Infrared Spectroscopy of Phenol−(H2O)n>10: Structural Strains in Hydrogen Bond Networks of Neutral Water Clusters

Spectroscopic and Theoretical Identifications of Two Structural Motifs of (H2O)10 Cluster

Precise characterization of archetypal systems of aqueous hydrogen-bonding networks is essential for developing accurate potential functions and universal models of water. The structures of water clusters (H2O)n (n = 2-9) have been verified recently through size-specific infrared spectroscopy with a vacuum ultraviolet free electron laser (VUV-FEL) and quantum chemical studies. For (H2O)10, the pentagonal prism and butterfly motifs were proposed to be important building blocks and were observed in previous experiments. Here we report the size-specific infrared spectra of (H2O)10 via a joint experimental and theoretical study. Well-resolved spectra provide a unique signature for the coexistence of pentagonal prism and butterfly motifs. These (H2O)10 motifs develop from the dominant structures of (H2O)n (n = 8, 9) clusters. This work provides an intriguing prelude to the diverse structure of liquid water and opens avenues for size-dependent measurement of larger systems to understand the stepwise formation mechanism of hydrogen-bonding networks.

Intra-cluster Charge Migration upon Hydration of Protonated Formic Acid Revealed by Anharmonic Analysis of Cold Ion Vibrational Spectra

The rates of many chemical reactions are accelerated when carried out in micron-sized droplets, but the molecular origin of the rate acceleration remains unclear. One example is the condensation reaction of 1,2-diaminobenzene with formic acid to yield benzimidazole. The observed rate enhancements have been rationalized by invoking enhanced acidity at the surface of methanol solvent droplets with low water content to enable protonation of formic acid to generate a cationic species (protonated formic acid or PFA) formed by attachment of a proton to the neutral acid. Because PFA is the key feature in this reaction mechanism, vibrational spectra of cryogenically cooled, microhydrated PFA·(H2O)n=1-6 were acquired to determine how the extent of charge localization depends on the degree of hydration. Analysis of these highly anharmonic spectra with path integral ab initio molecular dynamics simulations reveals the gradual displacement of the excess proton onto the water network in the microhydration regime at low temperatures with n = 3 as the tipping point for intra-cluster proton transfer.

Hydration dynamics and IR spectroscopy of 4-fluorophenol

Halogenated groups are relevant in pharmaceutical applications and potentially useful spectroscopic probes for infrared spectroscopy. In this work, the structural dynamics and infrared spectroscopy of para-fluorophenol (F-PhOH) and phenol (PhOH) is investigated in the gas phase and in water using a combination of experiment and molecular dynamics (MD) simulations. The gas phase and solvent dynamics around F-PhOH and PhOH is characterized from atomistic simulations using empirical energy functions with point charges or multipoles for the electrostatics, Machine Learning (ML) based parametrizations and with full ab initio (QM) and mixed Quantum Mechanical/Molecular Mechanics (QM/MM) simulations with a particular focus on the CF- and OH-stretch region. The CF-stretch band is heavily mixed with other modes whereas the OH-stretch in solution displays a characteristic high-frequency peak around 3600 cm-1 most likely associated with the -OH group of PhOH and F-PhOH together with a characteristic progression below 3000 cm-1 due to coupling with water modes which is also reproduced by several of the simulations. Solvent and radial distribution functions indicate that the CF-site is largely hydrophobic except for simulations using point charges which renders them unsuited for correctly describing hydration and dynamics around fluorinated sites. The hydrophobic character of the CF-group is particularly relevant for applications in pharmaceutical chemistry with a focus on local hydration and interaction with the surrounding protein.

Verification of Microwave Effects on Molecular Clusters by Using Supersonic Molecular Jets

The origin of the specific effect of microwaves on chemical reactions (the microwave effect) was investigated by examining the effect of microwaves on small groups of molecules such as clusters. The origin of the effect was verified by introducing 2.45 GHz microwaves into a system equipped with a supersonic molecular jet and a special microwave feedthrough to record the fluorescence excitation spectrum of molecules. The carrier gas was bubbled through water and introduced into a phenol-filled sample holder to generate phenol-water clusters. Subsequently, it was confirmed that exposure of the phenol-water clusters contained in the molecular jet ejected from the pulse valve to microwave radiation increased the fluorescence derived from the phenol monomer. This is considered to occur because the phenol-water clusters in the molecular jet absorb microwaves and collapse, thereby increasing the abundance of phenol monomers. This result suggests that microwaves affect not only bulk systems but also small groups of molecules, and that local selective heating, which is one of the causes of the microwave effect, may occur.

