Water Catalysis of the Reaction of Methanol with OH Radical in the Atmosphere is Negligible.

Faced with the contradictory results of two recent experimental studies [Jara-Toro et al., Angew. Chem. Int. Ed. 2017, 56, 2166 and Chao et al., Angew. Chem. Int. Ed. 2019, 58, 5013] of the possible catalytic effect of water vapor on CH3 OH + OH reaction, we report calculations that corroborate the conclusion made by Chao et al. and extend the rate constant evaluation down to 200 K. The rate constants of the CH3 OH + OH reaction catalyzed by a water molecule are computed as functions of temperature and relative humidity using high-level electronic structure and kinetics calculations. The Wuhan-Minnesota Scaling (WMS) method is used to provide accurate energetics to benchmark a density functional for direct dynamics. Both high-frequency and low-frequency anharmonicities are included. Variational and tunneling effects are treated by canonical variational transition state theory with multidimensional small-curvature tunneling. And, most significantly, we include multistructural effects in the rate constant calculations. Our calculations show that the catalytic effect of water vapor is not observable at 200-400 K.

Quantum Effects on H2 Diffusion in Zeolite RHO: Inverse Kinetic Isotope Effect for Sieving.

We use canonical variational theory (CVT) with small-curvature tunneling (SCT) contributions to investigate quantum effects on the H2 diffusion process in the pure-silica zeolite RHO. At low temperature we find an inverse kinetic isotopic sieving effect in that the heavier isotopic species diffuses faster than the lighter one. Three quantum effects contribute to this kinetic isotope effect (KIE). The first one is quantum mechanical tunneling; this-on its own-would lead to a normal kinetic isotopic sieving effect, in which lighter diprotium diffuses faster than dideuterium. The second factor, which we find to dominate in the present case, is zero-point energy (ZPE). Deuterium has a lower ZPE, which leads to a smaller effective barrier for tunneling because the transition state has a larger ZPE than the precursor stable state; this results in an inverse KIE. The third factor, the thermal vibrational energy (computed from the quantized vibrational partition function), also favors a normal KIE, but it is outweighed by the ZPE effect. The vibrations of the zeolite host framework are found to play an important role at low temperatures, and our calculations consider up to 7296 degrees of freedom at the equilibrium structure and the saddle point and up to 221 degrees of freedom along the reaction path. The importance of quantum considerations on the dynamics is elucidated by comparison to a purely classical treatment.

Computational kinetics of the hydrogen abstraction reactions of n-propanol and iso-propanol by OH radical.

Propanol (n-propanol or iso-propanol (i-propanol)) is a promising clean-burning oxygenated fuel component and fuel additive. Understanding its reactions with OH radical is of great significance in both combustion and atmospheric chemistry. In this work, we calculate the rate constants and branching ratios of the hydrogen abstraction reactions of n-propanol and i-propanol by OH radical in a broad temperature range of 63-2000 K using the competitive canonical unified statistical (CCUS) model. For both n-propanol and i-propanol, in both the high-pressure and low-pressure limits, the total reaction rate constants show a significant negative dependence on temperature in the low temperature regime and approach the capture rate for the formation of the pre-reactive complex when temperature is down to the ultracold regime. Several factors, tunneling, remarkable anharmonicity of high-frequency modes of transition states, and the presence of reaction channels with a negative free energy barrier, contribute to this phenomenon. We observe pressure-dependent branching fractions at T < ∼400 K for n-propanol or T < 200 K for i-propanol, and at higher temperatures, the branching fractions are independent of the pressure. The alpha-hydrogen (Hα) abstraction with a lower barrier is not always dominant as traditionally expected. The H-abstraction from the terminal carbon (Ht) of i-propanol, with a higher barrier, is dominant above 1000 K because of the remarkably larger effect of multi-structural and torsional (MS-T) anharmonicity. In the pressure-dependent ultra-low temperature regime and high-pressure limit, the beta-hydrogen (Hß) abstraction and the hydrogen abstraction from the hydroxyl group (HO) become dominant for n-propanol and i-propanol, respectively, mainly due to the tunneling effect.

