We have calculated the thermochemistry and rate coefficients for stable molecules and reactions in the title reaction families using CBS-QB3 and B3LYP/CBSB7 methods. The accurate treatment of hindered rotors for molecules having multiple internal rotors with potentials that are not independent of each other can be problematic, and a simplified scheme is suggested to treat them. This is particularly important for hydroperoxyalkylperoxy radicals (HOOQOO). Two new thermochemical group values are suggested in this paper, and with these values, the group additivity method for calculation of enthalpy as implemented in reaction mechanism generator (RMG) gives good agreement with CBS-QB3 predictions. The barrier heights follow the Evans-Polanyi relationship for each type of intramolecular hydrogen migration reaction studied.

The reactivity of a metal catalyst depends strongly on the adsorbate coverage, making it essential for the reactivity models to account for the in situ structures and properties of the catalyst under reaction conditions. The use of first principle based thermodynamic approaches to describe adsorbate–adsorbate interaction though attractive for its technical rigor is tedious and computationally demanding especially for metal nanoparticles. With the advent of empirical reactive force fields (ReaxFF), there is a great deal of interest to advance simulation approaches like hybrid grand canonical Monte Carlo reactive molecular dynamics (GCMC/RMD) that enable efficient use of ReaxFF to model the adsorptive states. The predictive ability of GCMC/RMD relies upon the quality of the force field, which in turn depends upon the training set used for its parametrization. To this end, we investigate the adsorption behavior of O and H over the Pt catalysts using the newly developed Pt/O/H ReaxFF. We assess the thermodynamic stability of Pt-adsorbates by GCMC/RMD and provide insight on the atomic composition of in situ catalysts. The theoretical adsorption isotherms of O and H are derived in many Pt surfaces over a wide range of reference gas pressures (e.g., 10–20 atm to 10 atm) relevant to the observed real catalysis, including the Pt(111), unreconstructed and reconstructed Pt(110) surfaces, and even Pt nanoparticles of different sizes and shapes. The force field is further evaluated to predict the relative binding energies of O on Pt(321) surface, while it has not been trained for this kinked surface. For both oxygen and hydrogen atoms, adsorption occurs initially at the Pt surface, followed by subsurface and bulk. Examination of the equilibrated structures discloses the contribution of different sites on the surface, subsurface, and the bulk regions during adsorption at various applications. The adsorption behavior obtained in this paper agrees with the DFT and/or the experimental data reported in the literature, which validates the Pt/O/H ReaxFF and demonstrates its applicability in catalytic reactions coupled with time acceleration tools. Based on the derived adsorption isotherm, one can infer the relative affinity of O, H, or OH species, and thus prepare appropriate structures at the specified reaction conditions for further investigation of the catalytic reactions by molecular dynamics and for designing experimental conditions for optimal catalyst performance.

Using reactive molecular dynamics (RMD), we present an atomistic insight into the interaction between water molecules and acidic centers of H-ZSM-5 zeolite. The reactive force field method, ReaxFF, was used to evaluate the adsorption and diffusion of water as well as to study the protonation of water molecules inside zeolite channels. The existing Si/Al/O/H parameters were refitted against DFT calculations to improve the ReaxFF description of interaction between water molecules and the acidic sites of zeolites. The diffusion coefficient of water in the zeolite obtained from refitted parameters is in excellent agreement with experimental results. The molecular dynamics (MD) simulations indicate that protonation of water molecules and acidity of the zeolite catalyst depend on water loadings and temperature and the observed trends compare favorably with existing experimental and theoretical studies. At higher water loadings, protonation of water molecules is more frequent leading to formation and growth of protonated water clusters inside zeolite channels. From the analysis of various reaction channels that were observed during the simulations, we found that such water clusters have relatively short life due to frequent interchange of protons and water molecules among the water clusters. Such proton hopping events play a key role in moving the protons between different acidic centers of zeolite. These simulations show the capability of ReaxFF in providing atomistic details of complex chemical interactions between the water phase and solid acid zeolites.

