Prediction of Experimentally Unavailable Product Branching Ratios for Biofuel Combustion: The Role of Anharmonicity in the Reaction of Isobutanol with OH

Isobutanol
is a prototype biofuel, and sorting out the mechanism
of its combustion is an important objective where theoretical modeling
can provide information that is unavailable and not easily obtained
by experiment. In the present work the rate constants and branching
ratios for the hydrogen abstraction reactions from isobutanol by hydroxyl
radical have been calculated using multi-path variational transition-state
theory with small-curvature tunneling. We use hybrid degeneracy-corrected
vibrational perturbation theory to show that it is critical to consider
the anharmonicity difference of high-frequency modes between reactants
and transition states. To obtain accurate rate constants, we must
apply different scaling factors to the calculated harmonic vibrational
frequencies at the reactants and at the transition states. The factors
determining the reaction rate constants have been analyzed in detail,
including variational effects, tunneling contributions, the effect
of multiple reaction paths on transmission coefficients, and anharmonicities
of low- and high-frequency vibrational modes. The analysis quantifies
the uncertainties in the rate calculations. A key result of the paper
is a prediction for the site dependence of hydrogen abstraction from
isobutanol by hydroxyl radical. This is very hard to measure experimentally,
although it is critical for combustion mechanism modeling. The present
prediction differs considerably from previous theoretical work.