Dangling OH Groups Research Articles

Conjoining the benefits of an additional phenyl ring in a simple benzophenone hydrazone-based platform @ discriminatory recognition of F− over CN− with quantitation of aqueous Cu2+: New tricks for an old dog

Herein, benzophenone hydrazone derived two π-conjugative networks viz.DMHMN and DMHMP have been judiciously designed for competitive recognition of F− (LOD: 6.8 ppm; Ka: 7.2 × 103 M−1) over CN− (LOD: ∼0.3 ppm; Ka: 20.3 × 104 M−1) along with Cu2+. Interestingly, the presence of an extra phenyl ring in DMHMN played a pivotal role for discriminative colorimetric recognition of F− over CN− despite having greater hydration energy of F−. Deliberate implantation of an extra phenyl ring leads to enhanced acidity of the benzophenone platform, rationalizing structure-performance synergies. While the presence of dangling OH group and azomethine functionality enables both the chemosensors to exhibit selective recognition toward Cu2+ in purely aqueous phase with the detection limit of ∼0.7 ppm and ∼0.16 ppm by DMHMP and DMHMN respectively, which are far below the safe limit set by World Health Organization (WHO). The sensing phenomenon by the redox non-innocent sensory receptors has been thoroughly investigated by UV–Vis, cyclic voltammetry and 1H NMR spectroscopic studies validated by density functional theory (DFT). Moreover, relay recognition of Al3+ by [DMHMN·F−] ensemble made it a suitable contender for complex logic circuitry fabrication like IMPLICATION/INHIBIT circuit. Additionally, the sensory probes have the capability of quantitative estimation of Cu2+ from unknown water samples, which was cross-examined with the help of the lab-based market available portable device. Lastly, smartphone assisted point-of-care testing (POCT) application has been performed for on-field detection of F− by DMHMN.

Impact of interfacial curvature on molecular properties of aqueous interfaces.

The curvature of soft interfaces plays a crucial role in determining their mechanical and thermodynamic properties, both at macroscopic and microscopic scales. In the case of air/water interfaces, particular attention has recently focused on water microdroplets, due to their distinctive chemical reactivity. However, the specific impact of curvature on the molecular properties of interfacial water and interfacial reactivity has so far remained elusive. Here, we use molecular dynamics simulations to determine the effect of curvature on a broad range of structural, dynamical, and thermodynamical properties of the interface. For a droplet, a flat interface, and a cavity, we successively examine the structure of the hydrogen-bond network and its relation to vibrational spectroscopy, the dynamics of water translation, rotation, and hydrogen-bond exchanges, and the thermodynamics of ion solvation and ion-pair dissociation. Our simulations show that curvature predominantly impacts the hydrogen-bond structure through the fraction of dangling OH groups and the dynamics of interfacial water molecules. In contrast, curvature has a limited effect on solvation and ion-pair dissociation thermodynamics. For water microdroplets, this suggests that the curvature alone cannot fully account for the distinctive reactivity measured in these systems, which are of great importance for catalysis and atmospheric chemistry.

Interfacial chemical reactivity enhancement.

Interfacial enhancements of chemical reaction equilibria and rates in liquid droplets are predicted using a combined theoretical and experimental analysis strategy. Self-consistent solutions of reaction and adsorption equilibria indicate that interfacial reactivity enhancement is driven primarily by the adsorption free energy of the product (or activated complex). Reactant surface activity has a smaller indirect influence on reactivity due to compensating reactant interfacial concentration and adsorption free energy changes, as well as adsorption-induced depletion of the droplet core. Experimental air-water interfacial adsorption free energies and critical micelle concentration correlations provide quantitative surface activity estimates as a function of molecular structure, predicting an increase in interfacial reactivity with increasing product size and decreasing product polarity, aromaticity, and charge (but less so for anions than cations). Reactions with small, neutral, or charged products are predicted to have little reactivity enhancement at an air-water interface unless the product is rendered sufficiently surface active by, for example, interactions with interfacial water dangling OH groups, charge transfer, or voltage fluctuations.

Theoretical Study of the Two-Dimensional Vibrational Sum Frequency Generation Spectroscopy of the Air-Water Interface at Varying Temperature and Its Connections to the Interfacial Structure and Dynamics.

