10.1021/jp000993n.s017 C.-C. Wu C.-C. Wu J. C. Jiang J. C. Jiang I. Hahndorf I. Hahndorf C. Chaudhuri C. Chaudhuri Y. T. Lee Y. T. Lee H.-C. Chang H.-C. Chang Characterization of Protonated Formamide-Containing Clusters by Infrared Spectroscopy and Ab Initio Calculations:  I. O-Protonation American Chemical Society 2000 vibrational predissociation spectroscopy HC protonated formamide clusters NH 4 ab initio calculations 2000-09-30 00:00:00 Dataset https://acs.figshare.com/articles/dataset/Characterization_of_Protonated_Formamide-Containing_Clusters_by_Infrared_Spectroscopy_and_Ab_Initio_Calculations_I_O-Protonation/3721326 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.