Bizarre ‘half-Möbius’ molecule has unusual properties chemists have never seen before

Researchers have created an unusually twisted molecule with an electronic structure never seen before.

The new molecular structure, called semi-Möbius topology, is “another knob we can turn to create and manipulate matter” and expands our fundamental understanding of physics and chemistry, co-lead author Igor Roncevic, lecturer in computational and theoretical chemistry at the University of Manchester in the UK, told Live Science.

A Möbius strip is a mathematically interesting shape created by twisting a ribbon 180 degrees and joining the ends, resulting in a single continuous surface. This strange inverted geometry has interesting implications for chemists, especially when considering the electronic and spatial properties of molecular structures.

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electronic rebellion

Typically, electrons are localized around a particular atom or bond, but a subset of cyclic compounds known as conjugated rings allows electrons to move freely throughout the loop above and below the atom. This delocalization makes the conjugated ring more stable than expected and also affects other properties such as color, optics, and reactivity.

However, in a Möbius molecule, the electron orbitals that hold the electrons are twisted 180 degrees from each other at the junction where their edges meet. Electrons can still move throughout the molecule, but at this junction some of the electron’s properties are effectively canceled out, resulting in completely contrasting properties and behavior throughout the molecule.

“In chemistry, it was thought that these were the only two options,” Roncevic said. “But our findings show that there is another option, a third option that can rotate by 90 degrees.”

To accomplish this, the team, co-led by IBM Zurich principal investigator Leo Gross, created two conjugated systems within a single ring of 13 carbon atoms. The ring contained two chlorine atoms bonded at positions 1 and 7, which separated these conjugated systems and unevenly separated the electrons on each side. One side of the ring held 13 electrons, while the other side held only 11 electrons.

“The problem is that electrons want to form pairs,” Roncevich said. “So what they do to make the pairs is twist the molecules.”

Therefore, the ring spontaneously twists 90 degrees, pushing one of the chlorine atoms up and the other down, aligning these two separate conjugated systems. This allows for mixing between the two systems, allowing electrons to be shared across the molecule.

“Right now, we don’t have two separate systems anymore. We have one 24-electron system,” Roncevic said. The resulting molecules therefore exhibit unique and characteristic electronic and magnetic properties that differ from standard and Möbius structures.

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One final twist

The restricted torsion angle of a semi-Möbius molecule produces two versions known as enantiomers.

The ring can be twisted to either the left or the right, so the resulting molecules are mirror images of each other, like the left and right hands. This property, technically known as chirality, is extremely important throughout chemistry and impacts everything from the synthesis of drug molecules to the manufacturing of OLEDs. Interestingly, by applying a small external voltage, the researchers were able to freely interconvert a single molecule between the two enantiomers. This has been extremely difficult to achieve using conventional chemistry.

The team backed up these experimental results with detailed calculations. The mind-boggling complexity of the half-Möbius electronic structure required state-of-the-art quantum computers. They published their findings in the journal Science on March 5th.

Looking ahead, the team will focus on exploring the fundamental theory and potential of these molecular structures.

“We actually created a molecule with a completely new electronic structure, and we want to see what else is possible,” Gross said. “You could also extend this to consider, for example, some half-Mobius twists and braids.”