Cooperativity of the Activated CH/π Interaction Probed through CH Stretching Vibrations in Phenol-(Acetylene)n (∼16 ≤ n ≤ ∼30) and (Acetylene)n+ (10 ≤ n ≤ 70) Clusters

The acidity of acetylene CH is stronger than that of alkane CH, and the attractive interaction between an acetylene CH with π-electrons, which shows a clear hydrogen bond property, is called activated CH/π interaction. In this study, cooperative enhancement of the activated CH/π interaction has been probed through the cluster size dependence of the red shift of the acetylene CH stretching vibrational band in neutral phenol-(acetylene)_n (∼16 ≤ n ≤ ∼30) and (acetylene)_n⁺ (10 ≤ n ≤ 70). In both the clusters, the characteristic asymmetric (red-shaded) shape of the CH stretch band has been observed. This band shape means that the magnitude of the activated CH/π interaction is enhanced by its cooperativity in the interior moiety of the cluster. The red-shifted component of the band extends with increasing cluster size, and the edge of this component seems to reach to the CH stretch band position of crystalline acetylene at the size of n = 20-30, indicating that dozens of molecules need to interact each other to maximize cooperativity in the activated CH/π interaction of acetylene. On the other hand, the peak position of the band does not converge to that of crystalline acetylene in the observed size range. The present result suggests that the spectral convergence of acetylene clusters to the bulk may occur in the cluster size range of hundreds or larger.

Reasonable design and sifting of microporous carbon nanosphere-based surface molecularly imprinted polymer for selective removal of phenol from wastewater

Highly selective surface molecularly imprinted polymer (SMIP) was prepared on glucose-derived microporous carbon nanospheres (GMCNs) by surface molecular imprinting technology for the removal of phenol from wastewater. GMCNs with rich pore structure and surface oxygenic functional groups were adopted as support materials, on which the active layers were constructed by grafting silane coupling agent 3-(methacryloyloxy) propyltrimethoxysilane. Then with phenol as template molecule, different types and amounts of functional monomer (including methacrylic acid and 4-vinylpyridine (4-VP)) were screened for optimizing imprinting conditions suitable for phenol adsorption, and a series of SMIP was obtained through crosslinking polymerization. The adsorption behaviors of SMIP were evaluated by UV spectrophotometry. The results show that, when 4-VP is used as functional monomer, the resultant 4-VP/SMIP exhibites an excellent adsorption capacity of 85.72 mg g^-1. The relative selectivity factor for phenol against hydroquinone, p-nitrophenol and p-tert-butylphenol is 8.38, 7.96 and 6.67, respectively, indicating outstanding adsorption capacity and selectivity of 4-VP/SMIP. The pseudo-second-order model and Langmuir‒Freundlich model fit better than other models for the adsorption of phenol. 4-VP/SMIP is promising for selective removal and enrichment recovery towards phenol in wastewater.

Variation of optical spectra of water clusters with size from many-body Green's function theory

Water clusters are an important species in the environment and atmosphere and take part in various chemical and biological reactions. How their optical properties vary with size is still an open question. Using the GW method and Bethe-Salpeter equation within the ab initio many-body Green's function theory, we study the electronic excitations in a series of water clusters (H₂O)_n with n = 1-48. We find that their absorption peaks blueshift with increasing cluster size due to the reducing electron-hole binding energy which arises from the enhanced electronic screening and gradually delocalized excitonic spatial distribution. The position of the first absorption peak has a close relation to the average number of hydrogen bonds per molecule. Off-diagonal matrix elements of the self-energy operator have pronounced effects on the unoccupied electronic levels and optical absorption for small clusters with n ≤ 10 when using density functional theory as the starting point for GW calculations. Although the optical absorption is predominated by delocalized excitons, highly localized excitons on a single water molecule are always present on the cluster surface in the vicinity of the absorption edge. These localized excitons may facilitate the photodissociation of water molecules. This can provide inspiration on the excited-state dynamics and photolysis in water clusters.

Hydrogen bond cooperativity and anticooperativity within the water hexamer

We propose a hierarchy of H-bond strength in terms of the single and double character of the involved donor and acceptors within different structures of (H₂O)₆.

An infrared spectroscopic and theoretical study on (CH3)3N–H+–(H2O)n, n = 1–22: highly polarized hydrogen bond networks of hydrated clusters

Highly polarized water networks are found in the micro hydaration of protonated trimethylamine.

