The vibrational spectra of the hydrated and methanol-solvated aminopyrazine, 2-aminopyridine and 3-aminopyridine in supersonic jets have been measured in terms of IR-UV double-resonance spectroscopy. Comparing the IR spectrum of aminopyrazine with those of 2-aminopyridine and 3-aminopyridine clusters, we determine the solvation structure of aminopyrazine to be a similar cyclic structure as hydrated 2-aminopyridine clusters [Wu, et al., Phys. Chem. Chem. Phys. 2004, 6, 515]. In the case of monohydrated aminopyrazine cluster, one of the normal modes composed of the hydrogen-bonded OH and NH stretching local modes gives the anomalously weak IR intensity, which is ascribed to the cancellation of the dipole moment change between the OH and NH stretching local modes. The solvated 3-aminopyridine clusters forms the hydrogen-bond between the pyridyl nitrogen atom and the OH group, but the amino group is indirectly affected to induce slight blue shift of the NH(2) stretches. This phenomenon is explained by inductive effect where the electron withdrawing from the amino group upon the solvation results in a "quinoid-like" structure of the amino group.

The cluster structures of hydrated aminopyrazines, APz−(H 2 O) n=2−4 , in supersonic jets have been investigated measuring the size-selected electronic and vibrational spectra and determined with the aid of quantum chemical calculations. The APz−(H 2 O) 2 structure is assigned as a cyclic N1 type where a homodromic hydrogen-bond chain starts from the amino group and ends at the 1-position nitrogen atom of the pyrazine moiety, corresponding to 2-aminopyridine−(H 2 O) 2 . On the other hand, APz−(H 2 O) n=3,4 has a linear hydrogen-bond network ending at the 4-position one (N4), which resembles 3aminopyridine−(H 2 O) n=3,4 . The hydrogen-bond network switching from the N1 type to the N4 one provides the accompanying red shifts of the S 1 −S 0 electronic transition that are entirely consistent with those of the corresponding 2-aminopyridine and 3aminopyridine clusters and also shows the drastically strengthened fluorescence intensity of origin bands in the electronic spectrum. The significant change in the excited-state dynamics is explored by the fluorescence lifetime measurement and the time-dependent density functional theory (TD-DFT) calculation. It is suggested that the drastic elongation of fluorescence lifetimes is due to the change in the electronic structure of the first excited state from nπ* to ππ*, resulting in the decreasing spin−orbit coupling to T 1 (ππ*).