Experimental determination of the high-temperature rate constant for the reaction of OH with sec-butanol

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

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.

Gas-phase rate coefficients for the OH + n-, i-, s-, and t-butanol reactions measured between 220 and 380 K: non-Arrhenius behavior and site-specific reactivity

Butanol (C4H9OH) is a potential biofuel alternative in fossil fuel gasoline and diesel formulations. The usage of butanol would necessarily lead to direct emissions into the atmosphere; thus, an understanding of its atmospheric processing and environmental impact is desired. Reaction with the OH radical is expected to be the predominant atmospheric removal process for the four aliphatic isomers of butanol. In this work, rate coefficients, k, for the gas-phase reaction of the n-, i-, s-, and t-butanol isomers with the OH radical were measured under pseudo-first-order conditions in OH using pulsed laser photolysis to produce OH radicals and laser induced fluorescence to monitor its temporal profile. Rate coefficients were measured over the temperature range 221-381 K at total pressures between 50 and 200 Torr (He). The reactions exhibited non-Arrhenius behavior over this temperature range and no dependence on total pressure with k(296 K) values of (9.68 ± 0.75), (9.72 ± 0.72), (8.88 ± 0.69), and (1.04 ± 0.08) (in units of 10(-12) cm(3) molecule(-1) s(-1)) for n-, i-, s-, and t-butanol, respectively. The quoted uncertainties are at the 2σ level and include estimated systematic errors. The observed non-Arrhenius behavior is interpreted here to result from a competition between the available H-atom abstraction reactive sites, which have different activation energies and pre-exponential factors. The present results are compared with results from previous kinetic studies, structure-activity relationships (SARs), and theoretical calculations and the discrepancies are discussed. Results from this work were combined with available high temperature (1200-1800 K) rate coefficient data and room temperature reaction end-product yields, where available, to derive a self-consistent site-specific set of reaction rate coefficients of the form AT(n) exp(-E/RT) for use in atmospheric and combustion chemistry modeling.