Atom scrambling of linear C₅ in the gas phase: a joint experimental and theoretical study

Abstract

The radical anion [CC13CCC]-•, with known bond connectivity, has been synthesized in the ion source of a VG ZAB 2HF mass spectrometer by the reaction of (CH3)3Si−CC13C(NNH-tosyl)-CC−Si(CH3)3 with HO- followed by F- (from SF6). The collision-induced mass spectrum of [CC13CCC]-• shows only one fragmentation: exclusive loss of 12C. Computational studies at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory indicate that this experimental observation means that [CC13CCC]-• does not rearrange under the conditions of collisional activation. The charge reversal [-CR+, synchronous charge stripping of the radical anion (by a vertical Franck−Condon process) to the radical cation], and the neutralization/reionization [-NR+, stepwise vertical oxidation of the radical anion to the neutral, then of the neutral to the radical cation] spectra of [CC13CCC]-• show partial and complete atom scrambling, respectively. Under the experimental conditions used, the neutral has a lifetime of 10-6 s before being converted to the corresponding radical cation. Computational studies at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory suggest that the major atom-scrambling pathway of the neutral involves cyclization of singlet CC13CCC to equilibrating and degenerate carbon-substituted rhombic structures that fragment to yield linear singlet 13C12C4 structures. Overall, the label is randomized. A similar mechanism is proposed to account for the partial atom scrambling of the radical cations formed in the -CR+ experiment from [CC13CCC]-•.