Quantum-Chemical Calculations of the Affinity of Neutral Molecules for Protons

Quantum-chemical calculations (B3LYP/6-31++G(d,p)) of proton affinities were performed for molecules of various structures in the gas phase. The errors in calculation results were estimated. The data on the proton affinity of several molecules for which experimental proton affinity values are unavailable were obtained. The hypothesis according to which the structure of heteroassociates in the HF-aprotic organic solvent systems depended on the proton affinity of solvent molecules was substantiated.     

First-Principles Investigations of Precursor Molecules for Microclusters

Details of the reactive processes by which TiC and TiN microclusters are formed from precursor molecules in the gas phase are not clearly understood. We have performed ab initio calculations based on density functional theory methods in order to get some insight into the chemical reactions of precursor molecules. Using microcanonical molecular-dynamics we have simulated scattering processes between molecules consisting of the elements Ti, Cl, N and H. The simulations show that fluctuations of the kinetic energy can be large enough to break up bonds at moderate temperatures.     

Quantum-Chemical Investigation of the Intermolecular Interaction Effect on the Spectral-Luminescent and Physical-Chemical Properties of Certain 8-Azasteroid Class Molecules

The effect of hydrogen bonds on the spectral-luminescent and proton-acceptor properties of 8-azagone-12,17-dione and its 2,3-dimethoxy substituent is examined. The method of molecular electrostatic potential is used for choosing a spatial model for complexes with the 1:2 mixture ratio. Hydrogen bonding of oxygen atoms of C and D cycles of both molecules with water molecules is shown to affect but slightly the spectral-luminescent properties. The effect of hydrogen bonds on the proton-acceptor properties of molecules both in the ground and fluorescent states is most pronounced: the proton-acceptor properties of oxygen atoms of the C, D, and methoxy groups decrease, particularly in the fluorescent state, while the same properties of the nitrogen atom increase. We can assume on the basis of these facts that the role of the nitrogen atom in the intermolecular interaction is increased when it is in the S ₁ state.     

Infrared and Quantum-Chemical Studies of Dimethylsilane

Infrared spectra of (CH₃)₂SiH₂ and (CH₃)₂SiD₂ have been reinvestigated. Quantum-chemical calculations of geometry and force fields have been made at the HF, MP2, and B3LYP levels, followed by scaling of the force field with 14 independent parameters. Several vibration frequencies are reassigned. Bond lengths, force constants, scale factors, and electrical properties are compared with those in other methylsilanes. Both Si-H and Si-C bonds are weakened by α-methyl substitution. The C-H bonds in the methyl groups differ slightly in length and stretching frequency in ways familiar in hydrocarbons. An increase in SiH stretching intensity per bond with methylation is predicted theoretically and found by experiment. The sizes of the negative charges associated with both stretching and bending of the SiH bonds in the methylsilanes increase with the Mulliken charge with progressive methylation.     

Luminescence Investigations of the Effect of the Structure of the Molecules on their Stability during Interaction with Electrons in the Gas Phase

Journal of Applied Spectroscopy - By luminescence spectroscopy of the decomposition products during excitation by monochromatic electrons with various energies in the gas phase it was shown that...     

Quantum-Chemical Investigation of Spectral-Luminescent and Physical-Chemical Properties of Aminopyridines

The spectral-luminescent properties of ortho-, meta-, and paraaminopyridines are examined using the method of intermediate neglect of differential overlap. The absorption spectra are interpreted. It is shown that the long wavelength band of all aminopyridines is formed by several electronic transitions of various nature and intensity. The nonequivalence of the NH-bonds of an amino group in o-aminopyridine is estimated from an analysis of bond populations and their changes under excitation. It is suggested that the nonequivalence is caused by the field effect due to the influence of an excess charge of the nitrogen atom in the orthoposition on the distribution of electron density in the region of localization of the amino group.     

Quantum-Chemical Investigation of Spectral-Luminescent and Physical-Chemical Properties of Benzamides

The effect of a halogen atom in orthoposition of a phenyl radical on the spectral, geometrical, and photophysical characteristics as well as on the proton-donor and proton-acceptor properties of the 2-Cl- and 2-Br-benzamides was studied by electron spectroscopy and quantum chemistry. The intersystem crossing is shown to be the major decay pathway for lower singlet states. A comparative analysis of the bond populations of free molecules of benzamide and its halogen derivatives suggests that NH bonds of an amino group are nonequivalent in the amides under study.     

