Unraveling Hidden Forces: How Big Substituents Affect Protonated Pyridines

Scientists have been digging into how big substituents, like those bulky tert-butyl groups, interact with each other in protonated pyridines. They used cool techniques like infrared multiphoton dissociation spectroscopy and trapped ion mobility spectrometry to check out these ions in the gas phase. By gradually increasing the size of the substituents from hydrogen (H) to methyl (Me) and finally to tert-butyl (tert-Bu), they could tweak the strength of the London dispersion interaction. This force pulls molecules together, and scientists wanted to see how well it's modeled in these ions. Alongside the experiments, they ran lots of calculations using a method called DFT-D3BJ, which includes corrections for dispersion forces. It turned out that this method might be overestimating how strongly alkyl groups stick together, especially big ones like tert-butyl. When other forces, like hydrogen bonding, are in the mix, the predictions get even shakier.