Helium hydride ion

The helium hydride ion (HeH+) holds a unique and profound place in cosmology as the presumed first molecule to form in the universe after the Big Bang. In the incredibly hot and dense early universe, as it expanded and cooled, fundamental particles began to combine. While atomic hydrogen and helium formed relatively quickly, the formation of molecules required a slightly lower temperature and density.

HeH+ is theorized to have been the first to achieve this molecular stability, comprising a helium atom bonded to a hydrogen atom, with the loss of an electron resulting in a net positive charge. This makes it the lightest heteronuclear ion, a critical benchmark in understanding the transition from a purely atomic state to a molecular one. Its existence is a direct consequence of the fundamental laws of physics governing atomic interactions and the expansion of the universe, providing a tangible link to the universe's nascent stages.

The study of HeH+ offers invaluable insights into the conditions of the early universe and the subsequent chemical evolution that led to the formation of stars, galaxies, and eventually, life.

The first laboratory synthesis of the helium hydride ion occurred in 1925, a significant achievement in early quantum chemistry. However, HeH+ is not a substance one can bottle up and sell. Its defining characteristic is its extreme reactivity, stemming from its potent acidic nature.

This reactivity makes it incredibly unstable in the presence of other molecules; it readily donates its proton (H+) to virtually any other species it encounters. Consequently, HeH+ cannot be prepared or stored in bulk under terrestrial conditions. Its isolation in a laboratory setting requires highly controlled environments, often involving low pressures and specific inert atmospheres to prevent immediate reaction.

This inherent instability, while posing challenges for terrestrial study, is precisely what makes its potential role in early cosmic chemistry so significant, as the conditions in the early universe were vastly different from those on Earth.

The helium hydride ion is recognized as the strongest known Brønsted-Lowry acid. This means it is exceptionally adept at donating a proton (H+). Its acidity far surpasses that of even the most potent superacids, such as fluoroantimonic acid (HSbF6).

This extraordinary acidity is a direct result of the strong electrostatic attraction between the positively charged helium nucleus and the electron cloud, which is heavily polarized towards the helium atom, leaving the hydrogen atom with a highly exposed and easily transferable proton. The theoretical pKa of HeH+ is estimated to be around -10, indicating an immense tendency to dissociate and release a proton. This property is not merely a scientific curiosity; it suggests that HeH+ would have played a crucial role in catalyzing early chemical reactions in the primordial universe, potentially facilitating the formation of more complex organic molecules essential for the emergence of life.

The presence of the helium hydride ion in the interstellar medium (ISM) has been a subject of theoretical speculation since the 1970s. Its predicted existence was based on models of cosmic chemistry and its role as a potential precursor to other molecules. However, direct observational evidence remained elusive for decades due to its transient nature and the challenges of detecting it in the cold, diffuse environments of space.

This changed dramatically in April 2019 when astronomers, utilizing the Stratospheric Observatory for Infrared Astronomy (SOFIA), a powerful airborne telescope, finally detected the spectral signature of HeH+. This landmark discovery confirmed its existence in the interstellar medium, specifically within the nebula NGC 7027. The detection provided crucial empirical data to validate cosmological models and offered a direct glimpse into the chemical processes occurring in the vast expanse between stars, solidifying HeH+'s importance in our understanding of cosmic evolution.