Captodative substituted C 4H 7O +2 carbenium ions and their relationship with the proton-bound complex CH 3CHO ţu [formula omitted] ţCCH 2CO

Abstract

The carboxy propylium ions CH 3CH 2 C + HCOOH, CH 3 C + HCH 2COOH and (CH 3) 2 C + 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 CH 2CHCH 2 C + (OH) 2. The major unimolecular dissociation products are CH 3CHCH C + O + H 2O and CH 3 C + O + CH 3CHO, together with some CH 3 C + HOH + CH 2CO. Loss of H 2O is proposed to occur directly from CH 3CH 2 C + HCOOH; formation of CH 3 C + O and CH 3 C + (H)OH involves rearrangement of CH 3CH 2 C + HCOOH to CH 3CH(OH)CH 2 C + O which, as shown by neutralization-reionization (NR) experiments collapses to the ion CH 3CHO ţu H + ⋯ CH 2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, CH 3 C + O, and m/z 45, CH 3 C + (H)OH, are dominant peaks; upon neutralization the bridged condensate falls apart to CH 3CHO and CH 3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, CH 3 C + O, is base peak in the normal electron impact mass spectrum of the hydroxy ester CH 3CH 2CH(OH)CH 2COOCH 3, methyl 3-hydroxy pentanoate, can now easily be rationalized.