Energetics and structural effects in the fragmentation of protonated esters in the gas phase

Abstract

A series of formate (methyl through butyl) and acetate (methyl through pentyl) esters have been protonated in the gas phase by the Brgnsted acids H3+, N2H+, C02H+, N20H+, and HCO+. Carbonyl oxygen protonation is 87-97 kcal mol-' exothermic for H,+ and 47-57 kcal mol-' exothermic for the weakest acid HCO+, permitting a study of the effect of protonation exothermicity on the decomposition modes of the protonated esters. With the exception of protonated methyl formate, three decomposition modes, (a) to (c) are observed.Reaction (a) is unimportant for formates; for acetates it is the sole decomposition channel for the methyl ester, but is less important for higher acetates. The dependence of the relative importance of this reaction mode on the protonation exothermicity indicates an activation energy considerably in excess of AHo, presumably because the reaction involves a symmetry-forbidden 1,3-H shift for the carbonyl protonated ester. For the higher acetates where the difference in the proton affinities of the carbonyl and ether oxygens is less, acyl ion formation results, in part, from protonation at the ether oxygen. For protonated methyl formate the major fragmentation reaction yields CH30H2+ + CO; this reaction also appears to have an activation energy considerably in excess of the AH,. For the remaining esters either reaction (b) or (c) is the major decomposition mode. The competition between these two channels depends strongly on the protonation exothermicity and the relative activation energies. From the reaction competition we conclude that 1,2-H shifts occur in the case of primary alkyl esters yielding more stable secondary or tertiary alkyl ions. This rearrangement appears to occur after the excess energy has been partitioned between the alkyl ion and the neutral acid since the extent of further fragmentation of the alkyl ion reflects the original structure of the alkyl group.

Gas phase fragmentation of protonated esters in air at ambient pressure through ion heating by electric field in differential mobility spectrometry

“…Propyl acetate " ", butyl acetate " ", pentyl acetate " ", hexyl acetate "♦", propyl propionate " "" propyl butyrate " ", ethyl propionate "᭹", and ethyl hexanoate " ". The most stable form of a protonated alkyl ester of a carboxylic acid has carbonyl protonation, but the accepted decomposition path forming the protonated acid demands first 1,3 proton migration to the alkoxy oxygen followed by 1,5 hydrogen migration [23][24][25][26]. The proposed mechanism for the decomposition of protonated propyl acetate to protonated acetic acid and propene is shown here with energy changes computed from literature values [24].…”

Destruction of dimethyl ether and methyl formate by collisions with He+

“…In the following, we will adopt the KIDA value for the total rate constant (i.e. α = 4.05 × 10 −9 , β=−0.50 and γ=0.00), while for the BRs we will use the values found by Herman & Harrison (1981). As before, to avoid introducing new trace ions in the network, we will consider only the two most abundant channels, namely those producing CH 3 OH + 2 and protonated MF.…”

“…However, Lawson et al (Lawson et al 2012), while attempting to measure the rate constants for dissociative electron-ion recombination of protonated MF, observed that significant fragmentation occurred when attempting to induce proton transfer from H 3 + to MF. In addition, according to a chemical ionization mass spectrometric experimental study (Herman & Harrison 1981), the outcome of the protonation reaction of MF by H + 3 is mostly dissociative and the observed relative abundances of ionized fragments are: 15.5% for H 5 C 2 O + 2 , 74% for CH 3 OH + 2 , 5.9% for CH 2 OH + and 4.4% for CH + 3 . Such experimental evidences are at odds with the prescriptions from KIDA, where the reaction is assumed to be completely non dissociative with k = 4.06 × 10 −9 cm 3 s −1 at 298 K (from the modified Arrhenius Eq.…”

Effect of reaction exothermicity on the proton transfer chemical ionization mass spectra of isomeric C5 and C6 alkanols

“…It should be noted that a significant fraction of the C,H9+ ion signal may originate by H 2 0 loss from the (M -H)+ ion rather than by reaction [3]. In a related study (8) of the CI spectra of pentyl acetates, where hydride abstraction is of lesser importance, we have observed lower yields of C5H9+ relative to C,H,,+. Third, those alcohols containing the -CH(OH)-CH, moiety show ion signals at mlz 45 and mlz 73 corresponding, respectively, to the CH,CH=OH+ and C3H7CH=OH+ ions.…”

“…3) while reactions [8] and [9], analogous to reactions [2] and [3] for C,H,,+, have been reported earlier (5) as higher energy fragmentation routes. In agreement with the results for the C,Hll+ ions, the extent of fragmentation of the C6H13+ ions decreases as one proceeds from primary to secondary to tertiary alcohols.…”