Molecular analysis of water clusters: Calculation of the cluster structures and vibrational spectrum using density functional theory

Density functional theory (DFT) is applied to obtain absorption spectra at THz frequencies for molecular clusters of H₂O. The vibrational modes of the clusters are calculated. Coupling among molecular vibrational modes explains their spectral features associated with THz excitation. THz excitation is associated with vibrational frequencies which are here calculated within the DFT approximation of electronic states. This is done for both isolated molecules and collections of molecules in a cluster. The principal result of the paper is that a crystal-like cluster of 38 water molecules together with a continuum solvent background is sufficient to replicate well the experimental vibrational frequencies.     

Au(II)化合物[Au(CH2)2PH2]2X2 (X＝F, Cl, Br, I)的量子化学理论研究

采用从头计算HF, MP2方法和密度泛函理论, 对Au(II)系列化合物[Au(CH₂)₂PH₂]₂X₂ (X＝F, Cl, Br, I)的几何结构、电子结构和振动频率进行了研究. 研究表明Au的5d和6s电子参与Au—Au以及Au—X之间的成键. Au—Au, Au—X键强烈的电子相关作用使HF方法不适于该体系的研究, BP86和B3LYP两种泛函给出较大的Au—Au和Au—X键长, 而MP2方法和局域的密度泛函方法则给出了合理的结构参数. 局域密度泛函方法计算得到的Au—Au键和 Au—X键振动频率也与实验数据符合较好. 还运用含时密度泛函理论计算了[Au(CH₂)₂PH₂]₂X₂的电子激发能, 对分子在紫外-可见光谱范围内的电子跃迁进行了分析, 考察了卤素配体对激发能的影响, 并结合分子轨道能级的变化对此给予了解释.     

Spectroscopic characterization of aminoacetonitrile,its ions and protonated aminoacetonitrile using quantum chemical methods

We present theoretical vibrational and absorption spectra of aminoacetonitrile, its cation, anion, cyanoprotonated, and aminoprotonated aminoacetonitrile. We used second‐order Moller–Plesset perturbation method (MP2) with TZVP basis set to obtain ground state geometries and vibrational spectra. Time dependent density functional theory method was used to obtain absorption spectra. Shifts in vibrational modes for aminoacetonitrile upon ionization and protonation are determined. The C≡N stretching mode which is the most important mode in detection of nitriles in space is more intense in aminoacetonitrile ions and its two protonated form and is less IR active for neutral aminoacetonitrile. The nature of electronic transition for these molecules is identified. All the electronic transitions for neutral aminoacetonitrile and its cation are the σ → σ* electronic transitions, whereas its anion and protonated aminoacetonitrile display the σ → σ* as well as π → π* transitions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

对苯二甲酸二苯酯类液晶化合物光谱和二阶非线性光学性质的理论研究

在密度泛函理论B3LYP/6-31G*水平上计算对苯二甲酸二苯酯类液晶化合物分子的几何结构、振动光谱、电子光谱和非线性光学性质, 分析讨论端接基对其光谱与非线性光学性质的影响. 结果表明, 端接基的引入对该类分子的几何结构影响不大. 烷氧基的链长对分子振动光谱的影响很小, 端基引入CN时, C＝O的伸缩振动频率蓝移9 cm^－1. TD-DFT计算表明, 最大吸收光谱源于分子中HOMO→LUMO的p→p*跃迁, 对应的最大吸收波长值在313～375 nm之间, 属于紫外区. 端接强供电子基团可以提高分子的二阶非线性光学性质.     

Structural and spectral properties of 3-substitutedphenyl-1,5-diphenylformazans: A quantum chemical study

The structural and optical properties of 3-substitutedphenyl-1,5-diphenylformazans are studied by quantum chemical methods. The density functional theory (DFT) is employed to optimize the ground state geometries of formazans substituted with different electron donating and withdrawing groups in both gas and solvent phases. The absorption spectra of formazan derivatives are calculated using time dependent density functional theory (TD-DFT). The polarizable continuum model (PCM) calculations of 3-substitutedphenyl-1,5-diphenylformazans are performed for bulk solvent effects. The geometrical parameters, vibrational frequencies, and relative stabilities of isomers of 3-substitutedphenyl-1,5-diphenylformazans are studied. The results obtained by TD-DFT calculations reveal that the substitution of electron withdrawing and donating substituents affects the absorption spectra of 3-substitutedphenyl-1,5-diphenylformazans. The calculated maximum absorption wavelengths (λ_max) are highly consistent with the experimental values as found from UV-vis spectra.     

