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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

Technical Report ·
OSTI ID:6771246
; ;
Rate constants and kinetic isotope effects for the title reactions were calculated 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 energy 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 300K). This indicates that although variational effects are small, tunneling makes a very important contribution to the rate constants, and it is found that the tunneling contribution is described quantitatively for all the reactions considered using the least-action ground state (LAG) transmission coefficient. The combination of improved canonical variational theory (ICVT) and LAG yields rate constants that have an average error (considering all the reactions and temperatures studied) of only 15% compared to the accurate quantal rate constants, and in only one case (D + H/sub 2/ at 200K) 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 worst comparisons occurring for intramolecular (X+HD) isotope ratios.
Research Organization:
Chemical Dynamics Corp., Upper Marlboro, MD (USA)
OSTI ID:
6771246
Report Number(s):
AD-A-175627/9/XAB
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
640304 -- Atomic
Molecular & Chemical Physics-- Collision Phenomena
74 ATOMIC AND MOLECULAR PHYSICS
ACCURACY
COLLISIONS
DEUTERIDES
DEUTERIUM
DEUTERIUM COMPOUNDS
DYNAMICS
ELEMENTS
ENERGY
HYDROGEN
HYDROGEN COMPOUNDS
HYDROGEN DEUTERIDE
HYDROGEN ISOTOPES
ISOTOPE EFFECTS
ISOTOPES
KINETICS
LIGHT NUCLEI
MECHANICS
MOLECULE COLLISIONS
MOLECULE-MOLECULE COLLISIONS
NONMETALS
NUCLEI
ODD-ODD NUCLEI
OXYGEN
POTENTIAL ENERGY
REACTION KINETICS
STABLE ISOTOPES
SURFACES
VARIATIONAL METHODS