COMPARATIVE QUANTUM-CHEMICAL STUDIES IN THE FRAMEWORK OF GAUSSIAN APPROXIMATIONS OF THE ACIDIC CHARACTERISTICS OF THE SIMPLEST ZEOLITE CLUSTERS AND OF THE SULFURIC-ACID MOLECULE

Different variants of the Gaussian approximation, giving the energetic characteristics of molecules with chemical accuracy (+/-2 kcal mol(-1 )), are applied to calculations of the deprotonation energy of the sul furic acid molecule in the gas phase as well as to the simplest cluste rs modeling the bridging hydroxyl groups in zeolites. The conclusion i s made that the bridging hydroxyls are more acidic than the sulfuric a cid molecule. The estimated range of deprotonation energy in zeolites (275+/-15 kcal mol(-1)) is in good agreement with experimental data an d with results of ab initio calculations for extended models including several tens of atoms. The effects of the quality of the basis set an d electron correlation on deprotonation energy are also discussed.