Spectroscopic properties of dipicolinic acid and its dianion

If robust spectroscopic techniques are to be developed for the detection and identification of pathogens, one must understand the relevant spectroscopic properties of the target molecules. In this paper, we employ density functional theory (DFT) to study the structural, electronic, vibrational, optical, and magnetic properties of dipicolinic acid (DPA) and its dianion DPA⁻². Our full geometrical optimization and Mulliken charge analysis show that DFT does not lead to the significant discrepancies between charges on symmetric carbon, hydrogen, and oxygen atoms that are found in less accurate calculations based on the complete active space MCSCF method. Our calculated vibrational frequencies, Raman spectra, and infrared spectra for ground-state DPA and DPA⁻² are in good agreement with experiment, and this is also true of the four calculated ¹³C NMR spectral lines (for α, β, γ, and carboxyl sites). Our time-dependent DFT study of the optical excitation and absorption of both DPA and DPA⁻² provides the first interpretation of the observed near ultraviolet absorption and fluorescence spectra. Finally, we discuss for the first time the effect of a solvent on the spectral properties of DPA.     

Comparative DFT study of crystalline ammonium perchlorate and ammonium dinitramide

The electronic structure, vibrational properties, absorption spectra, and thermodynamic properties of crystalline ammonium perchlorate (AP) and ammonium dinitramide (ADN) have been comparatively studied using density functional theory in the local density approximation. The results shows that the p states for the two solids play a very important role in their chemical reaction. From the low frequency to high frequency region, ADN has more motion modes for the vibrational frequencies than AP. The absorption spectra of AP and ADN display a few, strong bands in the fundamental absorption region. The thermodynamic properties show that ADN is easier to decompose than AP as the temperature increases.     

秋水仙碱立体异构体电子结构及圆二色谱性质的理论研究

采用量子化学密度泛函理论(DFT)在B3LYP/6-311++G二水平上对秋水仙碱四个立体异构体分子几何构型进行了优化,在优化的基础上进行了振动圆二色谱(VCD),紫外-可见光谱(UV-Vis)和电子圆二色谱(ECD)研究.为模拟真实条件,以水为溶剂,计算其对分子电子结构和光谱性质的影响.研究结果表明:秋水仙碱四个立体...     

Structural and spectral properties of 4-bromo-1-naphthyl chalcones: a quantum chemical study

The structural and optical properties of 4-bromo-1-naphthyl chalcones (BNC) have been studied by using quantum chemical methods. The density functional theory (DFT) and the singly excited configuration interaction (CIS) methods were employed to optimize the ground and excited state geometries of unsubstituted and substituted BNC with different electron withdrawing and donating groups in both gas and solvent phases. Based on the ground and excited state geometries, the absorption and emission spectra of BNC molecules were calculated using the time-dependent density functional theory (TDDFT) method. The solvent phase calculations were performed using the polarizable continuum model (PCM). The geometrical parameters, vibrational frequencies, and relative stability of cis- and trans-isomers of unsubstituted and substituted BNC molecules have been studied. The results from the TDDFT calculations reveal that the substitution of electron withdrawing and electron donating groups affects the absorption and emission spectra of BNC.     

呋喃查尔酮结构与电子光谱的密度泛函理论研究

在密度泛函理论的PBE1PBE/6-31G(d)水平上对呋喃查尔酮及其衍生物的几何结构进行优化计算.在获得基态稳定结构的基础上,应用含时密度泛函理论计算其电子吸收光谱,探讨了取代基和溶剂对电子吸收光谱的影响,计算结果与实验结果吻合很好,平均绝对偏差仅为3.3nm(0.04eV).结果表明,取代基的引入和溶剂极性的增大均使光谱发生红移.通过前线轨道分析,揭示了该类化合物的主要吸收峰均源自分子中HOMO→LUMO电子跃迁.     

Molecular structure, vibrational spectra and NBO analysis of phenylisothiocyanate by density functional method

The molecular geometry, vibrational frequencies and NBO analysis of phenylisothiocyanate (PITC) in the ground state have been calculated by using density functional theory calculation (B3LYP) with 6-311++G(d,p) basis set. The optimized geometrical parameters obtained by DFT calculations are in good agreement with experimental values. Comparison of the observed fundamental vibrational frequencies of the PITC and calculated result by density functional theory (B3LYP) indicates B3LYP is superior for molecular vibrational problems. The entropy of the title compound was also performed at HF/B3LYP/6-311++G(d,p) levels of theory. Natural bond orbital (NBO) analysis of title molecule is also carried out. A detailed interpretation of the IR and Raman spectra of PITC is reported on the basis of the calculated potential energy distribution (PED). The theoretical spectrogram for IR spectrum of the title molecule has been constructed.     

