Thermochemistry of HO₂ + HO₂ → H₂O₄: Does HO₂ Dimerization Affect Laboratory Studies?

Abstract

Self-reaction is an important sink for the hydroperoxy radical (HO2) in the atmosphere. It has been suggested (Denis, P. A.; Ornellas, F. R. J. Phys. Chem. A, 2009, 113 (2), 499-506) that the minor product hydrogen tetroxide (HO4H) may act as a reservoir of HO2. Here, we compute the thermochemistry of HO2 self-reactions to determine if either HO4H or the cyclic hydrogen-bound dimer (HO2)2 can act as reservoirs. We computed electronic energies using coupled-cluster calculations in the complete basis set limit, CCSD(T)/CBS[45]//CCSD(T)/cc-pVTZ. Our model chemistry includes corrections for vibrational anharmonicity in the zero-point energy and vibrational partition functions, core-valence correlation, scalar relativistic effects, diagonal Born-Oppenheimer, spin-orbit splitting, and higher-order corrections. We compute the Gibbs energy of dimerization to be (-20.1 ± 1.6) kJ/mol at 298.15 K (2σ uncertainty), and (-32.3 ± 1.5) kJ/mol at 220 K. For atmospherically relevant [HO2] = 10(8) molecules per cm(3), our thermochemistry indicates that dimerization will be negligible, and thus H2O4 species are atmospherically unimportant. Under conditions used in laboratory experiments ([HO2] > 10(12) molecules per cm(3), 220 K), H2O4 formation may be significant. We compute two absorption spectra that could be used for laboratory detection of HO4H: the OH stretch overtone (near-IR) and electronic (UV) spectra.

Permutation-Invariant-Polynomial Neural-Network-based Δ-Machine Learn-ing Approach: A Case for the HO2 Self-reaction and its Dynamics Study

“…Indeed, there exists a relatively stable intermediate between the two HO2 radicals, namely the H2O4 intermediate (the HO2 radical dimer) on the triplet state surface, whose energetics and spectroscopic characterization have also been scrutinized extensively by theory and experiment. [25][26][27][28][29][30][31][32] As shown in Figure 1, our calculations show that another high-energy pathway via H2OO + O2 leads to the same products H2O2 + O2. To the best of our knowledge, it is the first time that this reaction path is reported.…”

Permutation-Invariant-Polynomial Neural-Network-based Δ-Machine Learn-ing Approach: A Case for the HO2 Self-reaction and its Dynamics Study

“…Indeed, there exists a relatively stable intermediate between the two HO2 radicals, namely the H2O4 intermediate (the HO2 radical dimer) on the triplet state surface, whose energetics and spectroscopic characterization have also been scrutinized extensively by theory and experiment. [25][26][27][28][29][30][31][32] As shown in Figure 1, our calculations show that another high-energy pathway via H2OO + O2 leads to the same products H2O2 + O2. To the best of our knowledge, it is the first…”

Permutation-Invariant-Polynomial Neural-Network-Based Δ-Machine Learning Approach: A Case for the HO2 Self-Reaction and Its Dynamics Study

“…The aforementioned unique temperature- and pressure-dependent behaviors are related to the properties of the potential energy surface (PES), which governs the reaction mechanism, kinetics, and dynamics. Indeed, there exists a relatively stable intermediate between the two HO 2 radicals, namely the H 2 O 4 intermediate (the HO 2 radical dimer) on the triplet state surface, whose energetics and spectroscopic characterization have also been scrutinized extensively by theory and experiment. − As shown in Figure , our calculations show that another high-energy pathway via H 2 OO + O 2 leads to the same products H 2 O 2 + O 2 . To the best of our knowledge, it is the first time that this reaction path is reported.…”