Predictive Theoretical Kinetics of the Pressure-dependent Spin-forbidden Reaction O + CO → CO₂


The kinetics of the spin-forbidden reaction CO + O →CO2 is fully characterized theoretically. Global analytic representations of the lowest-energy singlet surface, the two lowest-energy triplet surfaces, and their spin-orbit coupling surfaces are obtained via dynamic weighted multireference electronic structure theory calculations and the interpolated moving least squares (IMLS) semiautomated surface fitting strategy. The analytic spin-orbit-coupled representation is used in full-dimensional electronically nonadiabatic molecular dynamics calculations, where the spin-forbidden dynamics is modeled using the coherent switches with decay of mixing (CSDM) multistate trajectory method. The trajectory calculations reveal direct (nonstatistical) and indirect (statistical) spin-forbidden reaction mechanisms. The resulting high pressure limit rate coefficient is more than an order of magnitude larger than the calculated value used in many detailed models of combustion and is larger than an application of "nonadiabatic transition state theory" (NA TST). This discrepancy is attributed to nonstatistical and multidimensional effects not present in the NA TST model. Pressure dependence is characterized via trajectory ensembles of CO2 + Kr collisions, which are used to parameterize a detailed model of energy transfer. In most kinetics applications where collisional energy transfer information is needed, it is estimated or determined empirically whereas here it is predicted without empiricism. The resulting energy transfer model is used to calculate the low-pressure limit rate coefficient, which is found to be ~3x larger than available experimental values at 3000-4600 K. Together, these calculations provide a complete first-principles (parameter-free) characterization of the kinetics of this system, which is particularly challenging as it involves spin-forbidden dynamics and pressure dependence.

Meeting Name

8th US National Combustion Meeting (2013: May 19-22, Park City, UT)



Keywords and Phrases

Calculations; Combustion; Electronic structure; Energy transfer; Kinetics; Molecular dynamics; Spin dynamics; Trajectories; Analytic representation; Collisional energy transfer; Electronic structure theory; Interpolated moving least-squares; Multidimensional effects; Non-adiabatic transitions; Nonadiabatic molecular dynamics; Trajectory calculations; Reaction kinetics

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Article - Conference proceedings

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© 2013 Western States Section/Combustion Institute, All rights reserved.

Publication Date

01 May 2013

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