Twisted Eigen Can Induce Proton Transfer at a Hydrophobic–Hydrophilic Interface

Abstract

The investigation of proton localization at a hydrophobic− hydrophilic interface is an important problem in chemical and materials sciences. In this study, protonated benzene (i.e., benzenium ion) and water clusters [BZH + W n (where n = 1−6)] are selected as prototype models to understand the interfacial interactions and proton transfer mechanism between a carbonaceous surface and water molecules. The excess protons can localize in the vicinity of the hydrophobic−hydrophilic interface, and these clusters are stabilized by various kinds of noncovalent interactions. Calculations are carried out using ab initio (MP2) and density functional theory B3LYP methods to shed more light on geometries, energetics, and spectral signatures of the protonated species [H + (H 2 O) n ] at the interfaces. These calculations revealed few low-lying isomers, which have not been reported earlier. Scrutiny of the results reveals that proton localization in the hydrophilic environment is more stable than the hydrophobic benzene πcloud. Furthermore, the occurrence of an O−H + •••π hydrogen bond significantly influences the O−H + •••O interactions in the water clusters and also intensively affects the vibrational modes of the Eigen cation. Thus, the aromatic π-clouds can stabilize the Eigen cation and at the same time, a twisted form of Eigen (one O−H + •••π → two O−H + •••π) can enhance the proton transfer through the water chain via a Grotthuss-type mechanism. The vibrational spectra of these clusters reveal that there is a large red-shifted frequency for the O−H + •••O, O−H + •••π, and O−H•••π modes of interaction. The energetic values and vibrational frequencies obtained from the B3LYP method are in close agreement with the MP2 level and experimental values, respectively.

Formation of Eigen or Zundel Features at Protonated Water Cluster–Aromatic Interfaces

“…For Bz-H + W 3 , we find nevertheless a cluster where the water molecule interacts with the π-cloud of Bz instead of H 3 O + interacting with this π-cloud as suggested earlier by Duncan and co-workers. As discussed by Maiyelvaganan et al, [21] the cluster found by Duncan and co-workers is less stable than the one we give here by ~6 kcal/mol as established at the CCSD(T)/CBS level. In the following we will consider the most stable one as displayed in Figure 2.…”

“…[10][11][12][13][14][15] Among them, H-bonding and charge induced interactions in protonated water clusters are ubiquitous in nature. [16][17][18][19][20][21] Nevertheless, the experimental determination of the structure and the stability of such complexes is very difficult and still challenging. [22] Instead, advanced computational techniques can provide reliable qualitative and quantitative insights on the structure, the rotational, vibrational, and electronic spectra of these molecular complexes, complementary to experimental works.…”

“…[26][27][28] Recent studies revealed that the excess proton is always stabilized in the aqueous medium and the stability of the clusters directly depends on the formation of an Eigen ion within the water network. [19,21] The full in-depth investigation, at the microscopic level, of protonated species at carbon surface material allows to understand the binding mechanism at this solid-liquid model interfaces.…”

“…The latter are classified as conventional and unconventional interactions [9] . Hydrogen bonding (H‐bonding), halogen bonding, charge induced interactions belong to the conventional type, whereas O−H⋅⋅⋅π, O−H + ⋅⋅⋅π, N−H⋅⋅⋅π, C−H⋅⋅⋅π, cation⋅⋅⋅π, anion⋅⋅⋅π, and weakly bound interactions such as C−H⋅⋅⋅O, lp⋅⋅⋅π, π⋅⋅⋅π and van der Waals (vdW) and dispersion interactions are considered as unconventional [10–15] . Among them, H‐bonding and charge induced interactions in protonated water clusters are ubiquitous in nature [16–21] .…”

A Deep Understanding on the Effective Generation of Twisted Intramolecular Charge Transfer by Protonation in Thiazolo[5,4-d]thiazole Derivatives

“…As one of the most concerned proton transfer processes, the excited-state intramolecular proton transfer (ESIPT), can present a series of unique photochemical properties through the significant phototautomerizations induced by the excitation-strengthened hydrogen bond. − It has been demonstrated that the synergistic effect of ESIPT and TICT could further efficiently stabilize the torsional configuration by reducing the molecule energy accompanied by the dissociation of the π conjugation and nearly complete charge transfer between two fragments connecting the rotation bond, thus further ensuring the preferential and efficient charge-transfer and separation. − This distinct characteristic of the ESIPT triggered TICT process has been typically investigated in a series of functionalized organic materials, such as photostable fluorophores, photosensitizers, and aggregation induced emission luminogens (AIEgens). − For instance, by regulating the polarity of the solvent, the planar local excited state of the benzotriazole based ultraviolet absorber could be preferentially stabilized by generating a highly polar twisted charger transfer state through ESIPT triggered TICT process, resulting in the enhancement of the photostability of the ultraviolet absorbers . However, although it has been extensively studied on the final ESIPT triggered TICT on simple fluorescent molecules, such as coumarin, benzotriazole, benzimidazole, and benzophenones, − it is still a challenging task to extend this strategy in the π-conjugated heterocycling molecules consisting of multiple proton acceptors, which is of crucial significance in promoting the performance of semiconductive organic materials.…”