Molecular structure, vibrational spectra and HOMO, LUMO analysis of yohimbine hydrochloride by density functional theory and ab initio Hartree-Fock calculations

Yohimbine hydrochloride (YHCl) is an aphrodisiac and promoted for erectile dysfunction, weight loss and depression. The optimized geometry, total energy, potential energy surface and vibrational wavenumbers of yohimbine hydrochloride have been determined using ab initio, Hartree–Fock (HF) and density functional theory (DFT/B3LYP) method with 6-311++G(d,p) basis set. A complete vibrational assignment is provided for the observed Raman and IR spectra of YHCl. The UV absorption spectrum was examined in ethanol solvent and compared with the calculated one in gas phase as well as in solvent environment (polarizable continuum model, PCM) using TD-DFT/6-31G basis set. These methods are proposed as a tool to be applied in the structural characterization of YHCl. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with frontier orbital gap are presented.     

Hydrogen bonding interactions between ethylene glycol and water: density,excess molar volume,and spectral study

Studies of the density and the excess molar volume of ethylene glycol (EG)-water mixtures were carried out to illustrate the hydrogen bonding interactions of EG with water at different temperatures. The re-sults suggest that a likely complex of 3 ethylene glycol molecules bonding with 4 water molecules in an ethylene glycol-water mixture (EGW) is formed at the maximal excess molar volume,which displays stronger absorption capabilities for SO2 when the concentration of SO2 reaches 400×10?6 (volume ratio) in the gas phase. Meanwhile,FTIR and UV spectra of EGWs were recorded at various EG concentra-tions to display the hydrogen bonding interactions of EG with water. The FTIR spectra show that the stretching vibrational band of hydroxyl in the EGWs shifts to a lower frequency and the bending vibra-tional band of water shifts to a higher frequency with increasing the EG concentration,respectively. Furthermore,the UV spectra show that the electron transferring band of the hydroxyl oxygen in EG shows red shift with increasing the EG concentration. The frequency shifts in FTIR spectra and the shifts of absorption bands in UV absorption spectra of EGWs are interpreted as the strong hydrogen bonding interactions of the hydrogen atoms in water with the hydroxyl oxygen atoms of EG.     

Predictions of the electronic absorption and emission spectra of luciferin and oxyluciferins including solvation effects

The ground and excited state properties of luciferin (LH(2)) and oxyluciferin (OxyLH(2)), the bioluminescent chemical in the firefly, have been characterized using the configuration interaction singles (CIS) and time dependent density functional (TDDFT) methods. The effects of solvation on the electronic absorption and emission spectra of luciferin and oxyluciferin are predicted with a self-consistent isodensity polarized continuum model of the solvent using both the configuration interaction singles model and time dependent density functional theory. The S(0)-->S(1) vertical excitation energies in the gas phase and in water are obtained with both methods. Optimizations of the excited state geometries permit the first predictions of the fluorescence spectra for these biologically important molecules. Shifts in both the absorption and emission spectra on proceeding from the gas phase to aqueous solution also are predicted.     

Revised vibrational band assignments for the experimental IR and Raman spectra of 2,3,4-trifluorobenzonitrile based on ab initio, DFT and normal coordinate calculations

In the present study, a systematic vibrational spectroscopic investigation for the experimental IR and Raman spectra of 2,3,4-trifluorobenzonitrile (TFB), aided by electronic structure calculations has been carried out. The electronic structure calculations – ab initio (RHF) and hybrid density functional methods (B3LYP) – have been performed with 6-31G* basis set. Molecular equilibrium geometries, electronic energies, IR intensities, harmonic vibrational frequencies, depolarization ratios and Raman activities have been computed. The results of the calculations have been used to simulate IR and Raman spectra for TFB that showed excellent agreement with the observed spectra. Potential energy distribution (PED) and normal mode analysis have also been performed. The assignments proposed based on the experimental IR and Raman spectra have been reviewed. A complete assignment of the observed spectra has been proposed.     

