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Fax: + 7 (095) 135 5328Regio-and stereoselectivity of [2+2]-photocycloaddition in complexes containing crown ether styryl dyes and alkaline-earth metal cations were studied using molecular mechanics. The main assumption is that the rate of the allowed concerted photocycloaddition correlates with the relative energies of the ground states of dimeric adducts, which are treated as prereaction complexes, and the resulting cyclobutanes. These energies are estimated by molecular mechanics within the MMX parameterization. The calculated characteristics of different dyes are studied as functions of the structure of the heterocyclic moiety, the structure and size of the N-substituent, the size of the crown ether cycle, and the nature of the metal cation. A comparison between the computational results and the experinaental data showed that the observed quantum yield of the reaction is governed by the relative energies of the dimeric complexes and the resulting cyclobutanes as well as by the mutual arrangement of dye molecules in the dimeric complexes. Both factors are closely interrelated. The approach used in this work furnishes an explanation for the experimental data and provides a guideline for supramolecular control of regio-and stereochemistry of cation-dependent [2+21-photocycloaddition.

According to the results of IR spectroscopic study and quantum--chemical calculations, hydroxo derivatives SiF4_x(OH)x are formed in the course of hydrolysis of silicon tetrafluoride in the presence of small amount of water along with hexafluorodisiloxane.Key words: silicon tetrafluoride, products of hydrolysis; IR spectroscopy, quantumchemical calculations.Silicon tetrafluoride is widely used for producing epitaxial layers of amorphous silicon and as a fluorinating agent in manufacturing SiO 2 fiber light pipes, t For these purposes SiF 4 with extremely low concentrations of oxygen-and/or hydrogen-containing impurities, z especially water and products of the hydrolysis of the main substance, is required. The most informative methods for the determination of impurities in SiF 4 are IR spectroscopy and mass-spectrometry. 3.4 It should be noted that the composition of the impurities in SiF 4 depends strongly on the method of preparation of the latter and the procedure for subsequent purification. Our preliminary study of IR spectra of gaseous SiF 4 synthesized by thermal decomposition of sodium hexafluorosilicate and containing water impurity showed the presence of an intense absorption band in the region near 3764 cm -t and a weaker band at 3860 cm -I. The aim of this work was to study the nature of the band at 3764 cm -I in the IR spectrum of silicon tetrafluoride experimentally and theoretically using ab initio quantum-chemical calculations. ExperimentalSilicon tetrafluoride was obtained by thermal decomposition of soclittm hexafluorosilicate and purified from volatile impurities by cryofiltration followed by rectification on a column with sampling the medium fraction.IR spectra of gaseous SiF 4 in the region 800--4500 cm -I were recorded on Specord MS0 and IFS-120HR instruments with a resolution of 4 cm -] and 0.1 cm -I, respectively. A multipass cell with a length of the optical path (/) of 100 cm and a cell with I = 20 cm with ZnSe windows were used. Spectra of solutions in liquid CCI 4 were obtained in a cell with an optical path length of I0 cm (ZnSe windows) equipped with an inlet system that allowed introducing gaseous SiF 4 and H20. Carbon tetrachloride was purified by rectification until a residual moisture content of 5 9 10 -5 tool.% was achieved.C_.aleulation IK'ocednre. Quantum-chemical calculations of the equilibrium molecular structure and vibrational spectra of possible products of hydrolysis of SiF 4 were carried out by the ab initio SCF method with inclusion of correlation energy according to the density functional theory (DFT) using the B3LYP mixed exchange-correlation functional in the 6-31 IG(d,p) basis set (five-component basis set functions). The geometry optimization and calculations of vibrational frequencies were carried out using the GAUSSIAN 94 program. 5.

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