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 rat ios, have been optimized by means of a semiclassical model potential explic itly derived for zeolites. For each structure, the relative stability and t he anharmonic vibrational stretching frequency omega(01)(OH) have been eval uated by means of the periodic ab initio CRYSTAL98 program, using the B3-LY P Hamiltonian. The spread of omega(01) among the structures is 45 cm-l, H-C HA(1:I) having the highest frequency and the largest stability per Al atom. Neat correlations between omega(01) and either the relative stability of t he corresponding H-CHA structures or the value of the electric field gradie nt at the H nucleus have been found to hold, in agreement with literature s uggestions. (C) 2000 Elsevier Science B.V. All rights reserved.