We performed a theoretical study of the temperature variation of two-dimensional vibrational sum frequency generation (2D-VSFG) spectra of the OH stretch modes at air-water interfaces in the mid-IR region. The calculations are performed at four different temperatures from 250 to 325 K by using a combination of techniques involving response function formalism of nonlinear spectroscopy, electronic structure calculations, and molecular dynamics simulations. Also, the calculations are performed for isotopically dilute solutions so that the intra- and intermolecular coupling between the vibrational modes of interest can be ignored. We have established the connections of temperature variation of various frequency- and time-dependent features of the calculated spectra to the changes in the underlying structure and dynamics of the interfaces. The results reveal that interfacial water is dynamically more heterogeneous than bulk water, with three dominant dynamical processes exhibiting their corresponding time-dependent features in the 2D-VSFG spectrum. These are the spectral diffusion of hydrogen-bonded OH groups at the interface, conversion of an initially hydrogen-bonded OH group to a dangling OH which is a stable state for surface water, unlike the bulk water, and the third one, which involves the conversion of an initially free or dangling OH group to its hydrogen-bonded state at the interface. The temporal appearance of the cross peaks corresponding to interconversion of the hydrogen-bonded state to the dangling state or vice versa of an interfacial OH group is found to take place at a slower rate than the dynamics of spectral diffusion of hydrogen-bonded molecules at the interface, which, in turn, is slower than the corresponding spectral diffusion of bulk water molecules. The temperature variation of these dynamic processes can be linked to the decay of appropriate hydrogen-bond and non-hydrogen-bond time correlation functions of interfacial water molecules for the different air-water systems studied in this work.

Enhanced early‐stage adsorption of chemical warfare agent simulant by MIL‐68‐(X%OH)

AbstractThe development of porous adsorbents for removing hazardous chemicals is of paramount significance in mitigating the risks posed by these substances. Among these hazardous chemicals, chemical warfare agents (CWAs) are of primary concern due to their significant threat. Here, we report the development of porous metal–organic framework‐based adsorbents designed to efficiently adsorb CWA simulant, particularly during the early stages of exposure. We selected MIL‐68, which has a rigid Kagomé structure, as a model system for investigating the adsorption of 2‐chloroethyl ethyl sulfide (CEES). The introduction of a dangling hydroxyl (OH) group within the MIL‐68 framework significantly enhances CEES adsorption during the early stages of exposure to CEES vapors. However, it is crucial to note that an excessive presence of dangling OH groups can negatively impact adsorption ability. Therefore, it is imperative to carefully control the amount of dangling OH groups introduced into the MIL‐68 framework to optimize CEES adsorption performance.

On the Trail of Molecular Hydrophilicity and Hydrophobicity at Aqueous Interfaces

Uncovering microscopic hydrophilicity and hydrophobicity at heterogeneous aqueous interfaces is essential as it dictates physico/chemical properties such as wetting, the electrical double layer, and reactivity. Several molecular and spectroscopic descriptors were proposed, but a major limitation is the lack of connections between them. Here, we combine density functional theory-based MD simulations (DFT-MD) and SFG spectroscopy to explore how interfacial water responds in contact with self-assembled monolayers (SAM) of tunable hydrophilicity. We introduce a microscopic metric to track the transition from hydrophobic to hydrophilic interfaces. This metric combines the H/V descriptor, a structural descriptor based on the preferential orientation within the water network in the topmost binding interfacial layer (BIL) and spectroscopic fingerprints of H-bonded and dangling OH groups of water carried by BIL-resolved SFG spectra. This metric builds a bridge between molecular descriptors of hydrophilicity/hydrophobicity and spectroscopically measured quantities and provides a recipe to quantitatively or qualitatively interpret experimental SFG signals.

Interfacial Water Structure of Binary Liquid Mixtures Reflects Nonideal Behavior.

The evaporation of molecules from water–organic solute binary mixtures is key for both atmospheric and industrial processes such as aerosol formation and distillation. Deviations from ideal evaporation energetics can be assigned to intermolecular interactions in solution, yet evaporation occurs from the interface, and the poorly understood interfacial, rather than the bulk, structure of binary mixtures affects evaporation kinetics. Here we determine the interfacial structure of nonideal binary mixtures of water with methanol, ethanol, and formic acid, by combining surface-specific vibrational spectroscopy with molecular dynamics simulations. We find that the free, dangling OH groups at the interfaces of these differently behaving nonideal mixtures are essentially indistinguishable. In contrast, the ordering of hydrogen-bonded interfacial water molecules differs substantially at these three interfaces. Specifically, the interfacial water molecules become more disordered (ordered) in mixtures with methanol and ethanol (formic acid), showing higher (lower) vapor pressure than that predicted by Raoult’s law.

