How two water molecules dance together-Although water is omnipresent, the interaction between individual water molecules is not yet fully understood

Water drop splash (stock image).Credit: © Romolo Tavani / Adobe Stock

https://www.sciencedaily.com/releases/2019/08/190813102348.htm

An international research team has gained new insights into how water molecules interact. For the first time, the researchers were able to completely observe all of the movements between the water molecules, known as intermolecular vibrations. A certain movement of individual water molecules against each other, called hindered rotations, is particularly important. Among other things, the findings help to better determine the intermolecular energy landscape between water molecules and thus to better understand the strange properties of water.

The team led by Professor Martina Havenith from Ruhr-Universität Bochum and Professor Joel Bowman from Emory University in Atlanta, together with colleagues from Radboud University in Nijmegen and Université de Montpellier, describe the work in the journal Angewandte Chemie International Edition on 27 July 2019.

Unknown interactions

Water is the most important solvent in chemistry and biology and possesses an array of strange properties -- for instance, it reaches its highest density at four degrees Celsius. This is due to the special interactions between the water molecules. "Describing these interactions has posed a challenge for research for decades," says Martina Havenith, head of the Bochum-based Chair of Physical Chemistry II and spokesperson for the Ruhr Explores Solvation (Resolv) Cluster of Excellence.

Experiments at extremely low temperatures

The team investigated the simplest conceivable interaction, namely between precisely two individual water molecules, using terahertz spectroscopy. The researchers send short pulses of radiation in the terahertz range through the sample, which absorbs part of the radiation. The absorption pattern reveals information about the attractive interactions between the molecules. A laser with especially high brightness, as is available in Nijmegen, was needed for the experiments. The researchers analysed the water molecules at extremely low temperatures. To do this, they successively stored individual water molecules in a tiny droplet of superfluid helium, which is as cold as 0.4 Kelvin. The droplets work like a vacuum cleaner that captures individual water molecules. Due to the low temperature, a stable bond occurs between two water molecules, which would not be stable at room temperature.

This experimental setup allowed the group to record a spectrum of the hindered rotations of two water molecules for the first time. "Water molecules are moving constantly," explains Martina Havenith. "They rotate, open and close." However, a water molecule that has a second water molecule in its vicinity cannot rotate freely -- this is why it is referred to as a hindered rotation.

A multidimensional energy map

The interaction of the water molecules can also be represented in the form of what is known as water potential. "This is a kind of multidimensional map that notes how the energy of the water molecules changes when the distances or angles between the molecules change," explains Martina Havenith. All the properties, such as density, conductivity or evaporation temperature, can be derived from the water potential. "Our measurements now allow the best possible test of all potentials developed to date," summarises the researcher.

How ions gather water molecules around them

This photo shows Gerhard Schwaab, Martina Havenith and Federico Sebastiani (from the left).

Credit: RUB, Marquard

https://www.sciencedaily.com/releases/2018/08/180809112504.htm

Charged particles in aqueous solutions are always surrounded by a shell of water molecules. However, much is still unknown about the nature of this so-called hydration shell. Using terahertz spectroscopy, chemists from Bochum have gained new insights into how an ion affects the water molecules in its environment. Prof Dr Martina Havenith, Dr Gerhard Schwaab and Dr Federico Sebastiani from the Chair of Physical Chemistry II of Ruhr-Universität Bochum (RUB) provide an overview of the results of the experiments in the journal Angewandte Chemie in July 2018.

"The hydration shell of ions is extremely important for understanding fundamental processes such as the transport of ions through membranes or batteries," says Martina Havenith, spokesperson of the Cluster of Excellence Ruhr Explores Solvation. "However, seemingly simple questions, like the size of the hydration shell or the occurrence of ion pair formation, still remain unanswered."

New spectroscopic methods developed

At the Ruhr-Universität Bochum, Martina Havenith's team approaches this question with spectroscopic methods developed in-house. The researchers send short pulses of radiation in the terahertz range, i.e. with a wavelength just under one millimetre, through the sample. The mixture absorbs the radiation to different degrees in different frequency ranges, which is made visible in the form of a spectrum. The spectrum, i.e. the absorption pattern, reveals something about the movement of certain bonds in the investigated molecules, for example about hydrogen bonds in a water network.

The Bochum group developed special techniques using low-frequency terahertz radiation to determine the size of the hydration shell, i.e. the number of water molecules that are affected by an ion. They mathematically break down the recorded absorption pattern into its components and can thus identify the parts in the spectrum that reveal something about individual ions or pairs of ions.

