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

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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, C3ν 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 (Δνs) 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 Rx‐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 Rx‐y is less, the stretching mode frequencies are inverted.