Wettability of graphene and interfacial water structure

<h2>Summary</h2> Understanding the wettability of graphene is important for various applications, such as water desalination, energy storage, and catalysis. However, the detailed water hydrogen-bonding structure at the water-graphene interface is not yet fully understood. Using vibrational sum-frequency-generation (VSFG) spectroscopy, we elucidate the water structure at a water-graphene interface. As the number of graphene layers increases, water molecules with dangling OH groups become more populated, indicating the transition from transparency to translucency. We compare the water contact angles on the graphene surfaces with VSFG results, showing an excellent correlation between the water adhesion energy of graphene and the fraction of dangling OH groups estimated from the VSFG spectrum. This observation suggests that the VSFG could be an incisive technique for measuring water's adhesion energy on any spatially confined interface where the water contact angle cannot be measured. We anticipate that the VSFG results will help to elucidate the wettability of low-dimensional materials. <h3>Video Abstract</h3>

Wettability of Graphene and Interfacial Water Structure

Understanding the graphene-water interaction, referred to as ‘wettability,’ is important for various applications, such as water desalination, filtration, energy storage, and catalysis. However, most studies on graphene's wettability have been performed with either macroscopic water contact angle measurements or molecular dynamics simulations. The detailed hydrogen-bonding network structure of water molecules at the graphene-water interface has not been fully understood at the molecular level. Here, using vibrational sum frequency generation (VSFG) spectroscopy, we elucidate the interfacial water structure and graphene hydrophobicity at a multilayer graphene-water interface. As the number of graphene layers increases, water molecules with dangling OH group become more populated. We compare the contact angles of water on the multilayer graphene surfaces with VSFG results. An excellent correlation between water adhesion energy of graphene and fraction of dangling OH groups estimated from the water OH stretch VSFG spectrum is established. This observation suggests that the VSFG could be an incisive technique for measuring water's adhesion energy on any spatially confined or blocked surface where the water contact angle cannot be measured. We further anticipate that the VSFG result on the transition from wetting transparency to translucency upon increasing the number of graphene layers will be used to understand the wettability of low-dimensional materials and the role of water structure on electric double layers of graphene-based electrodes.

Dehydrogenation of amine–boranes catalyzed by a [formula omitted] pincer iridium complex

Dibenzobarrelene-based PC s p 3 P pincer iridium complex bearing dangling CH2OH groups, ( ( PC s p 3 P CH 2 OH ) IrH ( Cl ) , catalyzes the dehydrogenation of amine–boranes at the reaction rate changing counter intuitively in the order: Me2NH·BH3 > > ButNH2·BH3 > NH3·BH3. The spectral (IR and NMR) data and DFT/M06 calculations have revealed that the binding of amine–boranes to the dangling OH group leads to an additional stabilization of the Ir···OH bond, thus hampering the dehydrogenation reaction, whereas the amine–borane coordination to iridium entails a fac- to mer-transformation of the complex and initiates the catalytic H2 evolution.

Impact of Electron Solvation on Ice Structures at the Molecular Scale.

Electron attachment and solvation at ice structures are well-known phenomena. The energy liberated in such events is commonly understood to cause temporary changes at such ice structures, but it may also trigger permanent modifications to a yet unknown extent. We determine the impact of electron solvation on D2O structures adsorbed on Cu(111) with low-temperature scanning tunneling microscopy, two-photon photoemission, and ab initio theory. Solvated electrons, generated by ultraviolet photons, lead not only to transient but also to permanent structural changes through the rearrangement of individual molecules. The persistent changes occur near sites with a high density of dangling OD groups that facilitate electron solvation. We conclude that energy dissipation during solvation triggers permanent molecular rearrangement via vibrational excitation.

Structure and stretching dynamics of water molecules around an amphiphilic amide from FPMD simulations: A case study of N,N-dimethylformamide

