ENHANCED HYDROGEN-BONDING OF WATER TO DIELS-ALDER TRANSITION-STATES -AB-INITIO EVIDENCE

Ab initio molecular orbital calculations reveal enhanced hydrogen bond ing of a water molecule to the transition states for the Diels-Alder r eactions of cyclopentadiene with methyl vinyl ketone (MVK) and acrylon itrile. The optimal interaction energies are found to be 1.5-2.0 kcal/ mol more favorable for hydrogen bonding to the oxygen or nitrogen in t he transition states than for the dienophiles. This supports the asser tion from a prior simulation study that the observed rate acceleration s for Diels-Alder reactions in aqueous solution arise from this hydrog en-bonding effect in addition to a relatively constant hydrophobic ter m.