COMPARISON OF AM1 AND PM3 SEMIEMPIRICAL TO AB-INITIO METHODS IN THE STUDY OF DIELS-ALDER REACTIONS OF BUTADIENE AND CYCLOPENTADIENE WITH CYANOETHYLENES
Bs. Jursic et Z. Zdravkovski, COMPARISON OF AM1 AND PM3 SEMIEMPIRICAL TO AB-INITIO METHODS IN THE STUDY OF DIELS-ALDER REACTIONS OF BUTADIENE AND CYCLOPENTADIENE WITH CYANOETHYLENES, Journal of molecular structure. Theochem, 115(3), 1994, pp. 249-257
Assuming a concerted synchronous mechanism with one transition state o
f the Diels-Alder reactions, the structures of the transition states a
nd the activation energies for the reactions of butadiene and cyclopen
tadiene with cyanoethylenes were calculated by AM1 and PM3 semiempiric
al methods. The structural parameters were compared with those obtaine
d by high level Gaussian calculations, whereas the activation energies
were compared both with the ab initio calculations and those obtained
experimentally. The structural properties calculated with PM3 methods
are in general in better agreement with the ab initio calculations. T
he low level ab initio calculations are in many cases worse than the s
emiempirical methods. All predicted activation energies with both semi
empirical methods are up to 300% higher than the experimental values.
The predicted reactivity is also opposite to the experimental data. On
ly the very high level Gaussian calculations are in good correlation w
ith experimental results. The predicted selectivity of the reaction is
also opposite to the experimental facts. Two explanations are offered
for this discrepancy: AM1 and PM3 methods cannot handle the calculati
on of the concerted Diels-Alder transition states and are not recommen
ded to be used for that. purpose, or this Diels-Alder reaction is not
concerted but is stepwise.