Double-contact ion-molecule binding: Infrared characterization of the ionic H bonds to formic acid in the I−⋅HCOOH complex

Abstract

We report mid-IR predissociation spectra of the I−⋅HCOOH⋅Arm(m=1–4) ion-acid complexes. The spectra are consistent with a planar structure where both hydrogens are engaged in ionic H bonds. Upon binding to the ion, the OH stretching fundamental displays a much more dramatic redshift (792 cm−1) than that of the CH stretch (99 cm−1), giving rise to a complex series of bands in the 2750–2950 cm−1 region. The contributions of the CH and OH stretches to the spectrum are isolated by recording spectra of the I−⋅DCOOH and I−⋅HCOOD species, which reveal that the OH stretching vibration is accompanied by combination bands involving soft modes while the CH stretch spectrum is dominated by a single feature. Some of the complexity in the I−⋅HCOOH spectrum arises from a strong Fermi resonance interaction between the v=1 level of the OH stretch and an overtone or combination band involving CH motion. We compare this behavior to that of the previously reported I−⋅CH3OH and I−⋅H2O complexes.