N,N-Dimethylformamide (DMF) is a unique tertiary amphiphilic amide, where the presence of a hydrophilic aldehyde group favors hydrogen bond acceptance, but two hydrophobic methyl substituents inhibit interaction with water molecules. As a result, the water molecules encounter two different environments in the vicinity of DMF molecule: around the hydrophobic nitrogen site; and near the hydrophilic carbonyl oxygen. We employ first principles molecular dynamics methods to simulate an aqueous solution of DMF using PBE functional with Grimme's D3 dispersion correction at 330 K. Investigations on the liquid structure to understand inter-atomic interactions in amides attract much attention. We calculated various structural and dynamical properties along with vibrational stretching frequency of water molecules to understand heterogeneously affected water molecules by an amphiphilic amide molecule. In solvated DMF, the first peak minimum of the N-OW and OC-OW radial distribution functions (‘w’ subscript denotes atoms of water) are located at 5.72 and 3.16 Å, respectively. These distance cutoffs decide the boundary of the solvation shell. At OC-HW distance 2.45 Å, the deep peak minimum indicates stable OC … HW hydrogen bond. Previous Monte Carlo simulations reported the presence of hydrogen bonds between the oxygen site of DMF and hydrogen of water. The time-series wavelet method was used to compute the time-dependent frequencies of the hydroxyl groups of water. The average frequency of the OH modes inside the CO solvation shell (~3364 cm−1) in DMF is higher than bulk (~3337 cm−1), and the trend matches with an aqueous solution of acetone. In nitrogen hydration shell, the intense band resembles the bulk frequency distribution, and a narrow distinctive peak at the high-frequency side (range ~3650–3750 cm−1) represents the non‑hydrogen bonded or dangling OH groups. Raman spectroscopy in hydrophobic TBA and air/water interface displayed dangling OH stretch peak at ~3660 cm−1 and ~3710 cm−1, respectively. Our calculation of the frequency distribution, frequency-frequency correlation function, and hydrogen bond dynamics show the water molecules at bulk behave as in pure water. Inside the solvation shell of aminic nitrogen, non‑hydrogen bonded OH modes with a dangling lifetime ~0.38 ps dominates over the water-water hydrogen bonds. The time-dependent decay of the frequency correlation inside CO solvation shell has three decay components, a rapid decay, then, an intermediate component (~1.52 ps) corresponding to the lifetime of the carbonyl-water hydrogen bond, and the longer timescale ~11.97 ps representing the residence time of water molecules in the vicinity of carbonyl oxygen. We find that the carbonyl-water hydrogen bond (OC … HW) is stronger than the water-water hydrogen bond. The methyl substituents on the nitrogen of DMF impose weak hydrophobic interaction, and the hydrophilic carbonyl oxygen forms a strong hydrogen bond with the neighboring water resulting in localized dynamics.

Aqueous hydroxyl group as the vibrational probe to access the hydrophobicity of amide derivatives

First principles molecular dynamics (FPMD) simulations of relatively dilute aqueous solutions of N-methylformamide (NMF), N, N-dimethylacetamide (DMA), N-methylacetamide (NMA) were carried out to elucidate the effects of variation of hydrophobicity of amide molecules on structure, dynamics and spectral properties of solvating water molecules. A quantitative analysis of dangling OH groups of water molecules, one of the consequences of hydrophobicity, around the amide molecules was performed to explore the structure of surrounding water molecules. We observe that DMA has the most hydrophobic character among the three amide molecules; the lifetime of dangling OH mode of water molecules and the number of such modes are found to be more inside the solvation shell of DMA as compared to NMA and NMF molecules. Overall this lifetime is more inside the solvation shell of amides compared to the bulk water molecules for all aqueous solutions of amides. Rotational dynamics calculation suggests significant retardation of OH bonds of water near the amide oxygen atom (O) due to the OwHw⋯O strong hydrogen bonds. A moderate slowdown of reorientational dynamics is also observed for OH modes that are close to the hydrophobic surface of DMA in its aqueous solution. Vibrational density of states (VDOS) and frequency distribution calculations point out the higher average stretching frequency (~3600–3850 cm−1) of free OH groups. Hydrogen bond lifetime calculations conceive that DMA, having three methyl groups, makes stronger hydrogen bonds through the CO moiety. Vibrational spectral diffusion of bulk water and solvation shell water molecules were also calculated in combination with wavelet transform and frequency-frequency autocorrelation functions. The CO group affected OH stretching frequency bands for aqueous solutions of NMF and NMA molecules are more pronounced than that of DMA. However, NH affected OH frequency bands are less pronounced for NMA and DMA due to the presence of more number of hydrophobic methyl groups. Going from NMF to DMA, the OH frequency bands inside the amide solvation shells shift towards the higher value due to the enhancement of hydrophobicity. The vibrational spectral diffusion of OH modes around CO and NH (N for DMA) groups, as well as for bulk water molecules, is also investigated. Three time scales were found for these calculations for all the cases. The fast time scales in the range ~50–100 fs are due to the amide-water intact hydrogen bonding, and two slower time scales in the range ~0.6–3.90 ps and ~10–20 ps were found. The time scales in the range of ~0.6 to 3.90 ps can be attributed to carbonyl-water and N-H-water hydrogen bonding, and the very long time scales are due to the escape dynamics water molecules from the solvation shell of CO and NH (N for DMA) groups.

