Dissociation of Ionized 1,2-Ethanediol and 1,2-Propanediol:  Proton-Transport Catalysis with Electron Transfer

Abstract

Ab initio molecular orbital (MO) calculations support the proposal that the key processes in the rearrangement of HOCH2CH2OH•+ and HOCH2CH(CH3)OH•+ (ionized 1,2-ethanediol and 1,2-propanediol) are sequential transfers of a proton and an electron taking place from one partner to the other in ion−dipole complexes rather than prompt hydrogen atom shifts taking place in distonic ions. Although the proposed distonic ions in the alternative mechanism (J. Am. Chem. Soc. 1992, 114, 2027) are thermodynamically remarkably stable species, a surprisingly large barrier exists for their interconversion by way of a 1,4-H atom shift. This large barrier results from significant distortion, from planarity, of the transition state. The rearrangement process of ionized 1,2-ethanediol and 1,2-propanediol can therefore best be described in terms of intramolecular catalysis (proton transport catalysis, Int. J. Mass Spectrom. Ion Processes 1992, 115, 95) in combination with an electron transfer taking place in intermediate ion−dipole complexes.

The remarkable dissociation chemistry of 2-aminoxyethanol ions NH2OCH2CH2OH+studied by experiment and theory

“…Previous work [3,4] has shown that metastable 1,2-ethanediol ions dissociate via the hydrogen-bridged intermediate, CH 2 O(H)· · ·H· · ·O CH 2…”

“…4 + ions are co-generated in this process but, as shown in Section 3.4, a pathway of much lower energy is available for this reaction.…”

“…In this study we have used tandem mass spectrometry based experiments in conjunction with model chemistry calculations to probe the rich dissociation chemistry of the metastable molecular ions NH 2 OCH 2 CH 2 OH •+ (AE-1). These ions dissociate via six competing pathways whereas the related 1,2-ethane diol ion HOCH 2 CH 2 OH •+ only loses HCO • in the metastable timeframe [3,4] via a mechanism that features proton-transport catalysis (PTC) [5] in hydrogen-bridged radical cations (HBRCs) [6] as key intermediates. It will be shown that HBRCs also play a key role in the various dissociation reactions of low-energy 2-aminoxyethanol radical cations.…”

“…The incipient HBRC-3b ions may lose NH 3 to yield ionized glycolaldehyde, or rearrange further via carbon-carbon bond cleavage into the ter-body complex HBRC-4. Ions HBRC-4 generated from AE-1 have a very high internal energy content (>56 kcal mol −1 , see the dashed line in Scheme 3b) and will therefore rapidly decompose into NH 4 + by the consecutive loss of HCO • + CH 2 O.…”

Isotopic labelling in mass spectrometry as a tool for studying reaction mechanisms of ion dissociations

“…Reversible rearrangement of EA-1 into the distonic ion EA-3 followed by isomerization into the ion-dipole complex This result, obtained from a double collision experiment, 29 was unexpected, not least because earlier it had been proposed that the hydrogen bonding between the OH groups in the neutral persists in the gas-phase ion and thus directs its dissociation chemistry, 31 a very reasonable (and therefore highly suspect) extrapolation from neutral chemistry. The solution to this problem again relied upon the results of computations, 32 where it was shown that the (predictable) 1,4-D shift, from oxygen to carbon, in the rearranged molecular ion, EG-2, required a large activation energy, making way for the less energy demanding internal catalysis mechanism, shown above in EG-4 ! EG-5, that places the hydroxyl D in the unexpected position (Scheme 3).…”