Methoxymethyl cation [CH₃OCH₂]⁺ revisited: Experimental and theoretical study

Abstract

The metastable dissociation of the methoxymethyl cation and a number of its deuterium and 13C variants was examined using a reverse-geometry double-focusing mass spectrometer. The loss of methane from the methoxymethyl cation clearly showed a composite peak shape which, when deconvoluted, revealed a bimodal kinetic energy release distribution in the resulting formyl cations. Labelling experiments revealed that the two carbon atoms and all hydrogens become equivalent on the time-scale of the unimolecular dissociation lifetime of the decomposing ion. A small deuterium isotope effect was found which can be rationalized on the basis of zero point energy effects. The bimodal kinetic energy release distribution was shown, with the aid of a four-sector instrument, to be due to the production of both formyl cation (with a large kinetic energy release) and isoformyl cation (with a much smaller kinetic energy release). The methoxymethyl cation was also prepared with a precisely defined amount of internal energy in a Fourier transform ion cyclotron resonance (FTICR) spectrometer by the reaction of methyl cation with formaldehyde. Experiments with I3C and deuterium labelling revealed that the dissociation to formyl cation of the methoxymethyl cations formed in the low-pressure FTICR cell by reaction of methyl cation with formaldehyde is accompanied by complete scrambling of the carbons and incomplete scrambling of the hydrogens. Ab initio calculations were carried out which identified and characterized each of the stable minima and transition states for the appropriate reactions. The calculations were fully consistent with the mechanism deduced on the basis of the experimental data.

Theoretical characterizations of novel C2H5O+ reactions

“…Audier et al [3] obtained a transition state similar to ours, although their CH s O angle was more bent than the one we found, and their C Me H t bond was slightly longer (2.837 Å versus 2.493 Å). (Audier et al used HF/6-31G(d) theory; our values are from QCISD/6-31G(d,p) theory).…”

“…The formation of H 3 O + from 2 has received considerable experimental attention [1][2][3], but to our knowledge has not been described by theory. Energies obtained by present work are given in Table 1; structures of intermediates and transition states comprise Fig.…”

“…In the ground state of 2, the methylene hydrogens are in the skeletal plane, and the methyl is bisected by this plane with one methyl hydrogen (H s ) in that plane cis to the CO bond. Methane elimination from CH 3 O + =CH 2 to generate HCO + is initiated by CH 3 -O bond stretching (a minor amount of HOC + is also produced [3]). The methyl rotates H s toward the oxygen as the CO bond lengthens.…”

“…Hypervalent entities may be common but little recognized in unimolecular gas phase ion chemistry. It is also not certain whether methane elimination from CH 3 O + =CH 2 (2) is ion-neutral complex-mediated [3] or not, making 1 and 2 together a useful model for exploring what determines the type of pathway an elimination takes. Therefore, we here characterize methane eliminations from C 2 H 5 O + isomers at higher levels of theory than accomplished previously and by intrinsic reaction coordinate calculations (IRC).…”

“…The isomerizations and dissociations of C 2 H 5 O + isomers are of long-standing interest in gas phase ion chemistry [1][2][3]; even so, they still have unusual and incompletely understood features. For example, in methane elimination from CH 3 CH=OH + (1), H migrates from O to the CC bond through a hypervalent stage by way of a methyl H [4].…”