ABINITIO CS TRANSITION-STATE FOR THE DIELS-ALDER REACTION OF ACETYLENE AND BUTADIENE

Ab initio calculations are reported for the Diels-Alder reaction between acetylene and butadiene. An activation energy of 60.9 kJ mol-1 and an enthalpy of -287.8 kJ mol-1 have been calculated at the MP4SDQ/6-31G//HF/6-31G level of theory. A full set of vibrational frequencies, calculated at the 4-31G level, exhibited a single imaginary frequency for the transition structure with Cs symmetry. The geometry of this transition structure was almost identical with the critical point determined at the HF/6-31G level of theory. An analysis of the distortion energy of the reactants in the forward reaction at infinite separation showed that the magnitudes of the repulsive and attractive electronic contributions to the transition barrier are approximately equal. The increase in energy of the filled π-orbital of acetylene not involved in bonding changes is a major contributor to the activation energy for the Diels-Alder reaction of butadiene with acetylene being greater than that of butadiene and ethylene. © 1990, American Chemical Society. All rights reserved.