Photochemistry in Interstellar Space and Reactions Generating Interstellar Molecules

Over 20 different molecules and radicals have so far been detected in interstellar space. The number of such molecules and radicals and their frequently complicated structure raise the question of the processes leading to their formation. Diatomic molecules and radicals could arise in gas-phase reactions. Formation of polyatomic molecules can be explained in terms of reactions proceeding on the surface dust particles and possibly involving interstellar radiation. The author demonstrates, with the aid of a model, that the molecular abundances and distributions within dust clouds in certain regions can be explained by photocatalytic reactions.     

Density matrix of crystalline systems. I. Long-range behavior and related computational problems

The one-electron density matrix (DM ) of crystalline systems is discussed, especially concerning its longrange behavior; reference is made throughout to systems treated at a Hartree–Fock–LCAO –SCF level of approximation. The analysis is performed on the assumption of generally smooth behavior of eigenvalues and eigenvectors in k (reciprocal) space, so that they can be expressed by means of a truncated Fourier expansion. This assumption allows us to obtain analytic approximations for the DM , on the basis of the information collected at a few, suitably selected sampling k points. It is therefore possible at the same time to discuss the influence of structural parameters (dimensionality of the system, existence and shape of the Fermi surface, structure of the chemical bonds) and to set up a computational scheme that is general and simple enough.     

Molecular silverware. I. General solutions to excluded volume constrained problems

General mathematical solutions to excluded volume constrained problems in computational chemistry are reported. The solutions have been used to create a new family of molecular modeling algorithms to facilitate the study of molecular interactions in condensed phases. The new algorithms, collectively known as Molecular Silverware, are for the most part interactive and designed for packing, solvating, and sampling molecules embedded in simple or complex topological environments. Multifolded, disconnected, or porous molecular structures are permitted. Molecular Silverware assists the preparation of Monte Carlo and molecular dynamics simulations at a small fraction of the total simulation time. Primary targets for applications include the study of molecular recognition mechanisms and the selective binding of DNA, RNA, peptides, saccharides and other biopolymers in solution as well as the prediction of phase separation behavior and physical properties of non-crystalline condensed phases such as bulk polymers, polymer blends, organic liquids, membranes, micelles, gels, crosslinked networks, glasses, and amorphous heterogeneous catalysts. As a result of this new approach to excluded volume constraints, the computer simulation of noncrystalline condensed phases is no longer hampered by the lack of a general and efficient method for the creation and configurational sampling of small and large molecular assemblies at high densities.     

Marangoni stresses and surface compression rheology of surfactant solutions. Achievements and problems

In the presence of soluble surfactants, the motion of liquid surfaces involves Marangoni effects. As a consequence, the surfaces exhibit elastic responses, even frequently behaving as rigid surfaces, especially at low surfactant concentration. The Marangoni effects can be conveniently quantified introducing surface viscoelastic compression parameters that characterize the mechanical response of the surface near equilibrium. Many experimental techniques allow measuring the viscoelastic parameters. However, many difficulties are encountered during the interpretation of the surface response in the various types of hydrodynamic velocity fields involved in the different techniques. The role of adsorption and desorption energy barriers appears crucial, despite the fact that little is known yet about their values. In this short review, we will present examples illustrating the different problems.     

Chromatographic solutions to pharmaceutical analytical problems

Summary Chromatography plays a major role in pharmaceutical analysis since classic methods such as titrimetry or direct spectroscopic analyses do not provide sufficient selectivity and/or detectability. Of various chromatographic methods, HPLC offers good resolution and great versatility in handling compounds that are thermally unstable, polar, and non-volatile at low temperatures. Discussed in this paper is an example of a pharmaceutical compound viz. baclofen [4-amino-3-(p-chlorophenyl)butyric acid], a compound with low UV absorptivitiy, where HPLC provides the desired selectivity and detectability to allow precise determinations in the presence of pharmaceutical excipients. Discussed here also are investigations on mechanism of separations by ion-exchange and ion-pair reversed-phase chromatography.Dedicated to Professor Leslie S. Ettre on the occasion of his 70th birthday.     

Numerical solutions for isoelectric focusing and isotachophoresis problems

This study combines an adaptive mesh redistribution (AMR) method and the space–time conservation element and solution element (CESE) method to construct a high-resolution scheme for the solution of electrophoresis pre-concentration and separation problems. In the proposed AMR–CESE scheme, the fine mesh points are moved toward the regions of discontinuity within the solution domain in accordance with the equidistribution principle. To reduce the numerical dissipation within the regions of the solution domain with a large spatial mesh, the spatial component of the CESE scheme is treated using a Courant–Friedrichs–Lewy (CFL) number insensitive scheme. The validity of the proposed approach is confirmed by comparing the results obtained for typical isoelectric focusing (IEF) and isotachophoresis (ITP) problems with those obtained from the conventional CESE scheme and the finite volume method (FVM), respectively. It is shown that the AMR–CESE scheme yields a better accuracy than uniform fixed-mesh solvers with no more than a minor increase in the computational cost.     

