Entropy-driven rearrangement of the water network at the hydrated amide group of the trans-formanilide-water cluster in the gas phase

Photoionization-induced rearrangement of the water network in the trans-formanilide 1 : 4 cluster, FA-(H2O)(4), has been investigated by using IR-photodissociation spectroscopy and quantum chemical calculations. The IR spectrum of FA-(H2O)(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]. However, the corresponding cyclic hydrogen-bonded structure in the D-0 state of [FA-(H2O)(4)](+) is a minor product arising from photoionization via the S-1-S-0 origin of FA-(H2O)(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 the water network in [FA-(H2O)(4)](+). Through DFT calculations, we conclude that the rearrangement occurs due to entropic effects.