The heats of formation of some protonated olefinic carbonyl compounds: [C₅H₉O]⁺ and [C₄H₇O₂]⁺ ions

Abstract

Abstract The proton transfer equilibrium reactions involving 3‐penten‐2‐one, 3‐methyl‐3‐buten‐2‐one, crotonic acid and methacrylic acid were carried out in an ion cyclotron resonance (ICR) spectrometer. The semiempirical method MNDO, used to estimate the heats of formation for 14 protonated [C 5 H 9 O] + and [C 4 H 7 O 2 ] + ions and the energetic aspect of the fragmentations of metastable [C 6 H 12 O] +. and [C 6 H 12 O 2 ] +. ions, leads to the conclusion that the ions corresponding to protonation at the carbonyl oxygen are the most stable. Thus the experimentally determined heats of formation of protonated olefinic carbonyl compounds can be attributed to the following structures: [CH 3 COHCHCHCH 3 ] + (δH f = 490 KJ mol −1 ), [CH 3 COHC(CH 3 )CH 2 ] + (δH f = 502 KJ mol −1 ), [HOCOHCHCHCH 3 ] + (δH f = 330 KJ mol −1 ) and [HOCOHC(CH 3 )CH 2 ] + (δH f = 336 KJ mol −1 ).