Trianionic 1,3,2-Dioxaborine-Containing Polymethines: Bright Near-Infrared Fluorophores

Trianionic polymethines of the A′-π-A-π-A-π-A′ type comprising dioxaborine rings (A) and different electron-accepting end groups (A′) have been synthesized. The obtained dyes absorb and emit light in the near-infrared region with remarkably high molar attenuation coefficients (ϵ up to 495 000 M⁻¹ cm⁻¹ in DMF) and fluorescence quantum yields (Φ_f up to 0.73 in DMF). Thus, the novel trianionic dyes stand among the brightest individual fluorophores known to date – with a magnitude of fluorescence brightness (ϵ⋅Φ_f) of 313 000 M⁻¹ cm⁻¹ in DMF. The synthesized dyes demonstrate a minor negative solvatochromism and small Stokes shifts. X-ray data reveal the nearly planar geometry of the trianionic chromophore. All the obtained compounds are stable in the solid state and in a solution, although the relative stability is much higher in polar aprotic than in protic solvents.     

NDDO MO Calculations

Siegbahn's potential model as extended by Ellison et al. is used with density matrix elements calculated by the NDDO/2 procedure, to correlate the K-shell binding energy shifts of C, N and O atoms in a few molecules containing only the first-row atoms. The correlation is not superior to that obtained with the CNDO/2 method when only the monopole term is retained in calculating the Madelung potential energy. However, the results are in excellent agreement with experiment when the two-parameters model including the dipole and quadrupole terms is used.     

MNDO Calculations of silicon-containing molecules

A comparison is made of MNDO and MINDO /3 calculations for saturated silicon-containing molecules, and with experimental values, for heats of formation, molecular geometries, charge distributions, and ionization potentials. Except for bond angles, it is found that with the published parameter values the MINDO /3 program gives more reliable results than MNDO . For unsaturated molecules, a comparison of bond lengths and stabilities of Si multiple bonds as given by the two programs and ab initio methods is made, and large discrepancies between predicted structures are pointed out. Some reasons for the dicrepancies are discussed.     

基态H_2~－（X~2_U~＋）的ACQM计算

基态Ｈ_２~－（Ｘ~2_U~＋）的ＡＣＱＭ计算俞华根（中国科学院，成都有机所，６１００６５）关键词：排列通道量子力学，通道波函数，１９７７年，Ｌｅｖｉｎ和Ｋｒｇｅｒ￣［１］首次将排列通道量子力学（ＡＣＱＭ）方法用于计算和Ｈ＿２基态势能曲线后，经改进的ＡＣ?..     

Calculations concerning Kato's equations

Calculations of Kato's equations have been carried out as a function of the coordinates of the system using a number of approximate wave functions in order to determine the effects of the energy, electron density, constraints, and the functional form of the wave functions on how well Kato's equations are obeyed. It is shown that by putting constraints on the functional form of some wave functions, the wave functions will give the exact value of Kato's equations.     

Ion-Exchange Calculations Using Spreadsheets

A simple set of calculations using a Microsoft Excel spreadsheet can help students determine ion-exchange equilibrium parameters, essential for the design of industrial equipment. Equilibrium parameters are evaluated from experimental isotherms obtained from different zeolites using the action-law model by means of nonlinear curve fitting (Marquardt method). Students will be able to discriminate between ideal and nonideal systems by observation of activity coefficients. The possibility of using the spreadsheet with different types of ion-exchange materials is also validated by comparing the model results with experimental data from papers available in the recent literature. Although the laboratory experience is laborious, it is very satisfactory.     

Uncertainties in Solubility Calculations

Summary.  When considering the possible migration of hazardous elements in groundwater, one has to take into account several phenomena, e.g. solubility, ion exchange, adsorption, matrix diffusion, and transport paths. Here, we focus upon the solubility which in turn depends on several more or less uncertain chemical properties. Uncertainties in the data during laboratory experiments aiming at measurements of thermodynamic constants may cause uncertainties in the amount of some species of several tenths of the relative mass fraction. The thermodynamic data may then be used for solubility calculations under different conditions and water compositions. Clearly, there are several uncertainties associated with solubility calculations in the rock-water system. First, there is the effect of uncertainties in thermodynamic data such as stability and solubility constants, and also enthalpies of reaction if the water is not at room temperature. Furthermore, there are the rock-water interactions which will change the water composition as different minerals come in contact with the water flowing through a system of fractures. Studies in mineralogy to an accuracy good enough for modeling of water evolution are difficult to perform, and therefore the mineral composition of the rock and thus the water composition should be treated as parameters subjected to uncertainties. In addition, there are also conceptual uncertainties with respect to input data. The calculation of a solubility should be an easy task for every chemist, but in fact results differing by orders of magnitude are found even when the modelers have used the same computer program and the same data. In this paper, uncertainties associated with solubility calculations are discussed. The results are exemplified on the calculated solubilities of some actinides in groundwater from crystalline rock. Received August 21, 2000. Accepted (revised) May 18, 2001     

