Communication: Energetics of reaction pathways for reactions of ethenol with the hydroxyl radical: The importance of internal hydrogen bonding at the transition state

We find high multireference character for abstraction of H from the OH group of ethenol (also called vinyl alcohol); therefore we adopt a multireference approach to calculate barrier heights for the various possible reaction channels of OH+ C₂ H³ OH. The relative barrier heights of ten possible saddle points for reaction of OH with ethenol are predicted by multireference Møller-Plesset perturbation theory with active spaces based on correlated participating orbitals (CPOs) and CPO plus a correlated π orbital (CPO+π ). Six barrier heights for abstracting H from a CH bond range from 3.1 to 7.7 kcal/mol, two barrier heights for abstracting H from an OH bond are both 6.0 kcal/mol, and two barrier heights for OH addition to the double bond are -1.8 and -2.8 kcal/mol. Thus we expect abstraction at high-temperature and addition at low temperature. The factor that determines which H is most favorable to abstract is an internal hydrogen bond that constitutes part of a six-membered ring at one of the abstraction saddle points; the hydrogen bond contributes about 3 kcal/mol stabilization.