Vibrational spectra of linear oligomers of carbonic acid: a quantum chemical study.

Gas phase quantum chemical calculations of linear, hydrogen bonded oligomers of carbonic acid have been carried out to examine the feasibility for such species to be the building blocks of crystalline carbonic acid. Infrared and Raman vibrational spectra have been calculated and are compared against experimentally known spectra for two polymorphs of carbonic acid. The calculated anharmonic frequencies of the linear oligomer agree well with the experimental data for the centrosymmetric β-carbonic acid, rather than with that for the α polymorph. These calculations strongly suggest that β-carbonic acid should consist of one-dimensional hydrogen bonded carbonic acid molecules in the anti-anti conformation.