Chemistry - the science of atoms, molecules, and matter - and Astronomy - the science of stars, galaxies and the Universe - are deeply related in two ways. First, in the early stage of the Universe, H and He were the only elements. Heavier nuclei of atoms like C, N, and O, essential in chemistry and biology, were produced much later in the core of stars, the only place where the temperature and pressure are sufficiently high to allow nuclear fusion. The natural abundances of elements are closely related to the evolution of stars and understood by nuclear astrophysics. Second and conversely, a star is produced from molecular clouds. Stars are formed through gravitational condensation of gas. To dispose the heat generated in the process, the presence of molecules is essential. In short stars and molecules are like the chicken and egg; without molecules there will be no stars, and without stars there will be no molecules. Because of its low temperature and low density, interstellar space is the most hostile environment for production of molecules, and yet it is known that the interstellar gas is more in molecular form than atomic. Ion-neutral reactions which do not have activation barriers play the dominant role for production of molecules and thus for star formation. Especially important is protonated molecular hydrogen, H3+, which is formed via the cosmic ray ionization H2 to H2+ followed by the reaction, H2 + H2+ → H3+ + H. The interstellar acid, H3+ donates a proton to neutral atoms and molecules X via proton hop reaction H3+ + X → H2 + HX+ and initiates chains of reactions. I shall discuss how this fundamental molecular ion H3+ has been discovered in space1 and how the related studies of molecular ions such as H3+, CH2+, CH3+, CH5+, C2H3+, NH2+, NH3+, H3O+, etc. have enriched chemistry2 and astronomy3.
1 T. R. Geballe and T. Oka, Nature, 384, 334 (1996)
2 T. Oka, The Encyclopedia of Mass Spectrometry, Vol. 1, Theory and Ion Chemistry, edit by M. L. Gross and R. Caprioli, Vol. Edit. P. B. Armentrout, Elsevier (2003), pp217-226.3 T. Oka, Proc. Natl. Acad. Sci. USA, 103, 12235 (2006)