Experiments on collisional energy transfer

King, Keith D.; Barker, John R.

doi:10.1016/b978-0-444-64207-3.00001-9

Cited by 8 publications

(9 citation statements)

References 188 publications

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“…Although one can use different theoretical 31 or experimental 32 methods to obtain the energy transfer parameters ⟨Δ E all ⟩ and α, kinetic modelers usually look at the literature for these values of the system under investigation. In many cases, this search fails and then two scenarios are possible.…”

Section: Theoretical and Computational Methodologymentioning

confidence: 99%

Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory

2020

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The system-specific quantum Rice–Ramsperger–Kassel (SS-QRRK) theory ( J. Am. Chem. Soc. 2016 , 138 , 2690) is suitable to determine rate constants below the high-pressure limit. Its current implementation allows incorporating variational effects, multidimensional tunneling, and multistructural torsional anharmonicity in rate constant calculations. Master equation solvers offer a more rigorous approach to compute pressure-dependent rate constants, but several implementations available in the literature do not incorporate the aforementioned effects. However, the SS-QRRK theory coupled with a formulation of the modified strong collision model underestimates the value of unimolecular pressure-dependent rate constants in the high-temperature regime for reactions involving large molecules. This underestimation is a consequence of the definition for collision efficiency, which is part of the energy transfer model. Selection of the energy transfer model and its parameters constitutes a common issue in pressure-dependent calculations. To overcome this underestimation problem, we evaluated and implemented in a bespoke Python code two alternative definitions for the collision efficiency using the SS-QRRK theory and tested their performance by comparing the pressure-dependent rate constants with the Rice–Ramsperger–Kassel–Marcus/Master Equation (RRKM/ME) results. The modeled systems were the tautomerization of propen-2-ol and the decomposition of 1-propyl, 1-butyl, and 1-pentyl radicals. One of the tested definitions, which Dean et al. explicitly derived ( Z. Phys. Chem. 2000 , 214 , 1533), corrected the underestimation of the pressure-dependent rate constants and, in addition, qualitatively reproduced the trend of RRKM/ME data. Therefore, the used SS-QRRK theory with accurate definitions for the collision efficiency can yield results that are in agreement with those from more sophisticated methodologies such as RRKM/ME.

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Section: Theoretical and Computational Methodologymentioning

confidence: 99%

Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory

2020

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“…Some studies characterized slices through the full four-dimensional function R ( E′ , J′ ; E , J ), while others, including our own, − reported low-order energy-transfer and/or angular-momentum-transfer moments of R . Experimental characterizations of R and its moments for highly vibrationally excited species have also appeared, and we note representative work from several groups. − …”

Section: Introductionmentioning

confidence: 94%

Microcanonical Rate Constants for Unimolecular Reactions in the Low-Pressure Limit

Jasper

2020

J. Phys. Chem. A

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Low-pressure-limit microcanonical rate constants, κ 0 (E,J), describe the rate of activating bath gas collisions in a unimolecular reaction and are calculated here using classical trajectories and quantized thresholds for reaction. The resulting semiclassical rate constants are twodimensional (in total energy E and total angular momentum J) and are intermediate in complexity between the four-dimensional state-to-state collisional energy and angular momentum transfer rate constant, R(E′,J′;E,J), and the highly averaged thermal rate constant, k 0 . Results are presented for CH 4 (+M), C 2 H x (+M), x = 3−6, and H 2 O (+M), where κ 0 (E,J) is shown generally to be a sensitive function of the bath gas, temperature, and initial state of the unimolecular reactant. Strong variations in κ 0 with respect to E and J lead to complex trends in relative microcanonical bath gas efficiencies. This underlying complexity may complicate the search for simple explanations for observed trends in relative thermal bath gas efficiencies. A different measure of the microcanonical collision efficiency that describes the energy range of activating collisions is introduced that supports the empirical decomposition of collisional activation into separable translational-to-vibrational and rotational-to-vibrational activation mechanisms. The two mechanisms depend differently on mass, temperature, and the J-dependence of the threshold energy for reaction, with rotational-to-vibrational activation favored for heavier baths and for reactions with rigid transition states. Finally, κ 0 is used to test the accuracy of several two-dimensional models for R that were proposed for use in master equation studies.

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“…The probability density for activating collisions is obtained from detailed balance and Equation (3). Although experimental data on large molecule energy transfer show that α is a function of both energy and temperature, 77 the energy dependence has a negligible effect when the thresholds for competing reactions are close in energy, as in the present reaction system. Because of the differences among the angular momentum treatments, U in Equation (3) is identified with either total energy, E , or active energy, ε , as appropriate.…”

Section: Theoretical Methodsmentioning

confidence: 63%

“…Collisional energy transfer in the context of unimolecular reaction systems is a complicated subject, which has been reviewed recently 77‐79 . For present purposes, we adopt the conventional exponential‐down model for the collision step‐size distribution:

P false(U^{'}, U false) = \exp [- \frac{U - U^{'}}{α}] for U \geq U^{'},

where U and U’ are the energies prior to, and following a collision, respectively, and parameter α is approximately equal to < ∆ E > down , the average energy transferred in deactivation collisions.…”

Section: Theoretical Methodsmentioning

confidence: 99%

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Semiclassical transition state theory/master equation kinetics of HO + CO: Performance evaluation

Barker

Stanton

Nguyen

2020

Int J of Chemical Kinetics

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Experiments on collisional energy transfer

Cited by 8 publications

References 188 publications

Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory

Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory

Microcanonical Rate Constants for Unimolecular Reactions in the Low-Pressure Limit

Semiclassical transition state theory/master equation kinetics of HO + CO: Performance evaluation

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