Conversion of linear to rhombic C-4 in the gas phase: A joint experimentaland theoretical study

Charge reversal (CR) and neutralization reionization (NR) experiments carri ed out on a 4-sector mass spectrometer demonstrate that isotopically labele d, linear C-4 anion rearranges upon collisional oxidation. The cations and neutrals formed in these experiments exhibit differing degrees of isotopic scrambling in their fragmentation patterns, indicative of (at least) partia l isomerization of both states. Theoretical studies, employing the CCSD(T)/ aug-cc-pVDZ//B3LYP/6-31G(d) level of theory, favor conversion to the rhombi c C-4 isomer on both cationic and neutral potential-energy surfaces with th e rhombic structures predicted to be slightly more stable than the linear f orms in each case. The combination of experiment with theory indicates that the elusive rhombic C-4 is formed as a cation and as a neutral following c harge stripping of linear C-4(-)