Anne Milet

Water surface is acidic

Water autoionization reaction 2H2O → H3O− + OH− is a textbook process of basic importance, resulting in pH = 7 for pure water. However, pH of pure water surface is shown to be significantly lower, the reduction being caused by proton stabilization at the surface. The evidence presented here includes ab initio and classical molecular dynamics simulations of water slabs with solvated H3O+ and OH− ions, density functional studies of (H2O)48H+ clusters, and spectroscopic isotopic-exchange data for D2O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H3O+ does, but OH− does not, display preference for surface sites, the H2O surface is predicted to be acidic with pH < 4.8. For similar reasons, the strength of some weak acids, such as carbonic acid, is expected to increase at the surface. Enhanced surface acidity can have a significant impact on aqueous surface chemistry, e.g., in the atmosphere.

Anisotropic intermolecular interactions in van der Waals and hydrogen-bonded complexes: What can we get from density functional calculations?

The applicability of various density functional theory (DFT) methods to describe the anisotropy of the intermolecular potential energy surfaces of hydrogen-bonded [OH−–H2O, (H2O)2] and van der Waals [CO–H2O, He–CO2] complexes has been tested by comparison with supermolecule CCSD(T) (coupled-cluster method restricted to single, double, and noniterative triple excitations) and perturbational SAPT (symmetry-adapted perturbation theory) results computed for the same geometries and with the same basis sets. It is shown that for strongly bound ionic hydrogen-bonded complexes, like OH−–H2O, hybrid approaches provide accurate results. For other systems, including the water dimer, the DFT calculations fail to reproduce the correct angular dependence of the potential surfaces. It is also shown that a hybrid functional adjusted to reproduce the CCSD(T) value of the binding energy for the water dimer produces results worse than the standard hybrid functionals for OH−–H2O, and fails to describe the correct anisotropy ...

Theoretical study of the protolytic dissociation of HCl in water clusters

Reaction mechanisms for the acidic dissociation of HCl in water clusters are considered. Intermediates in the reaction are obtained from stationary points on the potential energy surface of the systems HCl–(H2O)n with n=4 and 5. These points have been determined by the B3LYP density functional method in an aug-cc-pVDZ atomic orbital (AO) basis. The total energies of the stationary points are checked by the coupled cluster single-double-triple [CCSD(T)] method in the same AO basis. For the case of n=4 a multibody analysis of the interaction energies is performed by the CCSD(T) method as well as by symmetry adapted perturbation theory. The clusters have a completely dissociated form as their energetically lowest minimum.

Viologen-based redox-switchable anion-binding receptors

A series of viologen-based receptors have been synthesized and their anion-binding properties have been investigated by-NMR, UV-visible spectroscopy, electrochemistry and X-ray diffraction analyses. Linking two positively charged viologens through a propyl chain promotes a remarkable chelate-like binding of chlorides revealed by 1H-NMR spectroscopy. Of all the anionic species investigated, only fluoride is detectable by the naked eye and by electrochemical methods. The reduction-triggered formation of a π-dimer from two viologen-based cation radicals was also investigated by electrochemical and spectroelectrochemical methods and by theoretical calculations.

Theoretical study of the OH−(H2O)2 system: Nature and importance of three-body interactions

The nature and importance of nonadditive three-body interactions in the ionic OH−(H2O)2 cluster have been studied by supermolecule Mo/ller–Plesset (MP) perturbation theory and coupled-cluster method, and by symmetry-adapted perturbation theory (SAPT). The convergence of the SAPT expansion was tested by comparison with the results obtained from the supermolecule Mo/ller–Plesset perturbation theory calculations through the fourth order (MP2, MP3, MP4SDQ, MP4), and the coupled-cluster calculations including single, double, and approximate triple excitations [CCSD(T)]. It is shown that the SAPT results reproduce the converged CCSD(T) results within 10%. The SAPT method has been used to analyze the three-body interactions in the clusters OH−(H2O)n, n=2,3,4,10, with water molecules located either in the first or the second solvation shell. It is shown that at the Hartree–Fock level the induction nonadditivity is dominant, but it is partly quenched by the Heitler–London and exchange-induction/deformation terms. ...

