Many things around us are constantly changing through chemical reactions. While tens of thousands of types of chemical reactions occur in our daily lives, little is known about how these reactions actually take place. A South Korean research team has developed a new method to observe a wider range of chemical reaction processes. The Institute for Basic Science’s Center for Advanced Reaction Dynamics, which conducts world-class time-resolved diffraction experiments to elucidate the principles of chemical reactions, will outline concepts to explore this unknown world.

What is Ultrafast Structural Dynamics?

A chemical reaction involves changes where atoms that make up molecules at the angstrom level (one hundred millionth of a centimeter) form or break bonds. These chemical reactions, such as charging a smartphone, burning wood, or producing headache medicine sold at convenience stores, occur due to collisions between molecules moving at speeds of up to 500 m/s. Moreover, these phenomena of bond formation, breaking, and changing occur in the fleeting moment of a femtosecond (one quadrillionth of a second), beyond our perception. Therefore, observing 'Ultrafast Reaction Dynamics,' which forms the basis of chemical reactions during these brief periods, has been extremely challenging. To overcome this, tools that can observe such short times have been developed. Following the initiation of research on fast molecular changes and chemical reactions by Nobel laureate Ahmed Hassan ZEWAIL in 1999, many scientists have dedicated themselves to studying the fundamental principles of chemical reactions through ultrafast reaction dynamics.

There are several methods to study ultrafast reaction dynamics, relating to the various characteristic changes that occur as the structure or state of chemical substances changes. Typically, during chemical reactions, molecules change their structure and atomic arrangement, which alters properties such as light absorption or emission. This results in color changes and changes in speed or electrical properties of molecules. The method of observing these changes through light properties is called spectroscopy, while the method using changes in molecular speed due to changes in mass is called mass spectrometry. Among these, ultrafast structural dynamics refers to the method of observing the structure of substances, i.e., the arrangement of atoms within molecules, and its changes.

Metal particles like gold and silver, when reduced to nano-scale sizes smaller than the wavelength of light, exhibit colors different from what we commonly perceive. This phenomenon is due to the surface plasmon resonance effect. Metals possess free electrons, and when light shines on metal nanoparticles, the surface electrons resonate strongly with certain wavelengths of light, vibrating collectively as a group. At this point, the nanoparticles absorb that specific wavelength of light and exhibit a new, unconventional color. This color varies depending on the size or shape of the nanoparticle, leading gold nanoparticles to appear red. This altered color is then utilized in diagnostic kits.

Ultrafast Electron Diffraction (UED)

How can we observe the structure of a molecule? We use rulers to measure lengths by reading scales, measure distances by timing the return of light pulses, or magnify sizes with electron microscopes. However, it is not easy to observe length changes in atoms using rulers or electron microscopes due to the narrow distances between atoms and the rapid changes during chemical reactions. Therefore, tools that can convert these distances into observable values and capture very fast changes are necessary. The 'diffraction' technique transforms the distances between atoms into visible images.

[Figure 1] As the distance between atoms changes, the spacing of diffraction patterns changes. Wider atomic distances result in relatively denser diffraction patterns. By examining the spacing of light or electron beam patterns, it is possible to determine the distance between atoms and molecular structure information.

Diffraction techniques use the wave-particle duality of particles to reflect their structural information of as light patterns after colliding them with materials. X-rays, electron beams, and neutron beams are typically used to measure structural information of a material. As for ultrafast electron diffraction, it uses rapidly accelerated electrons to measure fast-changing molecular structures. Faster electrons can measure shorter lengths accurately. In this study, electrons were accelerated to 99% of the speed of light (0.99 c), allowing measurement of molecular structures with precision up to a few picometers (one trillionth of a meter).

[Figure 2] Photos of UED equipment at the SLAC National Accelerator Laboratory in the US (top) and the Korea Atomic Energy Research Institute (KAERI) in South Korea (bottom). (Source: SLAC National Accelerator Laboratory, KAERI Ultrafast Radiation Application Research Team)

Ionization and Photoexcitation

Reactions involving ions exhibit unique characteristics among various reactions. Ions are atoms or molecules that have gained or lost electrons and acquired electrical properties, which makes them likely to engage in strong chemical reactions due to electrostatic attraction or repulsion. These ions easily transform into other substances, causing chemical reactions like rusting metal or facilitating various biochemical reactions in our bodies.

However, the strong reactivity of ions has paradoxically made their study difficult. Preparing and controlling ion reactions for observation is challenging. This study used a 100 femtosecond ultrashort laser for photoexcitation, which creates ions by causing molecules to lose electrons rapidly through ionization. Using resonance-enhanced multiphoton ionization (REMPI), the researchers controlled photoexcitation to maintain stable ion states. The UED technique then allowed observation of the chemical reactions and structures of these ions.

[Figure 3] Diffraction patterns of ions generated through resonance-enhanced multiphoton ionization differ from those of neutral molecules due to changes in charge and structure. By examining these patterns, it is possible to capture the fleeting structural changes of ions and observe the progression of chemical reactions as if filming a movie.

A New Method to Observe Fleeting Changes in Ions in Real-Time

The recent study by the IBS Center for Advanced Reaction Dynamics is the first to observe the structural dynamics of molecular ions in real-time. This research provided a scientific foundation to observe a wider range of chemical reactions by presenting a method to observe the structural dynamics of ions in real-time. Using the innovative technology of MeV-UED (Mega-Electron-Volt Ultrafast Electron Diffraction), the researchers captured fine structural changes of ions in the gas phase with high precision and used REMPI to control the ionization process of molecules more precisely.

[Figure 4] Experiments confirmed no significant structural changes for about 3.6 picoseconds after formation of ion. The isomeric-dibromopropane cation (DBP+) forms after 15 picoseconds, and loosely bound bromine (Br) atoms in the DBP+ cation escape to form monobromopropane cation (MBP+) after 77 picoseconds. This direct observation of ion stabilization shows the final product has a bromonium structure, known as an organic reaction intermediate.

The final substance after ionization, MBP+, posseses bromonium ring structure, which are reaction intermediates in organic reactions involving of bromine (Br) and other halogen atoms. This shows that stable gas-phase ion molecules are closely related to organic reaction processes observed in the liquid phase.

This new approach has been crucial in understanding the detailed structural characteristics and dynamics of ions, previously considered impossible. This study enables a deeper understanding of gas-phase ions and proposes methodologies to study a broader range of reactions, providing new insights into chemical reaction principles, material property changes, and various phenomena such as chemical reactions occurring in space. This discovery advances the fundamental understanding of ion chemistry and is expected to significantly impact the design of diverse chemical reactions and space chemistry research in the future.

Reference: Heo, J. et al. “Capturing the generation and structural transformations of molecular ions”, Nature, 625, 710–714 (2024)