Photo-ionization induced rearrangement of a water network in trans-formanilide 1:4 cluster, FA-(H 2 O) 4 , has been investigated by using IR-photodissociation spectroscopy and quantum chemical calculations. The IR spectrum of FA-(H 2 O) 4 in the S 0 state shows that the observed cluster has a cyclic hydrogen-bonded structure where the CO group and the NH group of FA are bridged with four water molecules, consistent with the reported structure [E. G. Robertson, Chem. Phys. Lett. 2000, 325, 299].

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... However, the corresponding cyclic hydrogen-bonded structure in the D 0 state of [FA-(H 2 O) 4 ] + is a minor product arising from photoionization via the S 1 -S 0 origin of FA-(H 2 O) 4 . The dominant product has an extended H-bonded structure, where the intermolecular hydrogen bond between the hydrogen of the OH group of a water molecule and the CO group is dissociated. This is the first observation of a photoionization-induced rearrangement of water network in [FA-(H 2 O) 4 ] + . Through DFT calculations, we conclude that the rearrangement occurs due to the entropic effects.