Characterization of Protonated Formamide-Containing Clusters by Infrared Spectroscopy and Ab Initio Calculations:  I. O-Protonation

Characterization of protonated formamide clusters by vibrational predissociation spectroscopy confirms theoretical predictions that O-protonation occurs in preference to N-protonation in formamide. The confirmation is made from a close comparison of the infrared spectra of H⁺[HC(O)NH₂]₃ and NH₄⁺[HC(O)NH₂]₃ produced by a supersonic expansion with the spectra produced by ab initio calculations. For NH₄⁺[HC(O)NH₂]₃, prominent and well-resolved vibrational features are observed at 3436 and 3554 cm^-1. They derive, respectively, from the symmetric and asymmetric NH₂ stretching motions of the three formamide molecules linked separately to the NH₄⁺ ion core via three N−H⁺···O hydrogen bonds. Similarly distinct absorption features are also found for H⁺[HC(O)NH₂]₃; moreover, they differ in frequency from the corresponding vibrational modes of NH₄⁺[HC(O)NH₂]₃ by less than 10 cm^-1. The result is consistent with a picture of proton attachment to the oxygen atom, rather than the nitrogen atom in H⁺[HC(O)NH₂]₃. We provide in this work both spectroscopic and computational evidence for the O-protonation of formamide and its clusters in the gas phase.