Kinetic and Thermodynamic Stability of Acenes: Theoretical Study of Nucleophilic and Electrophilic Addition

To understand the reactivity of acenes, particularly pentacene, the addition of HCl and water to acenes
was studied for the benzene−nonacene series at the B3LYP/6-31G(d) level of theory. Surprisingly, the
reactivity of the acenes increases along the series up to hexacene and remains constant from hexacene
and above due to the biradical character of the ground state of higher acenes. While the exothermicity
of HCl and water additions are very similar, the activation barriers for HCl and water additions differ by
a constant factor of ca. 27 kcal/mol. The barrier for the addition of HCl varies from 44 kcal/mol for
benzene to 16−18 kcal/mol for pentacene−nonacene, whereas the barrier for the addition of water varies
from 71 kcal/mol for benzene to 43−46 kcal/mol for pentacene−nonacene. The transition states (TSs)
for the addition of water to acenes are relatively “late” on the reaction coordinate, compared to the
“earlier” TSs for the addition of HCl. There is a substantial substituent effect on the energy barriers for
these reactions. HCl behaves as an electrophile, with ρHCl (vs σp) = −4.48 and −3.39 for anthracenes
and pentacenes, respectively, while water behaves as a nucleophile, with ρH2O (vs σp) = 2.35 and 1.39
for anthracenes and pentacenes, respectively.