Periodic Reporting for period 4 - PROWAT (Proton conduction in structured water)
Reporting period: 2021-04-01 to 2022-03-31
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.
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 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.
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.
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 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.
1) Dynamics of hydrated proton clusters
We studied the vibrarioanl spectrum and 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 found that water molecules in the core and in the shell of the hydrated proton cluster show quite different vibrational frequencies and vibrational relaxation rates.
For the water molecules in the core we observe an ultrafast energy relaxation that leads to an ultrafast heating of the local aqueous environment of the proton. For the water molecules in the shellwe observe
a relatively slow vibrational relaxation with a T1 time constant ranging from 0.22 to 0.37 ps. (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 (J. Phys. Chem. B 122,10005-10013 (2018).
2) Dynamics of protons in water nanodroplets
We studied the vibrational and structural dynamics of hydrated protons in reverse micelles using polarization-resolved femtosecond infrared transient absorption spectroscopy. We find that in small nanodroplets with a diameter <4 nm proton hopping is more than 10 times slower than in bulk water. Even in big nanodroplets of ~8 nm diameter, we find that the rate of proton hopping is slowed down by at least 4 times in comparison to bulk water (manuscript in preparation). For the anionic reverse micelles we observe additional transient absorption bands that we assign to the complexes formed by protons and the negatively charged sulfonate group of the AOT surfactant molecules (manuscript in preparation). ACS Cent. Sci. 2020, 6 1150–1158 (2020)
3) Orientation and location of molecules and ions near the surface of water
We observed that the orientation of urea molecules close to a water surface is strongly dependent on the sign of the charge of the surface and closely follows the orientation of the neighboring water molecules. J. Phys. Chem. Lett. 12, 10823–10828 (2021).
We also found that protons are located surprisingly close to the surface of the solution, much closer than their negative counterions, like for instance Cl- or Br-. The different locations of the protons and their counterions create a strong electric field close to the surface that strongly orients water molecules.
4) Vibrational response of surface water
We found that the surface vibrational response of the water bending mode consists of both an interfacial dipole and a bulk quadrupolar response. We thus found that the bending mode of water can provide structural information on both the surface and the bulk of aqueous solutions. Phys. Rev. Lett. 127, 116001 (2021)
5) Observation of distinct conformers of the carboxylic acid group
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 The relative abundance of the conformers strongly depends on the polarity of the environment and on the possibility of forming an intramolecular hydrogen bond. J. Phys. Chem. Lett. 10, 12, 3217–3222 (2019); J. Phys. Chem. Lett. 11, 9, 3466–3472 (2020); Communications Chemistry 3, 84 (2020)
We studied the vibrarioanl spectrum and 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 found that water molecules in the core and in the shell of the hydrated proton cluster show quite different vibrational frequencies and vibrational relaxation rates.
For the water molecules in the core we observe an ultrafast energy relaxation that leads to an ultrafast heating of the local aqueous environment of the proton. For the water molecules in the shellwe observe
a relatively slow vibrational relaxation with a T1 time constant ranging from 0.22 to 0.37 ps. (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 (J. Phys. Chem. B 122,10005-10013 (2018).
2) Dynamics of protons in water nanodroplets
We studied the vibrational and structural dynamics of hydrated protons in reverse micelles using polarization-resolved femtosecond infrared transient absorption spectroscopy. We find that in small nanodroplets with a diameter <4 nm proton hopping is more than 10 times slower than in bulk water. Even in big nanodroplets of ~8 nm diameter, we find that the rate of proton hopping is slowed down by at least 4 times in comparison to bulk water (manuscript in preparation). For the anionic reverse micelles we observe additional transient absorption bands that we assign to the complexes formed by protons and the negatively charged sulfonate group of the AOT surfactant molecules (manuscript in preparation). ACS Cent. Sci. 2020, 6 1150–1158 (2020)
3) Orientation and location of molecules and ions near the surface of water
We observed that the orientation of urea molecules close to a water surface is strongly dependent on the sign of the charge of the surface and closely follows the orientation of the neighboring water molecules. J. Phys. Chem. Lett. 12, 10823–10828 (2021).
We also found that protons are located surprisingly close to the surface of the solution, much closer than their negative counterions, like for instance Cl- or Br-. The different locations of the protons and their counterions create a strong electric field close to the surface that strongly orients water molecules.
4) Vibrational response of surface water
We found that the surface vibrational response of the water bending mode consists of both an interfacial dipole and a bulk quadrupolar response. We thus found that the bending mode of water can provide structural information on both the surface and the bulk of aqueous solutions. Phys. Rev. Lett. 127, 116001 (2021)
5) Observation of distinct conformers of the carboxylic acid group
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 The relative abundance of the conformers strongly depends on the polarity of the environment and on the possibility of forming an intramolecular hydrogen bond. J. Phys. Chem. Lett. 10, 12, 3217–3222 (2019); J. Phys. Chem. Lett. 11, 9, 3466–3472 (2020); Communications Chemistry 3, 84 (2020)
An imporant discovery is that the proton conductivity of water strongly slows down when water gets confined to nanometric volumes. A surprising and important aspect of this discovery is that this already happens for water droplet diameters < 10 nm, in which case the droplet still contains 1000's of water molecules. This shows that even for these relatively large water sizes, there are changes in the hydrogen-bond structure and dynamics that affect the mobility of protons. This finding is highly relevant for all systems where water is confined to the nanometer scale, like in biological systems and in fuel cells. Within the project, we planned to develop techniques to measure the effect of nanoconfinement on the mobility of protons in water, but the outcome of this study was unexpected, especially because previous studies of other types of molecular dynamics in water (for instance the molecular reorientation of water molecules) showed that these dynamics only changed when water got extremely confined to sizes <1 nm. Aqueous proton transfer is clearly different, and thus appears to rely on very subtle structural dynamics of a large number of water molecules.
A second imporant and unexpected discovery is that the carboxylic acid group, which is ubiquitous in biological systems, shows distinct syn and anti conformers in water at room temperature. In the syn-conformer the O–H group is oriented at ~60∘ with respect to the C=O and in the anti-conformer the O–H is anti-parallel to the C=O. The expectation was that in water and at room temperature there would be a continuous distribution of conformers of the carboxylic acid group. We clearly demonstrated that this is not the case. The different conformers (syn and anti) have a quite different interaction with water and, even more importantly, these conformations do not rapidly exchange. The long-living presence of syn or anti conformers can thus have a pronounced effect on the conformation and reactivity of large (bio)molecules containing carboxylic acid groups, which may have biological implications that were not yet realized.
A second imporant and unexpected discovery is that the carboxylic acid group, which is ubiquitous in biological systems, shows distinct syn and anti conformers in water at room temperature. In the syn-conformer the O–H group is oriented at ~60∘ with respect to the C=O and in the anti-conformer the O–H is anti-parallel to the C=O. The expectation was that in water and at room temperature there would be a continuous distribution of conformers of the carboxylic acid group. We clearly demonstrated that this is not the case. The different conformers (syn and anti) have a quite different interaction with water and, even more importantly, these conformations do not rapidly exchange. The long-living presence of syn or anti conformers can thus have a pronounced effect on the conformation and reactivity of large (bio)molecules containing carboxylic acid groups, which may have biological implications that were not yet realized.