The formation of neutral CCC and its radical cation from the CCC radical anion in the gas phase. A joint experimental and theoretical study

Abstract

The radical anion [CC¹³C]⁻˙ has been produced by treatment of [(CH₃)₃SiCC¹³C(NNHSO₂C₆H₄-p-CH₃)Si(CH₃)₃] with HO⁻/F⁻ in the ion source of a mass spectrometer. The stable anion undergoes vertical two-electron oxidation [charge reversal (⁻CR⁺)] in a collision cell to give [CC¹³C]^+˙ which cyclises to the more stable [cyclo-CC¹³C]^+˙ over a barrier of only 11 kJ mol⁻¹ [calculated at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d) level of theory], effectively scrambling the three carbon atoms of the cation radical. One-electron Franck–Condon oxidation of [CC¹³C]⁻˙ yields neutral CC¹³C. Theoretical calculations suggest that neutral CCC may undergo a degenerate rearrangement through a cyclic C₃ transition state if the excess energy of CCC is ≥104 kJ mol⁻¹ (at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d) level of theory). It is likely that at least a proportion of the CC¹³C neutrals formed from [CC¹³C]⁻˙ should have sufficient energy to effect this reaction, resulting in the scrambling of the ¹³C label. The neutralisation/reionisation (⁻NR⁺) spectrum of [CC¹³C]⁻˙ ([CC¹³C]⁻˙ → CC¹³C → [CC¹³C]^+˙) shows a pronounced peak corresponding to the parent cation, confirming that neutral CC¹³C is stable for the time of the NR experiment (10⁻⁶ s). However due to total scrambling of the label in the cation, possible scrambling in the neutral CCC molecule cannot be probed by this experiment. The corresponding ⁻NR⁻ experiment of [CC¹³C]⁻˙ showed a recovery signal but the sensitivity of the instrument was not sufficient to detect the decomposition fragments of the final radical anion.