On the nature of the ‘methoxy’ cation

Ab initio molecular orbital calculations with large, polarization basis sets and incorporating valence‐electron correlation have been used to investigate the structure of the [COH3]+ ion formed at threshold from fragmentation of dimethyl ether in the mass spectrometer. The results indicate that this ion is the hydroxymethyl cation, [CH2OH]+, formed with excess energy in a two‐step rearrangement‐dissociation process involving an initial 1,2‐hydrogen shift in the dimethyl ether radical cation to yield an oxonium ion intermediate, [CH2OHCH3]+˙, followed by CO bond cleavage. The intermediate oxonium ion is predicted to lie slightly lower in energy than the dimethyl ether radical cation and represents a stable, previously unreported [C2H6O]+˙ isomer. It is suggested that at high energies, decomposition of the dimethyl ether radical cation involves dissociation followed by rearrangement.