# First-Principles Molecular Dynamics Simulations of Infrared and Raman Vibrational Spectra of H5O2 +, D5O2 +, DH4O2 +, and D4HO2 + from 50 to 300 K

## TL;DR

This paper uses simulations to study the vibrational spectra of water cluster ions at different temperatures, revealing new insights into their structural prevalence.

## Contribution

The study provides new temperature-dependent insights into the O···O stretch region and isotopologue prevalence using first-principles simulations.

## Key findings

- DH4O2+ (ext) is at least 90% prevalent between 50–300 K due to low zero-point energy.
- D4HO2+ (ext) becomes dominant over D4HO2+ (int) at 84.9 K and higher, contrary to previous assumptions.
- Rovibrational thermal averaging helps clarify complex spectra of partially deuterated isotopologues.

## Abstract

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.