The Chemistry of Interstellar Space

Within the last ten to twenty years, radioastronomers have discovered the existence of almost 100 different molecules in interstellar space. Ranging in complexity from two to thirteen atoms, these molecules are found in cold, rarefied regions called interstellar clouds, which are giant accumulations of gas and dust located in our galaxy as well as many others. Interstellar clouds are also the birthplaces of future generations of stars and are of great interest to astronomers. The observation of the large sample of gaseous molecules, detected mainly via their rotational spectral patterns, tells astronomers about the detailed physical conditions in interstellar clouds, and tells chemists about the extent of molecular synthesis possible under the seemingly harsh conditions of low temperature and density. The molecules are mainly organic in nature and comprise species known to be both stable and common in the laboratory as well as those both unstable and uncommon under terrestrial conditions, including radicals and molecular ions. Although the gas phase of interstellar clouds is well studied via spectroscopic techniques, the dust particles are much more poorly characterized via their scattering and absorption of visible radiation as well as some broad resonances in the ultraviolet and infrared regions of the spectrum. It is normally thought that these submicron-sized particles consist of cores that are composites of silicate and carbonaceous materials with mantles that contain material deposited from the gas such as ices of water, ammonia, and methane. In addition to the dust particles and gaseous molecules, there is some evidence for very large aromatic molecules (the so-called polycyclic aromatic hydrocarbons or PAH's) which occupy a nether region in between large gas-phase species and small dust particles. As our understanding of the chemical processes in interstellar clouds increases, it may be possible to speculate how large interstellar molecules can come into existence and whether or not there is a clear connection between interstellar chemistry and the start of life on earth.     

Derivatization of organometal(loid) species by sodium borohydride problems and solutions.

Like other derivatization techniques, hydride generation is a chemical reaction that produces side-reactions leading to analytical problems. Demethylation of dimethylarsinic acid was observed to be dependent upon the pH level of the hydride generation reaction mixture. If the reaction mixture was acidic, then in addition to (CH3)2AsH, the monomethyl arsenic hydride [(CH3)AsH2] could be detected. Demethylation and also the formation of an unidentified arsenic species were noted when trimethyl arsonic oxide was used as derivatization educt. All of these effects depend on the pH level of the hydride generation mixture. We observed significant levels of organometal(loid) species of elements such as Ge, As, Sn, Sb, Hg and Bi in blank hydride generation mixtures. The organometal(loid) contamination was irreproducible even during I day using a single solution of sodium borohydride in deionized water. We concluded that the organometal(loid) contamination arises directly from the derivatization agent, sodium borohydride, itself. Use of helium purging and various adsorptive materials to decontaminate the sodium borohydride solution prior to analysis did not result in a significant decrease in organometal(loid) contamination levels. Use of a palladium-cluster stabilised with 1,10-phenanthrolin as alternative hydride generation derivatization agent was not found to be suitable, since reaction yields were poor and transmethylation reactions were noted.     

The quality of QSAR models: problems and solutions

Assessment of the quality of goodness-of-fit and the confidence in predictivity (prediction power) are the main terms used to define the statistical quality of QSAR models. Three parts of this assessment can be defined as: (1) Measure of goodness-of-fit. (2) Validation of model stability. (3) Predictivity analysis. Currently there are no mandatory requirements for the validation methods to be used and rules for the quantitative confidence estimates. To compare the statistical quality of QSAR models it is necessary to have an overall statistical quality index which will depend on the goodness-of-fit, validation and predictivity results together. To do so it is necessary to define the set of mandatory parameters for all three parts of assessment listed above and develop the approach for overall quality estimates based on these parameters. It is also necessary to include into the overall index the penalty mechanism for parameter absence. The goal of the present study is to analyse parameters for all three parts of the QSAR model statistical quality assessment and investigate the flexible weighting approach for the overall statistical quality index development. Due the different statistical parameters traditionally used for assessment of goodness-of-fit it is necessary to create the mechanism, which allows flexible set of parameters to be used for the overall statistical quality index. Only after approval by scientific community and regulatory boards the final set of mandatory parameters can be selected.     

