S. Karabunarliev et M. Baumgarten, Using antiferromagnetic couplers for high-spin ground states in ion radicals, CHEM PHYS, 254(2-3), 2000, pp. 239-247
The presence of negatively spin-polarized peripheral sites in delocalized r
adical ions is exploited to propose a type of direct linkage leading to sta
bilization of high-spin states. Ion monoradicals of anthracene, pyrene, azu
lene and s-indacene are taken as spin-bearing units. Pairs with close charg
ing potentials are outlined as likely to form diradicals. Triplet stabiliza
tion is envisaged to occur from the exchange coupling of opposite-signed br
idgehead a-spin densities belonging to individual radical units, e.g., 2(9'
-anthryl)pyrene and 2,2'-bipyrenyl di-ions. Results from AM1-ROHF-CI (CAS[1
0,10], CAS[9,9]) computations are reported, where the computational approac
h is first checked for adequacy with regard to the spin-density distributio
n of the monoions, and then employed to assess the spin states of several i
onized dimers and trimers, some of which are synthetically available. The h
igh-spin states are found to be stabilized by up to similar to 300 cal mol(
-1), with the singlet-triplet or doublet-quartet gaps increasing for nonequ
ilibrium conformations approaching planarity. (C) 2000 Elsevier Science B.V
. All rights reserved.