Formation and isomerization of C120: considerations of symmetry, kinetics and thermodynamics

Abstract

AbstractAn orbital‐symmetry analysis (OCAMS) of the dimerization of C60 to its stable (D2h) dimer 1 indicates that the reaction cannot proceed along a totally symmetric pathway but that the reaction‐path symmetry must be reduced at least to C2h. Similarly, one‐step conversion of 1 to its isomer 3 (also D2h), which has been proposed as an intermediate for the intercalation of helium atoms into C60, is also limited to a C2h pathway. As neither of these pathways is geometrically appropriate, it is predicted that both reactions proceed stepwise. Semi‐empirical computations (AM1/UHF) confirmed this prediction: in the dimerization, a transoid singlet biradical (2) is formed, and then closes readily to 1 after rotation about its newly formed bond. An intermediate (4) of C2v symmetry is detected along the pathway for isomerization of 1 to 3. THERMO computations (AM1/RHF) indicated that both dimers should be in thermal equilibrium with C60 at moderate temperatures, but that their equilibrium concentrations in the gas phase would be exceedingly small under thermally accessible conditions. The limitations of the computational method employed and their bearing on the reliability of the results are discussed. Copyright © 2002 John Wiley & Sons, Ltd.