Spectroscopic Signatures of [H9O4]+ and [H13O6]+ Ions in a Polar Aprotic Environment Revealed Under DFT-PCM Approximation.

The structures, relative stability, infrared (IR) and Raman spectra of the most-stable forms of [H9O4]+ and [H13O6]+ ions in acetonitrile are computed using the B3LYP functional combined with the Polarizable Continuum Model approximation. These forms are hydrated [H3O]+ and [H5O2]+ cores. Of interest are two main environmental effects on the spectroscopic features of protonated water hydrates: (i) polarization of the solvent by the hydrate dipole moment; (ii) formation of H-bonds with bulky counterions (ClO4- and BF4-). The effect of the polarization on the structure of the [H3O]+ core strongly depends on the symmetry of the hydration shell. A distortion of a hydrated [H3O]+ easily changes its structure to the [H7O3]+ one that causes a change in the nature of the most IR-intensive bands. Thus, the specificity of this core can be easily lost that prevents identification of the corresponding species. By contrast, the [H5O2]+ core is more stable against distortion. It is characterized by the short O…O distance (< 2.45 Å), IR-intensive band near 1720 cm-1 and Raman-intensive line around 500 cm-1. The [H5O2]+ core remains identifiable even when protonated hydrate is involved in specific interactions with a bulky counterion. Geometrical criteria for identification of the [H3O]+, [H5O2]+ and [H7O3]+ cores are discussed.