DENSITY-FUNCTIONAL AND HARTREE-FOCK CALCULATIONS ON THE CYCLOPROPANE RING INTERMEDIATES INVOLVED IN THE ZEOLITE-CATALYZED SKELETAL ISOMERIZATION OF HYDROCARBONS AND IN THE CARBON-ISOTOPE SCRAMBLING IN 2-PROPYLCATION

Quantum-chemical calculations were carried out on the mechanism of the zeolite-catalyzed hydrocarbon skeletal isomerization via the cyclopro pane ring intermediates. According to the B3LYP/6-31G calculations, f ormation of cyclopropane from surface alkoxy species in zeolites occur s via a transition state whose hydrocarbon part resembles a corner-pro tonated cyclopropane (corner-PCP) ring. Two conformations of the trans ition state found differ in the orientation of the PCP portion with re spect to the acid site. The activation energy for the cyclopropane rin g closure reaction is found to be rather sensitive to the use of plana r symmetry constraints and to the level of calculations and less sensi tive to the level of the geometry optimization. Calculations on the me chanism of the carbon isotope scrambling in the free 2-propyl cation w ere also performed, at several theory levels up to the Gaussian-2 mode l. The relatively stable intermediates of this superacid-catalyzed rea ction are carbocations, in contrast to the zeolite-catalyzed isotope s crambling where the relatively stable intermediates are surface alkoxy species with the corner-protonated cyclopropane as a high-energy tran sition state.