VIBRONIC ORIGIN OF THE “SKEWED” ANTICLINE CONFIGURATION OF THE HYDROGEN PEROXIDE MOLECULE

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

COMPUTER-BASED PREDICTION OF TOXICITY USING THE ELECTRON-CONFORMATIONAL METHOD. APPLICATION TO FRAGRANCE ALLERGENS AND OTHER ENVIRONMENTAL POLLUTANTS

The electron-conformational (EC) method is employed for the toxicophore (Tph) identification and quantitative prediction of toxicity using the training set of 24 compounds that are considered as fragrance allergens. The values of a=LD50 in oral exposure of rats were chosen as a measure of toxicity. EC parameters are evaluated on the base of conformational analysis and ab initio electronic structure calculations (including solvent influence). The Tph consists of four sites which in this series of compounds are represented by three carbon and one oxygen atoms, but may be any other atoms that have the same electronic and geometric features within the tolerance limits. The regression model taking into consideration the Tph flexibility, anti-Tph shielding, and influence of out-of-Tph functional groups predicts well the experimental values of toxicity (R2 = 0.93) with a reasonable leaveone- out cross-validation

AB INITIO STUDY OF CHEMICAL ACTIVATION AND HYDROGENATION OF WHITE PHOSPHORUS IN REACTION WITH RHODIUM TRIHYDRIDE COMPLEX

The four-stage mechanism of reaction of the rhodium trihydride complex [(triphos)RhH3] (triphos=1,1,1-tris(diphenylphosphanylmethyl)ethane) with the white phosphorus molecule resulting in the phosphane and the cyclo-P3 complex [(triphos)M(η3-P3] is analyzed on the basis of ab initio calculations of reactants, products, and intermediate complexes of reaction. It is shown that generation of the transient complex [(triphos)RhH(η1:η1-P4)] followed by intramolecular hydrogen atom migration from the metal to one of the phosphorus atoms is the energetically favourable process. Calculations also show that P4 molecule is activated by coordination to the above complex: the metal-bonded P-P edge is broken, and the tetrahedron P4 is opened to form the butterfly geometry. This activation is realized mainly due to the one-orbital back donation of 4d-electron density from the atom of Rh to the unoccupied antibonding triple degenerate t1*-MO of P4

ACTIVATION OF ACETYLENE BY COORDINATION TO BIS-TRIPHENYLPHOSPHINE COMPLEX OF Pt(0): DFT STUDY

The present work is devoted to the theoretical study of the activation of the acetylene molecule coordinated in the [Pt(PPh3)2C2H2] complex. By means of DFT calculations it is shown that the geometrical and electronic characteristics of the C2H2 are essentially changed due to its coordination. The subsequent detailed analysis of the molecular orbitals (MO) of the active valence zone of this complex allows one to make important conclusion that this activation is being realized mainly due to the orbital back donation of 5d-electronic density from one of the occupied MOs of the complex [Pt(PPh3)2] to the unoccupied antibonding π*-MO of C2H2

TOXICOPHORES AND QUANTITATIVE STRUCTURE -TOXICITY RELATIONSHIPS FOR SOME ENVIRONMENTAL POLLUTANTS

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