On the preferred protonation site in furan and vinyl alcohol. An ab initio study
The geometries of furan and vinyl alcohol and their Cα-, Cβ-, and O-protonated forms have been optimized completely using the 3–21G basis set and analytic gradient (force) methods. The relative energies obtained with the 3-21G basis set were checked by 6-31G*//3-21G (furan and vinyl alcohol species) and 6-31G**//3-21G (vinyl alcohol and its protonated forms) computations. The Cα-protonated furan and the Cβ-protonated vinyl alcohol are found to be the most stable forms, in agreement with experimental results. The preferred reaction site (Cα or Cβ) upon electrophilic substitution has been rationalized in terms of simple resonance structure arguments supported by MCSCF calculations.