Resolving water molecules in hydration shell

The result: Hydration shells with a size between two and 21 water molecules were determined for more than 37 salts investigated. The number depends for instance on the size of the ion and its valency. Single-charged ions usually affect fewer water molecules than multiple-charged ions. "However, this is not entirely systematic, but also depends on the cation or anion present," explains Martina Havenith.

The researchers use their method to determine the so-called effective number of water molecules, which is the minimum number of water molecules that is affected by an ion, i.e. that cannot move as freely as the unaffected surrounding water. Due to the positive or negative charge of an ion, the water molecules with their partially positively charged hydrogen atoms or their partially negatively charged oxygen atom align themselves with the ion. "The effect of the ion on the water molecules gradually decreases with distance," Havenith explains. "Thus there is not always a clear boundary between affected and unaffected water molecules." The team therefore specifies a minimum number for the size of the hydration shell.

Ion pairs studied

However, the Bochum group dealt not only with individual ions, but also with pairs of cations and anions. The water molecules affect the formation of the ion pair. They can either form a joint hydration shell around the two partners or separate shells around cation and anion. The team is able to estimate how many water molecules these shells each consist of. "In order to know how many water molecules surround an iron chloride, it is not enough to know how many water molecules are affected by a single chloride ion and how many by a single iron ion," explains Havenith. This is not a simple additive process.

"In general, our results clearly show that cooperative effects rather than individual ion properties are decisive," sums up the researcher. It is therefore not enough to know a single ion property in order to predict how a salt will affect the water molecules in its environment. Instead, various parameters, such as the charge density or the combination of the cation-anion will determine whether an ion pair is formed.

Simulation results confirmed

The experimental data are suitable for theoretical simulations of other groups and can serve as input parameters for chemical process engineering.

---

Story Source:

Materials provided by Ruhr-University Bochum. Note: Content may be edited for style and length.

---

Journal Reference:

1. Martina Havenith-Newen, Gerhard Schwaab, Federico Sebastiani. Ion hydration and ion pairing as probed by THz spectroscopy. Angewandte Chemie International Edition, 2018; DOI: 10.1002/anie.201805261

Abstract-Detection of Nuclear Spin Isomers of Water Molecules by Terahertz Time-Domain Spectroscopy

Alexander A. Mamrashev,  Lev V. Maximov, Nazar A. Nikolaev,   Pavel L. Chapovsky,

http://ieeexplore.ieee.org/document/8098617/


Nuclear spin isomers of water molecules, i.e., ortho- and para-H2O, are quantum states of one of the most important molecules. Presently, our understanding of the water isomer properties, especially in the gas phase, remains insufficient. Scientific and practical applications of water spin isomers require the development of efficient methods of their detection in gaseous samples. In this paper, we apply the method of wideband terahertz time-domain spectroscopy to study rotational absorption spectra of ortho and para water isomers and measure their relative content in the ambient atmosphere. We find the ortho/para ratio to be 3.03 ± 0.03 close to the expected value of 3 when using the measurement spectral range 0.15-1.05 THz.

Abstract-Vibrational states of nano-confined water molecules in beryl investigated by first-principles calculations and optical experiments

M. A. Belyanchikov,  E. S. Zhukova,  S. Tretiak,  A. Zhugayevych,  M. Dressel,  F. Uhlig,  J. Smiatek,  M. Fyt,   V. G. Thomas, B. P. Gorshunov

http://pubs.rsc.org/en/content/articlelanding/2017/cp/c7cp06472a/unauth#!divAbstract

Using quantum mechanical calculations within density functional theory, we provide a comprehensive analysis of infrared-active excitation of water molecules confined in nanocages of a beryl crystal lattice. We calculate infrared-active modes including the translational, librational, and mixed-type resonances of regular and heavy water molecules. The results are compared to the experimental spectra measured for the two principal polarizations of the electric field: parallel and perpendicular to the crystallographic c-axis. Good agreement is achieved between calculated and measured isotopic shifts of the normal modes. We analyze the vibrational modes in connection with the structural characteristics and arrangements of water molecules within the beryl crystal. Specific atomic displacements are assigned to each experimentally detected vibrational mode resolving the properties of nano-confined water on scales not accessible by experiments. Our results elucidate the applicability and efficiency of a combined experimental and computational approach for describing and an in-depth understanding of nano-confined water, and pave the way for future studies of more complex systems.

