Flow tube and theoretical study of proton transfer reactions of C3H5+ ions

The allyl, CH2CHCH2+, and 2-propenyl, CH3CCH2+, ions have been observed as distinct isomeric species in a flowing afterglow/selected ion flow drift tu be (FA/SIFDT). Reaction with methanol is used as a diagnostic for distingui shing the two isomers. The isomeric ratio of allyl:2-propenyl formed via pr otonation of allene or propyne by a protonated base BH+, is shown to be dep endent on the proton affinity of the base B. Proton transfer from H3O+ to a llene produces the 2-propenyl cation only, whereas proton transfer from SO2 H+ to allene generates a mixture of allyl and 2-propenyl cations, enabling us to estimate the barrier height for the rearrangement allyl --> 2-propeny l as 110 +/- 30 kJ mol(-1). This is in accord with ab initio calculations p erformed at the G2(MP2) level of theory. The C3H5+ product of the reaction between C2H4.+ and C2H4 was identified as the 2-propenyl cation. Rate coeff icients are also reported for reactions of the allyl and 2-propenyl cations with several neutrals. (Int J Mass Spectrom 185/186/187 (1999) 253-261) (C ) 1999 Elsevier Science B.V.