Hydration-induced protomer switching in <i>p</i>-aminobenzoic acid studied by cold double ion trap infrared spectroscopy
Kyota Akasaka, Keisuke Hirata, Fuad Haddad, Otto Dopfer, Shun‐ichi Ishiuchi, Masaaki Fujii
Abstract
. The pure O-protomer is prepared by electrospray, and size-specific hydrated clusters are formed in a reaction ion trap. The resulting clusters are transferred into a second cryogenic ion trap and the distribution of O- and N-protomers is determined by mid-IR spectroscopy without broadening. The threshold to promote O → N protomer switching is indeed five water molecules. It is smaller than the value reported previously, and as a result, its pentahydrated structure does not support the Grotthuss mechanism proposed previously. The extent of O → N proton transfer is evaluated by collision-assisted stripping IR spectroscopy, and the N-protomer population increases with the number of water molecules. This result is consistent with the dominant population of the N-protomer in aqueous solution.