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

Abstract

The radical anion [CC 1 3 CCC] - . , with known bond connectivity, has been synthesized in the ion source of a VG ZAB 2HF mass spectrometer by the reaction of (CH3) 3 Si-C≡C 1 3 C(=NNH-tosyl)-C≡C-Si(CH 3 ) 3 with HO - followedby F - (from SF 6 ). The collision-induced mass spectrum of [CC 1 3 CCC] - . shows only one fragmentation: exclusive loss of 1 2 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 1 3 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 1 3 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 1 3 CCC to equilibrating and degenerate carbon-substituted rhombic structures that fragment to yield linear singlet 1 3 C 1 2 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 1 3 CCC] - . .