Theoretical investigation of the molecular, electronic structures and vibrational spectra of a series of first transition metal phthalocyanines

A theoretical investigation of the fully optimized geometries and electronic structures of metallophthalocyanines FePc, CoPc, NiPc, CuPc and ZnPc has been conducted with the density functional theory (DFT) method. A comparison between the different molecules for the geometry, molecular orbital, and atomic charge is made. The simulated order of the sizes of the central hole is FePc>CoPc>NiPcNiPc>CuPc>ZnPc, and the atomic charges of the central metal (M=Fe, Co, Ni, Cu, Zn) ions vary in the same order, FePc>CoPc>NiPcCoPc>FePc>CuPc>ZnPc, and the corresponding peaks predicted at 894, 896, 898, 882 and 871 cm(-1), respectively, also exhibit the same order as above-mentioned. Moreover, the lines of fit through plots of the experimental IR and Ra frequencies versus the calculated ones show very good correlations.     

DFT calculation of the chromyl nitrate, CrO2(NO3)2 The molecular force field

We have carried out a structural and vibrational theoretical study for chromyl nitrate. The density functional theory has been used to study its structure and vibrational properties. The geometries were fully optimised at the B3LYP/Lanl2DZ, B3LYP/6-31G* and B3LYP/6-311++G levels of theory and the harmonic vibrational frequencies were evaluated at the same levels. The calculated harmonic vibrational frequencies for chromyl nitrate are consistent with the experimental IR and Raman spectra in the solid and liquid phases. These calculations gave us a precise knowledge of the normal modes of vibration taking into account the type of coordination adopted by nitrate groups of this compound as monodentate and bidentate. We have also made the assignment of all the observed bands in the vibrational spectra for chromyl nitrate. The nature of the Cr-O and Cr<--O bonds in the compound were quantitatively investigated by means of Natural Bond Order (NBO) analysis. The topological properties of electronic charge density are analysed employing Bader's Atoms in Molecules theory (AIM).     

Theoretical Study on the Structures,Spectra and Thermodynamic Property of Herbicidal Monosulfuron

The structure optimization and frequency calculation have been carried out at the B3LYP/6-31G* level towards herbicidal monosulfuron using density functional theory.The computed results showed that the intramolecular hydrogen bond N-H…N can stabilize the molecule.IR spectra,Raman spectra and thermodynamic properties under different temperatures were also obtained.The first vertical excited state electronic transition energy was calculated by time-dependent density function theory,and absorption wavelength of the lowest energy excitation was obtained at 339.59 nm,belonging to the near UV.These results provided the basis for studies on compound's structure-activity relationship.     

Simulation of IR and Raman spectra of p-hydroxyanisole and p-nitroanisole based on scaled DFT force fields and their vibrational assignments

This work deals with the vibrational spectroscopy of p-hydroxyanisole (PHA) and p-nitroanisole (PNA) by means of quantum chemical calculations. The mid and far FT-IR and FT-Raman spectra were recorded in the condensed state. The fundamental vibrational frequencies and intensity of vibrational bands were evaluated using density functional theory (DFT) with the standard B3LYP/6-31G* method and basis set combination and were scaled using various scale factors which yield a good agreement between observed and calculated frequencies. The vibrational spectra were interpreted with the aid of normal coordinate analysis based on scaled density functional force field. The results of the calculations were applied to simulate infrared and Raman spectra of the title compounds, which showed excellent agreement with the observed spectra.     

DFT, FT-Raman and FT-IR investigations of 5-methoxysalicylic acid

FT-IR and FT-Raman spectra of 5-methoxysalicylic acid (5MeOSA) have been experimentally reported in the region of 4000–10 cm⁻¹ and 4000–50 cm⁻¹, respectively. The optimized geometric parameters, conformational equilibria, normal mode frequencies and corresponding vibrational assignments of 5MeOSA (C₈H₈O₄) are theoretically examined by means of B3LYP hybrid density functional theory (DFT) method together with 6-31++G(d,p) basis set. Furthermore, reliable vibrational assignments have made on the basis of potential energy distribution (PED) calculated and the thermodynamics functions, highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of 5MeOSA have been predicted. Calculations are employed for different conformations of 5MeOSA, both in gas phase and in solution. Solvent effects are investigated using chloroform and dimethylsulfoxide. All results indicate that B3LYP method is able to provide satisfactory results for predicting vibrational frequencies and the structural parameters, vibrational frequencies and assignments, IR and Raman intensities of 5MeOSA are solvent dependent.