Infrared (IR) spectroscopy of neutral and cationic dimers of tetrahydrofuran (THF) and tetrahydropyran (THP) is carried out to investigate isomerization reactions of these dimers following ionization. In the neutral state, both the THF and THP dimers tend to form the stacking structures bound essentially by dispersion. After ionization, both of these dimer cations form the proton transferred structures, in which the proton of the C_αH bond is intermolecularly transferred and shared between the oxygen atoms. Along with the pseudorotation of the five-member ring, the barrierless proton transfer from CH occurs in the ionized THF dimer, and the dimer finally forms the proton transferred structure. In the case of the THP dimer cation, the barrierless intracluster proton transfer occurs with the conformational change of the proton donor site because the acidic CH site of the THP cation strongly depends on its conformation. These isomerization processes involving proton transfer from CH are discussed based on IR spectroscopy and theoretical reaction path searches.

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