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.

通过振动预解离光谱对氟化甲酰胺簇进行表征，证实了理论预测：在氟化甲酰胺中，氧质子化相较于氮质子化更为优先。此证实通过对比超音速膨胀产生的H+[HC(O)NH2]3和NH4+[HC(O)NH2]3的红外光谱与基于从头算计算得到的光谱而得出。对于NH4+[HC(O)NH2]3，观察到在3436和3554 cm-1处的显著且清晰分辨的振动特性，这些特性分别源于三个氟化甲酰胺分子中对称和非对称的NH2伸缩振动，它们分别通过三个N−H+···O氢键与NH4+离子核心相连。类似地，对于H+[HC(O)NH2]3，也发现了明确的吸收特性；此外，它们的频率与NH4+[HC(O)NH2]3相应的振动模式相比，差异小于10 cm-1。这一结果与氢质子附着于氧原子而非氮原子的情景相符。本研究提供了气相中氟化甲酰胺及其簇的氧质子化的光谱和计算证据。