Generation of neutrals from ionic precursors in the gas phase. The rearrangement of CCCCCHO to HCCCCCO

Abstract

The neutrals HCCCCCO and CCCCCHO have been studied by experiment and by molecular modelling at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. Neutral HCCCCCO has been made by one-electron reduction of [HCCCCCO]⁺ in the dual collision cell of a VG ZAB 2HF mass spectrometer. The isomer CCCCCHO is also formed in the dual collision cell, but this time by one-electron oxidation of the anion [CCCCCHO]⁻. Comparison of the CID and ⁺NR⁺ mass spectra of [HCCCCCO]⁺ indicates that neutral HCCCCCO, when energised, retains its structural integrity. If the excess energy of HCCCCCO is ≥ 170 kJ mol⁻¹, decomposition can occur to give HCCCC and CO (calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory). The situation with the isomer CCCCCHO is different. Comparison of the ⁻CR⁺ and ⁻NR⁺ spectra of [CCCCCHO]⁻ shows that both neutral and cationic forms of CCCCCHO partially rearrange to a species which decomposes by loss of CO. The peak corresponding to loss of CO is more pronounced in the ⁻NR⁺ spectrum, indicating that the rearrangement is more prevalent for the neutral than the cation. Theoretical calculations suggest that the species losing CO could be CCCCHCO or HCCCCCO, but that HCCCCCO is the more likely. The lowest-energy rearrangement pathway occurs by successive H transfers, namely CCCCCHO → CCCCHCO → CCCHCCO → HCCCCCO. The rearrangement of CCCCCHO to HCCCCCO requires CCCCCHO to have an excess energy of ≥ 94 kJ mol⁻¹. The species HCCCCCO formed by this exothermic sequence (214 kJ mol⁻¹) has a maximum excess energy of 308 kJ mol⁻¹: this is sufficient to effect decomposition to HCCCC and CO.