Ab Initio Aqueous Thermochemistry:  Application to the Oxidation of Hydroxylamine in Nitric Acid Solution

Abstract

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.

Brick by brick computation of the gibbs free energy of reaction in solution using quantum chemistry and COSMO‐RS

“…We have benchmarked the accuracy that can be achieved by different level of theory separately for the three parts. The findings for the benchmark systems and the example shown in the previous section are in accordance with recent studies on reaction energies and activation energies . It can be concluded that a well‐chosen combination of DFT and high level WFT for the gas phase and thermodynamic energy contribution with COSMO‐RS calculated free energy of solvations can yield and within an error range of 10 kJ/mol.…”

“…The findings for the benchmark systems and the example shown in the previous section are in accordance with recent studies on reaction energies and activation energies. 81,[84][85][86][87][88][89] It can be concluded that a well-chosen combination of DFT and high level WFT for the gas phase and thermodynamic energy contribution with COSMO-RS calculated free energy of solvations can yield DG soln and within an error range of 10 kJ/mol. An even better prediction quality drawing close to "chemical" accuracy < 5 kJ/mol, can be achieved if no major rearrangements of the molecules geometric and electronic structures occur during the course of the reaction, as in the hydrogenation example presented above.…”

Operando Modeling of Multicomponent Reactive Solutions in Homogeneous Catalysis: from Non‐standard Free Energies to Reaction Network Control

“…These factors can be accounted for by using the implicit COSMO‐RS solvation model . For example, Green et al employed this approach to investigate the autocatalytic oxidation of hydroxylamine in nitric acid solutions and analyze differences between calculated and experimentally obtained energy functions (Δ G , Δ H , Δ S ) …”

“…[34,35] For example, Green et al employed this approach to investigate the autocatalytic oxidation of hydroxylamine in nitric acid solutions and analyze differences between calculated and experimentally obtained energy functions (ΔG, ΔH, ΔS). [36] In this work, we employ a composite DFT-COSMO-RS scheme to carry out a rapid and detailed analysis of conditiondependencies of the thermodynamic parameters in complex multicomponent reactive solutions. As a showcase, we focus on a model homogeneous catalytic reaction that is the basepromoted CO 2 reduction to formates catalyzed by homogeneous Ru pincer catalysts ( Figure 1a).…”

Identification of Thermal Decomposition Products and Reactions for Liquid Ammonium Nitrate on the Basis of Ab Initio Calculation

“…Various reliable models of gas‐phase reactions have been proposed to date, all of which work to explain observed combustion behaviors . More recently, models for the liquid‐phase reactions of energetic salts have also been developed . However, there are no detailed reaction models for the condensed phase reactions of AN, only semidetailed mechanisms .…”