Captodative substituted C4H7O+2 carbenium ions and their relationship with the proton-bound complex CH3CHO ţuH+ ţCCH2CO

The carboxy propylium ions CH3CH2C+HCOOH, CH3C+HCH2COOH and (CH3)2C+COOH have been generated in the gas phase by loss of I. from the respective ionized iodo acids. The straight chain isomers interconvert rapidly with themselves and with CH2CHCH2C+(OH)2. The major unimolecular dissociation products are CH3CHCHC+O + H2O and CH3C+O + CH3CHO, together with some CH3C+HOH + CH2CO. Loss of H2O is proposed to occur directly from CH3CH2C+HCOOH; formation of CH3C+O and CH3C+(H)OH involves rearrangement of CH3CH2C+HCOOH to CH3CH(OH)CH2C+O which, as shown by neutralization-reionization (NR) experiments collapses to the ion CH3CHO ţuH+ ⋯ CH2CO, formally proton bound acetaldehyde/ketene. It is this species which is sampled in metastable ion (MI), collisional activation (CA) and NR experiments. Thus in the CA mass spectra of the carboxypropylium ions m/z 43, CH3C+O, and m/z 45, CH3C+(H)OH, are dominant peaks; upon neutralization the bridged condensate falls apart to CH3CHO and CH3O which is, thermodynamically and kinetically, the most favourable dissociation. Our work shows that insight into the mechanisms of fast isomerization reactions of molecular ions may greatly assist the confident interpretation of the mass spectra of unknowns. Thus for example the puzzling observation that m/z 43, CH3C+O, is base peak in the normal electron impact mass spectrum of the hydroxy ester CH3CH2CH(OH)CH2COOCH3, methyl 3-hydroxy pentanoate, can now easily be rationalized.