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Fast Structural Dynamics in Concentrated HCl Solutions: From Proton Hopping to the Bulk Viscosity
Laura Kacenauskaite1,2, Max Moncada Cohen1, Stephen J Van Wyck1
1Department of Chemistry, Stanford University, Stanford, California 94305, United States.
Journal of the American Chemical Society
|April 29, 2024
Summary
Concentrated HCl solutions show unique structural dynamics, linking proton hopping and bulk viscosity to a slowest relaxation time (t3). This slowest dynamic component increases with concentration, unlike in pure water.
Area of Science:
- Physical Chemistry
- Solution Dynamics
- Spectroscopy
Background:
- Proton hopping and hydronium ion spectroscopy in concentrated HCl are well-studied.
- Structural dynamics of concentrated HCl solutions remain less understood.
- Comparing HCl to NaCl solutions helps elucidate the role of the hydronium cation.
Purpose of the Study:
- Investigate the concentration-dependent structural dynamics of HCl solutions.
- Compare HCl dynamics to NaCl solutions using optical heterodyne detected-optical Kerr effect (OHD-OKE).
- Relate observed dynamics to proton hopping and bulk viscosity.
Main Methods:
- Optical heterodyne detected-optical Kerr effect (OHD-OKE) measurements.
- Analysis of concentration-dependent dynamics from 0.8 m to 15.5 m HCl.
- Comparison with literature data from ab initio MD simulations and 2D IR chemical exchange experiments.
Main Results:
- HCl and NaCl solutions exhibit triexponential OHD-OKE signal decays, unlike biexponential decay in pure water.
- A slowest decay constant (t3) linearly correlates with bulk viscosity.
- Proton hopping times align with the slowest structural dynamics relaxation time (t3).
Conclusions:
- Structural dynamics of hydronium/chloride/water clusters govern proton hopping and bulk viscosity.
- The slowest relaxation time (t3) increases with HCl concentration, absent in pure water.
- Fastest decay constants (t1, t2) in HCl resemble pure water's and increase slightly with concentration.
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