The Formation of Protonated Dimethyl Ether from the Metastable Molecular Ions of 1-Methoxy-2-Propanol,CH 3 OCH 2 CH(OH)CH 3

The unimolecular metastable decomposition of 1-methoxy-2-propanol, CH₃OCH₂CH(OH)CH₃ (mol. wt. 90) induced by electron ionization, has been investigated by use of mass-analyzed ion kinetic energy (MIKE) spectrometry and D-labeling in conjunction with thermochemical data. In the metastable time window, the molecular ions decompose almost exclusively into ions at m/z 47 [i.e. protonated dimethyl ether, CH₃O⁺(H)CH₃] by the loss of a C₂H₃O radical species following a double hydrogen atom transfer (DHT). Until now, only one DHT mechanism has been proposed, involving and accounting for the loss of an acetyl radical, C₂H₃O. In the present study it is shown that more DHT mechanisms are operative, leading to the losses of isomeric C₂H₃O radicals. The results obtained are best explained by the formation of the key intermediate ion–molecule complexes [CH₃OCH₃^+•, CH₃CHO] and [CH₃OCH₃, CH₂=C(H)OH^+•] following unimolecular metastable dissociation of the molecular ion. Subsequent hydrogen atom abstraction channels by CH₃OCH₃^+• in the former complex and proton abstraction channels by CH₃OCH₃ in the latter complex lead eventually to the formation of protonated dimethyl ether with m/z 47.