Strain Energy Calculations of Caged Silanes

Abstract

Strain energies (SE) of typical caged silanes were evaluated at the B3LYP/6-31+G** and B3LYP/aug-cc-pVDZ levels, in combination with the isodesmic and homodesmotic reactions. The SE values of Si₄H₄ and Si₈H₈ are 512 kJ/mol and 336–338 kJ/mol, respectively, at the B3LYP/6-31+G** level, whereas the SE values of the caged silanes with five-number or larger rings are very small. In comparison, the SE value of Si₈H₈ is much smaller than 656–707 kJ/mol of cubane (C₈H₈), since the Si?Si bond length (2.274 Å) of Si₈H₈ is much larger than the corresponding C?C bond length of cubane. The large electron charge density at body-center of Si₄H₄ causes strong repulsion between the cage center and Si?Si bond, which leads to the Si?Si bond bending. The electron charge density at the face center of Si₄H₄ is more than two times of that of Si₈H₈, which also contributes significantly to the large SE difference between Si₄H₄ and Si₈H₈.     

Calculations on the photodissociation of acetone

CNDO/2 approximate molecular orbital calculations were performed on the ground and excited states of acetone. A possible reaction pathway for the photodissociation was investigated by calculating the energy of the ³πσ^* state as a function of the carbon-carbon internuclear distance. The results suggest that as the dissociation progresses the methyl group appears to “roll-off” around the oxygen atom.     

Calculations of silicooxygen ring vibration frequencies

In the work model calculations of the vibrations of ideally isolated silicooxygen rings (using PM3 method) have been carried out. three-, four-, and six-membered rings have been considered. It has been found that that the three-membered silicooxygen rings are flat and practically undeformed showing D_3h symmetry. The rings of higher number of ring members (i.e. n>3) are deformed to some extent. The deformation reveals itself most significantly in the Si–O–Si bond angles distribution. In the case of all the rings the bridging Si–O–Si bonds are ca. 0.02–0.04 Å shorter than the non-bridging Si–O⁻ bonds. Hypothetical IR spectra for all the rings considered have been also calculated. Analysis of these hypothetical spectra leads to the conclusion that the whole spectrum can be divided into four wavenumbers regions, 1200–1100 cm⁻¹ stretching Si–O(Si) vibrations; 1000–800 cm⁻¹ stretching Si–O⁻ vibrations; 800–600 cm⁻¹; the region in which a band characteristic of silicooxygen rings appears, and below 600 cm⁻¹ bending ⁻O–Si–O⁻ and (Si)O–Si–O(Si). It has been also found that as the number of ring members increases the ‘ring band’ shifts to lower wavenumbers: 725 cm⁻¹ for three-membered rings, 650 cm⁻¹ for four-membered rings and 610 cm⁻¹ for six-membered rings. Calculated spectra have been compared with the experimental spectra of cyclosilicates. They showed good agreement in the 1200–600 cm⁻¹ region. In the experimental spectra as well as in the calculated ones, with increasing the number of ring members the ‘ring band’ shifts towards lower wavenumbers.     

Simple Statistical Calculations of Entropy Changes

The simple statistical treatment of the temperature equilibration of two two-level systems provides an easily understandable example of changes in entropy during reversible and irreversible processes. This treatment yields the fundamental formula dS = dq_rev/T; thus, it provides a useful link between the classical (macroscopic) and statistical (microscopic) view of thermodynamic processes.     

Calculations on the diphenylmethyl anion

Semiempirical and ab initio calculations for the diphenylmethyl anion and related species are reported. MINDO/3 calculations indicate a coplanar anion with an enlarged bond angle of ～139° to counteract the steric repulsions. MNDO calculations reveal an expanded bond angle with somewhat greater twist (21°). The ab initio calculations (STO -3G level) reveal an expanded central bond angle with an intermediate degree of phenyl twist. The results are compared with experimental data for this and related anions.     

Calculations of thermal functions of group-III nitrides

The change of thermal functions (ΔH ⁰(T), ΔS ⁰(T), ΔG ⁰(T)) and formation functions (ΔH _f⁰(T), ΔG _f⁰(T), K _f(T)) with temperature for gallium nitride and indium nitride have been formulated based on the reliable experimental data obtained by the use the same equipment in one laboratory.     

Theoretical Calculations of Combination of Hydrogen Atoms

A resonance-complex set was developed to investigate the process of combination of hydrogen atoms with a modified resonance-complex theory and Monte-Carlo trajectory method over the temperature range 40-300 K. The obtained ratios of ortho to para at 300 K and at 77 K agree satisfactorily with experimental results. Unlike previous results, a maximum of the rate coefficient as a function of temperature is found near 150 K.