Gas-Grain Modeling of Interstellar O2

Molecular oxygen (O\begin{document}$ _2 $\end{document}) is essential to human beings on the earth. Although elemental oxygen is rather abundant, O\begin{document}$ _2 $\end{document} is rare in the interstellar medium. It was only detected in two galactic and one extra-galactic region. The inconsistency between observations and theoretical studies is a big challenge for astrochemical models. Here we report a two-phase modeling research of molecular oxygen, using the Nautilus gas-grain code. We apply the isothermal cold dense models in the interstellar medium with two typical sets of initial elemental abundances, as well as the warm-up models with various physical conditions. Under cold dense conditions, we find that the timescales for gas-phase CO, O\begin{document}$ _2 $\end{document} and H\begin{document}$ _2 $\end{document}O to reach peak values are dependent on the hydrogen density and are shortened when hydrogen density increases. In warm-up models, O\begin{document}$ _2 $\end{document} abundances are in good agreement with observations at temperatures rising after 10\begin{document}$ ^5 $\end{document} yr. In both isothermal and warm-up models, the steady-state O\begin{document}$ _2 $\end{document} fractional abundance is independent of the hydrogen density, as long as the temperature is high enough (\begin{document}$ > $\end{document}30 K), at which O\begin{document}$ _2 $\end{document} is prevented from significant depleting onto grain surface. In addition, low density is preferable for the formation of O\begin{document}$ _2 $\end{document}, whether molecular oxygen is under cold conditions or in warm regions.     

Prebiotic chemistry in eutectic solutions at the water-ice matrix

A crystalline ice matrix at subzero temperatures can maintain a liquid phase where organic solutes and salts concentrate to form eutectic solutions. This concentration effect converts the confined reactant solutions in the ice matrix, sometimes making condensation and polymerisation reactions occur more favourably. These reactions occur at significantly high rates from a prebiotic chemistry standpoint, and the labile products can be protected from degradation. The experimental study of the synthesis of nitrogen heterocycles at the ice-water system showed the efficiency of this scenario and could explain the origin of nucleobases in the inner Solar System bodies, including meteorites and extra-terrestrial ices, and on the early Earth. The same conditions can also favour the condensation of monomers to form ribonucleic acid and peptides. Together with the synthesis of these monomers, the ice world (i.e., the chemical evolution in the range between the freezing point of water and the limit of stability of liquid brines, 273 to 210 K) is an under-explored experimental model in prebiotic chemistry.     

Dynamical problems in the theory of polymer solutions

Qualitative discrepancies are found between what is predicted by available theory and what is actually observed, for several concentration regimes of the dynamical properties of polymer solutions. The difficulties are most severe, from the standpoint of experiment or simulation as well as theory, for the entanglement concentration regime. However, the classical problems of chain polymers in dilute solution are not fully understood. For example, the constants of proportionality that relate hydrodynamic radii to the radius of gyration, in the nondraining limit and in theta solvents, may not be universal constants. That is, the proportionality constants may vary with polymer and solvent species. Discrepancies between theory and experiment are discussed for the two different systems, dilute chains and semidilute rods. Speculation is offered on the resolution of these difficulties.     

星际分子微波波谱研究进展

介绍了星际分子研究的概况,对微波波谱作为研究星际分子的重要方法的特点、特殊方法和技术以及取得的进展进行了综述,特别对近年来的微波波谱星际分子研究进行了分类介绍,提出了目前研究的难点并展望了发展趋势.     

Structure and Stability of Interstellar Molecule C_3S

ＩｎｔｒｏｄｕｃｔｉｏｎＳｍａｌｌｃｌｕｓｔｅｒｓｃｏｎｔａｉｎｉｎｇｃａｒｂｏｎａｎｄｓｕｌｆｕｒ,ｓｕｃｈａｓＣＳ ,Ｃ2 ＳａｎｄＣ3 Ｓ ,ｗｈｉｃｈｐｏｓｓｅｓｓｌａｒｇｅｐｅｒｍａｎｅｎｔｄｉｐｏｌｅｍｏｍｅｎｔｓａｎｄｈａｖｅｂｅｅｎｉｄｅｎｔｉｆｉｅｄｉｎｔｈｅｃａｒｂｏｎｓｔａｒＩＲＣ+ 10°2 16ａｎｄｉｎｔｈｅＴａｕｒｕｓｃｏｌｄｍｏｌｅｃｕｌａｒｄｅｎｓｅｃｌｏｕｄＴＭＣ 1,1 7ｈａｖｅａｔｔｒａｃｔｅｄｍｕｃｈａｔｔｅｎｔｉｏｎｂｅｃａｕｓｅｏｆｔｈｅｉｒｉｍｐｏｒｔａｎｔｒｏｌｅｓ…     

A matrix solution to linearized reaction-diffusion boundary value problems

The balance equations of multicomponent diffusion with chemical reactions of linearized kinetics valid for diluted species have been given in the vectoAn illustrative example taken from the absorption—desorption process occurring in fermentation broths has been analyzed according to the approach dev     

Singlet H-cCNC-C：A Potential Interstellar Molecule

A new planar isomer of HNC3 system,H-cCNC-C,is theoretically predicted by means of B3LYP and CCSD(T) methods.The suggested species can isomerize into other five kinetically more stable isomers,which have been experimentally identified,with relatively higher reaction barriers.In view of its higher kinetic stability,we can reasonably believe that the obtained species H-cCNC-C can be experimentally observed in future studies.