Unimolecular Metastable Decomposition of Ionized 2-Hydroxy-2-methylpropanoic Acid and Its Methyl Ester.

Abstract

Unimolecular metastable decompositions of ionized 2-hydroxy-2-methylpropanoic acid, (CH3)2C(OH)COOH (2), and its methyl ester, (CH3)2C(OH)COOCH3 (1), have been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling technique. The abundances of the molecular ions of 1 and 2 are too weak to measure their MIKE spectra. The source-generated m/z 59 ions, (CH3)2C+(OH), decompose into the ions at m/z 41 and 31 by losing H2O and C2H4, respectively, in the metastable time window. The ion abundance ratio ([m/z 31]/[m/z 41]) was 1.6 for 1 and was 2.2 for 2. This contrasts to the case of CH3CH2C+(OH) from 2-hydroxybutanoic acid, CH3CH2CH(OH)COOH (5) which leads predominantly to H2O loss ([m/z 31]/[m/z 41] is 0.12). A partial hydrogen exchange between the hydroxyl and methyl groups occurs prior to the H2O and C2H4 losses from the m/z 59 ion for 1, but the extents are different from each other. The extent of the exchange increases with decreasing the mean internal energy of the precursor ion. Most of the [M-ĊH3]+ ions at m/z 103 and 89 for 1 and 2 decompose into the ions at m/z 75 and 61, respectively, by the loss of CO. These reactions occur through a 1,2-skeletal rearrangement of OCH3 or OH. In the MIKE spectrum, the abundance of ion with a low value of ΣΔHf is larger than that with a high ΣΔHf (ΣΔHf is the sum of heats of formation of the fragment ion and the neutral species lost.). In contrast with the case of methyl and ethyl 2-hydroxypropanoates, CH3CH(OH)COOCH3 (3) and CH3CH(OH)COOCH2CH3 (4), which are isomers of 2 and 1, so-called double hydrogen atom migration was absent in the cases of 1 and 2.