CALCULATION OF THERMODYNAMIC PARAMETERS OF SYNTHESIS REACTIONS OF CYCLODI-, CYCLOTRI-AND CYCLOTETRA DIMETHYLSILOXANES

DOI: 10.6060/tcct.20165911.5354
Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2016. V. 59. N 11. P. 27-32

Reactions of synthesis of cyclodimethylsiloxanes (cyclodi-, cyclotri- and cyclotetradi-methylsiloxanes) from dimethylsiloxane were studied by the methods of quantum chemistry and chemical thermodynamics. The temperature dependence of the thermodynamic functions of reaction components (E – internal energy, S – entropy, H – enthalpy, G – Gibbs energy, Cp – heat capacity at constant pressure and (G-H298)/T – reduced Gibbs energy) was estab-lished according to quantum-chemical calculations which were carried out with the optimiza-tion of geometrical parameters of the molecules and the control of vibrational frequencies using statistical thermodynamics methods. The data on chemical hardness η of the molecules derived from the calculated energy of the frontier orbitals, are presented. It was found that only (CH3)2SiO LUMO energy En+1 was negative, thus, that molecule was electrophilic; the other molecules with positive value of energy were nucleophilic. The value of the chemical hardness η for all molecules exceeded 1 eV, that meant they were hard electro-/nucleophiles. The statistical and quantum-chemical methods of calculation of the thermodynamic functions of the molecule [(CH3)2SiO]4 at T = 298.15 K were compared. It was shown that the results of the calculations by the statistical method were in good agreement with the corresponding quantum-chemical data. The Gibbs energy G of the reactions considering the electron energy of the components Eelec was calculated. The favorable conditions for the reactions proceed were estimated by the value of the Gibbs energy G. It was established that the cyclotetradi-methylsiloxane formation was the most preferable at all the examined temperatures. The da-ta obtained on the dimethylsiloxanes structure optimization indicated, that bond length Si – C varied slightly from molecule to molecule. The lengths of Si – O bonds varied depending on the geometrical structure of molecules. The paper showed that modern quantum-chemical methods provided valuable information which is necessary to analyze the geometric and en-ergy characteristics of the silicon compounds. It was noticed that it was possible to use data on thermodynamic functions obtained by quantum chemical calculations for the analysis of silicon compounds reactions in the absence of published data.

Key words: cyclic siloxanes, cyclotetradimethylsiloxane, quantum-chemical calculations, ther-modynamic parameters

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2016, Т. 59, № 11, Стр. 27-32

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