Investigation of hydrogen bonding in neat dimethyl sulfoxide and binary mixture (dimethyl sulfoxide + water) by concentration-dependent Raman study and ab initio calculation

In this study,our vibrational spectroscopic analysis is made on hydrogen-bonding between dimethyl sulfoxide and water comprises both experimental Raman spectra and ab initio calculations on structures of various dimethyl sulfoxide/water clusters with increasing water content.The Raman peak position of the v(S=O) stretching mode of dimethyl sulfoxide serves as a probe for monitoring the degree of hydrogen-bonding between dimethyl sulfoxide and water.In addition,the two vibrational modes,namely,the CH 3 symmetric stretching mode and the CH 3 asymmetric stretching mode have been analysed under different concentrations.We relate the computational results to the experimental vibrational wavenumber trends that are observed in our concentration-dependent Raman study.The combination of experimental Raman data with ab initio calculation leads to a better knowledge of the nature of the hydrogen bonding and the structures of the hydrogen-bonded complexes studied.     

In situ study of the metal–insulator transition in VO2 by EELS and ab initio calculations

Changes in the dielectric properties during the thermochromic transition of commercial VO₂ powders were determined in situ, by analyzing the low-loss region of the electron energy-loss spectroscopy (EELS) spectra in a transmission electron microscope at room temperature (insulator phase) and 100 °C (metallic phase). A comparison of experimental EELS spectra and ab initio density-functional theory calculations (WIEN2k code) within the generalized gradient approximation (GGA) is presented. A characteristic peak around 5.6 eV appears in the energy-loss function in metallic phase, which is absent in insulator phase. The origin of the characteristic peak is analyzed by means of energy-band structure calculations.     

Hydrogen bonding in the pyrimidine/ formamide system: a concentration‐dependent Raman and DFT study

High‐resolution Raman spectra of pyrimidine (PD) and formamide (FA) mixtures with different compositions recorded in the ring breathing region of PD (ν₁ ∼ 991 cm⁻¹) are presented. The dilution of PD with FA leads to the appearance of a new band at ν₁′ ∼ 994 cm⁻¹, which is assigned to hydrogen‐bonded PD:FA species. From a quantitative analysis of the concentration‐dependent Raman spectra, the average number of FA molecules in the first solvation sphere of PD is determined as being equal to 2. This value is supported by density functional theory (DFT) calculations: a symmetric 1:2 complex is the most stable species among various hydrogen‐bonded PD:FA clusters with stoichiometries ranging from 1:1 to 1:4. A qualitative explanation for the blue shift of the ν₁ mode upon complexation is given. Additionally, we have observed not only similarities but also some differences with respect to the PD:water system. Copyright © 2010 John Wiley & Sons, Ltd.     

Self‐association and hydrogen bonding of propionaldehyde in binary mixtures with water and methanol investigated by concentration‐dependent polarized Raman study and DFT calculations

The Raman spectra of neat propionaldehyde [CH₃CH₂CHO or propanal (Pr)] and its binary mixtures with hydrogen‐donor solvents, water (W) and methanol (M), [CH₃CH₂CHO + H₂O] and CH₃CH₂CHO + CH₃OH] with different mole fractions of the reference system, Pr varying from 0.1 to 0.9 at a regular interval of 0.1, were recorded in the ν(CO) stretching region, 1600–1800 cm⁻¹. The isotropic parts of the Raman spectra were analyzed for both the cases. The wavenumber positions and line widths of the component bands were determined by a rigorous line‐shape analysis, and the peaks corresponding to self‐associated and hydrogen‐bonded species were identified. Raman peak at ∼1721 cm⁻¹ in neat Pr, which has been attributed to the self‐associated species, downshifts slightly (∼1 cm⁻¹) in going from mole fraction 0.9 to 0.6 in (Pr + W) binary mixture, but on further dilution it shows a sudden downshift of ∼7 cm⁻¹. This has been attributed to the low solubility of Pr in W (∼30%), which does not permit a hydrogen‐bonded network to form at higher concentrations of Pr. A significant decrease in the intensity of this peak in the Raman spectra of Pr in a nonpolar solvent, n‐heptane, at high dilution (C = 0.05) further confirms that this peak corresponds to the self‐associated species. In case of the (Pr + M) binary mixture, however, the spectral changes with concentration show a rather regular trend and no special features were observed. Copyright © 2010 John Wiley & Sons, Ltd.     

