Ethylene Glycol Ions Dissociate by Tunneling through an H-Atom Transfer Barrier:  A DFT and TPEPICO Study

Density functional theory (DFT) and threshold photoelectron−photoion coincidence spectroscopy (TPEPICO) have been used to investigate the dissociation dynamics of the ethylene glycol ion. A total of 13 isomers of the ethylene glycol ion (C₂H₆O₂^+•) and the transition states connecting them were obtained at the B3LYP/6-31G(d) level. The TPEPICO experimental results show that the CH₃OH₂⁺ ion, produced by a double hydrogen transfer, and the CH₂OH⁺ ion, produced by direct C−C bond cleavage, are the two dominant products. The H₂O loss channel, the lowest dissociation energy channel according to the DFT calculations, is suppressed because of a high barrier leading to its formation. The time-of-flight distributions of the CH₃OH₂⁺ ion at low energies are asymmetric, which indicates that this ion is produced from a slowly dissociating (metastable) parent ion. A two-well−two-channel model is proposed to describe the isomerization and dissociation process. The simulations combined with RRKM theory suggest that the production of the CH₃OH₂⁺ ion involves a hydrogen-bridged reaction intermediate, and its slow production is caused by tunneling through the isomerization barrier. This mechanism is supported by data for deuterated ethylene glycol. The 0 K appearance energy for the CH₂OH⁺ ion is determined to be 11.08 ± 0.04 eV, from which the 298 K heat of formation of the ethylene glycol molecule is determined to be −383.1 ± 4.5 kJ/mol, in agreement with other experimental values.