Theoretical study of deuterium isotope effects in the reactions H/sub 2/ + CH/sub 3/ and H + CH/sub 4/

Journal Article · Thu Jan 19 04:00:00 EST 1984 · J. Phys. Chem.; (United States)

DOI:https://doi.org/10.1021/j150646a013· OSTI ID:6922807

Schatz, G C; Wagner, A F; Dunning, Jr, T H

This paper presents ab initio potential surface parameters and transition state theory (TST) rate constants for the reaction H/sub 2/ + CH/sub 3/ ..-->.. H + CH/sub 4/, its reverse, and all the deuterium isotopic counterparts associated with it and its reverse. The potential surface parameters are derived from accurate POLCI calculations and include vibrational frequencies, moments of inertia, and other quantities for CH/sub 3/, H/sub 2/, CH/sub 4/, and the H-H-CH/sub 3/ saddle point. TST rate constants are calculated from standard expressions and the Wigner tunneling correction. For H/sub 2/ + CH/sub 3/ and H/sub 2/ + CD/sub 3/, agreement of the rate constant with experiment is good over a broad temperature range, suggesting that the calculated 10.7 kcal/mol barrier is accurate to within about 0.5 kcal/mol barrier. Agreement with experiment for H + CH/sub 4/ using the calculated 13.5 kcal/mol reverse reaction barrier is poorer; a 12.5 kcal/mol barrier is found to provide a more reasonable estimate of the true barrier. The analysis of isotope effects in the H + CH/sub 4/ reaction is restricted to examining the branching ratios between H and D atom abstraction in the reaction of H with the mixed species CH/sub 3/D, CH/sub 2/D/sub 2/, and CHD/sub 3/. A combination of reaction path multiplicity, favorable zero point energy shifts, and a greater likelihood of tunneling causes H atom abstraction to predominate over D atom abstraction in H + CH/sub 3/D and H + CH/sub 2/D/sub 2/, but for H + CHD/sub 3/, it was found that the H atom and D atom abstraction rate constants cross near 700 K, with H atom abstraction dominating at low temperatures and D atom at high. 9 figures, 7 tables.

• Research Organization: Argonne National Lab., IL
• DOE Contract Number: W-31109-ENG-38
• OSTI ID: 6922807
• Journal Information: J. Phys. Chem.; (United States), Journal Name: J. Phys. Chem.; (United States) Vol. 88:2; ISSN JPCHA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400202* -- Isotope Effects
Isotope Exchange
& Isotope Separation
ALKANES
DEUTERIUM
ELEMENTS
HYDROCARBONS
HYDROGEN
HYDROGEN ISOTOPES
ISOTOPE EFFECTS
ISOTOPES
KINETICS
LIGHT NUCLEI
METHANE
NONMETALS
NUCLEI
ODD-ODD NUCLEI
ORGANIC COMPOUNDS
QUANTUM NUMBERS
REACTION KINETICS
STABLE ISOTOPES