A QUANTUM-CHEMICAL STUDY OF ADSORBED NONCLASSICAL CARBONIUM-IONS AS ACTIVE INTERMEDIATES IN CATALYTIC TRANSFORMATIONS OF PARAFFINS .2. PROTOLYTIC DEHYDROGENATION AND HYDROGEN-DEUTERIUM HETERO-ISOTOPE EXCHANGE OF PARAFFINS ON HIGH-SILICA ZEOLITES

HF/3-21G quantum-chemical analysis of the protolytic attack of acid pr otons in zeolites at the C-H bonds in methane and ethane indicated tha t the resulting transition states depend on the sign of the bond polar ization. If a hydride ion is split off from the paraffin, then the tra nsition state resembles the adsorbed carbonium ion and the reaction re sults in molecular hydrogen and in formation of the surface alkoxy gro up. The case, when a proton tends to split off from the paraffin, corr esponds to the hetero-isotope exchange of paraffins with surface OH gr oups. This is a concerted acid-base reaction with a transition state d ifferent from adsorbed carbonium ion.