Interaction of ammonia with a zeolitic proton: ab initio quantum-chemical cluster calculations

The interaction of NH3 and a zeolitic cluster as well as the protonation of NH3 by zeolitic protons were studied by quantum-chem. calcns. on small clusters at different levels of approxn. The focus was on a comparison of results obtained by the different methods. The clusters were studied at the SCF level as well as at the correlated level. Electron correlation is included through second-order Moeller-Plesset perturbation theory. The basis-set superposition error (BSSE) was avoided by using the counterpoise scheme. Monodentate singly bonded NH3 (NH3 attached to 1 O atom) forms a strong H bond with the zeolitic OH group. This bond has a strength of 60 or 67 kJ/mol, depending on the geometry of the zeolitic cluster. This value is approx. half the exptl. heat of desorption. For this case, the O-N distance is very short (2.74 or 2.73 .ANG.) and the intermol. O-H-N stretching frequency is calcd. to be 185 or 193 cm-1. The latter values agree reasonably with exptl. data. Upon complexation with NH3 and OH stretching frequency shows a red shift of 551 cm-1. Proton transfer from the zeolitic cluster to NH3 is calcd. to be unfavorable by 52 kJ/mol, as long as NH4+ is considered to be monodentate coordinated. The description of the H bonded form is only slightly dependent on the basis set used. However, the proton-transfer energy does depend strongly on the basis set used. Electron correlation makes the proton transfer more favorable. The BSSE has a large influence on the description of the structures, esp. if electron correlation is included. Although electron correlation has a non negligible effect on the proton-transfer energy, some conclusions can be drawn from SCF calcns. on doubly and triply coordinated NH4+. The calcd., energy of adsorption now is approx. twice that calcd. for the H bonded and singly coordinated NH3 and close to exptl. obsd. values of NH3 adsorption. These results indicate that these adsorption modes are prefered over the singly bonded form. These forms are preferred because of the favorable electrostatic stabilization of NH4+ when bonded to the cluster by 2 or 3 H bond