Positrons, typically linked to their destructive annihilation when they meet electrons, have now been shown to possess the potential for forming chemical bonds that improve molecular stability, as per research conducted with quantum Monte Carlo simulations. This investigation, directed by Andrés Reyes at the National University of Colombia, questions the conventional understanding of chemical bonding.
As the antimatter equivalents of electrons, positrons have the same mass yet carry an opposite charge. The annihilation process involving positrons and electrons is widely recognized; however, their capacity to establish transient yet significant bound states with electrons, atoms, or molecules has been less investigated. Traditionally, positrons have been studied in contexts of bonding with negatively charged ions, where they can develop stable complexes. Nevertheless, the recent research has revealed the possibility of positron bonding with neutral atoms, indicating a shift from typical expectations regarding chemical bonding mechanisms.
Atoms of alkaline earth metals, known for creating weak dimers, are ideal subjects for examining positron bonding with neutral atoms. These atoms, which generally form electronic chemical bonds, are not expected to favor the weaker positron bonds. Despite this, utilizing the highly precise quantum Monte Carlo method, Reyes and his colleagues aimed to discover if a positron could bind two beryllium atoms together, investigating the evolution of the potential energy curve during the process.
The team expected the positron to merely enhance the weak electronic bond between beryllium atoms. Surprisingly, their simulations revealed two different bonding mechanisms. At larger Be–Be separations (3.5–4.0Å), the positron located itself in the internuclear region, effectively taking over the electronic bond completely. This shift demonstrated the positron taking a central position in sustaining the bond, a result that Reyes described as unexpected in a conversation with Chemistry World.
A second unforeseen bonding mechanism was identified at shorter distances (less than 3.5Å), where the positron migrated outward, leading to increased molecular stability as the nuclei moved closer together. This resulted in a bond significantly stronger than previously recorded, highlighting a new interaction not observed in prior studies.
Dario Bressanini from the University of Insubria emphasized the need to reassess the idea of a chemical bond when positrons are present, characterizing this counterintuitive phenomenon as exciting within the discipline. The potential for positronic compounds continues to challenge established chemical knowledge.
While still theoretical at this stage, the beryllium dimer stands out as a promising candidate for experimental investigation of positron bonds, should conditions like ultracold temperatures be achieved. Ken Jordan from the University of Pittsburgh highlights the difficulties in capturing such fleeting interactions, commenting on the necessity of cryogenic conditions and sophisticated experimental techniques to observe these novel positronic complexes.