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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Structure and Hydrogen Bonding of Different Isomers of 2-Aminopyridine·NH_3 Studied by IR/R2PI
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Structure and Hydrogen Bonding of Different Isomers of 2-Aminopyridine·NH_3 Studied by IR/R2PI

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摘要

The electronic and vibrational spectra of the 1:1 clusters of 2-aminopyridine (2AP) with ammonia have been measured using resonant two-photon ionization (R2PI) and IR/R2PI vibrational spectroscopy. Density functional theory calculations were performed to identify structures and assign the vibrational spectra in the NH stretch region. The two lowest-energy isomers have been identified. In isomer I, which is the global minimum energy structure, ammonia forms a strong H bond with the amino group and a weak one with the aromatic nitrogen. The vibrations of the groups involved in this H bonding exhibit red shifts of 110 cm~(-1) for the donating amino N-H and of 69 cm~(-1) for the donating N-H group of ammonia. The degeneracy of the two asymmetrical vibrations of ammonia is removed in the cluster, with the corresponding bands at 3438 and 3406 cm~(-1). In isomer II ammonia forms an H bond to the N-H group which points away from the aromatic nitrogen. This bond is weaker than the corresponding one in isomer I. The stretching vibration of the H bonded N-H in the amino group exhibits a red shift of 66 cm~(-1), whereas the vibrations of ammonia are very similar to those of isolated ammonia. This gives evidence that ammonia is a pure H bond acceptor. The exclusiveness of these structural isomers is due to the dominant role of the proton affinity of ammonia in the H bond. The spectroscopic results are well supported by the calculations performed at the density functional level of theory. In a similar spectroscopic study of 2AP with one or two water molecules, water forms in-plane bridges between the ring nitrogen and the amino group. However, the H bond with the ring nitrogen is dominant and only one lowest-energy isomer could be found for each cluster size (Wu et al. Phys. Chem. Chem. Phys. 2004, 6, 515).

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