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-(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]. 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 the water network in [FA-(H(2)O)(4)](+). Through DFT calculations, we conclude that the rearrangement occurs due to entropic effects.

Stepwise Microhydration of Aromatic Amide Cations: Formation of Water Solvation Network Revealed by Infrared Spectra of Formanilide+–(H2O)n Clusters (n ≤ 5)

“…In contrast to n = 1 and 2, Ar tagging has a strong impact on the appearance of the IRPD spectrum of FA + −(H 2 O) 3 4 , a previous study predicted six isomers within 350 cm −1 above the most stable one, 37 all having a structure in which a H-bonded water network is attached to the NH group. These structures are also calculated here at the ωB97X-D level (Figure 8), using the same notation (I−VI).…”

“…Interestingly, the energetic order of the six considered isomers changes drastically depending on whether one compares the energies (E 0 ) or the free energies evaluated at 298 K (G 0 ). 37 The relative energies of isomers I−VI are +47, +249, +370, 0, +133, and +697 cm −1 with respect to isomer IV, while the corresponding relative free energies are −637, −687, −503, 0, −443, +100 cm −1 . Hence, while at T = 0 K isomer IV is clearly the most stable structure, at elevated temperature it becomes quite unfavorable with respect to the isomers I−III and V. Because the H-bonded network and their connection to the amide is quite different for the isomers, their IR spectra predicted in the N−H and O−H stretch ranges are different, too (Figure 9).…”

Investigation of the hydrated 7-hydroxy-4-methylcoumarin dimer by combined IR/UV spectroscopy

“…[45][46][47][48][49] Among these investigations rearrangement reactions within the ions have been investigated both with ns lasers 51-58 as well as time-resovled studies using ps lasers. [59][60][61] These investigations describe a migration of argon or molecules such as water, methanol, and methane in clusters with small aromatic molecules such as formanilide, 55 acetanilide, 60 4-aminobenzonitrile, 58,61 phenol, 57, 59 aminophenol, 52, 53 benzene, 51 tryptamine, 56 or phenylglcyine. 54 To the best of our knowledge no investigations on neutral or ionic 7H4MC under solvent free, isolated conditions have been performed.…”

Watching Water Migration around a Peptide Bond

“…These severe deficiencies urgently call for the development and application of more sophisticated theoretical approaches and emphasize the importance of the current experimental results. Experimentally, the next step includes the characterization of microhydrated AA‐(H 2 O) n clusters to probe the dynamics of a larger hydration network around proteins at the molecular level,6b as well as an extension to polypeptides.…”