Mechanisms of sequential ion-molecule reactions in protonated methanol using mass spectrometry, ab initio methods, and statistical modeling

We present mass spectrometry results for reactions of protonated methanol clusters, (CH3OH)nH+. Mass spectra indicate a preference for association over condensation at our experimental conditions and are used in conjunction with computational methods to probe the reaction mechanisms involved. We find that the reaction for the protonated monomer with neutral methanol consists of two entrance complexes that are in equilibrium due to a very small barrier between them. Statistical modeling indicates that competition between proton transfer, condensation, and association are dictated by the depth of the proton-bound complex and the height of the SN2 transition state. For the reaction of the protonated dimer we determine condensation is not energetically favorable at thermal energies as a solvation effect raises the SN2 barrier. Geometries for protonated methanol clusters (CH3OH)nH+ up to n = 6 are also provided, which allow us to examine the role of entropy and hydrogen bonding in these structures

Temperature and Isotope Dependent Kinetics of Nickel-Catalyzed Oxidation of Methane by Ozone

The temperature dependent kinetics of Ni+ + O3 and of NiO+ + CH4/CD4 are measured from 300 to 600 K using a selected-ion flow tube apparatus. Together, these reactions comprise a catalytic cycle converting CH4 to CH3OH. The reaction of Ni+ + O3 proceeds at the collisional limit, faster than previously reported at 300 K. The NiO+ product reacts further with O3, also at the collisional limit, yielding both higher oxides (up to NiO5+ is observed) as well as undergoing an apparent reduction back to Ni+. This apparent reduction channel is due to the oxidation channel yielding NiO2+∗ with sufficient energy to dissociate. 4NiO+ + CH4 (CD4) (whereas 4NiO+ refers to the quartet state of NiO+) proceeds with a rate constant of (2.6 ± 0.4) × 10-10 cm3 s-1 [(1.8 ± 0.5) × 10-10 cm3 s-1] at 300 K and a temperature dependence of ∼T-0.7±0.3 (∼T-1.1±0.4), producing only the 2Ni+ + 1CH3OH channel up to 600 K. Statistical modeling of the reaction based on calculated stationary points along the reaction coordinate reproduces the experimental rate constant as a function of temperature but underpredicts the kinetic isotope shift. The modeling was found to better represent the data when the crossing from quartet to doublet surface was incomplete, suggesting a possible kinetic effect in crossing from quartet to doublet surfaces. Additionally, the modeling predicts a competing 3NiOH+ + 2CH3 channel to become increasingly important at higher temperatures