We study nanoclusters of Mg-rich olivine and pyroxene (having (MgO)6(SiO2)3 and (MgO)4(SiO2)4 compositions) with respect to their reactivity towards hydrogen atoms, using density functional calculations. Ultrasmall silicate particles are fundamental intermediates in cosmic dust grain formation and processing, and are thought to make up a significant mass fraction of the grain population. Due to their nanoscale dimensions and high surface area to bulk ratios, they are likely to also have a disproportionately large influence on surface chemistry in the interstellar medium. This work investigates the potential role of silicate nanoclusters in vital interstellar hydrogen-based chemistry by studying atomic H adsorption and H2 formation. Our extensive set of calculations confirm the generality of a Brønsted-Evans-Polanyi (BEP) relation between the H2 reaction barrier and the 2Hchem binding energy, suggesting it to be independent of silicate dust grain shape, size, crystallinity and composition. Our results also suggest that amorphous/porous grains with forsteritic composition would tend to dissociate H2, but relatively Mg-poor silicate grains (e.g. enstatite composition) and/or more crystalline/compact silicate grains would tend to catalyse H2 formation. The high structural thermostability of silicate nanoclusters with respect to the heat released during exothermic H2 formation reactions is also verified.
Thermal rate constants for chemical reactions using the corrections of zero curvature tunneling (ZCT) and of small curvature tunneling (SCT) methods are reported. The general procedure is implemented and used with high-quality ab initio computations and semiclassical reaction probabilities along the minimum energy path (MEP). The approach is based on a vibrational adiabatic reaction path and is applied to the H + Si(CH3)4 → H2 + Si(CH3)3CH2 reaction and its isotopically substituted variants. All of the degrees of freedom are optimized, and harmonic vibrational frequencies and zero-point energies are calculated at the MP2(full) level with the cc-pVTZ basis set. Single-point energies are calculated at a higher level of theory with the same basis set, namely, CCSD(T,full). The influence of the basis set superposition error (BSSE) on the energetics is tested. The method is further exploited to predict primary and secondary kinetic isotope effects (KIEs and SKIEs, respectively). Rate constants computed with the ZCT and SCT methods over a wide temperature range (180-2000 K) show important quantum tunneling effects at low temperatures when compared to rates obtained from the purely classical transition-state theory (TST) and from the canonical variational transition state theory (CVT). For the H + Si(CH3)4 reaction, they are given by the following expressions: k(TST/ZCT) = 9.47 × 10(-19) × T(2.65) exp(-2455.7/T) and k(CVT/SCT) = 7.81 × 10(-19) × T(2.61) exp[(2704.2/T) (in cm(3) molecule(-1) s(-1)). These calculated rates are in very good agreement with those from available experiments.
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