Drastic Change in Electronic Transition upon Hydrogen Bond Network Switching in 3-Aminopyridine-(H₂O) _n Clusters

The hydration structures of 3-aminopyridine (3AP)-(H₂O) _n ( n = 2-4) in supersonic jets have been investigated by measuring the electronic and vibrational spectra with the aid of quantum chemical calculations. The S₁-S₀ electronic transition of 3AP-(H₂O)₂ is observed at a slightly red-shifted position from 3AP-(H₂O)₁, while further hydration induces drastic red shifts and complicated vibrational structures. We assign the cluster structures of 3AP-(H₂O)₂ as a cyclic structure composed of the homodromic hydrogen bond (H-bond) chain connecting the pyridyl CH bond acting as the proton donor toward a pyridyl nitrogen acceptor. For 3AP-(H₂O) _n ( n = 3, 4), on the other hand, the initial donor site in the H-bond network changes from a pyridyl CH group to an amino group. The observed red shift derived from H-bond network switching can be reproduced very well with the S₁-S₀ origin band estimation obtained by applying geometry optimization and subsequent harmonic vibrational analysis of (TD-)DFT calculations to each electronic state of the isomer structure. It is suggested that the drastic red shift of the electronic transition upon H-bond network switching is due to a much more stabilized "quinoid-like" structure in the ππ* state by the H-bond formation of an amino group.