Calculations showed that adsorption complexes can be formed on two types of adsorption sites: Ca(2+) in the center of six-member rings and Ca(2+) in eight-member rings. Only the former adsorption complexes were experimentally observed in IR spectra; they are characterized by H-H stretching frequency of 4083 cm(-1) and ΔH(0) = -12 kJ/mol in agreement with calculated results.