The librational and vibrational spectra of water in natrolite, Na2Al2Si3O10·2H2O compared with ab-initio calculations Original Research Article

The librational spectrum of water in natrolite at 25 K is presented for the first time, and experimental results as obtained by inelastic neutron scattering and IR are compared with theoretical ab-initio calculations of a 2Na–H2O cluster alone. Calculations using various basis sets show the necessity for augmented basis sets in the determination of the librational frequencies. The experimental librational frequencies are 517, 557 and 701 cm−1 and the theoretical frequencies obtained for the 6-311G**(2d,2p), with MP2 electron correlation, are 460.8, 508.7 and 669.4 cm−1 for rocking, twisting and wagging, respectively. The calculated frequencies are approximately stable above the 6-31G* level. We also consider the anharmonicity of the energy well, and show that the harmonic approximation is valid to 2% in the frequency. We then consider a harmonic model of hydrogen bonding accounting for the observed structure and show this to be incompatible with the observed frequencies.