Even-numbered hydrogen ion generated for the first time, defying theoretical structure predictions

Of the hydrogen ion’s many forms, the trihydrogen cation (H₃⁺) appears to be the most abundant ion in the universe, responsible for several pathways for building complex molecules in interstellar space. Compared to their odd-numbered counterparts, however, hydrogen ions made up of an even number of protons occur much less frequently and remain poorly understood. Work in infrared spectroscopy addressing these issues has revealed the structure of one even-numbered hydrogen ion, the H₆⁺ cation.

McDonald et. al. generated a H₆⁺ ion in the gas phase, selected it with mass spectrometry, and characterized its infrared absorption spectrum. The group created the ion using pulsed-discharge supersonic expansion and measured its vibrational spectrum in the region from 2,050 to 4,550 cm⁻¹.

Until recently, most computational chemistry theories supported the notion that hydrogen ion clusters form by bonding diatomic hydrogen to a central trihydrogen cation “seed,” giving rise to odd-numbered clusters. The authors’ previous work, finding that H₅⁺ instead exhibits a strongly coupled system with highly anharmonic vibrational motions, served as inspiration to study the H₆⁺ cation.

They discovered that the H₆⁺ cation occurs in two isomers: a structure with a charged diatomic hydrogen molecule cation flanked by two H₂ molecules; and another one in which the trihydrogen cation acts as the core, and H₂ and a single proton occupy opposite sides. The spectral pattern of the D_2d structure was roughly five times as intense as the C_s structure.

The group hopes their findings will inspire the research community to explore more sophisticated theoretical techniques for treating the vibrational motions of such anharmonic molecules.

Source: “Communication: Infrared photodissociation spectroscopy of the H₆⁺ cation in the gas phase,” by David McDonald II, J. Philipp Wagner, and Michael A. Duncan, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5043425 .