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  • Title: Roaming-like Mechanism for Dehydration of Diol Radicals.
    Author: Asatryan R, Pal Y, Hachmann J, Ruckenstein E.
    Journal: J Phys Chem A; 2018 Dec 27; 122(51):9738-9754. PubMed ID: 30484647.
    Abstract:
    Diol radicals (DRs) are important intermediates in biocatalysis, atmospheric chemistry, and biomass combustion. They are particularly generated from photolysis of halogenated diols and addition of hydroxyl radical to a double bond of unsaturated alcohols, such as lignols. The energized DRs further isomerize/decompose to form products, including water. Aqueous-phase dehydration in radiolytic and biomimetic systems typically occurs at low temperatures, with or without catalysis, whereas the gas-phase dehydration is usually considered energetically unfavorable. In the present study, we propose a new low-energy, roaming-like mechanism based on a detailed dispersion-corrected DFT and ab initio level analysis of the gas-phase dehydration of DRs obtained from the combination of OH radicals with allyl alcohol (AA, CH2═CHCH2OH)-the simplest relevant model of the unsaturated alcohols. The roaming pathways involve a nearly dissociated OH-group, which subsequently abstracts an H atom of the remaining fragment to form water and [C3H5O] radical via a transition state (TS) with energy close to the C-O bond fission asymptote. Two types of roaming-like first-order saddle points (SP) are identified for unimolecular dehydration of 1,2- and 1,3-DR radical adducts involving either both hydroxyl groups of diol radicals to generate an oxygen-centered radical, or β-OH group and a skeletal α-hydrogen atom of the 1,2-DR to form a resonantly stabilized hydroxyallyl radical. Two higher energy conventional (tight) transition states, along with the pathways to 1,2-OH-migration, as well as direct H-abstraction, are also identified and analyzed. Most of the traditional density functional theory methods that have been successfully employed in the literature to locate so-far-known roaming SPs were also able to identify the new mechanism, in accord with dispersion-corrected double hybrid B2PLYP-D3(BJ) and mPW2PLYPD methods involving MP2-correlation corrections. However, the MP2 method itself failed to locate any of them, which seems to be typical for MP2 method for loose TS structures, confirmed here for a flat region of PES connecting direct and roaming saddle points. However, MP2 method correctly locates an identical roaming SP for a larger p-coumaryl alcohol model involving hydroxyphenyl substituent at Cγ atom of AA. Two types of interfragmental interactions are identified that stabilize the roaming SPs: (a) H-bonding of the leaving OH radical either with the H atom of the remaining OH group, or with π-cloud of the double bond; (b) direct interaction of π-electrons with the lone-pair electrons of the heteroatom in the leaving OH group through the TS-ring. The alternative TSs are qualitatively characterized by "collinearity" angle of the OH radical attack on the O-H/C-H bonds of the substrate in abstraction-like O-H-O geometry, attributed to the improved orbital overlaps. The proposed mechanism presents broader implications to signify, particularly, a larger role in atmospheric and combustion processes, especially biomass pyrolysis.
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