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质子化甲酰胺簇进行表征，证实了理论预测：在甲酰胺中，氧质子化优先于氮质子化。此证实通过将超音速膨胀产生的H+[HC(O)NH2]3和NH4+[HC(O)NH2]3的红外光谱与从头计算产生的光谱进行密切比较而得出。对于NH4+[HC(O)NH2]3，在3436和3554 cm-1处观察到显著的且分辨良好的振动特征。这些特征分别源于与NH4+离子核心通过三个N−H+···O氢键分别连接的三个甲酰胺分子的对称和非对称NH2伸缩振动。类似地，对于H+[HC(O)NH2]3，也发现了独特的吸收特征；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。这一结果与质子附着于氧原子而非氮原子的情景相一致。在本研究中，我们既提供了气相中甲酰胺及其簇的氧质子化的光谱学证据，也提供了计算证据。