Using ab-initio calculations at the MP2/6-311G(3df, 2pd), a very high level theory, we obtained minimum energy structures for the C2H5O radical, products and transition states of reactions on the potential energy surface. | JOURNAL OF SCIENCE OF HNUE DOI Chemical and Biological Sci. 2015 Vol. 60 No. 9 pp. 44-50 This paper is available online at http THEORETICAL STUDY OF TEMPERATURE-AND PRESSURE- DEPENDENT RATE CONSTANT FOR UNIMOLECULAR DECOMPOSITION REACTIONS OF C2H5O RADICAL Nguyen Ngoc Tri1 2 and Nguyen Ngoc Ha1 1 Faculty of Chemistry Hanoi National University of Education 2 Faculty of Chemistry Quy Nhon University Abstract. Using ab-initio calculations at the MP2 6-311 G 3df 2pd a very high level theory we obtained minimum energy structures for the C2H5O radical products and transition states of reactions on the potential energy surface. The decomposition reactions of the C2H5O radical include three reactions C2H5O CH3 HCHO 1 C2H5O CH3CHO H 2 and C2H5O CH2OCH2 H 3 . Remarkably the TS-3 transition state and 3 reaction are not found on the previous studies. The rate constants at the high-pressure limit of the 1 2 and 3 reactions were calculated using the transition state theory TST and the Rice-Ramsperger-Kassel-Marcus RRKM theory. The pressure-dependent rate constants of decomposition reactions were analyzed and solved using the RRKM theory and the master equation method. The temperature and pressure- dependent rate constant of three reactions were k T P 1 x - T k T P 2 x T and k T P 3 x T . The result shows that at low-temperature the rate constants of reactions 1 and 2 depend on low-pressure and are nearly independent of high-pressure while the rate constant of reaction 3 depends on both low and high pressure. At high- temperature the rate constants of the three reactions depend on pressure. Keyword Unimolecular reaction RRKM temperature- and pressure-dependent rate constant. 1. Introduction Alkoxy radicals are important immediates in reaction processes organic chemistry reactions and atmospheric chemistry. Moreover those are oxygen-containing .