Abstract
Disubstituted trialkyloxonium ions R 1OCH 2O +(R 1)CH 2OR 2 (R 1, R 2 = CH 3 or C 2H 5) have been prepared by chemical ionization of dimethoxy- and diethoxymethane individually or as a mixture, and their fragmentation has been studied by means of metastable ion and collision-induced dissociations. It is found that when a methoxymethyl group is attached to the charged oxygen atom, the oxonium ions can fragment by C–O bond cleavage to generate a methoxymethyl cation/dialkoxymethane ion–neutral complex, in which methyl cation transfer occurs to expel neutral formaldehyde. When an ethoxymethyl group is connected with the central oxygen atom, a reaction channel involving loss of C 2H 4O is observed and found to be insensitive to collisions. This process is proposed to involve isomerization prior to fragmentation leading to methylated dialkoxymethanes coordinated with neutral acetaldehyde in ion–neutral complexes; these ion–neutral complexes are estimated to be 35 kJ mol −1 more stable than the original oxonium ions.
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