Quantum-Chemical Simulation of the Solvent Effect on Spontaneous Emission of Singlet Oxygen

A molecular simulation of the solvent effect on radiative rate constant k _r of singlet oxygen is carried out. This study included a search for the most probable conformations of the complexes of molecules of singlet oxygen and ten solvents and calculation of dipole moments M of transitions a¹Δ _g –b¹Σ _g ⁺ (M _a–b ) and a¹Δ _g –X³Σ _g ^- (M _a–X ) of the oxygen molecule for them. Averaging of M _a–b by conformations, taking into account the probability of their formation for complexes without atoms with a large atomic number (Cl, S), yields values that, as a rule, correlate well with the behavior of k _r in the experiment. Taking into account the possibility of decreasing the distance (compared to equilibrium) between molecules in a collision complex at room temperature made it possible to achieve satisfactory agreement of the calculated and experimental data also for complexes with CCl₄, C₂Cl₄, and CS₂. The obtained data indicate that a number of factors affect k _r . The correlation of k _r with molecular polarizability in a number of cases is due, on the one hand, to its effect on the strength of dispersion interactions in the complex and, on the other hand, to the fact that it to some extent reflects the position of the upper filled orbitals of the solvent molecule. Both factors affect the degree of mixing of the π orbitals of the singlet oxygen molecule with the orbitals of the solvent molecule, which, as was found earlier, facilitates the activation of the a¹Δ _g –b¹Σ _g ⁺ transition and the borrowing of its intensity by the a¹Δ _g –X³Σ _g ^- transition.     

Quantum-Chemical Study of the Nature of Electronically Excited States of Hydroxyaromatic Compounds

Russian Physics Journal - Spectroscopy methods make it possible to record the features of specific photo-induced processes occurring inside and between molecules and, moreover, chemical and...     

A Quantum-Chemical Investigation of the Geometry and NMR Chemical Shifts of Bilirubin

A computational investigation using density-functional-theory methods has been performed concerning the structure and nuclear magnetic resonance (NMR) chemical shifts of bilirubin with a special emphasis on the hydrogen bonds. Solid-state effects on the NMR spectra are investigated by considering a trimeric model derived from the available X-ray structure. Satisfactory agreement between theory and experiment is found with ring-current effects playing only a minor role for the interpretation of the solid-state NMR spectra. Authors' address: Jürgen Gauss, Institut für Physikalische Chemie, Universit?t Mainz, 55099 Mainz, Germany     

拟除虫菊酯光解产物的红外图谱研究

本文研究了胺菊酯、甲醚菊酯、丙烯菊酯和氯氰菊酯在液相中的光解作用，并用离心薄层层析方法对光解混合物进行了分离。对光解产物的ＦＴＩＲ红外光谱谱图和标样的ＦＴＩＲ红外光谱谱图的研究表明，光解产物的ＦＴＩＲ红外光谱图发生了较大变化，有羟基和环氧化结构产生，即有氧化产物生成；甲醚菊酯和丙烯酯光解产物的元素分析结果与表明产物中氧的含量有明显提高、证实氧参与了拟除虫菊酯的光解过程。     

基于量子化学计算的谷氨酰胺太赫兹吸收谱模拟

利用量子化学计算方法模拟物质的太赫兹吸收谱,可以为目标物质的太赫兹吸收特征匹配分子振动模式,对深刻理解谱的形成机理十分必要。模拟结果的可靠性,主要取决于目标物质初始构型的搭建和振动模式计算方法的选择。首先利用太赫兹时域光谱技术获取了谷氨酰胺固态样品的太赫兹吸收谱,为了在理论模拟过程中体现考虑分子间作用的程度,构建了三种常用于有机物太赫兹吸收谱模拟的谷氨酰胺初始构型,单分子、二聚体、晶胞。使用量子化学计算程序基于密度泛函理论对三种初始构型进行了结构优化和振动模式计算。将计算结果通过洛伦兹线型函数拟合为吸收谱与实验吸收谱进行对比发现,二聚体构型的模拟吸收谱从吸收峰个数上优于单分子构型,在此基础上,晶胞构型模拟结果从吸收峰峰位上又较二聚体构型有了明显的改进,随着初始构型考虑分子间作用的程度提高模拟结果逐步逼近实验吸收谱。在得到了可靠的理论模拟吸收谱的基础上,成功对谷氨酰胺固态样品在0.3～2.6 THz范围内的三个吸收峰匹配了晶胞内各分子的集体振动模式。研究表明,在固体样品的太赫兹吸收谱理论模拟中,在计算能力允许的情况下,应尽可能选择全面反映分子间作用力的晶胞构型作为计算的初始构型。     