Structural evolution and solvation of the OH radical in ionized water radical cations (H2O)n+, n = 5–8

Structural evolution of ionized water radical cations (H₂O)_n⁺, n = 5–8, is studied by ab intio methods.

Infrared spectroscopy of large-sized neutral and protonated ammonia clusters

Size selective IR spectroscopy shows the nature of hydrogen bond networks in neutral and protonated ammonia clusters.

Characterization of a solvent-separated ion-radical pair in cationized water networks: infrared photodissociation and Ar-attachment experiments for water cluster radical cations (H2O)n+(n = 3-8)

We present infrared spectra of nominal water cluster radical cations (H(2)O)(n)(+) (n = 3-8), or to be precise, ion-radical complexes H(+)(H(2)O)(n-1)(OH), with and without an Ar tag. These clusters are closely related to the ionizing radiation-induced processes in water and are a good model to characterize solvation structures of the ion-radical pair. The spectra of Ar-tagged species show narrower bandwidths relative to those of the bare clusters due to the reduced internal energy via an Ar-attachment. The observed spectra are analyzed by comparing with those of the similar system, H(+)(H(2)O)(n), and calculated ones. We find that the observed spectra are attributable to ion-radical-separated motifs in n ≥ 5, as reported in the previous study (Mizuse et al. Chem. Sci.2011, 2, 868-876). Beyond the structural trends found in the previous study, we characterize isomeric structures and determine the number of water molecules between the charged site and the OH radical in each cluster size. In all of the characterized cluster structures (n = 5-8), the most favorable position of OH radical is the next neighbor of the charged site (H(3)O(+) or H(5)O(2)(+)). The positions and cluster structures are governed by the balance among the hydrogen-bonding abilities of the charged site, H(2)O, and OH radical. These findings on the ionized water networks lead to understanding of the detailed processes of ionizing radiation-initiated reactions in liquid water and aqueous solutions.

Structures and dissociation channels of protonated mixed clusters around a small magic number: infrared spectroscopy of ((CH3)3N)n-H(+)-H2O (n = 1-3)

The magic number behavior of ((CH(3))(3)N)(n)-H(+)-H(2)O clusters at n = 3 is investigated by applying infrared spectroscopy to the clusters of n = 1-3. Structures of these clusters are determined in conjunction with density functional theory calculations. Dissociation channels upon infrared excitation are also measured, and their correlation with the cluster structures is examined. It is demonstrated that the magic number cluster has a closed-shell structure, in which the water moiety is surrounded by three (CH(3))(3)N molecules. The ion core (protonated site) of the clusters is found to be (CH(3))(3)NH(+) for n = 1-3, but coexistence of an isomer of the H(3)O(+) ion core cannot be ruled out for n = 3. Large rearrangement of the cluster structures of n = 2 and 3 before dissociation, which has been suggested in the mass spectrometric studies, is confirmed on the basis of the structure determination by infrared spectroscopy.

Zn2+ has a primary hydration sphere of five: IR action spectroscopy and theoretical studies of hydrated Zn2+ complexes in the gas phase

Complexes of Zn(2+)(H(2)O)(n), where n = 6-12, are examined using infrared photodissociation (IRPD) spectroscopy, blackbody infrared radiative dissociation (BIRD), and theory. Geometry optimizations and frequency calculations are performed at the B3LYP/6-311+G(d,p) level along with single point energy calculations for relative energetics at the B3LYP, B3P86, and MP2(full) levels with a 6-311+G(2d,2p) basis set. The IRPD spectrum of Zn(2+)(H(2)O)(8) is most consistent with the calculated spectrum of the five-coordinate MP2(full) ground-state (GS) species. Results from larger complexes also point toward a coordination number of five, although contributions from six-coordinate species cannot be ruled out. For n = 6 and 7, comparisons of the individual IRPD spectra with calculated spectra are less conclusive. However, in combination with the BIRD and laser photodissociation kinetics as well as a comparison to hydrated Cu(2+) and Ca(2+), the presence of five-coordinate species with some contribution from six-coordinate species seems likely. Additionally, the BIRD rate constants show that Zn(2+)(H(2)O)(6) and Zn(2+)(H(2)O)(7) complexes are less stable than Zn(2+)(H(2)O)(8). This trend is consistent with previous work that demonstrates the enthalpic favorability of the charge separation process forming singly charged hydrated metal hydroxide and protonated water complexes versus loss of a water molecule for complexes of n ≤ 7. Overall, these results are most consistent with the lowest-energy structures calculated at the MP2(full) level of theory and disagree with those calculated at B3LYP and B3P86 levels.