Abstract-Theoretical Study on Electronic and Vibrational Properties of Hydrogen Bonds in Glycine-Water Clusters

• Yulei Shi^a, 
• Zhiyuan Zhang^b, c, 
• Wanrun Jiang^b, c, 
• Zhigang Wang^b, c, , , 

• ^a Beijing Key Laboratory for Terahertz Spectroscopy and Imaging, Key Laboratory of Terahertz Optoelectronics, Ministry of Education, Department of Physics, Capital Normal University, Beijing 100048, China.
• ^b Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China
• ^c Jilin Provincial Key Laboratory of Applied Atomic and Molecular Spectroscopy (Jilin University), Changchun 130012, China

http://www.sciencedirect.com/science/article/pii/S000926141730578X

The hydrogen bond (H-bond) in organic-water molecules is essential in nature, and it present unique properties distinct from those in pure water or organic clusters. Combining with the charge-transfer and energy decomposition analyses, we investigated the penetrating molecular-orbitals in glycine-water clusters, which give evidences of the covalent-like characteristics of H-bonds in this system. Besides, the infrared spectral features provide a rare opportunity to discover the exceedingly-evident redshifts of symmetric stretching modes (Symst) in water on forming H-bond, in contrast to the slightly-redshifted asymmetric stretching modes (Asyst) in water. To explain these intriguing behaviors, we further analyzed the nuclear vibrating patterns, which clearly reveal that H-bond retains two unexpected effects on nuclear motions in water: i) Intensifying donor Symst, and ii) Inhibiting donor Asyst. Furthermore, we also quantified the impact of anharmonic quantum fluctuations on each hydrogen bond. For the stretching modes involved in H-bonds, red shifts up to more than one hundred wave numbers are observed under anharmonic vibration, explicitly indicating the increased “covalency” of H-bonds. These finds shed light on the essential understanding of H-bonding comprehensively, and should provide incentives for future experimental studies

Mapping changes in the dynamics and structure of water molecules in the vicinity of solutes

Martina Havenith has quickly implemented the first research idea, which she wanted to realise with funds from a prestigious grant from the European Research Council. Credit: RUB, Marquard

 https://phys.org/news/2017-05-dynamics-molecules-vicinity-solutes.html#jCp

Chemists at Ruhr-Universität Bochum have developed a new method that allows them to map changes in the dynamics and structure of water molecules in the vicinity of solutes. With this technique, called terahertz calorimetry, they investigated the properties of the hydration shell of dissolved alcohol molecules. In the future, they want to also use the method for water mapping around more complex systems such as enzymes, which can be important for drug design.

The results were published by Prof Dr Martina Havenith, chair for Physical Chemistry II and spokeswoman for the cluster of excellence Resolv, with Dr Fabian Böhm and Dr Gerhard Schwaab in the journal Angewandte Chemie.

Method can now be applied in real time

Fundamental biological processes such as enzymatic catalysis or molecular binding occur in aqueous phase. Calorimetry serves as a powerful biophysical tool to study the molecular recognition and stability of biomolecular systems by measuring changes in thermodynamic state variables, e.g. upon protein folding or association, for the purpose of deriving the heat transfer associated with these changes. Calorimetry determines, enthalpy and entropy, which are measures of the heat transfer and disorder in the system.

Calorimetry is restricted to timescales of 1 to 100 seconds. In contrast, spectroscopic processes, which are based on short laser pulses, are able to perform measurements on the time scale of a millionth or a billionth of a second. The Bochum-based chemists showed that both approaches are complimentary.

"By establishing a terahertz calorimeter in a proof of concept experiment, we have achieved the first aim that we had been working on using the Advanced Grant funds from the European Research Council," explains Martina Havenith.

Determining the structure of the watery envelopes

A shell of surrounding water molecules, the hydration shell, forms around any dissolved molecule. The solute affects the regular network of hydrogen bridges between the water molecules, causing the water in the hydration shell to behave differently to the free water. The structure of the hydration shell depends on the shape and the chemical composition of the dissolved molecule.

Havenith's team investigated the hydration shell of five different alcohol chains and were able to classify differently structured hydration water by terahertz calorimetry. Exposure to terahertz pulses provides fingerprints of the vibrations within the water network. This, in turn, allows the researchers to deduce fundamental quantities such as entropy and enthalpy.

"The method allows us for the first time to spectroscopically map entropy and enthalpy around solutes, which are crucial parameters to characterize molecular recognition," summarises Havenith.

 Explore further: Fingerprint of dissolved glycine in the Terahertz range explained

More information: Martina Havenith-Newen et al. Hydration water mapping around alcohol chains by THz-calorimetry reveal local changes in heat capacity and free energy upon solvation, Angewandte Chemie International Edition (2017). DOI: 10.1002/anie.201612162