Ab initio molecular orbital calculations were performed and thermochemical parameters estimated for 46 species involved in the oxidation of hydroxylamine in aqueous nitric acid solution. Solution-phase properties were estimated using the several levels of theory in Gaussian03 and using COSMOtherm. The use of computational chemistry calculations for the estimation of physical properties and constants in solution is addressed. The connection between the pseudochemical potential of Ben-Naim and the traditional standard state-based thermochemistry is shown, and the connection of these ideas to computational chemistry results is established. This theoretical framework provides a basis for the practical use of the solution-phase computational chemistry estimates for real systems, without the implicit assumptions that often hide the nuances of solution-phase thermochemistry. The effect of nonidealities and a method to account for them is also discussed. A method is presented for estimating the solvation enthalpy and entropy for dilute aqueous solutions based on the solvation free energy from the ab initio calculations. The accuracy of the estimated thermochemical parameters was determined through comparison with (i) enthalpies of formation in the gas phase and in solution, (ii) Henry's law data for aqueous solutions, and (iii) various reaction equilibria in aqueous solution. Typical mean absolute deviations (MAD) for the solvation free energy in room-temperature water appear to be ~1.5 kcal/mol for most methods investigated. The MAD for computed enthalpies of formation in solution was 1.5-3 kcal/mol, depending on the methodology employed and the type of species (ion, radical, closed-shell) being computed. This work provides a relatively simple and unambiguous approach that can be used to estimate the thermochemical parameters needed to build detailed ab initio kinetic models of systems in aqueous solution. Technical challenges that limit the accuracy of the estimates are highlighted.

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Reactive force field methods such as AIREBO, ReaxFF and COMB, are extremely useful for studying physical and chemical interactions between molecules and materials. However, many chemical reactions have relatively high activation energies, putting them beyond the times-scale of conventional molecular dynamics (MD) at modest temperatures. To capture the low-temperature long-lived radical chemistry in atomistic simulations, we have developed a new transition detection scheme for performing Reactive Parallel Replica Dynamics (RPRD) simulation enabling an extended MD time-scales, essentially up to a microsecond using ReaxFF. In the newly implemented event detection scheme, the transition events are identified whenever there is a change in connectivity of any atom. 1-Heptene pyrolysis is chosen as a model system, and RPRD simulations are performed at temperatures as low as 1350K for up to 1 μs for a system consisting of 40 heptene molecules. The chemical mechanism and the product distribution that were obtained from the RPRD simulation are in reasonable agreement with shock tube experiments.

We developed the ReaxFF force field for Pt/Ni/C/H/O interactions, specifically targeted for heterogeneous catalysis application of the Pt-Ni alloy. The force field is trained using the DFT data for equations of state of PtNi, PtNi and PtNi alloys, the surface energy of the PtNi(111) (x = 0.67-0.83), and binding energies of various atomic and molecular species (O, H, C, CH, CH, CH, CO, OH, and HO) on these surfaces. The ReaxFF force field shows a Pt surface segregation at x ≥ 0.67 for the (111) surface and x ≥ 0.62 for the (100) surface in vacuum. In addition, from the investigation of the preferential alloy component of the adsorbates, it is expected that H and CH on the alloy surface to induce a segregation of Pt whereas the oxidation intermediates and products such as C, O, OH, HO, CO, CH, and CH are found to induce Ni segregation. The relative order of binding strengths among adsorbates is a function of alloy composition and the force field is trained to describe the trend observed in DFT calculations, namely, H < HO < CH ≈ O ≈ CO < OH < CH < C ≈ CH on PtNi, H < HO < CO < O ≈ CH < OH < CH < CH < C on PtNi, and H < HO < O < CO < CH < OH < CH < C ≈ CH on PtNi. Using this force field, we performed the grand-canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations for a PtNi slab and a truncated cuboctahedral nanoparticle terminated by (111) and (100) faces, to examine the surface segregation trend under different gas environments. It is found that Pt segregates to the alloy surface when the surface is exposed to vacuum and/or H environment while Ni segregates under the O environment. These results suggest that the Pt/Ni alloy force field can be successfully used for the preparation of Pt-Ni nanobimetallic catalysts structure using GCMC and run MD simulations to investigate its role and the catalytic chemistry in catalytic oxidation, dehydrogenation and coupling reactions. The current Pt/Ni force field still is found to have difficulties in describing the observed segregation trend in Ni-rich alloy compositions (x < 0.6), suggesting the need for additional force field training and evaluation for its application to describe the characteristics and chemistry of Ni-rich alloys.