, piperidine, 4-methylpiperidine, N-methylpiperazine, morpholine and hexamethyleneimine) are theoretically investigated using the density functional theory (DFT) and Hartree-Fock (HF) levels of theory with the standard 6-31G* basis set. The calculated vibrational frequencies are evaluated via comparison with experimental values. The vibrational spectral data obtained from solid-phase FT-IR spectra are assigned modes based on the results of the theoretical calculations. The UV-Vis spectrum of each compound is measured in various solvents with a wide range of polarity to examine the role of solvent chemistry on observed spectral changes. The calculated spectra are in good agreement with the experimental spectra.