The hydrated proton H+(H2O)n. II. A spectroscopic study and normal coordinate treatment of the oxonium ions H3O+ and D3O+

Abstract

A spectroscopic study and a normal coordinate treatment (NCT) of the oxonium ions H₃O⁺ and D₃O⁺ in solid H₃O⁺CH₃C₆H₄SO₃⁻ and D₃O⁺CH₃C₆H₄SO₃⁻ were made. Neutron diffraction studies indicated a pyramidal, C_3ν symmetry for the oxonium ions. Based on this symmetry, the observed frequencies for H₃O⁺ and D₃O⁺ were assigned and a NCT attempted using the general valence force field and the Urey‐Bradley force field. Comparison of the fundamental vibrations in various oxonium compounds was made. It was demonstrated that a relationship between the OH stretching frequencies in H₃O⁺ and D₃O⁺ ions vs R (the O⋯O distance) exists, as well as a relationship between the stretching frequency shift (Δν_s) and R. The failure of past investigators to recognize this trend may have contributed to the confusion regarding the assignments. This is the first confirmation of such a trend existing in oxonium compounds. It is suggested that the stretching mode frequencies in H₃O⁺ and D₃O⁺ are dependent on R. When R_x‐y is greater than the X–H⋯Y van der Waals radius sum, the asymmetric stretching vibration (ν₃,e) is at higher frequency than the symmetric stretching vibration (ν₁,a₁); where R_x‐y is less, the stretching mode frequencies are inverted.