Multi-path variational transition state theory for chiral molecules: the site-dependent kinetics for abstraction of hydrogen from 2-butanol by hydroperoxyl radical, analysis of hydrogen bonding in the transition state, and dramatic temperature dependence of the activation energy

Abstract

The goal of the present work is modeling the kinetics of a key reaction involved in the combustion of the biofuel 2-butanol. To accomplish this we extended multi-path variational transition state theory (MP-VTST) with the small curvature tunneling (SCT) approximation to include multistructural anharmonicity factors for molecules with chiral carbons. We use the resulting theory to predict the site-dependent rate constants of the hydrogen abstraction from 2-butanol by hydroperoxyl radical. The generalized transmission coefficients were averaged over the four lowest-energy reaction paths. The computed forward reaction rate constants indicate that hydrogen abstraction from the C-2 site has the largest contribution to the overall reaction from 200 K to 2400 K, with a contribution ranging from 99.9988% at 200 K to 88.9% at 800 K to 21.2% at 3000 K, while hydrogen abstraction from the oxygen site makes the lowest contribution at all temperatures, ranging from 2.5 × 10⁻⁹% at 200 K to 0.65% at 800 K to 18% at 3000 K. This work highlights the importance of including the multiple-structure and torsional potential anharmonicity in the computation of the thermal rate constants. We also analyzed the role played by the hydrogen bond at the transition state, and we illustrated the risks of (a) considering only the lowest-energy conformations in the calculations of the rate constants or (b) ignoring the nonlinear temperature dependence of the activation energies. A hydrogen bond at the transition state can lower the enthalpy of activation, but raise the free energy of activation. We find an energy of activation that increases from 11 kcal mol⁻¹ at 200 K to more than 36 kcal mol⁻¹ at high temperature for this radical reaction with a biofuel molecule.