Gas-phase protonation of benzocycloalkenes

The gas-phase basicity (GB) of four benzocycloalkenes (benzocyclobutene, indan, tetralin and benzocycloheptene) has been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry, allowing proton transfer equilibria to be established with reference bases of known GB. The values obtained are all very similar, 776.8-778.8 kJ mol(-1) at 300 K, practically equal within experimental error and close to the GB of o-xylene. In this respect, any specific behavior which might be ascribed to the size of the fused ring is not manifested. H/D exchange reactions with CH3OD have been used to probe the structure of the protonated benzocycloalkenes. The positive-ion chemistry of protonated benzocyclobutene is characterized by its kinetic instability towards rearrangement into a more stable isomer(s). The isomerization is prompted either by an exothermic protonation process, for example using i-C3H7+ as proton donor, or within an ion neutral complex with a polar molecule.