What is the most stable structure of the molecule with the sum formula C₆H₁₂O₆?
Glucose is the first molecule that comes to mind, but this is certainly not the most stable. Identifying the lowest-energy structure for a certain sum formula, or “Guinness molecule”, is important in the areas of chemical energy conversion, astrophysics, or even fermentation.

In his Editorial in Angewandte Chemie, Martin A. Suhm, Göttingen University, Germany, discusses some Guinness molecules. Some structures like H₂O are straightforward. For carbohydrates (CH₂O)_n, formalydehyde may seem the obvious Guinness molecule, when n = 1. In fact, when zero-point energy is taken into account, the van der Waals complex between H₂ and CO is the winner. For n = 2, the complex between methane and carbon dioxide is more stable than CH₃COOH, which in turn is more stable than HOCH₂−CHO. This could explain the low abundance of acetic acid in space.

The search for Guinness molecules takes into account subjects such as quantum mechanics and spectroscopy and thus is a highly useful teaching tool for helping students think about relative bond strengths.

• Guinness Molecules: Identifying Lowest-Energy Structures,
  Martin A. Suhm,
  Angew. Chem. Int. Ed. 2013.
  DOI: 10.1002/anie.201309063