# Cryogenic Vibrational Spectroscopy of the Deprotonated Dimer of Phosphoric Acid

## TL;DR

This paper investigates the structure of a deprotonated dimer of phosphoric acid using advanced spectroscopy techniques and compares the results with theoretical predictions.

## Contribution

The study provides a clear structural assignment of the deprotonated dimer of phosphoric acid using experimental and theoretical methods.

## Key findings

- Two nearly isoenergetic structures were predicted for H3PO4·H2PO4–.
- Experimental spectra allowed for a clear identification of the actual structure.
- The identified structure may influence interactions of phosphoric acid in condensed phases.

## Abstract

Phosphate-containing molecules are ubiquitous in nature,
where
they play crucial roles in biochemical processes. Further, they are
of technical importance, for example, in certain batteries and in
fuel cells, where a unique property of phosphoric acid is exploitedits
exceptionally high proton conductivity. Proton transport in phosphoric
acid is known to involve proton shuttling; however, the elementary
steps involved are not clear. To elucidate the hydrogen bonding preferences
of phosphoric acid, we investigate the dihydrogen phosphate anion
as well as the deprotonated dimer of phosphoric acid (H3PO4·H2PO4
–) in the gas phase using infrared
action spectroscopy in helium nanodroplets and infrared D2-tagging photodissociation spectroscopy, and the experimental spectra
are compared to theoretical ones. Theory finds for H3PO4·H2PO4
– two different structures that are
predicted to be nearly isoenergetic. The comparison to the experimental
spectra, however, allows for a clear assignment and structure identification.
The resulting structure has an interesting binding motif, which might
be of relevance to interactions of phosphoric acid in the condensed
phase and which can serve as a benchmark for quantum chemical calculations.