The Formation of Protonated Dimethyl Ether from the Metastable Molecular Ions of 1-Methoxy-2-Propanol, CH₃OCH₂CH(OH)CH₃

Abstract

The unimolecular metastable decomposition of 1-methoxy-2-propanol, CH 3 OCH 2 CH(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 3 O + (H)CH 3 ] by the loss of a C 2 H 3 O 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 2 H 3 O. In the present study it is shown that more DHT mechanisms are operative, leading to the losses of isomeric C 2 H 3 O radicals. The results obtained are best explained by the formation of the key intermediate ion-molecule complexes [CH 3 OCH 3 +• , CH 3 CHO] and [CH 3 OCH 3 , CH 2 =C(H)OH +• ] following unimolecular metastable dissociation of the molecular ion. Subsequent hydrogen atom abstraction channels by CH 3 OCH 3 +• in the former complex and proton abstraction channels by CH 3 OCH 3 in the latter complex lead eventually to the formation of protonated dimethyl ether with m/z 47.

Complex Acts of Knowing: Paradox and Descriptive Self‐Awareness

“…and 2 + . , respectively, because methyl loss from a methoxy group is not common 11, 12. The 57 Da (15 + 42 Da) loss from 1 + .…”

9 Organic gas phase ion chemistry

“…The gas phase reactivity of conventional radical cations as well as their distonic ion counterparts continues to generate considerable interest, especially since these species play a key role in the fragmentation reactions of ions generated under EI conditions. Although the EI literature is so vast that no attempt is made to review the 2001 contributions here, we do note the following: (i) several groups are carrying out careful labelling studies which reveal that many fragmentation reactions of ions under EI conditions are very complex, often involving hydrogen transfers and skeletal or other isomerizations; 100 (ii) hydrogen and alkyl shifts are often involved which can cause isomerization between conventional radical cations and distonic ion structures; 101 (iii) ion-neutral complexes often play an important role in EI fragmentation reactions 102 as well as the collision induced dissociation (CID) reactions of radical cations; 103 (iv) high-resolution analysis of kinetic energy distributions of fragment ions provides detailed information on their energy content and on the competition between fast and slow fragmentation; 104 (v) several rearrangement and migration reactions under EI conditions have been discussed 105-108 including a gas phase Newman-Kwart rearrangement reaction; 109 (vi) the benzylium/tropylium ion structure and mode of formation saga continues; 110 and (vii) one cannot assume that a fragment ion at a particular m/z value in an EI mass spectrum has only one structure. 111 This Section begins with a review of studies that have considered both conventional radical cations and distonic ion structures, before moving onto studies which have considered only the reactivity of conventional radical cation structures or the reactivity of distonic ions.…”

Unimolecular Metastable Decomposition of Bis(2,2,2-trifluoroethyl) Ether, CF3CH2OCH2CF3, and Ethyl 2,2,2-Trifluoroethyl Ether, CH3CH2OCH2CF3, upon Electron Ionization.

“…This result might indicate that the rule which a reaction with a lower SDH f shows a higher ion abundance, is only valid for reactions proceeding via a similar decomposition mechanism. 28) A proposed fragmentation mechanism for the m/z 163 ions into the m/z 83 and 61 ions is depicted in Scheme 1.…”