DFT STUDY OF THE DIELS-ALDER REACTIONS BETWEEN ETHYLENE WITH BUTA-1,3-DIENE AND CYCLOPENTADIENE

A set of DFT methods (Xa, HFB, S-VWN, B-LYP, B-HandH and Becke3-LYP) h as been used for the calculation of the transition states and energy b arriers of the Diels-Alder reaction of ethylene with buta-1,3-diene an d cyclopentadiene. The pure DFT methods overestimate the bond lengths, while the hybrid methods give values much-closer to those obtained by conventional ab initio methods. The ratio of the sigma-forming bonds to the pi-breaking bonds is in excellent correlation to the predicted electronic energy barriers for these reactions. The local spin density approximation (S-VWN) fails completely giving a negative value for th e classical energy barrier height for the addition of ethylene to buta -1,3-diene. On the other hand, the vibrational adiabatic barrier heigh ts predicted by B-LYP/6-31G(*) and the hybrid Becke3-LYP/6-31G(**) th eoretical models are in excellent agreement with available experimenta l data for both reactions.