Gas phase generation of the neutrals H_2CCCCO, HCCCCDO and CCCHCHO from anionic precursors. Rearrangements of HCCCCDO and CCCHCHO. A joint experimental and theoretical study

摘要

The reaction between O~(-·) and MeO-CH_2-C≡C-CDO in the ion source of a VG ZAB 2HF mass spectrometer gives a number of product anions including [H_2CCCCO]~(-·) and [HCCCCDO]~(-·) (in the ratio 1 : 5). Neutralisation-reionisation (~-NR~+) of [H_2CCCCO]~(-·) results in the sequential two-electron vertical oxidation [H_2CCCCO]~(-·) → H_2CCCCO → [H_2CCCCO]~(+·). Singlet H_2CCCCO lies 158 kJ mol~(-1) below the triplet [at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory]. The majority of neutrals H_2CCCCO are stable for the microsecond duration of the NR experiment, but some are energised and decompose to give H2CCC and CO. A similar ~-NR~+ experiment with [HCCCCDO]~(-·) yields neutrals HCCCCDO, some of which are excited and rearrange. Calculations show that it is the singlet form of HCCCCHO which rearranges (the singlet lies 36 kJ mol~(-1) above the ground state triplet): the rearrangement occurs by the sequencial H transfer process, HCCCCHO → HCC(CH)CO ← H_2CCCCO. Neutral HCCCCHO needs an excess energy of only 43 kJ mol~(-1) to effect this reaction, which is exothermic by 230 kJ mol~(-1). Both HCC(CH)CO and H_2CCCCO formed in this way should have sufficient excess energy to cause some loss of CO. The anions [CC(CH)CHO]~(-·) and [CC(CD)CHO]~(-·) are formed in the ion source of the mass spectrometer by the reactions of HO~- with Me_3SiC≡C-CH=CHOMe and Me_3SiC≡C-CD=CHOMe respectively. ~-NR~+ of these anions indicate that energised forms of CC(CH)CHO and CC(CD)CHO may rearrange to isomer(s) which decompose by loss of CO. Singlet CC(CH)CHO rearranges to HCC(CH)CO and H_2CCCCO, both of which are energised and fragment by loss of CO.