Infrared depletion spectroscopy and structure of the 2-aminopyridine dimer

The electronic and vibrational spectra of the 2-aminopyridine (2AP) dimer have been measured using resonant two-photon ionization (R2PI) and IR ion depletion (IR/R2PI) spectroscopy. Only two vibrational bands in the N-H stretch region have been observed at 3319 and 3529 cm(-1), which are red-shifted by 120 and 17 cm(-1), respectively, relative to the symmetric and antisymmetric N-H vibrations of the monomer. This provides evidence of the cyclic structure of this dimer with two antiparallel N-H...N hydrogen bonds. In addition, ab initio calculations at the B3LYP/6-311++G(d,p) level have been carried out. The cyclic structure is also the calculated minimum energy conformation. Each NH2 group forms a sigma-hydrogen bond with the ring nitrogen of the partner. Compared to the dimers of 2-pyridone ((2PY)(2)) and 2-hydroxypyridine ((2HP)(2)) and to the mixed dimer ((2PY).(2HP)), the hydrogen bonds in (2AP)(2) are weaker. This is confirmed by the differences in the vibrational spectra and by the computational results. The binding energy of the 2AP dimer is the lowest among these dimers, and its hydrogen bonds are the longest. The weaker hydrogen bonds in 2AP may be rationalized by the nonplanar structure of the molecule in the ground state. The theoretical results are in good agreement with the experimental data.