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<sup>+</sup>[HC(O)NH<sub>2</sub>]<sub>3</sub> and NH<sub>4</sub><sup>+</sup>[HC(O)NH<sub>2</sub>]<sub>3</sub> produced by a supersonic expansion with the spectra produced by ab initio calculations. For NH<sub>4</sub><sup>+</sup>[HC(O)NH<sub>2</sub>]<sub>3</sub>, prominent and well-resolved vibrational features are observed at 3436 and 3554 cm<sup>-1</sup>. They derive, respectively, from the symmetric and asymmetric NH<sub>2</sub> stretching motions of the three formamide molecules linked separately to the NH<sub>4</sub><sup>+</sup> ion core via three N−H<sup>+</sup>···O hydrogen bonds. Similarly distinct absorption features are also found for H<sup>+</sup>[HC(O)NH<sub>2</sub>]<sub>3</sub>; moreover, they differ in frequency from the corresponding vibrational modes of NH<sub>4</sub><sup>+</sup>[HC(O)NH<sub>2</sub>]<sub>3</sub> by less than 10 cm<sup>-1</sup>. The result is consistent with a picture of proton attachment to the oxygen atom, rather than the nitrogen atom in H<sup>+</sup>[HC(O)NH<sub>2</sub>]<sub>3</sub>. We provide in this work both spectroscopic and computational evidence for the O-protonation of formamide and its clusters in the gas phase.