Thermal Isomerizations of Diethynyl Cyclobutadienes and Implications for Fullerene Formation.
The mechanism by which carbon condenses to form PAHs or fullerenes is a problem that has garnered considerable theoretical and experimental attention. The ring-coalescence and annealing model for the formation of C(60) involves a [2 + 2] cycloaddition reaction of a cyclopolyyne to form a tetraalkynyl cyclobuta-1,3-diene intermediate, followed by a Bergman cycloaromatization reaction of the enediyne moiety. Intramolecular trapping of the incipient p-benzyne diradical across a diyne moiety of the macrocyclic ring affords an aromatic ring that must undergo further intramolecular reactions via polyradical intermediates to produce a condensed graphitic structure or fullerene. Computational studies of a model system for the intriguing tetraalkynylcyclobuta-1,3-diene intermediate, however, reveal that the corresponding p-benzyne diradical lies in a shallow minimum with a very low barrier to ring opening to cyclooctadienediyne. This pathway has not been previously considered in the mechanism for carbon condensation.
Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706-1322, United States.
Brian J
BJ
Esselman
Department of Chemistry, Purdue University , 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States.
Frank L
FL
Emmert
Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706-1322, United States.
Andrew J
AJ
Wiederhold
Department of Chemistry, Purdue University , 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States.
Stephanie J
SJ
Thompson
Department of Chemistry, Purdue University , 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States.
Lyudmila V
LV
Slipchenko
Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706-1322, United States.
Robert J
RJ
McMahon