Reactivity of the gaseous radical cations of trimethylenemethane and 2-isopropylidenecyclopentane-1,3-diyl

The radical cation of trimethylenemethane (TMM.+) and a derivative, 2-isopr opylidenecyclopentane-1,3-diyl, were generated by electron ionization of me thylenecyclopropane (MCP) and electron-ionization-induced loss of nitrogen from 7-isopropylidene-2,3-diazabicyclo[2.2.1]hept-2-ene, respectively, in a Fourier transform ion cyclotron resonance mass spectrometer. The two radic al cations were isolated and their products and reaction rates with various gaseous reagents were measured. TMM.+ displays radical-type reactivity, e. g., it slowly abstracts a hydrogen atom from benzeneselenol and a thio- or selenomethyl radical from dimethyl disulfide and dimethyl diselenide, respe ctively. This reactivity distinguishes it from the isomeric 1,2- and 1,3-bu tadiene radical cations. Surprisingly, however, the TMM.+ undergoes electro n transfer reactions in addition to radical reactions. The difference betwe en the adiabatic recombination energy of TMM.+ (9.2 +/- 0.1 eV) and the adi abatic ionization energy of MCP (9.4 +/- 0.1 eV; the earlier literature est imate is less than or equal to 9.57 eV), which were experimentally determin ed in this work, indicates that the above electron abstraction reactions li kely involve cyclization to yield MCP land not TMM) as the final neutral pr oduct. In sharp contrast to TMM.+, the sterically hindered 2-isopropylidene cyclopentane-1,3-diyl radical cation is unreactive toward most of the reage nts studied, including benzeneselenol, dimethyl disulfide, and dimethyl dis elenide. The recombination energy of this TMM-type radical cation land thus , the ionization energy of the 3-isopropylidenecyclopentane-1,3-diyl biradi cal) was measured to be 7.8 +/- 0.1 eV.