Activation barriers in the Diels-Alder reactions of acetylenic systems
have been calculated and discussed using the ab initio G2MS and IMOMO
(G2MS:MP2) methods. Activation energies for the reaction of acetylene
+ butadiene and acetylene + cyclopentadiene are predicted to be 25.8 a
nd 22.0 kcal/mol, respectively. Although the acetylene + butadiene tra
nsition state is more favorable in the interaction energy than the eth
ylene + butadiene transition state by about 3 kcal/mol due to secondar
y pi orbital interactions, the stiffer acetylene + butadiene system re
quires about 5 kcal/mol more reactant distortion energy. Thus, overall
, the acetylene + butadiene barrier is about 2 kcal/mol higher and the
predicted rate is slower than ethylene + butadiene, consistent with e
xperiment. For the reaction of acetylenedicarboxylic acid and cyclopen
tadiene, which possesses a very asymmetric transition state, the best
IMOMO(G2MS:MP2) method predicts an activation barrier of 9.2 kcal/mol
and reveals significant electronic effects of substituents outside the
model system of acetylene + butadiene.