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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - PROWAT (Proton conduction in structured water)

Teaser

Summary

Aqueous proton transfer is an extremely important and ubiquitous process in nature. All living systems rely
on proton transfer across membranes for the production and storage of energy. Aqueous proton transfer also
constitutes a crucial step in the functioning of hydrogen fuel cells. In most cases, the proton is transferred through
water media that are highly confined in one or more dimensions and for which the hydrogen-bond structure
can be quite different from that of bulk water.

It has been shown that in bulk water protons are being transported by a mechanims that by a special mechanism
that is often referred to as Grotthuss conduction. In this process the proton charge is transferred to
other hydrogen atoms in the hydrogen-bonded network by an interconversion of chemical bonds and hydrogen
bonds. Clearly, this mechanism strongly relies on the structure and dynamics of the hydrogen-bond network
of water. As a result, confinement and structuring of the water medium as in nanochannels or near surfaces,
is expected to have a profound effect on the mobility of protons.

The effects of surfaces on the rate of aqueous proton transfer are complex and can be of conflicting nature.
For instance, surfaces can destabilize the water hydrogen-bond network, which could speed up the proton conduction.
However, such a destabilization also reduces the connectivity required for Grotthuss conduction, which could slow
down the proton transfer. Recently, new experimental techniques like heterodyne detected sum-frequency
generation (HD-SFG) and two-dimensional HD-SFG (2D-HD-VSFG) have been developed that are capable of probing
the structural dynamics of water molecules and proton-hydration structures near surfaces.

In this project we will use these and other advanced spectroscopic techniques to study the rate and molecular
mechanisms of proton transfer through structured aqueous media. These systems include aqueous solutions
of different solutes, water near the of surfaces graphene and electrically switchable monolayers,
and the aqueous nanochannels of metal-organic frameworks. These studies will provide a fundamental
understanding of the molecular mechanisms of aqueous proton transfer in natural and man-made
(bio)molecular systems, and can lead to the development of new proton-conducting membranes and
nanochannels, with applications in more efficient fuel cells. The obtained knowledge may also lead to new strategies
to control proton mobility, e.g. by electrical switching of the properties of the water network at surfaces
and in nanochannels, i.e. to field-effect proton transistors.

Work performed

Over the last years we studied the effect of spatial confinement and surface-water interactions on
the water structure and the rate and mechanism of aqueous proton transfer for different
types of confined systems.

1) Dynamics of protons in structured aqueous solutions
We studied the structuring of water in binary mixtures of isotopically diluted H2O and lutidine (2,6-dimethylpyridine) by measuring
the vibrational energy relaxation and orientational dynamics of HDO molecules with femtosecond mid-infrared pump-probe spectroscopy.
We observed that with increasing lutidine concentration the vibrational relaxation rate decreases. At high lutidine concentrations the relaxation
also becomes frequency dependent: the red wing of the OD-stretch absorption band relaxes faster (1.5â€“2 ps) than the blue wing (2â€“3.5 ps).
The reorientation of the HDO molecules shows two components, with time constants of 2.5 ps and >10 ps. The latter slow component is attributed to HDO molecules solvating the hydrophobic groups of lutidine. The anisotropy of the transient absorption signal persists after relaxation of the OD stretch vibration, which shows that the vibrational relaxation leads to a local hot state. The anisotropy of this local hot state decays with a time constant of ~5 picoseconds, likely as a result of a structural reorganization of the binary water-lutidine mixture (manuscript in preparation).

We also studied the ultrafast relaxation dynamics of water clusters containing protons that are dissolved in acetonitrile. These dynamics were measured with femtosecond mid-infrared pump-probe spectroscopy.
We observed a strong dependence of the transient absorption dynamics on the frequency of excitation. When we excited the OH vibrations of the hydrated proton clusters with frequencies â‰¤3100 cmâ€“1,
we observed an ultrafast energy relaxation that leads to an ultrafast heating of the local aqueous environment of the proton. This response is assigned to the OH vibrations of the water molecules
in the core of the hydrated proton cluster. When we excited with frequencies â‰¥3200 cmâ€“1, we observed a relatively slow vibrational relaxation with a T1 time constant ranging from 0.22 Â± 0.04 ps for
an excitation frequency of 3200 cmâ€“1 to 0.37 Â± 0.02 ps for an excitation frequency of 3520 cmâ€“1. We assigned the latter response to water molecules in the outer part of the hydrated proton cluster.
(published as O.O. Sofronov et al., J. Phys. Chem. B 123, 6222-6228 (2019)).

We also investigated the structure and dynamics of proton solvation structures in mixed water/dimethyl sulfoxide (DMSO) solvents using two-color mid-infrared femtosecond pump-probe spectroscopy. At a water fraction below 20%, protons are mainly solvated as (DMSO-H)(+) and (DMSO-H)(+) -H2O structures. We find that excitation of the OH-stretch vibration of the proton in (DMSO-H)(+)-H2O structures leads to an ultrafast contraction of the hydrogen bond between (DMSO-H)(+) and H2O. This excited state relaxes rapidly with T1 = 95 +/- 10 fs and leads in part to a strong local heating effect and in part to predissociation of the protonated cluster into (DMSO-H)(+) and water monomers (published as O.O. Sofronov et al. J. Phys. Chem. B 122,10005-10013 (2018)).

Finally, we investigated the molecular geometry of the carboxyl group of formic acid in acetonitrile and aqueous solutions at room temperature with two-dimensional infrared spectroscopy (2D-IR). We found that the carboxyl group adopts two distinct configurations: a configuration in which the carbonyl group is oriented antiparallel to the hydroxyl (anti-conformer), and a configuration in which the carbonyl group is oriented at an angle of 60 with respect to the hydroxyl (syn-conformer). These results constitute the first experimental evidence that carboxyl groups exist as two distinct and long-living conformational isomers in aqueous solution at room temperature (published as G. Giubertoni et al., J. Phys. Chem. Lett. 10, 3

Final results

The reported studies on the water nanodroplets provided novel insights in the effect of confinement on the mobility of the protons. Confinement was observed to lead to a
very strong reduction of the rate at which the proton is transferred. The slowing down due to confinement is observed to be dramatically stronger for the mobility of the proton in water than for
the water molecules themselves, which was extent unexpected. This finding demonstrates that proton transfer is a highly collective process involving the motion and hydrogen-bond
dynamics of many water molecules. Another unexpected and novel results was that carboxylic acid groups exist in two distinct conformers, even in water at room temperature.
In one of these conformers the O-H and C=O groups are syn oriented, in the other they are anti oriented.

In the remainder of the project we will study the effect of confinement on the mobility of protons for aqueous nanochannels. As model nanochannel systems we will use metal-organic-frameworks.
We will also continue the study of the structuring effect of hydrophobic and hydrophilic molecules on the surface of water. To this purpose we will develop time-resolved surface sum-frequency spectroscopy and
we will use this technique to study the dynamics of water molecules at the surface, and ultimately the rate and mechanism at which protons are conducted along the water surface.
Finally, we will study the effect of switching the hydrophobic/hydrophilic character of a layer near an aqueous surface on the proton transfer at this surface.