The research team led by Academician Yang Xueming and Researcher Xiao Chunlei from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, collaborated with the theoretical team of Academician Zhang Donghui and Researcher Zhang Zhaojun, to achieve precise control over the stereo dynamics of chemical reactions.

On one hand, the experimental research team developed a new method that can give hydrogen molecules specific quantum states while adjusting the orientation of their chemical bonds.

This enabled the controlled collision of hydrogen atoms with deuterated hydrogen molecules in a specific orientation, thereby allowing precise control over the process and outcome of chemical reactions.

Combining this method of actively controlling chemical reactions has created new opportunities for a better understanding of chemical reactions in the future.

On the other hand, the theoretical research team used quantum dynamics calculations to not only accurately reproduce the experimental observations but also elucidate the microscopic processes and mechanisms of the reactions from the perspective of quantum mechanics.Combining experiments with theory, this research has achieved a high level in both experimental and theoretical aspects, and the reviewers have evaluated the research as a "milestone in reaction dynamics research."

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Unveiling the "Black Box": The Challenges of Precise Study of Chemical Reactions

In the simplest chemical reaction, H+H2 → H2+H, which involves three hydrogen atoms, the challenges faced in studying it precisely are beyond many people's expectations.

On one hand, the key to understanding a chemical reaction lies in understanding how the old chemical bonds of the reactants break and how the new chemical bonds of the products are formed.On the time scale, this process occurs extremely rapidly, with the corresponding time scale being on the order of femtoseconds or picoseconds.

On the spatial scale, chemical reactions occur at the atomic level, and directly measuring the state and behavior of reactants and products in the form of individual atoms or molecules is also a great challenge.

On the other hand, reactants and products in the form of individual atoms or molecules also have specific internal structures and corresponding states of motion, and cannot be treated as simple mass points.

For example, a hydrogen molecule is formed by the chemical bond between two hydrogen atoms, and its shape is similar to a dumbbell. The chemical bond is similar to a spring, which can stretch and contract, corresponding to different vibrational states of the hydrogen molecule. In addition, this "dumbbell" of the hydrogen molecule will also have different rotational states.

During the collision process of hydrogen molecules and other atomic molecules, hydrogen molecules with different vibrational and rotational states have different energies and modes of motion. These factors will have an impact on the chemical reaction process that cannot be ignored.In addition, during the collision process between hydrogen molecules and other atomic molecules, their "shape" and "structure" must also be considered.

For example, if another reactant attacks from the middle of the "dumbbell" or from one side of the "dumbbell," the collision process and outcome are likely to be completely different.

The focus of steric dynamics lies in the role that the spatial orientation of the reactant molecules plays in the reaction.

For a long time, how to use the shape and spatial orientation of molecules to finely control the process and outcome of chemical reactions has been one of the frontier issues in the study of chemical dynamics.Controlling Microscopic Molecules with Lasers to Achieve Precise Control of Quantum States and Orientation of Reactants

Starting from 2010, the research group began to attempt to regulate chemical reactions by controlling the initial vibrational state of hydrogen molecules.

In 2013, they excited the vibrational quantum state (v=1, j=0) of the hydrogen molecule isotope (HD). Here, v represents the vibrational quantum number of the HD molecule, while j represents the rotational quantum number.

The research group found that in the F+HD → HF+D reaction, when the HD molecule vibrates, the reaction occurs through a new pathway. Interestingly, even if the same amount of energy is provided to the reaction system in the form of collision energy, the reaction cannot occur through this pathway.

In the Cl+HD → HF+D reaction, when the HD molecule vibrates, the reaction occurs through a resonance state with an extremely short lifetime, which is generated by the mechanism of chemical bond softening.The two important studies mentioned above were published in Science in 2013 and 2015, respectively, which laid a solid theoretical and experimental foundation for the stereodynamic study in this research [2,3].

However, there are still unresolved scientific issues in previous studies. Specifically, in the experiments at that time, the spatial orientation of the HD molecule was random. That is to say, the distribution of the chemical bond direction of the HD molecule was statistically isotropic.

Therefore, it was impossible to study the role of the molecular shape and the direction of collision in chemical reactions at that time.