In situ high-pressure X-ray diffraction experiments and ab initio calculations of Co2P

In situ high-pressure experiments of Co2P are carried out by means of angle dispersive X-ray diffraction with diamond anvil cell technique. No phase transition is observed in the present pressure range up to 15 GPa at room temperature, even at high temperature and 15 GPa. Results of compression for Co2P are well presented by the second-order Birch-Murnaghan equation of state with V0 = 130.99(2)3 (1=0.1 nm) and K0 = 160(3) GPa. Axial compressibilities are described by compressional modulus of the axis: Ka = 123(2) GPa, Kb = 167(8) GPa and Kc = 220(7) GPa. Theoretical calculations further support the experimental results and indicate that C23-type Co2P is stable at high pressure compared with the C22-type phase.     

Concentration dependent Raman and IR study on salicylaldehyde in binary mixtures

A vibrational spectroscopic study of binary mixtures of salicylaldehyde (SA) in three different solvents (polar and nonpolar) is presented. The vibrational modes ν(CO), hydroxyl stretching mode (C OH) and aldehydic (C H) stretching vibration were analyzed. Changes in wavenumber position and full width half maximum have been explained for neat as well as binary mixtures with different volume fractions of the reference system, SA, in terms of inter‐ and intramolecular hydrogen bonding. The IR spectra of these mixtures have also been taken and compared with the Raman data. The spectral changes have been well explained using the concentration fluctuation model and solute–solvent interaction. Copyright © 2007 John Wiley & Sons, Ltd.     

Quantum-chemical calculations of the structure,vibrational spectra,and torsional and inversion potentials of methylcarbamate

We present results of ab initio and DFT calculations of the structure, potential functions of the methyl group internal rotation and the amino group inversion, and vibrational frequencies and intensities in IR and Raman spectra of methylcarbamate. The calculations were carried out using different basis sets in the HF, MP2, and DFT/B3LYP approximations. The influence of both the basis set size and the allowance for electronic correlation on peculiarities of the structure of the amino group in methylcarbamate has been analyzed. It is shown that the B3LYP/6-311++G(2d, p) and B3LYP/cc-pVDZ calculations reproduce highly accurately experimental geometric parameters of methylcarbamate. Parameters of torsional and inversion potentials and characteristics of vibrational spectra calculated in different approximations show satisfactory agreement with experimental values. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 349–357, May–June, 2009.     

Solvent dependent study of carbonyl vibrations of 3‐phenoxybenzaldehyde and 4‐ethoxybenzaldehyde by Raman spectroscopy and ab initio calculations

A Raman spectroscopy investigation of the carbonyl stretching vibrations of 3‐phenoxybenzaldehye (3Phbz) and 4‐ethoxybenzaldeheyde (4Etob) was carried out in binary mixtures with different polar and nonpolar solvents. The purpose of this study was twofold: firstly, to describe the interaction of the carbonyl groups of two solute molecules in terms of a splitting in the isotropic and anisotropic components and secondly, to analyze their spectroscopic signatures in a binary mixture. Changes in wavenumber position, variation in the anisotropic shift and full width half maximum were investigated for binary mixtures with different mole fractions of the reference systems. In binary mixtures, the observed increase in wavenumber with solvent concentration does not show linearity, indicating the significant role of molecular interactions on the occurrence of breaking of the self‐association of the solute. In all the solvents, a gradual decrease in the anisotropic shift reflects the progressive separation of the coupled oscillators with dilution. Γ_i(η_c), 3Phbz—solvent mixtures, exhibit a gradual decrease with decrease in the concentration of the solute which is an evidence on the influence of micro viscosity on linewidth. For 4Etob, the carbonyl stretching vibration shows two well‐resolved components in the Raman spectra, attributed to the presence of two distinct carbonyl groups: hydrogen‐bonded and free carbonyl groups. The intensity ratio of the carbonyl stretching vibration of these two types of carbonyl groups is studied to understand the dynamics of solute/solvent molecules owing to hydrogen bond interactions. Ab initio calculations were employed for predicting relevant molecular structures in the binary mixtures arising from intermolecular interactions, and are related to the experimental results. Copyright © 2009 John Wiley & Sons, Ltd.     

Summation of Parquet diagrams as an ab initio method in nuclear structure calculations

In this work we discuss the summation of the Parquet class of diagrams within Green’s function theory as a possible framework for ab initio nuclear structure calculations. The theory is presented and some numerical details are discussed, in particular the approximations employed. We apply the Parquet method to a simple model, and compare our results with those from an exact solution. The main conclusion is that even at the level of approximation presented here, the results shows good agreement with other comparable ab initio approaches.     