Quantum-Chemical Study of Stressed Polyethylene and Butadiene Rubber Chain Scission

The thermal decomposition of polyethylene and butadiene rubber chains in the presence of a tensile force acting along the axis of the molecule was simulated. The reaction of an isolated chain was considered. The chain models were the octane and 2,6-octadiene molecules. A deformation was introduced in the problem by fixing nonequilibrium distances between the terminal carbon atoms. The reaction coordinate (the middle C–C bond length R) was scanned at a fixed length of the molecule (L). That is, the potential energy surface section of the reaction was constructed at L = const. The reaction sensitivity to deformation was evaluated by B3LYP, LC-ωPBE, CCSD(T), CASSCF, and MP2 quantum-chemical calculations. All these calculations showed that the molecule elongated by ~1 Å for polyethylene, but shortened by 0.3–0.5 Å for 2,6-octadiene during chain scission. This means that the tensile deformation accelerates the decomposition of polyethylene, but decelerates the decomposition of butadiene rubber.     

Excited State Properties of Fluorine-Substituted Hexabenzocoronene: A Quantum-Chemical Characterization

The first fluorine-substituted hexabenzocoronene has been synthesized and its electronic structure and optical properties have been reported [Q. Zhang, et al., Org. Lett.7 (2005) 5019]. In this letter, the electronic structure and excited state properties of the fluorine-substituted hexabenzocoronene are studied with quantum chemistry method as well as the transition and the charge difference densities. The transition densities show the orientations and strength of the dipole moments and the charge difference densities reveal the orientation and results of the intramolecular charge transfer. The calculated transition energies and oscillator strengths are consistent with the experimental data, and the theoretical results of transition and charge difference densities are valuable to understanding the excited state properties of the fluorine-substituted hexabenzocoronene.     

Excited Atoms in the Flash Photolysis of some Germanium Compounds

We have recently reported measurements of the rates of the collisional relaxation of various atoms amongst the spin-orbit components of their ground electronic states; an approximate correlation exists with other types of molecular energy transfer.^1-3 We present here an outline of some qualitative observations of the production and relaxation of Ge(4¹D₂) and Ge(4³P_j) in the flash photolysis of a number of germanium compounds. Atomic germanium vas selected for study because the ground state comprises a ‘normal’ Landé multiplet and should exhibit ‘Inverted’ relaxation as compared for example to Fe(a⁵D_j) which equilibrates initially in the high energy substates. The low lying states of germanium are listed In table 1.⁴ The flash photolysis apparatus was of standard design^1-3 and spectra were recorded with a Hilger medium quartz spectrograph.     

Excited State Properties of Fluorine-Substituted Hexabenzocoronene： A Quantum-Chemical Characterization

The first fluorine-substituted hexabenzocoronene has been synthesized and its electronic structure and optical properties have been reported [Q. Zhang, et al., Org. Lett. 7 （2005） 5019]. In this letter, the electronic structure and excited state properties of the fluorine-substituted hexabenzocoronene are studied with quantum chemistry method as well as the transition and the charge difference densities. The transition densities show the orientations and strength of the dipole moments and the charge difference densities reveal the orientation and results of the intramolecular charge transfer. The calculated transition energies and oscillator strengths are consistent with the experimental data, and the theoretical results of transition and charge difference densities are valuable to understanding the excited state properties of the fluorine-substituted hexabenzocoronene.     

阻抑动力学同步荧光法测定环境水样中苯胺

在稀硫酸介质和CTMAB存在下 ,痕量苯胺对溴酸钾氧化荧光素的反应具有灵敏的阻抑作用 ,荧光素的氧化产物有强荧光 ,但其激发光谱和发射光谱严重重叠 ,文章采用同步荧光光谱法减小了谱带重叠 ,获得了很好的灵敏度和选择性 ,据此建立了阻抑动力学同步荧光法测定苯胺的新方法。方法的线性范围是 0～10 μg·L- 1 ,检测限为 0 3μg·L- 1 。用于环境水样和实验室废水中苯胺含量的测定 ,结果满意