AB-INITIO STUDY OF THE ADDUCTS OF CARBON-MONOXIDE WITH ALKALINE CATIONS

The interaction between CO (either via the C or the O end) and the alk aline cations (Li+, Na+, K+, Rb+, and Cs+) has been studied by means o f six nb initio methods, featuring the classical Hartree-Fock, the sec ond order Moller-Plesset treatment of electron correlation, one local density functional and two gradient-cornered methods as well as a quad ratic configuration interaction inclusive of single and double substit utions with a noniterative triples contribution to the energy. Basis s ets adopted for CO, Li+, Na+, and K+ and the corresponding adducts are of triple-xi valence quality augmented with a double set of polarizat ion functions (d on C and O; p on the cations). For Rb+ and Cs+, Hay-W adt effective core potential basis sets have been adopted. Calculated features are the binding energy, the frequency and intensity of the CO stretch, the bending mode, the cation-carbon (or oxygen) stretch, and the equilibrium geometry. Gradient-corrected density functional metho ds yield results nearly as good as the most expensive correlated metho d based on configurations interaction. A number of correlations are es tablished among the observables. The role of electrostatics in the int eraction is analyzed both by studying the molecular electrostatic pote ntial of CO and by replacing the cation with a proton in the same posi tion. Binding through the C end is invariably preferred, though, with increasing size of the cation, binding through the O end become progre ssively less unfavored. Experimental data concerning alkaline-cation s ubstituted zeolites are compared with computational results, and an ov erall agreement is observed. (C) 1996 American Institute of Physics.