Chemistry Journal of Moldova

DOI: dx.doi.org/10.19261/cjm.2008.03(1).12

Graphical Abstract: The vibronic origin of instability of the symmetrical forms (D¥ h, C2h and C2v) of the hydrogen peroxide molecule H2O2 was revealed using ab initio calculations of the electronic structure and the adiabatic potential energy curves. The vibronic constants in this approach were estimated by fitting of the ab initio calculated adiabatic potential in the vicinity of the high-symmetry nuclear configurations to its analytical expression. It was shown that the equilibrium “skewed” anticline shape of the C2 symmetry can be realized in two ways: D¥h ® C2v® C2 or D¥h ® C2h® C2 with the decreasing of the adiabatic potential energy at every step.


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DOI: dx.doi.org/10.19261/cjm.2008.03(1).13

Graphical Abstract: The electron-conformational (EC) method is employed to reveal the toxicophore and to predict aquatic toxicity quantitatively using as a training set a series of 51 compounds that have aquatic toxicity to fish. By performing conformational analysis (optimization of geometries of the low-energy conformers by the PM3 method) and electronic structure calculations (by ab initio method corrected within the SM54/PM3 solvatation model), the Electron-Conformational Matrix of Congruity (ECMC) was constructed for each conformation of these compounds. The toxicophore defined as the EC sub-matrix of activity (ECSA), a sub-matrix with matrix elements common to all the active compounds under consideration within minimal tolerances, is determined by an iterative procedure of comparison of their ECMC’s, gradually minimizing the tolerances. Starting with only the four most toxic compounds, their ECSA (toxicophore) was found to consists of a 4x4 matrix (four sites with certain electronic and topologic characteristics) which was shown to be present in 17 most active compounds. A structure-toxicity correlation between three toxicophore parameters and the activities of these 17 compounds with R2=0.94 was found. It is shown that the same toxicophore with larger tolerances satisfies the compounds with les activity, thus explicitly demonstrating how the activity is controlled by the tolerances quantitatively and which atoms (sites) are most flexible in this respect. This allows for getting slightly different toxicophores for different levels of activity. For some active compounds that have no toxicophore a bimolecular mechanism of activity is suggested. Distinguished from other QSAR methods, no arbitrary descriptors and no statistics are involved in this EC structure-activity investigation.


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DOI: dx.doi.org/10.19261/cjm.2008.03(1).15

Graphical Abstract: This review presents the basic concepts of the methods used for investigation of fast reactions kinetics, such as: flow methods, with particular emphasis on the stopped-flow approach, NMR, ESR, electrochemical methods, with particular emphasis on the time resolved Fourier Transform electrochemical impedance spectroscopy, flash photolysis, and several others. It offers a brief description of fast reactions commonly encountered in chemical systems, providing an insight into the possibilities of performing kinetic investigations of such reaction systems.


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DOI: dx.doi.org/10.19261/cjm.2008.03(1).17

Graphical Abstract: Differential scanning calorimetry has been used to study the α-relaxation (glass transition) in virgin polystyrene (PS), PS-clay nanocomposite, amorphous indomethacin (IM), maltitol (Mt) and glucose (Gl). Variation of the effective activation energy (E) throughout the glass transition has been determined by applying an advanced isoconversional method to DSC data on the glass transition. The relaxations have been characterized by determining the effective activation energies (E) and evaluating the sizes of cooperatively rearranging regions at the glass transition (Vg). The values of Vg have been determined from the heat capacity data. The α-relaxation demonstrates markedly larger values of E (~340 vs ~270 kJ mol-1) for the PS-clay system than for virgin PS. For IM in the glass transition region, the effective activation energy of relaxation decreases with increasing temperature from 320 to 160 kJ mol-1. In the Tg region E decreases (from~250 to ~150 kJ mol-1 in maltitol and from~220 to ~170 kJ mol-1 in glucose) with increasing T as typically found for the α-relaxation. It has been found that in the sub-Tg region E decreases with decreasing T reaching the values ~60 (glucose) and ~70 (maltitol) kJ -1 that are comparable to the literature values of the activation energies for the β-relaxation. Heat capacity measurements have allowed for the evaluation of the cooperatively rearranging region in terms of the linear size The PS-clay system has also been found to have a significantly larger value of Vg, 36.7 nm3 as compared to 20.9 nm3 for PS. Heat capacity measurements of IM have allowed for the evaluation of the cooperatively rearranging region (CRR) in term of linear size (3.4 nm) and the number of molecules (90). The size of CRR have been determined as 3.1 (maltitol) and 3.3 (glucose) nm.


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