Rate constants for hydrogen abstraction by H(O) over dot 2 from n-butanol

Rate constants of hydrogen-atom abstraction from n-butanol by the HO2 radical have been calculated. Conventional transition state theory employing rigid-rotor harmonic-oscillator approximations for all but the torsional degrees of freedom is used with tight transition states. The PitzerGwinn-like approximation using Fourier fits to internal rotations was applied to determine the one-dimensional hindered potentials. Asymmetric Eckart barriers were used to model tunneling in one-dimensional through saddle points. Activation entropies for all of the reaction channels have been determined. Hydrogen bonds formed in the transition states lead to ring structures, which lower the energy barrier and thus an increase in the rate constant for abstraction. Conversely, entropy is lost when the ring structure is formed and this decreases the frequency factor for abstraction; therefore, both of these effect influence the rate constants in opposite ways. Abstraction of an a hydrogen atom is dominant throughout the whole temperature range, and the branching ratio decreases from 96.1% at 500 K to 46.6% at 2000 K. As the carbon chain lengthens, the influence from the OH group lessens and hence d hydrogens behave in a similar fashion to primary H-atoms in n-butane. The estimated uncertainty for the individual rate constants is a factor of 2.5. Computed total, kt, and individual rate constants, based on the CCSD(T)/cc-pVTZ//MP2/6-311G(d,p) potential energy surface, in the temperature range of 5002000 K for n-butanol + HO2 are reported as follows (cm(3) mol(-1) s(-1)): (C) 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 155164, 2012