In order to give the molecule a specific spatial orientation in the experiment, it is necessary to make it rotate. Therefore, in the new experiment, the HD molecule is no longer prepared in the quantum state (v=1, j=0), but (v=1, j=2). The rotational quantum number j=2 indicates that the HD molecule is in an excited state of rotation.

Xiao Chunlei explained: "This is like placing a gyroscope in space, its direction may be random."Once the gyroscope starts spinning and has angular momentum, its axis direction will not change without the action of an external force, due to the conservation of momentum. Therefore, the gyroscope is no longer isotropically distributed, and statistically, it has a specific orientation.

Based on this principle, researchers have prepared molecules with different spatial orientations using lasers in experiments, and studied the reaction H+HD→H2+D through high-resolution crossed molecular beam experiments at collision energies of 0.50, 1.20, and 2.07 electron volts, respectively.

According to the relevant results, they found that the difference in the orientation of HD will lead to significant differences in the reaction results.

Subsequently, the research team theoretically simulated the phenomena in the experiments using quantum dynamics simulation, and also used the theory of polarized differential cross-sections to clearly reveal the stereodynamic effects in the reaction, as well as the key role of quantum interference phenomena in the vertical collision configuration reaction.

"Although there have been other experiments controlling the orientation of molecules in the field before, the level of control has not been so refined," said Xiao Chunlei.Furthermore, he pointed out that the study, through the "double-barreled" approach of high-resolution experiments and high-precision quantum dynamics theory, has revealed the role of stereo orientation in the classic reaction system of H+HD, providing a good example for understanding the stereodynamic effects in chemical reactions.

Ultimately, the related paper, titled "Stereodynamical control of the H + HD → H2 + D reaction through HD reagent alignment," was published in Science [1].

Wang Yufeng and Huang Jiayu, doctoral students at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, are co-first authors. Academicians Yang Xue Ming, Zhang Donghui, and researchers Zhang Zhaojun and Xiao Chunlei serve as co-corresponding authors.

"Only by preparing instruments that others do not have can we see phenomena that they cannot see."Although the framework of quantum mechanics has been established, and in many cases, it is possible to accurately simulate physical processes based on quantum mechanics, it is important to understand that in the field of chemistry, it is still difficult to fully achieve precise simulation and prediction based solely on quantum mechanics.

For complex systems such as chemical reactions, their Schrödinger equations do not have exact analytical solutions and can only be approximated through numerical methods. Obtaining accurate results presents a significant challenge.

Precisely because of this, many reactions in organic and inorganic chemistry still rely on the experience and rules of researchers for related understanding and prediction.

The goal of reaction dynamics research is to combine theory and experiment to develop chemistry into a precise and predictable discipline.

In this study, researchers started from the principles of quantum mechanics, not only accurately calculating detailed information such as reaction pathways, product angles, and quantum states, but also clearly revealing the mechanisms of chemical reactions and the physical picture of how molecular orientation plays a role in the reaction process.In terms of application fields, taking drug design as an example, the process of developing new drugs usually takes a long time and requires researchers to invest a lot of time and energy.

If the mutual promotion of theory and experiment can develop an accurate theory that can precisely simulate and predict the interactions between various drug molecules and protein molecules, it will greatly reduce the cost of drug development and improve the efficiency of drug design. This is also one of the ultimate goals in the field of reaction dynamics.

Xiao Chunlei graduated from the Department of Physics at the University of Science and Technology of China and obtained his Ph.D. at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences. During his junior year in college, he had the opportunity to visit and learn from the research group of Academician Yang Xueming. When he saw such precise equipment, he was instantly "brightened up".

Since then, Xiao Chunlei has focused his research on using the crossed molecular beam method and advanced laser technology. He has carried out a series of experimental studies in the direction of elementary reaction dynamics and won the Special Award of the President of the Chinese Academy of Sciences upon his doctoral graduation.

In this research, the experimental breakthrough could not have been achieved without the high-energy, narrow linewidth, and high-stability optical parametric oscillator they independently developed and built, which is the key to the efficient vibrational excitation of hydrogen molecules.Xiao Chunlei stated: "Teacher Yang has repeatedly instructed us that 'only by creating instruments that others do not have can we observe phenomena they cannot see.' Our research this time has once again confirmed this statement."