Photoionization-Induced Water Migration in the Hydratedtrans-Formanilide Cluster Cation Revealed by Gas-Phase Spectroscopy and Ab Initio Molecular Dynamics Simulation

Abstract

Photoionization-induced water migration in the trans-formanilide-water 1:1 cluster, FA-(H(2)O)(1), has been investigated by using IR-dip spectroscopy, quantum chemical calculations, and ab initio molecular dynamics simulations. In the S(0) state, FA-(H(2)O)(1) has two structural isomers, FA(NH)-(H(2)O)(1) and FA(CO)-(H(2)O)(1), where a water molecule is hydrogen-bonded (H-bonded) to the NH group and the CO group, respectively. In addition, the S(1)-S(0) origin transition of FA(CO)-(H(2)O)(2), where a water dimer is H-bonded to the CO group, was observed only in the [FA-(H(2)O)(1)](+) mass channel, indicating that one of the water molecules evaporates completely in the D(0) state. These results are consistent with a previous report [Robertson, E. G. Chem. Phys. Lett., 2000, 325, 299]. In the D(0) state, however, [FA-(H(2)O)(1)](+) produced by photoionization via the S(1)-S(0) origin transitions of FA(NH)-(H(2)O)(1) and FA(CO)-(H(2)O)(1) shows essentially the same IR spectra. Compared with the theoretical calculations, [FA-(H(2)O)(1)](+) can be assigned to [FA(NH)-(H(2)O)(1)](+). This means that the water molecule in [FA-(H(2)O)(1)](+) migrates from the CO group to the NH group when [FA-(H(2)O)(1)](+) is produced by photoionization of FA(CO)-(H(2)O)(1). [FA-(H(2)O)(1)](+) produced by photoionization of FA(CO)-(H(2)O)(2) also shows the IR spectrum corresponding to [FA(NH)-(H(2)O)(1)](+). In this case, the water migration from the CO group to the NH group occurs with the evaporation of a water molecule. Ab initio molecular dynamics simulations revealed the water migration pathway in [FA-(H(2)O)(1)](+). The calculations of classical electrostatic interactions show that charge-dipole interaction between FA(+) and H(2)O induces an initial structural change in [FA-(H(2)O)(1)](+). An exchange repulsion between the lone pairs of the CO group and H(2)O in [FA-(H(2)O)(1)](+) also affects the initial direction of the water migration. These two factors play important roles in determining the initial water migration pathway.

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

“…36,60 Surprisingly, no clusters in the size range n = 3 and n ≥ 5 have been identified in any of the molecular beam experiments. 36,60,62,63 Ionization into the 2 A″ ground electronic state (D 0 ) of FA + is accomplished by removal of a bonding π-electron from the HOMO, which is delocalized over the phenyl and amide groups. 68,71 As a consequence, the t-FA + and c-FA + rotamers are planar, and their energy difference increases by 302 ± 7 cm −1 .…”

“…59,60 Interestingly, clusters of FA with polar (L = H 2 O) and nonpolar ligands (L = Ar) have so far only been observed for the more stable t-FA rotamer. 36,37,40,41,60,62,63,69,70 Nonpolar ligands, such as Ar, prefer π-stacking to the phenyl ring of FA, because dispersion forces with the aromatic π-electron system dominate the attraction. 69 In contrast, for polar hydrophilic and protic ligands, such as H 2 O, the most attractive binding sites are offered by the amide group via H-bonding.…”

“…69 In contrast, for polar hydrophilic and protic ligands, such as H 2 O, the most attractive binding sites are offered by the amide group via H-bonding. 36,40,60,62,63,70 In the most stable t-FA−H 2 O(NH) isomer, H 2 O binds as proton acceptor to the acidic NH group with a binding energy of D 0 = 5.65 ± 0.30 kcal/mol (1976 ± 105 cm −1 ), whereas in the only marginally less stable t-FA−H 2 O(CO) isomer the H 2 O ligand binds as a proton donor to the CO group with D 0 = 5.40 ± 0.28 kcal/mol (1889 ± 98 cm −1 ), 40 as shown by their isomerselective IR spectra. 36,60 The barriers for isomerization have been determined in elegant spectroscopic experiments as 892 ± 96 cm −1 (CO → NH) and 869 ± 119 cm −1 (NH → CO).…”

“…70 Two isomers have been identified for t-FA−(H 2 O) 2 by IR−UV hole burning and UV rotational contour spectroscopy. 36,60,62 By comparison to calculations, their cyclic structures have been determined in which (H 2 O) 2 dimers are attached either to the NH or CO site of t-FA and close an intermolecular H-bonded solvent ring to either the aromatic π-electron system or the amide CH group, respectively. 36,60,62 The t-FA−(H 2 O) 4 cluster appears as a quite stable single isomer in the molecular beam, with a H-bonded (H 2 O) 4 chain bridging the gap between the NH and CO groups of t-FA.…”

“…36,60,62 The t-FA−(H 2 O) 4 cluster appears as a quite stable single isomer in the molecular beam, with a H-bonded (H 2 O) 4 chain bridging the gap between the NH and CO groups of t-FA. 36,60 Surprisingly, no clusters in the size range n = 3 and n ≥ 5 have been identified in any of the molecular beam experiments. 36,60,62,63 Ionization into the 2 A″ ground electronic state (D 0 ) of FA + is accomplished by removal of a bonding π-electron from the HOMO, which is delocalized over the phenyl and amide groups.…”