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Hydroxide diffuses slower than hydronium in water because its solvated structure inhibits correlated proton transfer

Summary

Ab initio molecular dynamics at hybrid DFT quality with van der Waals (Tkatchenko–Scheffler) and self-interaction corrections compares hydronium vs hydroxide structural diffusion in bulk water, focusing on temporal correlations of proton transfers and the solvation motifs that gate OH⁻ mobility. The study addresses why OH⁻ and H₃O⁺ can show different macroscopic transport signatures even though both participate in Grotthuss-style shuttling: the analysis emphasizes when successive transfers occur together versus independently, not only average hopping rates.

Methods

From the Nature Chemistry article PDF (pdf_path); values follow the Methods section there. The authors compare multiple DFT flavors (PBE, PBE-TS, PBE0-TS) to separate dispersion and hybrid effects on solvation structure and dynamics.

  • DFT / AIMD code: Quantum ESPRESSO; exact exchange for PBE0 via Wannier-based formulation; Tkatchenko-Scheffler (TS) vdW treated self-consistently (PBE0-TS). Plane-wave cutoff 72 Ry; Troullier-Martins norm-conserving pseudopotentials; gamma-only Brillouin sampling.
  • Cells: Pure water 128 H2O, cubic edge 15.7 A; H3O+(aq) (63 H2O + one excess proton: 127 H, 63 O), edge 12.4 A; OH-(aq) (63 H2O + OH-: 127 H, 64 O), edge 12.4 A (targets ambient liquid density as stated). Periodic boundary conditions on cubic supercells for each composition.
  • Car-Parrinello MD: Fictitious electronic mass 150 a.u.; electronic mass preconditioning cutoff 6 Ry; NVT at 330 K; Nose-Hoover chain thermostats (one chain per atom, four thermostats per chain). Timestep 3.5 a.u. (~0.08 fs). Hydrogen masses set to deuterium (2.0135 amu) for sampling; oxygen 15.9995 amu. Trajectory lengths: H3O+ 28 / 45 / 32 ps (PBE / PBE-TS / PBE0-TS); OH- 54 / 55 / 38 ps; bulk water 14 / 14 / 25 ps (PBE / PBE-TS / PBE0-TS). H-bond definition: O-O < 3.5 A, H-O-O angle < 30 deg; O-H covalent cutoff 1.24 A.
  • Barostat / pressure: N/A — fixed-volume NVT Car–Parrinello trajectories without NPT barostat coupling to a pressure target.

Findings

  • Mechanism / outcomes: Hydronium diffusion remains associated with correlated, relatively frequent Grotthuss hopping among Eigen- and Zundel-like motifs, whereas hydroxide shows less temporally correlated transfers gated by a hypercoordinated solvation shell (Nature Chemistry Results).
  • Comparisons: The study compares PBE, PBE-TS, and PBE0-TS DFT flavors to isolate dispersion and hybrid effects on both structure and dynamics, relating trends to experimentally inferred OH⁻ versus H₃O⁺ transport asymmetry.
  • Sensitivity: Functional choice shifts solvation motifs and hopping kinetics; 330 K NVT sampling and heavy-hydrogen masses modify absolute diffusion numbers relative to 298 K laboratory NMR.
  • Limitations / outlook: AIMD cost limits system size and trajectory length; the Discussion cites literature on nuclear quantum effects that are not the headline correction here.
  • Corpus honesty: Numerical settings mirror the Methods section of the PDF at pdf_path; confirm Ry cutoffs and thermostat chain depth if reproducing with newer Quantum ESPRESSO builds.

Limitations

  • AIMD cost limits system size and sampling depth; nuclear quantum effects are discussed in the literature cited by the paper but are not the central headline here.