Assessment of Gaussian-4 theory for the computation of enthalpies of formation of large organic molecules

Gas-phase enthalpies of formation of 122 relatively large organic molecules with up to 15 non-hydrogen atoms have been calculated at the Gaussian-4 (G4) level of theory using the atomization reaction procedure. The calculated values were compared with experimental data published mainly last years. Particular attention has been given to nitro compounds and nitrogen, oxygen, and sulfur containing heterocyclic compounds. The expected accumulation of systematic errors as the molecular size increases was not observed with increasing the number of non-hydrogen atoms from 6 to 15. The largest mean absolute deviation between experimental and G4 enthalpies of formation, 10.7 kJ/mol, was revealed for nitro compounds. All theoretical values for nitro compounds were underestimated by 5–15 kJ/mol. The best agreement with experiment with mean absolute deviation of 4.5 kJ/mol was observed for compounds which types were widely presented in the original test set of G4 method. The mean absolute deviations for nitrogen heterocycles (6.8 kJ/mol) and oxygen and sulfur heterocycles (9.1 kJ/mol) are noticeably larger. Experimental enthalpies of formation of four compounds (N,N-dinitromethanamine, 2,3,5,6-tetrachloronitrobenzene, 2-methyl-2H-tetrazole, and proline) were suggested to be unreliable from comparison with the G4 values calculated from atomization energies and isodesmic reactions.     

A wide-range, low-cost 150 bp ladder for sizing DNA fragments between 150 and 4500 bp

Agarose gel electrophoresis, a very routine procedure, requires molecular weight standards; these are usually manufactured from plasmid or viral DNA fragments, or more recently, from PCR products of defined sizes. We describe here the preparation of a molecular weight standard from a completely different DNA source - the uniquely organized genome of the beetle Tenebrio molitor. The standard can be used to accurately size DNAs between 150 and 4500 bp, a useful range of sizes for many agarose gel electrophoresis applications, including separation of PCR products and plasmid cloning targets. In addition, it is easy to prepare, inexpensive, and rivals the best of the commercial ladders.     

Behaviour of pure water and water mixture with benzene or chloroform adsorbed onto ordered mesoporous silicas

Structural characteristics of synthesized ordered mesoporous silicas MCM-41, MCM-48 and SBA-15 were studied using XRD, nitrogen adsorption and FTIR methods. Pure water and mixtures with water/benzene and water/chloroform-d adsorbed onto silicas were studied by ¹H NMR spectroscopy with layer-by-layer freezing-out of bulk and interfacial liquids. Concentrated aqueous suspensions of MCM-48 and SBA-15 were studied by thermally stimulated depolarization current (TSDC) method. Benzene and chloroform-d can displace a portion of water to broad pores from the pore walls and from narrower pores, especially in the case of a large excess of an organic solvent. This process is accompanied by diminution of both interaction energy of water with an adsorbent surface and freezing temperature depression of adsorbed water. The effect of nonpolar benzene on pore water is much stronger than that of weakly polar chloroform-d. Modifications of the Gibbs-Thomson relation to describe the freezing point depression of mixtures of immiscible liquids confined in pores allow us to determine distribution functions of sizes of structures with unfrozen pore water and benzene. Former address: Pisarzhevskii Institute of Physical Chemistry, 31 Prospect Nauki, Kiev, Ukraine     

A new method for [c,d]pyridine peri-annelation: synthesis of azapyrenes from phenalenes and their dihydro derivatives

An effective synthesis of various azapyrenes from phenalenes and their dihydro derivatives has been developed using 1,3,5-triazines in polyphosphoric acid (PPA).     

Colloidal FeIII,MnIII, CoIII,and CuII Hydroxides Stabilized by Starch as Catalysts of Water Oxidation Reaction with One Electron Oxidant Ru(bpy)33+

Colloidal catalysts for oxidation of water to dioxygen, which are stable on storage and under the reaction conditions, are synthesized based on Co^III, Mn^III, Fe^III and Cu^II hydroxides. Stabilization of the colloids with dextrated starch allows the process of hydroxide ageing to be stopped at the stage of the formation of primary nuclei (ca. 2–3 nm from transmission electron microscopy data). Molecular mechanics and dynamic light scattering studies indicate a core-shell type structure of the catalysts, where the hydroxide core is stabilized by the molecular starch network (ca. 5–7 nm). The colloidal catalysts are highly efficient in oxidizing water with one electron oxidant Ru(bpy)₃³⁺ at pH 7 to 10. The influence of pH, catalyst concentration, and buffer nature on the oxygen yield is studied. The maximal yields are 72, 53, and 78 % over Fe-, Mn- and Co-containing catalysts, respectively, and turnover numbers are 7.8; 54 and 360, respectively. The Cu-containing catalyst is poorly effective to the water oxidation (the maximal yield is 28 % O₂). The synthesized catalysts are of interest for stopped-flow kinetic studies of the mechanism of the water oxidation and as precursors for anchoring nanosized hydroxides onto various supports in order to develop biomimetic systems for artificial photosynthesis.     

The effect of preparation method on the defect structure and luminescence properties of γ-Al2O3

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