A FTIR search for linkage isomerism of CN(-) ions adsorbed on oxides and zeolites.

Adsorption of HCN and DCN was studied at 150-600 K on TiO(2), ZnO, MgO and a set of X zeolites by means of FTIR spectroscopy. At the lowest temperatures adsorption is non-dissociative and occurs on surface OH groups of oxides, cations and coordinately unsaturated oxygen atoms. Bands at 2243-2147 cm(-1) assignable to CN(-) ions start to grow in the spectra of ZnO at 170 K, at 250 K for MgO and at 300 K for TiO(2), where bands of polymeric species appear simultaneously. The structure of bands near 2200 cm(-1) in the spectra of the studied oxides does not display a temperature dependence characteristic of isomeric forms. Reversible temperature changes of the band of CN(-) ions in Na-, K-, Rb- and CsX zeolites exposed to HCN provide evidence for linkage isomerism, when the ions bound to alkali cations via C- or N- atoms co-exist in the thermodynamic equilibrium. C-Bonded species account for the stronger band at 2200-2140 cm(-1), while the low-frequency shoulder of this band, which appears and grows at elevated temperatures, is due to the energetically less favorable N-bonded form. The enthalpy of isomerization, estimated from the temperature dependence of band intensities, decreases from ca 20 kJ mol(-1) or more for Na(+) and K(+) to 7.6 kJ mol(-1) for Cs(+), in accordance with the electrostatic model.