Wn(n=3—27)原子团簇结构的第一性原理计算

使用密度泛函理论下的第一性原理方法,对W_n原子团簇(n=3—27)的结构特性进行了理论计算. 得到了W_n团簇(n=3—7)的最低能量结构和(n=8—27)的局域能量极小的典型结构. 使用凝胶模型,提出的电子组态1s²1p⁶1d¹⁰2s²1f¹⁴2p⁶3s ^{关键词： W团簇 结构性质 稳定性 从头计算}     

Hydration and translocation of an excess proton in water clusters: Anab initio molecular dynamics study

The hydration structure and translocation of an excess proton in hydrogen bonded water clusters of two different sizes are investigated by means of finite temperature quantum simulations. The simulations are performed by employing the method of Car-Parrinello molecular dynamics where the forces on the nuclei are obtained directly from ‘on the fly’ quantum electronic structure calculations. Since no predefined interaction potentials are used in this scheme, it is ideally suited to study proton translocation processes which proceed through breaking and formation of chemical bonds. The coordination number of the hydrated proton and the index of oxygen to which the excess proton is attached are calculated along the simulation trajectories for both the clusters.     

Formation and dissociation of protonated cytosine–cytosine base pairs in i-motifs by ab initio quantum chemical calculations

Formation and dissociation mechanisms of C-C＋ base pairs in acidic and alkaline environments are investigated, employing ab initio quantum chemical calculations. Our calculations suggest that, in an acidic environment, a cytosine monomer is first protonated and then dimerized with an unprotonated cytosine monomer to form a C-C＋ base pair; in an alkaline environment, a protonated cytosine dimer is first unprotonated and then dissociated into two cytosine monomers. In addition, the force for detaching a C-C＋ base pair was found to be inversely proportional to the distance between the two cytosine monomers. These results provide a microscopic mechanism to qualitatively explain the experimentally observed reversible formation and dissociation of i-motifs.     

Effects of conformation and hydrogen bonding on the CC and NCN stretching Raman bands of N‐deuterated histidinium

Histidine is an important and versatile amino acid residue that plays a variety of structural and functional roles in proteins. Although the Raman bands of histidine are generally weak, histidine in the N‐deuterated cationic form with imidazole N_π D and N_τ D bonds (N‐deuterated histidinium) gives two strong Raman bands assignable to the C₄C₅ stretch (ν_CC) and the N_π C₂ N_τ symmetric stretch (ν_NCN) of the imidazole ring. We examined the Raman spectra of N‐deuterated histidinium in 12 crystals with known structures. The observed ν_CC and ν_NCN wavenumbers were analyzed to find empirical correlations with the conformation and hydrogen bonding. The effect of conformation on the vibrational wavenumber was expressed as a threefold cosine function of the C_α C_β C₄C₅ torsional angle. The effect of hydrogen bonding at N_π or N_τ was assumed to be proportional to the inverse sixth power of the distance between the hydrogen and acceptor atoms. Multiple linear regression analysis clearly shows that the conformational effect on the vibrational wavenumber is comparable for ν_CC and ν_NCN. The hydrogen bond at N_π weakly lowers the ν_CC wavenumber and substantially raises the ν_NCN wavenumber. On the other hand, the hydrogen bond at N_τ strongly raises the ν_CC wavenumber but does not affect the ν_NCN wavenumber. These empirical correlations may be useful in Raman spectral analysis of the conformation and hydrogen bonding states of histidine residues in proteins. Copyright © 2010 John Wiley & Sons, Ltd.     

Absorption, fluorescence studies and ab initio calculations on binary mixture of p-dimethylaminobenzaldehyde

Absorption as well as fluorescence emission studies of p-dimethylaminobenzaldehyde (DMABA) in solvents with different polarity have been reported by varying the concentration of the solute. Dual fluorescence corresponding to the non-polar (NP) and twisted intramolecular charge transfer (TICT) states has been observed. The optimized geometry of DMABA was evaluated using ab-initio theory at various levels. The optimized geometries of the hydrogen bonded complexes with the solvent molecules were also calculated with the theory Hartree Fock at the basis set 6-31+G (HF/6-31+G). The results have been used to understand the structure of the molecule and the spectral changes in terms of hydrogen bonding and solute–solvent interaction.     

