Intermediate Complexes in the Gas-Phase Reaction of the Tert -C 4 H 9 + Cation with Water

Both ion cyclotron resonance (ICR) and mass-analyzed ion kinetic energy (MIKE) experiments indicate that a slow H/D exchange occurs when the tert-C₄H₉⁺ cation reacts with D₂O. This can be rationalized by the interconversion between [tert-C₄H₉⁺, H₂O] and [isobutene, H₃O⁺] complexes. Semi-empirical calculations show that the [tert-C₄H₉⁺, H₂O] complexes clearly correspond to several minima on the potential energy surface (PES). The most stable of these complexes is also found to be an energy minimum by ab initio calculations. The corresponding structure is highly stabilized. Its interaction energy is 44.4 kJ mol⁻¹ calculated by the G2(MP2,SVP) method. In contrast, it depends on the calculation method whether the [isobutene, H₃O⁺] complex is found to have an energy minimum on the PES or not. This area of the PES is rather flat and probably corresponds to a plateau or to a broad transition state allowing isomerization of the α-complex with H-permutation. Since this plateau has an energy value very near the [tert-C₄H₉⁺, H₂O] final state, the β-structure [isobutene, H₃O⁺] can operate in the H/D exchange reaction but is rate limiting in this process.