The vibrational spectrum of H2O2+⋅ radical cation: An illustration of symmetry breaking

Theoretical Rotational and Vibrational Spectral Data for the Hypermagnesium Oxide Species Mg2O and Mg2O. Chemphyschem 2024:e202400479. [PMID: 38801234 DOI: 10.1002/cphc.202400479] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

While magnesium is astronomically observed in small molecules, it largely serves as a contributor to silicate grains, though how these grains form is not well-understood. The smallest hypermagnesium oxide compounds (Mg2 ${{}_{2}}$ O/Mg2 ${{}_{2}}$ O+ ${{}^{+}}$ ) may play a role in silicate formation, but little vibrational reference data exist. As such, anharmonic spectroscopic data are computed forX ˜ 1 Σ g + ${{{\tilde{\rm {X}}}}^1 {\rm{\Sigma }}_g^+ }$ Mg2 ${{}_{2}}$ O,a ˜ 1 Σ u + ${{{\tilde{\rm {a}}}}^1 {\rm{\Sigma }}_u^+ }$ Mg2 ${{}_{2}}$ O, andX ˜ 2 Σ g + ${{{\tilde{\rm {X}}}}^2 {\rm{\Sigma }}_g^+ }$ Mg2 ${{}_{2}}$ O+ ${{}^{+}}$ using quartic force fields (QFFs). Explicitly-correlated coupled-cluster QFFs for the neutral species perform well, implying that full multireference treatment may not be necessary for such systems if enough electron correlation is included. Equation-of-motion ionization potential (EOMIP) methods forX ˜ 2 Σ g + ${{{\tilde{\rm {X}}}}^2 {\rm{\Sigma }}_g^+ }$ Mg2 ${{}_{2}}$ O+ ${{}^{+}}$ QFFs circumvent previous symmetry breaking issues even in explicitly-correlated coupled-cluster results, motivating the need for EOMIP treatments at minimum for such systems. All three species are found to have high-intensity vibrational frequencies. Even so, the highly intense frequency (X ˜ 1 Σ g + ${{{\tilde{\rm {X}}}}^1 {\rm{\Sigma }}_g^+ }$ Mg2 ${{}_{2}}$ O: 894.7 cm-1/11.18 μm;a ˜ 1 Σ u + ${{{\tilde{\rm {a}}}}^1 {\rm{\Sigma }}_u^+ }$ Mg2 ${{}_{2}}$ O: 915.0 cm-1/10.91 μm) for either neutral state may be astronomically obscured by the polycyclic aromatic hydrocarbon 11.2 μm band. Mg2 ${{}_{2}}$ O+ ${{}^{+}}$ may be less susceptible to such obfuscation, and itsν 1 ${{\nu }_{1}}$ intensity is computed to be a massive 4793 km mol-1.

Ion and radical chemistry in (H2O2)N clusters

We investigate the ionization induced chemistry of hydrogen peroxide in (H₂O₂)_N clusters generated after the pickup of individual H₂O₂ molecules on large free Ar_M, M[combining macron]≈ 160, nanoparticles in molecular beams. Positive and negative ion mass spectra are recorded after an electron ionization of the clusters at energies 5-70 eV and after a slow electron attachment (below 4 eV), respectively. The spectra demonstrate that (H₂O₂)_N clusters with N≥ 20 are formed on argon nanoparticles. This is the first experimental report on hydrogen peroxide clusters in molecular beams. The major negative cluster ion series (H₂O₂)_nO₂^- indicates O₂^- ion formation. The dissociative electron attachment to H₂O₂ molecules in the gas phase yielded only OH^- and O^- (Nandi et al., Chem. Phys. Lett., 2003, 373, 454). These ions and the series containing them are much less abundant in the clusters. We propose a sequence of ion-molecule and radical reactions to explain the formation of O₂^-, HO₂^- and other ions observed in the negatively charged cluster ion series. Since hydrogen peroxide plays an important role in many areas of chemistry from the Earth's atmosphere to biological tissues, our study opens new horizons for experimental investigations of hydrogen peroxide chemistry in complex environments.