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.