Anion Binding of One-, Two-, and Three-Armed Thiourea Receptors Examined via Photoelectron Spectroscopy and Quantum Computations

Abstract

A benzene ring substituted with 1-3 thiourea containing arms (1-3) were examined by photoelectron spectroscopy and density functional theory computations. Their conjugate bases and chloride, acetate and dihydrogen phosphate anion clusters are reported. The resulting vertical and adiabatic detachment energies span from 3.93 - 5.82 eV (VDE) and 3.65 - 5.10 (ADE) for the deprotonated species and 4.88 - 5.97 eV (VDE) and 4.45 - 5.60 eV (ADE) for the anion complexes. These results reveal the stabilizing effects of multiple hydrogen bonds and anionic host-guest interactions in the gas phase. Previously measured equilibrium binding constants in aqueous dimethyl sulfoxide for all three thioureas are compared to the present results and cooperative binding is uniformly observed in the gas phase but only for one case (i.e., 3 • H2PO4-) in solution.

Formation of (HCOO–)(H2SO4) Anion Clusters: Violation of Gas-Phase Acidity Predictions

“…Recently more examples have been reported, but this time affecting the anionic complexes. One of these cases involves thiourea/acetic acid complexes, which exhibit deprotonation of acetic acid despite the unfavorable differences in PA values. It was suggested that the resulting complex gains additional stability due to creation of additional intermolecular hydrogen bonds.…”

Spectroscopic Signature of Proton Location in Proton Bound HSO4–·H+·X– (X = F, Cl, Br, and I) Clusters

“…Gas-phase proton affinity (PA), a thermodynamic quantity measuring the proton acceptance ability of an isolated base irrelevant to the surrounding environment, seems at first to provide a more appropriate criteria than acidity-basicity for predicting the preferential proton locations. , Indeed, considerable experimental evidence has been obtained in accordance with PA predictions. − However, such an approach has been questioned in several studies of cationic systems, − and recently also in anionic proton bound clusters. − Those studies suggested that when the partner molecules are in close contact and experience strong mutual interactions, individual PA values will not convey a correct picture. We have recently shown that in an HSO 4 – ·H + ·HCOO – cluster, even the PA of HCOO – is about 36 kcal/mol larger than that of HSO 4 – , the cluster will still preferably adopt the form of H 2 SO 4 ·HCOO – , due to the resulting substantial electron delocalization and formation of two strong hydrogen bonds .…”

Thiourea‐Based Tritopic Halogen‐Bonding Acceptors

“…Thiourea based tritopic assemblies have shown a lot of promise, for example, as an ionophores transferring ions and small molecules across hydrophobic membrane [1] . Incompletely N ‐substituted thiourea units offer interesting capabilities to these molecules acting as anion receptors due to their ability to form hydrogen bonds via NH‐groups [2–4] . Previous studies have shown that thiourea moieties are rather rigid and well suited to form halogen bonds, which offers possibilities to these molecules to form larger assemblies trough intermolecular weak interactions [5–7] .…”