Imidazoles and triazoles are essential chemical compounds used in many medicines, including drugs used to defeat various pathogen-induced infections and cancer. Besides these applications, both imidazoles and triazoles are used not only in humans but also to protect crops against fungi.

However, despite their high effectiveness, they can easily end up in water or soil, leading to environmental pollution and uncontrolled development of fungi resistant to fungicides. Removing these chemicals from the environment is far from easy for the high stability of the compounds.

Therefore, novel ways to degrade imidazoles and triazoles are widely studied to improve and deeply understand the mechanisms that stand behind the bonds breaking in both compounds, especially in developing effective wastewater treatment. To control the structure of obtained molecules under the application of external stimuli, mechanistic insight for detailed description on a molecular level is needed.

A recent study published in the Journal of the American Chemical Society by an international research team, led by Dr. Dariusz Piekarski from the Institute of Physical Chemistry, Polish Academy of Sciences and Dr. Jaroslav Kočišek from the Czech Academy of Sciences presents significant advancements for molecular chemistry, revealing the role of hydrogen and bromine in the dissociation dynamics of triazole anions. Researchers show in detail how to break these molecules using low-energy electrons.

When a compound like triazole captures one of these low-energy electrons, it forms a short-lived charged version of itself that finally breaks up. But not all triazoles undergo such a reaction in the same way. This process depends on the molecular structure, especially the position of hydrogen atoms.

In the present study, researchers looked at two versions of a bromine-substituted triazole. Researchers employed dissociative electron attachment (DEA) to study the behavior of the specific site in two nearly identical molecules, like 3-bromo-1H-1,2,4-triazole and 3-bromo-4H-1,2,4-triazole (4HBrT) that differ only in the position of one hydrogen atom, under the exposition to low-energy electrons.

By combining the empirical studies with sophisticated theoretical calculations based on the potential energy surfaces, molecular dynamics, and analytic continuation methods, they tracked the atom’s position change and the lifetimes of transient negatively charged molecules with remarkable precision.

Such combining of experiments with quantum chemistry shows that hydrogen position change has a direct influence on the molecular dynamics after an interaction with low-energy electrons, where even a single electron induces subtle structural differences, resulting in dramatically different molecular dynamics.

Hydrogen position controls the character of the resonant states. While the singly occupied molecular orbital SOMO of 1HBrT is highly symmetric with a short lifetime against both dissociation of bromine or loss of electron, the 4HBrT SOMO state is asymmetric, resulting in induced dance of the bromine atom around the rest of the molecule.

Quantum chemical calculations show that bromine migrates more easily when the hydrogen is in the 4-position than an 80 times lighter hydrogen atom, forming a stable noncovalent complex around the triazole ring. The hydrogen position defines whether the bromine atom will move around a molecule or break away directly during a molecular breakup reaction.

For 4HBrT, the dissociation of the bromine atom proceeds via a delayed mechanism, where the bromine forms temporarily weakly bonded species, stabilizing the transient negative ion and elongating its lifetime. This results in an intermediate metastable state before hydrogen bromide (HBr) is formed.

In contrast, the 1HBrT, a different position of the H-atom facilitates easy and direct cleavage of the C–Br bond, allowing bromine to dissociate without interaction with the remaining triazole ring structure.

The research findings provide new insights into the behavior of transient negative ions and could have far-reaching implications for pharmaceutical development and environmental chemistry. They found that the bromine atom not only makes it easier for the molecule to grab an electron, but also helps stabilize different forms of the molecule, depending on where the hydrogen is placed in the ring via different timescale-lived resonant states called transient negative ions.

“Like two keys that at first glance look the same but open completely different doors,” explains Dr. Piekarski.

This “roaming” of bromine completely reverses the molecular breaking patterns, facilitating the release of hydrogen bromide HBr, whereas, in the 1H-form, the direct bromine dissociation dominates. This fundamental scientific achievement challenges conventional chemical knowledge in several ways, not only for the bromine orbiting around a molecule.

More surprisingly, bromine roaming happens in negative charge states prior to the electron auto-detachment process. Second, the study reveals that even a tiny shift in the position of the hydrogen atom can completely alter this reaction pathway. Third, it shows that Br^– ions can form weak, noncovalent bonds around the triazole ring, creating a much more stable complex that holds the electrons much longer than expected.

“The idea that we can control the movement of heavy atoms through something as simple as placing hydrogen in a given position is exciting and offers new possibilities for chemical design,” remarks Dr. Piekarski.

Now, it is possible to steer halogenated molecular targets’ breakups in desired directions just by positioning hydrogen atoms. These findings point out how even subtle structural differences can guide chemical reactions in unexpected directions. Their study opens new directions for controlled molecular manipulation in chemistry and material science, demonstrating the value of low-energy electron studies in probing dynamic molecular behavior.

The demonstrated work points to a pathway for more effective breakdown of stable, pollution-prone compounds in the environment and an understanding of how drug-like molecules behave under certain conditions, which is crucial for drug design. Future studies will explore whether similar phenomena occur while induced with different radiation sources and in other halogenated compounds.