Improper hydrogen bonding and motional narrowing in binary mixtures of 2‐ and 3‐bromopyridine in methanol probed by polarized Raman study and DFT calculations

A concentration‐dependent Raman study of the ν(C Br) stretching and trigonal bending modes of 2‐ and 3‐Br‐pyridine (2Br‐p and 3Br‐p) in CH₃OH was performed at different mole fractions of the reference molecule, 2Br‐p/3Br‐p, from 0.1 to 0.9 in order to understand the origin of blue/red wavenumber shifts of the vibrational modes due to hydrogen‐bond formation. The appearance of additional Raman bands in these binary systems at ∼617 cm⁻¹in the case of 2Br‐p and at ∼618 cm⁻¹ in the case of 3Br‐p compared to neat bromopyridine derivatives were attributed to specific hydrogen‐bonded complexes formed in the mixtures. The interpretation of experimental results is supported by density functional calculations on optimized geometries and vibrational wavenumbers of 2Br‐p and 3Br‐p and a series of hydrogen‐bonded complexes with methanol. The parameters obtained from these calculations were used for a qualitative explanation of the blue/red shifts. The wavenumber shifts and linewidth changes for the ν(C Br) stretching and trigonal bending modes as a function of concentration reveal that the caging effects leading to motional narrowing and diffusion‐causing line broadening are simultaneously operative, in addition to the blue shift caused due to hydrogen bonding. Copyright © 2007 John Wiley & Sons, Ltd.     

Ab initio molecular dynamics investigations of structural, electronic and dynamical properties of water in supercritical carbon dioxide

Ab initio molecular dynamics simulations of a solitary perdeuterated water molecule solvated in supercritical carbon dioxide have been performed along an isotherm at three different densities. Electron donor-acceptor interactions between the oxygen atom of water and the carbon atom of CO₂ as well as hydrogen bonded interactions between the two molecules have been shown to play a dominant role in the solvation. The mean dipole moment of the water molecule increases with the density of the solution, from a value of 1.85 D at low density to around 2.15 D at the highest density. The increase in the solvent density causes the water molecule to exhibit a range of behavior, from a free molecule to one that interacts strongly with CO₂. A blue shift in the bending mode of water has been observed with increasing solvent density. The carbon dioxide molecules which are present in the first neighbor shell of water are found to exhibit larger propensity to deviate from a linear geometry in their instantaneous configurations.      

Simulation of the fben d transitions of lanthanide ions in YPO4 using quantum-chemical calculations

We constructed an effective one-electron Hamiltonian by using the 4f/5d energies and eigenvectors obtained from the first-principles calculation with the relativistic self-consistent discrete variational Slater software package (DV-Xα). From the effective Hamiltonian, we obtained the crystal-field and spin-orbit interaction parameters for the 4f and 5d electrons of lanthanide ions (Ce³⁺, Pr³⁺, Nd³⁺ and Eu³⁺⁾ doped in YPO₄, and these parameters were used to calculate the 4f^N-4f^N-15d transition. Comparison with experiments shows that the obtained parameters are reasonable and the excitation spectra can be well predicted.     

菱铁矿FeCO3高压相变与性质的第一性原理研究

采用平面波赝势方法对菱铁矿FeCO₃高压下的晶体结构, 电子构型和电子结构进行了第一性原理计算研究. 研究过程中考虑了菱铁矿FeCO₃真实的反铁磁(AFM) 自旋有序态, 模拟静水压环境, 从零压逐步加压到500 GPa. 在40---50 GPa压力范围内, FeCO₃发生了从高自旋(HS) AFM态到低自旋(LS) 非磁性(NM) 态的磁性相变, 伴随着晶胞体积坍塌10.5%. FeCO₃在相变前后均是绝缘体, 但是相变后的LS-NM态的Fe²⁺ 离子的3d电子局域化程度更强, 能隙随着压力的进一步增大而逐步增大, 离化程度更高, 直到500 GPa没有发生金属绝缘体相变.     

WnNim(n+m=8)团簇的极性和光谱性质的理论研究

采用密度泛函理论(DFT)中的B3LYP方法,在LANL2DZ基组水平上对WnNim(n+m=8)团簇的各种可能构型进行了几何参数全优化,得到了它们的基态构型;并对基态构型的偶极距、极化率、红外光谱和拉曼光谱性质进行了分析,结果表明:团簇WnNim(n+m=8)都具有极性,富W团簇非线性光学效应强,容易被外加场极化;振动频率主要分布在0-350 cm-1范围内,团簇W4Ni4因其振动方式的特殊性,红外光谱和拉曼光谱在频率421.971 cm-1处,都有明显强峰;团簇W5Ni3因其结构的高对称性在振动光谱中出现多处共振现象.     

Signature of the hydrogen-bonded environment of liquid water in X-ray emission spectra from first-principles calculations

Based on ab initio molecular dynamics simulations and density functional theory, we performed a systematic theoretical study to elucidate the correlation between the H-bonded environment and Xray emission spectra of liquid water. The spectra generated from excited water molecules embedded in an intact H-bonded environment yield broader spectral peaks and a larger spectral range than the spectra generated from water molecules in a broken H-bonded environment. Such differences are caused by the local electronic structures on the excited water molecules within the core-hole lifetime that evolve differently through the rearrangement of neighboring water molecules in different H-bonded environments.