4+2+] Diels-Alder Cycloaddition to C60 and C70 Radical Cations in the Gas Phase: A Comparison with Solution

Abstract

Results of an experimental study using the selected-ion flow tube (SIFT) technique are reported for ionmolecule reactions of &'+ with a variety of cycloalkenes and acyclic and cyclic dienes at 294 f 2 K in helium gas at a pressure of 0.35 f 0.0 1 Torr. Addition is observed only with 1,3-~yclopentadiene and 1,3-~yclohexadiene. Rate coefficients were measured to be 1.0 and 1.5 X 10-l1 cm3 molecule-' s-I, respectively. No reactions, k < 3 X 10-12 cm3 molecule-' s-', were observed with 1,3-butadiene, isoprene, 1,3-pentadiene, cyclopentene, cyclohexene, furan, and 1 ,Ccyclohexadiene. These results provide indirect evidence for the occurrence of a Diels-Alder cycloaddition with 1,3-~yclopentadiene and 1,3-~yclohexadiene. The addition reactions of these two molecules with c60'+ were found to be 5 f 1 and 6 f 2, respectively, faster than the addition reactions with C~O*+ for which rate coefficients of 2.0 and 2.5 X cm3 molecule-' s-l, respectively, were measured. This relative reactivity, which in the case of 1,3-~yclopentadiene is remarkably similar to a reported relative reactivity of 7:l measured in toluene solution at 293 K, has been interpreted in terms of the hybridization of the C atoms in and c70. It is now well established that fullerene molecules undergo Diels-Alder cycloaddition reactions in the condensedphase with a variety of dienes, including cyclopentadiene, furan, isobenzofuran, anthracene, and 2,3-dimethylbuta- 1 ,3-diene.1.2 For example, up to six molecules of cyclopentadiene have been observed to add to in benzene.3 Very recent kinetic studies of the reactions of c.50 and C70 with cyclopentadiene in excess toluene between 291 and 354 K have indicated activation energies for these two reactions and have shown that Ca reacts about 7 times faster than C70 at 293 K.4 An analogous behavior has not yet been established for the gas phase, although it is interesting to note that a CcCsH6 Diels-Alder adduct produced in a lowpressure benzene flame has recently been reported.5 Here we present results of experimental investigations of the gas-phase reactivity of radical cations of and C70 toward acyclic and cyclic dienes and related compounds. (Because of their charge, cations of C~O and C70 are more easily monitored in the gas phase than their neutral counterparts.) The results reveal a remarkable selectivity of c60 toward 1,3-~yclopentadiene and 1,3-cyclohexadiene, and so provide indirect evidence for the occurrence of gas-phase Diels-Alder cycloadditions with these two molecules to yield structures I and 11, respectively.