Scientists propose a new micro-mechanism for the dissolution of hydrogen chlorid

Xie Fei is a young scholar from Shaodong City, Hunan Province. He graduated from Huazhong University of Science and Technology with a bachelor's degree and a Ph.D. from the University of Alberta in Canada, and later conducted postdoctoral research at the German Electron Synchrotron.

Not long ago, he published a first-author paper in Science, which included only three authors including himself, and the related experiments were completed using only tap water.

Thanks to this achievement, he also received an offer from the University of Science and Technology of China and will return to China to take up the job after completing his postdoctoral research.

Regarding this paper, he said: "The reviewers believe that this study has provided a very credible answer to the basic problems that have troubled physical chemists for decades, and they believe that we have put forward a clear mechanism for the micro-solvation of hydrogen chloride to form hydrochloric acid, correcting the search direction of predecessors and pointing out the focus for future research."

In the study, he proposed a microscopic mechanism about the basic chemical process, providing new ideas for understanding the microscopic behavior of molecular systems, which has potential to promote the development of chemistry, physics, materials, and other fields.The microwave spectroscopy instruments involved in this study have a very broad application prospect, but their industrialization and commercialization are still in the initial stage. "At the same time, trying to localize is also one of the future important tasks for Chinese microwave spectroscopists," said Xie Jian.

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There are 5 water molecules and 3 hydrogen bonds.

According to the introduction, the purpose of this study is to explore the transition process of the chemical and physical properties of substances from a microscopic perspective to a macroscopic perspective.

At the microscopic level, the academic community mainly focuses on the electronic structure of individual atoms and molecules, the formation of chemical bonds, and their reaction processes under different conditions, while also covering the application of quantum mechanics.

In fact, for modern electronic structure and modern molecular dynamics calculations, they can already provide relatively accurate descriptions, which can help understand the basic properties and basic behaviors of substances.In recent years, as the research of scholars in the field gradually shifts to a macroscopic scale, people have found that the properties of matter begin to exhibit more complex and more diverse characteristics.

At this time, if we still use existing theories, it is difficult to make precise descriptions from the perspective of first principles.

Therefore, the team hopes to use experimental methods to observe the aggregation and growth process of molecular clusters, in order to deduce and explain the microscopic mechanisms of the macroscopic properties of matter.

For example, deducing and explaining the chemical reaction dynamics and thermodynamic properties of molecules in solution systems, atmospheric aerosols, and ice surfaces.

Thus providing more scientific foundations for fields such as atmospheric chemistry, astrochemistry, material design, and environmental protection.To be specific, this study aims to answer a very fundamental question in chemistry:

We learned in high school that when hydrogen chloride gas dissolves in water, it dissociates to produce ions, thereby forming hydrochloric acid. These ions are very active and can participate in and catalyze a series of reactions.

For example, they participate in the corrosion of metals, the formation of atmospheric pollution, and the digestion of food in the human stomach. So, how many water molecules are needed at least for the dissociation of a hydrogen chloride molecule?

This seems to be a simple question, but it has puzzled the academic community for decades.

Taking a cup of hydrochloric acid solution at room temperature as an example, there are about hundreds of trillions of molecules inside, moving randomly at a speed of hundreds of meters per second, and frequently colliding with each other.Under these circumstances, it is difficult to capture the interactions between a few molecules. Previously, physical chemists commonly used cooling methods to isolate clusters formed by several molecules in a vacuum for observation of their behavior.

Of course, at this scale (several angstroms = several 0.1 nanometers), the behavior of matter typically includes molecular vibrations, molecular rotations, nuclear spins, electron orbits, etc., and is therefore a quantized, i.e., discontinuous energy process.

To this end, the research team used supersonic expansion technology to cool and separate the structure of hydrochloric acid-water clusters in a vacuum.

At this time, hydrochloric acid and water molecules will expand from high pressure to a vacuum chamber through a small hole. During this process, collisions will occur between the molecules and the inert carrier gas.

These collisions will convert the internal energy of the molecules into highly directional kinetic energy, thereby cooling the molecules to 1-2 Kelvin. At such low temperatures, the molecules begin to condense and form clusters.In the vacuum chamber, these clusters can persist for about several hundred microseconds. Within this time window, the team irradiated hydrochloric acid water clusters with microwave radiation to obtain their rotational spectral fingerprint characteristics.

These fingerprint features carry detailed structural information of the clusters, and through this information, different structural arrangements can be clearly identified.

The key to this study is that the nuclear spin of the chlorine atom is coupled with the overall rotational energy levels of the molecular cluster, leading to the presence of hyperfine structures in the rotational spectrum.

This hyperfine structure is strongly influenced by the electrons surrounding the chlorine atomic nucleus.

Based on this, the research group analyzed the hyperfine structure and obtained information on whether the hydrochloric acid molecular units in these clusters are covalently bonded or dissociated (ionic).By precisely measuring the perturbation of the overall rotational transition of the molecular cluster caused by the spin of the chlorine atomic nucleus, the team determined the structure of the hydrochloric acid water cluster and the corresponding distribution of the electron cloud outside the chlorine atomic nucleus.

Through this, they proposed a highly credible microscopic mechanism for the solvation of hydrochloric acid. This answers the aforementioned question: that is, five water molecules can induce the dissociation of the hydrogen chloride molecule through the direct action of three hydrogen bonds, thereby forming an ion pair.

Bravely "sowing," there will be a harvest.

In fact, in 2022, Xie et al. had already used the same method to study the hydrolysis process of a weak base molecule containing an amino group.

However, even up to the heptahydrate cluster, they still did not directly observe the formation of ions.However, by analyzing the nuclear quadrupole coupling constants of nitrogen atoms, they discovered the dissociation tendency of the amino group.

Naturally, they deduced that for a strong base or acid, under the action of several water molecules, this dissociation tendency would be more pronounced, and might even directly form ion pairs.

So in 2023, they conducted experimental measurements and related analyses, and found evidence of the dissociation of hydrochloric acid with five water molecules.

And before this paper was published, the controversy over the microsolvation process of hydrochloric acid had been ongoing in the spectroscopy community for decades.

Many peers with a good reputation in the field had tried more than a decade ago, but did not find the relevant spectroscopic evidence.So, at first, they were a bit hesitant about whether to carry out the research, as there was a high probability of wasting experimental resources and scientific effort.

However, the research results on weak bases in 2022 gave us confidence and inspired us to complete this study, said Xie Fu.

Recently, the relevant paper was published in Science with the title "Electric nuclear quadrupole coupling reveals dissociation of HCl with a few water molecules" [1].

Xie Fu is the first author and co-corresponding author, and Professor Melanie Schnell of the German Electron Synchrotron is the co-corresponding author.

As for the micro-solvent process of various types of acid or base molecules, Xie Fu and others expect to observe similar dissociation trends or behaviors.Of course, specific information requires one-on-one measurement and research. "The workload is enormous, and we also hope to share it with our peers," said Xie Fan.

It is also reported that one of the main bottlenecks in microwave spectroscopy at present is the annotation of spectral characteristic fingerprints.

Especially the data from broadband microwave instruments is large, and the microwave spectrum resolution is high (~kHz), which will lead to a very high spectral information entropy.

The use of neural network-based feature recognition technology is expected to break through this bottleneck, which is also one of Xie Fan's follow-up research plans.