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Test of variational transition state theory and multidimensional semiclassical transmission coefficient methods against accurate quantal rate constants for H + H/sub 2//HD, D + H/sub 2/, and O + H/sub 2//D/sub 2//HD, including intra- and intermolecular kinetic isotope effects

Journal Article · · J. Am. Chem. Soc.; (United States)
DOI:https://doi.org/10.1021/ja00271a015· OSTI ID:5658959
; ;
Rate constants and kinetic isotope effects for the title reactions have been calculated by using accurate quantum dynamical methods and used to test the accuracy of corresponding rate constants from conventional and variational transition state theory. The quantum dynamical rate constants are estimated to be within 35% of the exact rate constants for the potential surfaces chosen for this comparison. For all the reactions considered, the conventional and variational transition state theory rate constants with unit transmission coefficient are found to be very close to each other (better than 7%) but in poor agreement with the accurate quantum results (off by factors of 6-22 at 300 K). This indicates that although variational effects are small, tunnelling makes a very important contribution to the rate constants, and it is found that this tunnelling contribution is described quantitatively for all the reactions considered with use of the least action ground state (LAG) transmission coefficient. The combination of improved canonical variational theory (ICVT) and LAG yields rate constants which have an average error (considering all the reactions and temperatures studied) of 15% compared to the accurate quantum rate constants, and in only one case (D + H/sub 2/ at 200 K) does the ICVT/LAG rate constant differ by more than 35% from the accurate value. The comparison of ICVT/LAG kinetic isotope effects is found to be similarly good, with the worst comparisons occurring for intramolecular (X + HD) isotope ratios.
Research Organization:
Chemical Dynamics Corp., Columbus, OH
DOE Contract Number:
AC02-79ER10425
OSTI ID:
5658959
Journal Information:
J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 108:11; ISSN JACSA
Country of Publication:
United States
Language:
English

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Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400202* -- Isotope Effects
Isotope Exchange
& Isotope Separation
CHEMICAL REACTION KINETICS
DATA
DEUTERIDES
DEUTERIUM
DEUTERIUM COMPOUNDS
ELEMENTS
HYDROGEN
HYDROGEN COMPOUNDS
HYDROGEN DEUTERIDE
HYDROGEN ISOTOPES
INFORMATION
ISOTOPE EFFECTS
ISOTOPES
KINETICS
LIGHT NUCLEI
MECHANICS
MEDIUM TEMPERATURE
NONMETALS
NUCLEI
NUMERICAL DATA
ODD-ODD NUCLEI
OXYGEN
QUANTUM MECHANICS
REACTION KINETICS
STABLE ISOTOPES
THEORETICAL DATA
TUNNEL EFFECT