Conversion of neutral C2COC2 to C4CO. Potential interstellar molecules

Both [C4CO](-.) and [C2COC2](-.) are formed in the ion source of a VG ZAB 2 HF mass spectrometer by the respective processes HO- + Me3Si-C=C-C=C-CO-CMe 3 --> [C4CO](-.) + Me3SiOH + Me3C., and Me3Si-C=C-CO-C=C-SiMe3 + SF, + e -- > [C2COC2](-.) + 2Me(3)SiF + SF4. The second synthetic pathway involves a d ouble desilylation reaction similar to that first reported by Squires. The two radical anion isomers produce different and characteristic charge rever sal spectra upon collisional activation. In contrast, following collision i nduced charge stripping, both radical anions produce neutral C4CO as eviden ced by the identical neutralisation reionisation (-NR+) spectra. The exclus ive rearrangement of (C2COC2)-C-13 to (C4CO)-C-13 indicates that C-12-O bon d formation is not involved in the reaction. Ab initio calculations (at the RCCSD (T)/aug-cc-pVDZ//B3LYP/6-31G* level of theory) have been used to inv estigate the reaction coordinates on the potential surfaces for both single t and tripler rearrangements of neutral C2COC2. Singlet C2COC2 is less stab le than singlet C4CO by 78.8 kcal mol(-1) and requires only 8.5 kcal mol(-1 ) of additional energy to effect conversion to C4CO by a rearrangement sequ ence involving three C-C ring opening/cyclisation steps. (C) 2000 Elsevier Science B.V.