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

Abstract

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 S₀ 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 D₀ state of [FA–(H₂O)₄]⁺ is a minor product arising from photoionization via the S₁–S₀ 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.