The unimolecular chemistry of [1,2‐propanediol]⁺˙: a rationale in terms of hydrogen‐bridged radical cations

Abstract

By combining results from a variety of mass spectrometric techniques (metastable ion, collisional activation, collision-induced dissociative ionization, neutralization-reionization spectrometry and appearance energy measurements) and the classical method of isotopic labelling, a unified mechanism is proposed for the complex unimolecular chemistry of ionized 1,Zpropanediol. The key intermediates involved are the stable hydrogen-bridged radical cations [CH,=C(H)-0 * -* H -* -O(H)CH ,I+', which were generated independently from [4-metboxy, l-butanol]" (loss of C,H,) and [l-methoxyglyceroll" (loss of CH,O), [CH,-C=O * --He * -O(H)CH ,I+' and the related ion-dipole complex [CH,=C(OH)CH,/H,O]+'. The latter species serves as the precursor for the loss of CH,' and in this reaction the same non-ergodic behaviour is observed as in the loss of CH,' from the ionized enol of acetone.

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

“…The sequence depicted in Scheme (proposal A) has been the subject of a recent experimental and computational study, and a brief discussion will suffice. Unlike 1,2-ethanediol, the reactions for 1,2-propanediol all take place below or at the dissociation limit; that is, all species involved must have energies at or below that for CH 3 OH 2 + + CH 3 CO • = 0. First, the charge-transfer complex (CT) was located and its energy was calculated by the procedure described in ref .…”

“…As was stated above, CH 3 OH 2 + and HCO • are the sole dissociation products for ionized 1,2-ethanediol. By contrast, 1,2-propanediol undergoes, in addition to loss of CH 3 CO • , five other dissociations. , Apparently, the additional methyl substituent makes available dissociation routes and products that are not possible for ionized 1,2-ethanediol …”

“…The next-higher homologue of 1 , ionized 1,2-propanediol, HOCH 2 CH(CH 3 )OH •+ ( 1p ), also dissociates by DHT, yielding CH 3 OH 2 + + CH 3 CO • . This process appears to be remarkably similar to that for 1 ; for example, the isotopomer CH 3 CH(OD)CH 2 OD •+ specifically forms CH 2 DOHD + to the exclusion of CH 3 OD 2 + …”

An Experimental and Theoretical Study of the Long-Lived Radical Cation of CH3OCH2CH2OH

“…Such nonconventional radical cations include hydrogen-bridged radical cations and distonic radical cations ( i . e ., ionized ylides, zwitterions, or biradicals). − Both types of ions have been established to represent stable gas-phase species. ,,6b, …”