Atom Scrambling of Linear C5 in the Gas Phase: a Joint Experimental and Theoretical Study

The radical anion [CC 13 CCC] -•, with known bond connectivity, has been synthesized in the ion source of a VG ZAB 2HF mass spectrometer by the reaction of (CH 3 ) 3 Si−C⋮C 13 C( NNH-tosyl)-C⋮C−Si(CH 3 ) 3 with HO - followed by F - (from SF 6 ). The collision-induced mass spectrum of [CC 13 CCC] -• shows only one fragmentation: exclusive loss of 12 C. Computational studies at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory indicate that this experimental observation means that [CC 13 CCC] -• 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 [CC 13 CCC] -• 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 CC 13 CCC to equilibrating and degenerate carbon-substituted rhombic structures that fragment to yield linear singlet 13 C 12 C 4 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 [CC 13 CCC] -• .