Roaming Pathway Leading to Unexpected Water + Vinyl Products in C₂H₄OH Dissociation

Abstract

We report molecular dynamics simulations of the unimolecular dissociation of energetic C2H4OH radicals using a full-dimensional potential energy surface (PES) fit to density functional theory (DFT) and coupled cluster (CCSD) calculations. Quasiclassical trajectories are initiated on the surface using microcanonical sampling at a total energy of 85.0 kcal/mol (44.1 kcal/mol relative to the zero-point energy). The trajectories reveal a roaming channel leading to the formation of water. The transition state (TS) corresponding to direct water production is energetically inaccessible. However, the roaming pathway finds a lower-energy path via frustrated dissociation to hydroxyl that makes water + vinyl production feasible. The trajectory calculations suggest that the roaming pathway constitutes a minor (a few percent) channel of the overall C2H4OH dissociation. The mechanism of the roaming reaction is analyzed in terms of the geomteric proximity of the TSs leading to OH loss and internal abstraction as well as the partitioning of vibrational energy into different modes.

Global permutationally invariant potential energy surface for ozone forming reaction

“…[2][3][4][5][6][7] For the molecules where all atoms are identical (allotropes) this should be the method of choice. 8 It allows reducing significantly the number of ab initio data points needed for accurate surface representation, as well as automatically takes care of the intrinsic molecular symmetry.…”

Exploring Multiple Potential Energy Surfaces: Photochemistry of Small Carbonyl Compounds

“…This speculation was confirmed in 2004 by combined experimental and theoretical studies [95]. Since its discovery, this channel named "roaming" has been a hot topic as a new type of reaction mechanism [96][97][98], and it has been found in many reactions having implications to atmospheric chemistry [99][100][101][102][103][104][105][106][107][108] and combustion chemistry [88,109,110]. The IC was postulated to occur in the potential well of H 2 CO in many previous studies.…”

Photoproduct Channels from BrCD2CD2OH at 193 nm and the HDO + Vinyl Products from the CD2CD2OH Radical Intermediate

“…Recent studies, both theoretical and experimental, have attempted to determine branching ratios of the product channels of the reaction, beginning at the CH 2 CH 2 OH (β-hydroxyethyl) radical adduct. 23,29,30 The CH 2 CH 2 OH radical intermediate may be produced from the photodissociation of 2-haloethanols. Hintsa et al 31 were the first to investigate the photodissociation of 2-bromoethanol at 193 nm, showing that a large fraction of β-hydroxyethyl radicals were formed stable to subsequent dissociation to OH + ethene and attributing it to a large partitioning of energy into rotational energy of the radicals.…”

“…We proposed that the dissociation of the radical to H 2 O + C 2 H 3 observed in our experiments might involve a frustrated dissociation of the C−O bond toward the [CH 2 CH 2 ]··· OH exit channel followed by an H atom abstraction by the OH moiety instead of returning to the CH 2 CH 2 OH well. In classical trajectory calculations on the global potential energy surface for this reaction, Kamarchik et al 30 observed a roaming mechanism for the H 2 O + vinyl product channel beginning from the radical intermediate. However, their calculations at 44 kcal/mol predicted a smaller branching to these products than was observed experimentally.…”