Vibrational study of Li6P6O18.3H2O and ab initio calculations in P6O18 and P6O18.3H2O.

The infrared and polarized Raman spectra of the trigonal Li(6)P(6)O(18).3H(2)O crystal are reported. The results are analysed using several group theory approaches, in terms of internal and external modes of the highly symmetric P(6)O(18) cyclophosphoric ring and water molecules. Equilibrium geometries and vibrational spectra of P(6)O(18) units, free and in interaction with water molecules (P(6)O(18).3H(2)O) have been determined by ab initio calculations using the basis set 6-31+G(d) of Hartree Fock method. Experimental frequencies and polarisation conditions are remarkably consistent with ab initio calculations. A detailed description of the normal modes of vibration of these systems is presented.