H-chabazite with variable Si/Al ratio: stability and OH vibrational frequency computed in a periodic LCAO B3-LYP approach

The structure of H-chabazites (H-CHA) with 1:1, 3:1, 5:1 and 11:1 Si/Al ratios, have been optimized by means of a semiclassical model potential explicitly derived for zeolites. For each structure, the relative stability and the anharmonic vibrational stretching frequency omega(01)(OH) have been evaluated by means of the periodic ab initio CRYSTAL98 program, using the B3-LYP Hamiltonian. The spread of omega(01) among the structures is 45 cm-l, H-CHA(1:I) having the highest frequency and the largest stability per Al atom. Neat correlations between omega(01) and either the relative stability of the corresponding H-CHA structures or the value of the electric field gradient at the H nucleus have been found to hold, in agreement with literature suggestions. (C) 2000 Elsevier Science B.V. All rights reserved.