Response to Comment on Autoionization at the surface of neat water: is the top layer pH neutral, basic, or acidic? by J. K. Beattie, Phys. Chem. Chem. Phys., 2007, 9, DOI: 10.1039/b713702h

In this Response we define the sublayers of the water/vapor interface for clarity. We point out that the experimental evidence for surface enhancement of hydronium but not hydroxide is consistent across a broad concentration range and discuss the limitations of the molecular dynamics simulation at pH = 7 while noting that the computational results are only weakly dependent on concentration.

Ynol ethers from dichloroenol ethers: mechanistic elucidation through 35Cl labeling.

The mechanism of ynol ether formation from dichloroenol ethers, a decades-old transformation, has been studied by experimental and theoretical techniques to determine the relative importance of the Fritsch-Buttenberg-Wichell rearrangement (alpha-elimination) and beta-elimination in the evolution of the intermediate carbenoid.

Reaction of nitrones with silyl ketene acetals: A DFT study

Theoretical calculations at the DFT level indicate that the reaction of a nitrone with a silyl ketene acetal proceeds, contrary to previous suggestions, through a classical 1,3‐dipolar cycloaddition, followed by a silyl group transfer step to give the open‐chain product. Introduction of a diffuse basis set is necessary to describe properly nitrones. The influence of a Lewis acid has been studied.

Activation of carbonyl bonds by quaternary ammoniums and a (Na+:crown-ether) complex: investigation of the ring-opening polymerization of cyclic esters

Quaternary ammoniums associated with bis(trifluoromethane)sulfonimide (NTf2) or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF) counterions were readily prepared from commercially available tertiary amine, amidine, guanidine and pyridine. As predicted by molecular modelling, these ammonium salts proved to be efficient multiple H-bond donor catalysts in the ring-opening polymerization of cyclic esters (lactide, δ-valerolactone and e-caprolactone). In addition, a sodium(I) complex of [15-c-5] crown-ether paired with NTf2 or BARF was shown to activate the cyclic monomers through a cation–dipole interaction. These simple and non-protonated ionic structures appeared as attractive alternative organocatalysts to classical H-bond donors for the activation of carbonyl bonds.

Structure and properties of the weakly bound trimer (H2O)2HCl. Theoretical predictions and comparison with high-resolution rotational spectroscopy

Abstract In this paper we report ab initio predictions of the minimum energy structure of the (H 2 O) 2 HCl cluster, its stationary points, low-energy tunneling pathways, the dipole moment, and the nuclear quadrupole coupling constants. Structures corresponding to the stationary points were optimized with the second-order Moller–Plesset theory, while the corresponding interaction energies and binding energies were computed using the coupled-cluster method restricted to single, double, and non-iterative triple excitations. It is shown that the non-additive interactions play an important role. The contribution of the three-body term represents as much as 13–20% of the total interaction energy. The nature of the intermolecular interactions in the cluster was investigated by symmetry-adapted perturbation theory. As expected, the complex is mostly stabilized by the electrostatic and induction interactions, but the dispersion term is far from negligible. It is found that the potential energy surface of this cluster shows three low-energy pathways connecting two enantiomeric minimum energy structures. The height of the barriers separating these minima suggests that it should be possible to observe spectroscopic transitions resulting from the tunneling between the equivalent minima. From the study of these low-energy rearrangement processes we determined the permutation-inversion group of the complex, classified its vibration–rotation–tunneling states, and determined the electric dipole selection rules and spin-statistical weights governing the intensity pattern in the spectra. The results of the theoretical predictions are compared with the experimental data from the microwave measurements [Z. Kisiel et al., J. Chem. Phys. 112 (2000) 5767; Chem. Phys. Lett. 325 (2000) 523].