UNIMOLECULAR CHEMISTRY OF METHYL FORMATE CATION RADICAL - A COMBINED THEORETICAL AND EXPERIMENTAL-STUDY

The unimolecular chemistry of solitary methyl formate cation radical (1) has been studied by a combination of ab initio molecular orbital calculations executed at the MP3/6-31G**//6-31G* + ZPVE level of theory and mass spectrometry based experiments. The minimal energy requirement path corresponds to the loss of CO to produce CH3OH.+ via a remarkably atom-specific process. The calculations indicate that the reaction proceeds via the hydrogen-bridged complex CH3OH...C = O.+ (5). The lowest energy pathway for the formation of 5 has been calculated to occur via a methoxy shift and simultaneous H transfer to the migrating moiety. The minimum rate for the loss of CO from 5 is small, and an intense metastable peak is produced. At slightly higher internal energies another process becomes energetically feasible: The dissociation to CH2OH+ + HCO., which too is a remarkably atom-specific reaction. Our calculations indicate that this dissociation follows a multistep path. First 1 rearranges via a hydrogen 1,4-shift to the distonic ion HC(OH)OCH2.+, 2, which in turn isomerizes to the hydrogen-bridged complex HC = O...H...O = CH2.+, 6. The rate-determining step for this sequence, 2 --> 6, is calculated to lie 4.4 kcal/mol above the dissociation limit. According to 180-labeling experiments, the oxygen atoms retain their positional integrity, and so ions 6 do not undergo a degenerate isomerization via a hydrogen 1,5-shift. At still higher energies (those corresponding to the 70-eV electron impact mass spectrum) another process leads to CH2OH+ ions, namely, the consecutive fragmentation HC(O)OCH3.+ --> CO + CH3OH.+ --> CH2OH + H.. Contrary to earlier suggestions, methoxy ions, CH3O+, are not produced from methyl formate ions. The sequence 1 --> 2 --> 6 --> CH2OH+ + HCO. is mechanistically related to that of the higher homologue; i.e., ionized methyl acetate, CH3C(O)OCH3.+, which inter alia prior to dissociation to .CH2OH/CH3C = O+ isomerizes to CH3C = O...H...O = CH2.+.