The formation of protonated dimethyl ether from the metastable molecular ions of 1-methoxy-2-propanol, CH_3OCH_2CH(OH)CH_3

摘要

The unimolecular metastable decomposition of 1-methoxy-2-propanol, CH_3OCH_2CH(OH)CH_3 (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, CH_3O~+(H)CH_3] by the loss of a C_2H_3O 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, C_2H_3O. In the present study it is shown that more DHT mechanisms are operative, leading to the losses of isomeric C_2H_3O radicals. The results obtained are best explained by the formation of the key intermediate ion-molecule complexes [CH_3OCH_3~(+.), CH_3CHO] and [CH_3OCH_3, CH_2=C(H)OH~(+.)] following unimolecular metastable dissociation of the molecular ion. Subsequent hydrogen atom abstraction channels by CH_3OCH_3~(+.) in the former complex and proton abstraction channels by CH_3OCH_3 in the latter complex lead eventually to the formation of protonated dimethyl ether with m/z 47.