First-Principles Molecular Dynamics Simulations of Infrared and Raman Vibrational Spectra of H ₅ O ₂ ⁺ , D ₅ O ₂ ⁺ , DH ₄ O ₂ ⁺ , and D ₄ HO

We report molecular dynamics simulations of infrared (IR) and Raman spectra of H₅O₂⁺ and its deuterium-substituted analogs. We use the well-tested HBB potential and dipole surfaces along with a recently fitted polarizability surface of CCSD(T)/aug-cc-pVTZ quality. The focus of the present work is to provide new insights into the O···O stretch region, located in the [500-700] cm^-1 spectral range, by means of analyzing the spectra over a broad range of temperatures: from 50 to 300 K. Also, rovibrational thermal averaging was performed to untangle the unusually complex spectra of partially deuterated isotopologues DH₄O₂⁺ and D₄HO₂⁺ with H and D minority species in both internal (int) and external (ext) positions. Our findings show that DH₄O₂⁺ (ext) is at least 90% prevalent at the [50-300] Kelvin temperatures, predominantly due to its low zero-point energy (ZPE). Furthermore, contrary to previous reports that the mixed isotopologue D₄HO₂⁺ (int) should be favored over D₄HO₂⁺ (ext) due to its lower ZPE, the present calculations indicate that while the D₄HO₂⁺ (ext) species is "invisible" at lower temperatures, it overtakes D₄HO₂⁺ (int) as the dominant species at 84.9 K and higher.