Structural and vibrational study of [Mo7O24]6- and [W7O24]6-.

Density functional methods have been used to investigate the structure and the vibrational modes of [M7O24]6- isopolyanions of molybdenum and tungsten. Relativistic effects have been considered through the zeroth-order regular approximation (ZORA) and interactions with an aqueous environment modeled by the COSMO approach. A structural study of the two compounds has been performed, and the geometrical parameters obtained are in good agreement with experimental data. However, when the solvent is introduced in the model, deviations are found, especially for some tungsten-oxygen bonds which involve pseudoterminal oxygens. Thus, different computational strategies have been tested to reject any reliance on the COSMO model and the optimization algorithms. The variations compared to solid-state bond lengths appear to be due to the solvent. Infrared and Raman spectra have been also calculated in the gas phase and in water leading, for the first time, to a detailed assignment of the vibrational frequencies. The vibrational contributions of the aminopyridinium counterion [C5H7N2]+ have been isolated, improving the assignment of experimental spectra. Inclusion of solvent causes a shift toward lower frequencies and an increase in the intensity of the peaks. Spectra obtained using pseudo-gas-phase calculations reproduce the experimental data most satisfactorily, especially when the experiments are performed on the solid state.