Hydration-Shell Vibrational Spectroscopy.

Hydration-shell vibrational spectroscopy provides an experimental window into solute-induced water structure changes that mediate aqueous folding, binding, and self-assembly. Decomposition of measured Raman and infrared (IR) spectra of aqueous solutions using multivariate curve resolution (MCR) and related methods may be used to obtain solute-correlated spectra revealing solute-induced perturbations of water structure, such as changes in water hydrogen-bond strength, tetrahedral order, and the presence of dangling (non-hydrogen-bonded) OH groups. More generally, vibrational-MCR may be applied to both aqueous and nonaqueous solutions, including multicomponent mixtures, to quantify solvent-mediated interactions between oily, polar, and ionic solutes, in both dilute and crowded fluids. Combining vibrational-MCR with emerging theoretical modeling strategies promises synergetic advances in the predictive understanding of multiscale self-assembly processes of both biological and technological interest.

Intramolecular OH stretching analysis of hydrated lysozyme in presence of trehalose by IR spectroscopy

The present work reports the analysis of the intramolecular OH stretching band obtained by InfraRed spectroscopy measurements. In order to characterize the effect of trehalose on the hydration properties of lysozyme the so-called two-state model is adopted for the analysis of the intramolecular OH stretching band. This latter assumes that, provided that the trehalose OH stretching contribution is subtracted, water molecules can be partitioned into two different states of inter-molecular bonding: molecules with two OH groups both hydrogen-bonded within a tetrahedral network, and molecules with one or two dangling OH groups. What emerges from this study is that trehalose significantly influences the hydrogen bond network of water and its temperature behaviour. Such a result confirms that the trehalose induced strengthening of the hydrogen-bond network leads to a stabilization of the lysozyme structure. Moreover, the analysis of the spectra temperature dependence shows a trehalose-induced higher thermal restraint of the lysozyme-trehalose-water system in respect to the lysozyme-water mixture.

Heterogeneous Occupancy and Vibrational Dynamics of Spatially Patterned Water Molecules.

We performed first-principles molecular dynamics simulations of relatively dilute aqueous solutions of sulfate and thiosulfate dianions to analyze the structure, dynamics, and vibrational spectral properties of water molecules around the solute, especially the spatially patterned solvent molecules in the first solvation layer and the extended layers. This study also involves the investigation of dynamics of dangling OH groups in these layers and their role in patterning the water molecules around the dianions. Structural evaluation of the systems is carried out by radial distribution functions, number integrals, and spatial distribution functions. The lifetime of dangling OH groups inside the solvation shell is compared more to that of the bulk. By constructing the O-H groups in three ensembles (S1, S2, and S3) around the anion, we show that the frequency distribution of OH modes in the S1 ensemble show red-shifting for both sulfate and thiosulfate. The O-H groups in the S2 ensemble of the sulfate-water system show red-shifting by 10 cm-1, while in the case of thiosulfate-water, these O-H groups show blue-shifting by 8 cm-1. The water molecules in S1 and S2 subensembles have slower dynamics compared to those in the bulk (S3). The dynamics of various kinds of hydrogen bonds were characterized by hydrogen bond population correlation functions. The spectral diffusion of solvation shell O-H modes was performed through a frequency-time correlation function. We find a significant amount of orientational retardation of water molecules in the S1 layer and moderate retardation in the S2 layer as compared to that in the bulk, S3 layer. All these findings, the red shift of the OH stretching frequency in S1 and S2 layers, slowing down of the orientational dynamics of OH vectors in S1 and S2 layers, and less diffusivity of water in S1 and S2 layers, show the long-range kosmotropic effect of multivalent sulfate and thiosulfate oxyanions. Due to the long-range effect, heterogeneous occupancy of water molecules is observed, and the water molecules are found to arrange in a patterned manner in the vicinity of anions with varied local density.

Interaction of Water with the Gypsum (010) Surface: Structure and Dynamics from Nonlinear Vibrational Spectroscopy and Ab Initio Molecular Dynamics.