Kinetics of the Methanol Reaction with OH at Interstellar, Atmospheric, and Combustion Temperatures.

The OH radical is the most important radical in combustion and in the atmosphere, and methanol is a fuel and antifreeze additive, model biofuel, and trace atmospheric constituent. These reagents are also present in interstellar space. Here we calculate the rate constants, branching ratios, and kinetic isotope effects (KIEs) of the hydrogen abstraction reaction of methanol by OH radical in a broad temperature range of 30-2000 K, covering interstellar space, the atmosphere, and combustion by using the competitive canonical unified statistical (CCUS) model in both the low-pressure and high-pressure limits and, for comparison, the pre-equilibrium model. Coupled cluster CCSD(T)-F12a theory and multi-reference CASPT2 theory were used to carry out benchmark calculations of the stationary points on the potential energy surface to select the most appropriate density functional method for direct dynamics calculations of rate constants. We find a significant effect of the anharmonicity of high-frequency modes of transition states on the low-temperature rate constant, and we show how tunneling leads to an unusual negative temperature dependence of the rate constants in the range 200 K > T > 100 K. The calculations also demonstrate the importance of the extent of stabilization of the pre-reactive complex. The capture rate for the formation of the complex is the dominant dynamical bottleneck for T < 100 K, and it leads to weak temperature dependence of the rate below 100 K in the high-pressure-limit of the CCUS model. We also report the pressure dependence of branching ratios (which are hard to measure so theory is essential) and the KIEs, and we report an unusual nonmonotonic variation of the KIE in the high-pressure limit at low temperatures.

Large Anharmonic Effects on Tunneling and Kinetics: Reaction of Propane with Muonium.

Calculations of kinetic isotope effects (KIEs) provide challenging tests of quantal mass effects on reaction rates, and muonium KIEs are the most challenging. Here, we show that it can be very important to include reaction-coordinate-dependent vibrational anharmonicity along the whole reaction path to calculate tunneling probabilities and KIEs. For the reaction of propane with Mu, this decreases both the height and width of the vibrationally adiabatic potential barrier, with both effects increasing the rate constants. Our results agree well with the experimental observations.

Anharmonic kinetics of the cyclopentane reaction with hydroxyl radical.

Cyclopentane is one of the major constituents of transportation fuels, especially jet fuel and diesel, and also is a volatile organic compound with a significant presence in the atmosphere. Hydrogen abstraction from cyclopentane by hydroxyl radical plays a significant role in combustion and atmospheric chemistry. In this work we study the kinetics of this reaction at 200-2000 K using direct dynamics calculations in which the potential energy surface is obtained by quantum mechanical electronic structure calculations. The forward and reverse barrier heights and reaction energies obtained by the CCSD(T)-F12a/jun-cc-pVTZ coupled cluster calculations are used as a benchmark to select an accurate electronic structure method among 36 combinations of exchange-correlation functional and basis set. The selected M06-2X/MG3S method shows the best performance with a mean unsigned deviation from the benchmark of only 0.22 kcal mol-1 for reaction energies and barrier heights. A quadratic-quartic function is adopted to describe the ring bending potential of cyclopentane, and the quartic anharmonicity in the bending mode is treated by a one-dimensional Schrödinger equation using both Wentzel-Kramers-Brillouin (WKB) and Fourier Grid Hamiltonian (FGH) methods. The torsional anharmonicity in the transition state is treated in turn by the free rotor approximation, the one-dimensional hindered rotor approximation, and the multi-structural torsional anharmonicity method. Rate constants of the title reaction are computed by canonical variational transition state theory including tunneling by the multi-dimensional small-curvature tunneling approximation (CVT/SCT). The final rate constants include the quasiharmonic, quadratic-quartic, and torsional anharmonicity. Our calculations are in excellent agreement with all the experimental data available at both combustion and atmospheric temperatures with a deviation of less than 30%.