THEORETICAL-STUDY OF THE HYDRATION REACTION OF KETENE

The addition of monomeric and dimeric H2O to ketene has been studied by ab initio molecular orbital calculations using a 6-31G* basis set and energy calculations up to MP4(SDTQ). Complete geometry optimizations relaxing all internal coordinates were performed. Our calculations indicate that a concerted addition of dimeric H2O to the carbon-oxygen double bond, yielding an enediol, is energetically slightly preferred to a similar addition to the carbon-carbon double bond, yielding acetic acid. The energy barriers obtained at the highest level of calculation, MP4(SDTQ)/6-31G*//RHF/6-31G*, were 17.4 and 19.4 kcal/mol for the two additions, respectively. After a zero-point energy (ZPE) correction was introduced, the energies were 19.9 and 21.8 kcal/mol, respectively. The potential barriers to addition of monomeric H2O were found to be substantially higher at the same calculational level, the values being 37.8 and 41.4 kcal/mol for the addition to the carbon-oxygen double bond and to the carbon-carbon double bond, respectively. The ZPE correction modified these values to 39.7 and 4 1.0 kcal/mol, respectively. The conversion of enediol to acetic acid through intramolecular hydrogen migration has an activation energy of 45.7 kcal/mol relative to the diol, calculated at the highest level and corrected for ZPE. A systematic search for stationary points representing noncyclic transient species for the system ketene and monomeric H2O revealed only a weakly bound complex with a C(alpha)-O(water) distance of 3.02 angstrom.