Density-functional methods for the study of the ground-state vibrations of the guanidinium ion

Sixteen density functional methods (including four hybrid methods) with the 3–21G and the 6–31G(d, p) basis set of atomic functions are used to predict the structure and vibrations of the guanidinium ion C(NH₂)₃]⁺. The study was done with the ion both in a vacuum and in an aqueous solution. To account for the solvation effect on the vibrating behavior of the ion, the solvent was modeled in two ways of increasing complexity: First, the guanidinium was inserted into a cavity of a continuous medium (dielectric constant ϵ = 78) and, second, six explicit water molecules were considered around the ion and the whole aggregate inserted into the cavity of the continuum. The conformation corresponding to the energy minimum is predicted to have D₃ symmetry rather than D_3h. The harmonic vibrational frequencies obtained have a mean absolute deviation from the experimental data of about one-half the value achieved by pure Hartree-Fock methods. Isotopic substitution calculations were also carried out and shifts obtained are in good agreement with experience and so are the assignments of the observed bands to the vibrational normal modes. The study of the solvent effect shows the existence of modes that are not affected by hydration and some improvement in the values predicted, especially for low-frequency vibrations. © 1997 John Wiley & Sons, Inc.