E.s.r. studies of the reactions of atomic oxygen and hydrogen with simple hydrocarbons

Abstract

Electron spin resonance has been used to study the reaction of oxygen atoms in a nitrogen carrier and of hydrogen atoms in an argon carrier with simple hydrocarbons in a fast flow system at total pressures around 2 mm Hg. The reaction between oxygen atoms and acetylene occurs predominantly by the mechanism: O+C₂H₂= CH₂+CO (3) O+CH₂= CO+2H (6) CH₂+C₂H₂= C₃H₄(12) where k₃=(9.2 ± 0.4)× 10¹⁰ cm³ mole^–1 sec^–1 and k₆=(2.7 ± 1.0)k₁₂ both at 298°K. The initial step in the reaction of oxygen atoms with methyl acetylene is O+CH₃ . C CH = CH₃ . CH : + CO (13) with k₁₃=(4 ± 1)× 10¹¹ cm³ mole^–1 sec^–1 at 298°K; the total rate constant for the disappearance of oxygen atoms being 3–4 times greater.

The mechanisms of the O+C₂H₄ and O+CH₄ reactions have been deduced from their overall stoichiometries and the hydrogen atom yields. For the initial steps O+C₂H₄= CH₃+HCO (15) O+CH₄= CH₃+OH (23)k₁₅=(3.2 ± 0.4)× 10¹¹ cm³ mole^–1 sec^–1 at 298°K and k₂₃=(7 ± 2)× 10¹² exp (–7700 ± 300/RT) cm³ mole^–1 sec^–1 between 450 and 600°K. The addition of hydrogen atoms to methyl acetylene and to ethylene were second order processes, H+C₃H₄= C₃H₅(4) H+C₂H₄= C₂H₅(17) with k₄=(2.4 ± 0.3)× 10¹¹ cm³ mole^–1 sec^–1 and k₁₇=(8.8 ± 0.4)× 10¹⁰ cm³ mole^–1 sec^–1 at 298°K.