Water-mineral interfaces are important for several environmental, industrial, biological, and geological processes. Gypsum, CaSO4·2H2O, is a widespread mineral of high technological, medical, and environmental relevance, but little is known about its surface structure and its interaction with water. A molecular-level understanding of gypsum/water interface is given here by a combined experimental/theoretical study. We investigate the structure and dynamics of water adsorbed from vapor on the gypsum (010) single-crystal surface at room temperature, combining sum-frequency generation (SFG) vibrational spectroscopy experiments and ab initio molecular dynamics (AIMD) simulations. The SFG spectra of gypsum at low relative humidity (RH) show an anisotropic arrangement of structural water molecules and the presence of dangling OH groups. The AIMD simulations allow a detailed assignment of the SFG spectra and show that the cleaved (010) surface rearranges to have only 25% of the OH groups pointing away from the surface. At higher RHs, the first adsorbed water layer binds to these OH groups and forms an anisotropic arrangement, but with the amount of free OH groups significantly suppressed and without any significant diffusion. Upon adsorption of a second water layer, although the topmost layer of molecules is more disordered and dynamic than the previous one, its structure is still influenced by the gypsum surface underneath because it has a much reduced amount of free OH groups with respect to the free surface of water, and a slower surface diffusion with respect to bulk water. The theoretical results corroborate the experimental ones and provide an accurate atomic characterization of the surface structure.

Adsorption of Methylene Fluoride and Methylene Chloride at the Surface of Ice under Tropospheric Conditions: A Grand Canonical Monte Carlo Simulation Study

The adsorption of two halogenated methane derivatives, namely, methylene fluoride and methylene chloride, at the surface of Ih ice is studied by grand canonical Monte Carlo simulations under tropospheric conditions. The adsorption isotherms of the two molecules, differing only in the halogen atom type, are found to be markedly different from each other. Thus, while methylene fluoride exhibits multilayer adsorption and its adsorption isotherm belongs to class II according to the IUPAC convention, methylene chloride does not show considerable adsorption at the ice surface, as its condensation well precedes the saturation of even the first adsorbed molecular layer. Interestingly, both the surface orientation and the binding energy of the two types of adsorbed molecules are rather similar to each other; first layer molecules form one single hydrogen bond with the dangling OH groups of the ice surface. The strong differences in the adsorption behavior of methylene fluoride and methylene chloride are traced bac...

Structure and Dynamics of Water Dangling OH Bonds in Hydrophobic Hydration Shells. Comparison of Simulation and Experiment

Molecular dynamics and electric field strength simulations are performed in order to quantify the structural, dynamic, and vibrational properties of non-H-bonded (dangling) OH groups in the hydration shell of neopentane, as well as in bulk water. The results are found to be in good agreement with the experimentally observed high-frequency (∼3660 cm(-1)) OH band arising from the hydration shell of neopentanol dissolved in HOD/D(2)O, obtained by analyzing variable concentration Raman spectra using multivariate curve resolution (Raman-MCR). The simulation results further indicate that hydration shell dangling OH groups preferentially point toward the central carbon atom of neopentane to a degree that increases with the lifetime of the dangling OH.

Protruding interfacial OH groups and ‘on-water’ heterogeneous catalysis

The key aspect of the remarkable organic catalysis that is observed to occur at theorganic/water phase boundary, the so-called ‘on-water’ catalysis (Narayan et al 2005Angew. Chem. 44 3275), was recently proposed to be the protruding OH groups of watermolecules at the interface that interact with the transition state (TS) via hydrogenbonding and lower activation barriers (Jung and Marcus 2007 J. Am. Chem. Soc. 1295492). In particular, the cycloaddition reaction of quadricyclane (Q) with dimethylazodicarboxylate (DMAD) on-water was calculated to be more than 100 000 timesmore efficient in terms of rate constant than the neat reaction. In this paper, wereview and consider a related reaction of Q with dimethyl acetylenedicarboxylate,where nitrogen, a good H-bond acceptor, in DMAD is replaced by carbon, a poorH-bond acceptor. A very low rate acceleration of acetylenedicarboxylate on-waterrelative to the neat reaction is obtained theoretically, as compared to DMADon-water, due to the relatively low H-bonding ability of acetylenedicarboxylate withwater at the TS relative to the reactants. We suggest that there may also bean ‘intrinsic steric effect’ or orientational advantage in the on-water catalysisin general, and both electronic and steric effects may be in operation for thesmaller on-water catalysis for the cycloaddition reaction of quadricyclane andacetylenedicarboxylate. A preliminary quantum mechanical/molecular mechanical(QM/MM) simulation including 1264 water molecules for the on-water reaction ofDMAD + Q also suggests that there are indeed approximately two–four more H-bonds between theTS and the dangling OH groups than between the reactants and the surface.