Metastable Decompositions of gem-Dimethoxyalkanes, (RR'C(OCH3)2), upon Electron Impact. II. 1,1- and 2,2-Dimethoxypropanes.

Abstract

Unimolecular metastable decompositions of 1,1-dimethoxypropane (CH3CH2CH(OCH3)2, 1) and 2,2-dimethoxypropane ((CH3)2C(OCH3)2, 2) upon electron impact have been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling technique. The molecular ions of 1 and 2, even at 20eV electron energy, are formed with extremely low abundance. Sequential transfers of a methyl group and a hydrogen atom to an ether oxygen are observed during the decomposition of [1-H]+ (CH3CH2C(OCH3)=O+CH3, m/z 103), [2-CH3]+ (CH3C(OCH3)=O+CH3, m/z 89), and [1-C2H5]+ (CH(OCH3)=O+CH3, m/z 75) to protonated dimethyl ether (m/z 47). The relative importance of this process increases with decreasing the length of the alkyl chain, R, in RC(OCH3)=O+CH3 (R=C2H5, CH3, and H). On the other hand, the abundance of the ion (RCO+) generated by the loss of CH3OCH3 from these ions with transfer of only a methyl group increases with increasing R. The relative abundance of the ions in the MIKE spectra is rationalized with energetic consideration, i.e, the total heat of formation (ΣΔHf) of the ion plus the neutral fragment, or proton affinity (PA) of two components in an intermediate proton-bridged complex.