Structure, stability, and dynamics of hydrogen polyoxides

Abstract

ABSTRACT:Hydrogen polyoxides H 2 O n have been theoretically studied in gas phase and in aqueous solution. Structures up to n ¼ 4 have been detected experimentally. Larger systems are speculative till now but previous calculations predicted stable structures. Minimum energy geometries and formation enthalpies for n ¼ 3-10 have been calculated in gas phase using high-level ab initio calculations that include DFT, QCISD, and CCSD(T) methods. Formation enthalpies are negative for n < 5, close to zero for n ¼ 5, and positive for n > 5. They go up by $12-16 kcal/mol per O atom although the increase is not regular; it presents an oscillatory behavior that is clearly connected with odd/even effects reported for OO bond lengths. Computations in aqueous solution focus on free energies of solvation, solute-solvent hydrogen bonds strength, and solvent effects on polyoxide structure. We use two solvent models. A dielectric continuum implicit model is used to estimate free energies of solvation at an affordable cost. An explicit model is then used combining a QM/MM description of the solution and molecular dynamics (MD) simulations for polyoxides n ¼ 3-6. The solute is described at the DFT level whereas solvent molecules are treated with an empirical force field. The analysis of the solute dynamics in solution shows that the systems are very good proton donors but poor proton acceptors. The structure with n ¼ 5 exhibits a particular behavior that seems to be related with its larger statistical probability of forming intramolecular hydrogen bonds. Most bonds are lengthened in solution, especially the polar HAO bonds and the inner OAO bonds. Remarkably, the amplitude of OAO bond length fluctuations has been shown to depend primarily on its position along the chain, that is, the distance with respect to the terminal HAO bond, and less on chain length. For n ¼ 6, the central OAO bond fluctuates between 1.2 and 1.9 Å . Chains beyond n ¼ 6 should be extremely labile since fluctuations of the central OAO bonds should be even larger. Hence, if formed, the corresponding lifetimes should be extremely short.

Theoretical investigation of the structures and spectroscopic properties of (H2O4)n (n = 1–4) clusters

“…The electronic structures of H 2 O n ( n = 3–10) were theoretically analyzed by Martins‐Costa et al They concluded that the hyperconjugation effect was responsible for the helical structure of these molecules . The mechanism of the gas‐phase ozone‐water reaction by isomerization and decomposition of the H 2 O‐O 3 complex involving the H 2 O 4 intermediate was investigated by Vahedpour et al The structures and enthalpies of H 2 O n ( n = 3–10) in the gas phase and solution phase were predicted using density functional theory (DFT) and CCSD(T) methods by Ruiz‐López and coworkers . The structure and spectroscopic properties of H 2 O 4 were predicted at a very high level of theory by Hollman and Schaefer .…”

Theoretical characterization of hydrogen pentoxide, H2O5

“…This trend can be appreciated in Figure where we plotted Δ H f,298 ° (H 2 O 6 ) vs. number of oxygen atoms for each polyoxide. It is important to note that Martins‐Costa showed that starting at H 2 O 6 , the enthalpies of formation of polyoxides do not longer increase linearly but exhibit some oscillatory behavior. This effect can be attributed to the chain parity effect due to hyperconjugation .…”

“…[5] This trend can be appreciated in Figure 2 where we plotted DH o f;298 (H 2 O 6 ) vs. number of oxygen atoms for each polyoxide. It is important to note that Martins-Costa [39] showed that starting at H 2 O 6 , the enthalpies of formation of polyoxides do not longer increase linearly but Table 6, we present the enthalpies of formation of the polyoxides determined at the G4 and CCSD(T) levels of theory, we also included all the minor corrections applied to reach the CCSD(T) value. At the latter level of theory, we determined DH o f;298 (H 2 O 5 ) to be 1.4 AE 1.5 kcal/mol.…”

“…The findings by Levanov et al were in excellent agreement with the theoretical results reported for H 2 O 3 and H 2 O 4. For the next member in the H 2 O n series, n = 5 there have been some investigations about its stability, structure, and enthalpy of formation . However, the infrared spectrum has not been studied in detail and its enthalpy of formation is waiting for a CCSD(T)/CBS study.…”

“…[5,35] For the next member in the H 2 O n series, n ¼ 5 there have been some investigations about its stability, structure, and enthalpy of formation. [38][39] However, the infrared spectrum has not been studied in detail and its enthalpy of formation is waiting for a CCSD(T)/CBS study. For this reason, we decided to continue our investigations on hydrogen polyoxides [3][4][5] and focus our attention on H 2 O 5 .…”