Acidity of the Bronsted acid sites of zeolites

The acidity of Bronsted acid sites of the (H3SiO)(n)O-q (X3-n-qSi)(OH) AlpY 3-m-q(OSiH2)(r)(OSiH3)(m) zeolitic clusters, with X = (H,OH), Y = (H,OH), ( n, m) = (0, 1, 3), r = (0,1, 8), and (p, q) = (0, 1), is evaluated in terms of the intrinsic acidity of the OH groups. is defined as the ratio between the OH distance and the frequency of the OH vibration mode and has been ca lculated at ab initio Hartree-Fock (HF) and second-order Moller-Plesset (MP 2) levels and the standard 6-31+G(d,p) basis set. The acidity is rationaliz ed in terms of the topology of the charge density p(r) calculated at the po sition of the critical point of the OH bond p(r(c)) of the acid sites. It i s shown that there exists a linear relationship between and the charge conc entration p(r,), with a correlation coefficient of r = -0.98 for HF results , and r = -0.96 for MP2 ones. (C) 2001 John Wiley & Sons, Inc.