Gas-phase protonation of benzocycloalkenes

Eur. J. Mass Spectrom. 10, 881–888 (2004)
DOI: 10.1255/ejms.705

Gas-phase protonation of benzocycloalkenes

Barbara Chiavarino, Maria Elisa Crestoni and Simonetta Fornarini^*
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive, Università di Roma “La Sapienza”, P. le A. Moro 5, I- 00185 Roma, Italy. E-mail: simonetta.fornarini@uniroma1.it

ABSTRACT:

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.

Keywords: FT-ICR mass spectrometry, proton transfer reactions, benzocycloalkenes, H/D exchange reactions, ion neutral complex, gas-phase basicities

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