UC researchers defy chemistry’s ‘rules of attraction’

UC researchers defy chemistry’s ‘rules of attraction’

Researchers at the University of Canterbury have discovered a very unusual chemical system that breaks the chemical 'rules of attraction', in which two positively charged molecules form a closely bound sandwich complex. So they set out to investigate: how can this happen? Do we need to rewrite the textbooks?

One of the core rules of chemistry is that opposites attract. In the same way that magnets repel if aligned with north poles facing, but attract when aligned north-to-south, molecules typically attract one another if they are oppositely charged, while molecules with like charges repel.

The academic paper about this odd behaviour that defies the rules, Cyclopropenium Cations Break the Rules of Attraction to Form Closely Bound Dimers, written by Andrew J. Wallace, Chaminda D. Jayasinghe, Matthew I. J. Polson, Owen J. Curnow, and Deborah L. Crittenden of UC’s Department of Chemistry, College of Science, was recently published in the prestigious Journal of the American Chemical Society (JACS).

The UC researchers discovered a number of factors are responsible for this seemingly odd behaviour that broke this core rule, according to Chemistry senior lecturer Dr Deborah Crittenden:

· Firstly, the positive charge needs to be able to spread out across the molecule, to minimise (but not eliminate) those repulsive interactions.

· Then, the molecules have to have quite a large contact area, so that they can experience attractive interactions caused by electrons from one molecule subtly influencing the behaviour of the electrons on the other molecule. This effect is referred to in Chemistry by many names; van der Waals interactions, dispersion, intermolecular electron correlation, or induced dipole/induced dipole interactions, and is usually thought to be very weak. However, for the cyclopropenium dimer these 'weak' interactions outweigh the so-called 'strong' electrostatic interactions (described above), allowing the molecules to become attracted to one another when they get close enough together.

· Finally, the sandwich motif gets 'locked in' by the recruitment of stabilising negatively charged atoms, which surround the doubly positively charged dimer and prevent it from falling apart, stabilising it through the ‘normal’ electrostatic interactions expected from textbook definitions of forces behind intermolecular interactions.

“Overall, this unusual observation has led to a deeper and richer understanding of how molecules behave and interact, and shows the limitations of blindly applying the simple 'rules' of attraction presented in textbooks,” Dr Crittenden says.

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