A molecular dynamics study of the nonlinear spectra and structure of charged (101) quartz/water interfaces.

The relationship between the nonlinear spectra and the structure of charged (101) α-quartz/water interfaces was investigated by classical molecular dynamics simulations. The results of the simulations show that the layered organization of interfacial water is only slightly perturbed by the surface charge and the ionic strength of the aqueous phase. Molecules next to the surface, in a bonded interfacial layer (BIL), tend to direct one of their OH bonds towards the surface, while the water orientation further from the surface, in a diffuse layer (DL), is essentially isotropic. A Stern layer of solvated cations is built up already on a neutral surface and the deprotonation of silanols leads to the formation of an inner Helmholtz plane of cations directly interacting with the SiO- surface sites. A flip of the molecular orientation in the DL could be inferred from the analysis of the structural characteristics of concentrated NaCl solution in contact with a highly charged surface. The computed spectra of χ(2) susceptibility show a double band structure typical of the spectra of silica/water interfaces and the calculations reproduce the experimentally observed variation of the spectral intensity with the pH of the liquid phase. The analysis of results suggests that the behaviour stems from the interplay between the orientational and induced components of the χ(2) susceptibility of the BIL and DL. The dependence of the Im[χ(2)] spectra of the BIL on the surface charge yields the spectrum of H2O molecules directly interacting with SiO- sites. The spectrum is in excellent agreement with the experimental spectrum of the topmost water of a silica/water interface [Urashima et al., J. Phys. Chem. Lett., 2018, 9, 4109].

Structure and sum-frequency generation spectra of water on neutral hydroxylated silica surfaces.

Structural organization and vibrational sum-frequency generation (VSFG) spectra of water on crystalline and amorphous neutral silica surfaces were investigated by classical molecular dynamics simulations. The liquid phase represented with neat water and 1 M NaCl solution was analysed in terms of bonded interfacial layer (BIL), diffuse layer (DL) and bulk region. The simulations show that the structure of BIL depends on the surface morphology and density of surface OH groups. The water-silanol H-bond network and BIL structure are mainly insensitive to the presence of ions in the liquid phase. Molecules in DL of SiO2/neat water interfaces preferentially orient their OH bonds towards the surfaces. This effect is directly related to an effective negative charge of formally neutral surfaces. Ions of the electrolyte solution affect the intermolecular structure in DL by screening the surface electric field and by the chaotropic effect. Calculated phase-sensitive VSFG (Im[χ(2)]) spectrum of BIL features low-frequency negative and high-frequency positive bands. Characteristics of the positive band reflect the strength of water-surface interactions and surface crystallinity, while the position and shape of the negative band are common to all interfaces. The Im[χ(2)] spectrum of DL is dominated by a contribution from the third-order χ(3) susceptibility with the sign of the contribution directly related to the sign of electrostatic potential in the interfacial region. The DL spectrum is strongly affected by the presence of solvated ions. The computed intensity and Im[χ(2)] spectra of the amorphous silica/NaCl solution interface are in a good agreement with the conventional and phase-sensitive experimental VSFG spectra of fused SiO2/water system at low pH, in contrast to the spectra of the amorphous silica/neat water interface. Origins of the discrepancy are discussed.

Structure and sum-frequency generation spectra of water on uncharged Q4 silica surfaces: a molecular dynamics study.

The structural characteristics and sum-frequency generation (SFG) spectra of water near neutral Q4 silica surfaces were investigated by molecular dynamics simulations. The interactions of water molecules with atoms of the solid were described by different potential models, in particular by the CLAYFF [Cygan et al., J. Phys. Chem. B, 2004, 108, 1255] and INTERFACE [Heinz et al. Langmuir, 2013, 29, 1754] force fields. The calculations of the contact angle of water have shown that the silica surface modeled with CLAYFF behaves as macroscopically hydrophilic, in contrast to the surface described with the INTERFACE model. The hydrophilicity of CLAYFF stems from too attractive electrostatic surface-water interactions. Regardless of the surface's affinity for water, the aqueous phase has a layered structure in the direction perpendicular to the surface with density fluctuations decaying within a distance of 10 Å from the surface. The orientational ordering of H2O molecules was found to be more short-range than the density fluctuations, especially for the hydrophobic surfaces. Modeling the SFG spectra has shown that the spectra of all studied hydrophobic silica-water interfaces are similar and have features in common with the spectrum of the water-vapor interface. The spectra fairly agree with experimental results obtained for the silica-water interface at low pH conditions [Myalitsin et al., J. Phys. Chem. C, 2016, 120, 9357]. The spectral response for the hydrophobic interface was computed to primarily arise from the topmost molecules of the first layer of interfacial water. In contrast, the SFG signal from the hydrophilic silica-water interface is accumulated over a greater distance extending for several water layers due to more long-range perturbation of the structure by the surface.

A Spectroscopic Study of Tautomeric Equilibrium of Salicylideneaniline in ZSM-5 Zeolites.

Salicylideneaniline (SA) sorbed in cation-exchanged M-ZSM-5 (M = H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ and Zn2+) zeolites was studied by spectroscopic techniques assisted by quantum-chemical calculations. The nature of extra-framework cations present in the zeolite void was found to affect the spectral signature of the sorbed SA molecule that points to the shift of tautomeric equilibrium between the enol and keto forms. Small size cations, such as H⁺ and Li⁺, stabilize a cis-keto SA tautomer along with a enol one in the zeolite structure. The calculations indicate that the sorbed cis-keto tautomer may have the dipole large enough to be considered as a zwitterion. New features appearing in the spectra with the increase of the cation size were attributed to the presence of trans-keto SA tautomer, which up to now has been observed only in time-resolved spectroscopic experiments. A strong interaction of the molecule with cations in Zn-ZSM-5 zeolite results in the chelation of enol SA with the divalent Zn2+ ions. The results of the study suggest that the tautomeric equilibrium of molecules belonging to the Schiff base family can be tuned by the confinement in the nanoporous materials via a choice of topology of zeolite framework and the nature of extra-framework cations.

Unraveling the Structure-Raman Spectra Relationships in V2O5 Polymorphs via a Comprehensive Experimental and DFT Study.

Vanadium pentoxide polymorphs (α-, ß-, γ'-, and ε'-V2O5) have been studied using the Raman spectroscopy and quantum-chemical calculations based on density functional theory. All crystal structures have been optimized by minimizing the total energy with respect to the lattice parameters and the positions of atoms in the unit cell. The structural optimization has been followed by the analysis of the phonon states in the Γ-point of the Brillouin zone, and the analysis has been completed by the computation of the Raman scattering intensities of the vibrational modes of the structures. The optimized structural characteristics compare well with the experimental data, and the calculated Raman spectra match the experimental ones remarkably well. With the good agreement between the spectra, a reliable assignment of the observed Raman peaks to the vibrations of specific structurals units in the V2O5 lattices is proposed. The obtained results support the viewpoint on the layered structure of vanadium pentoxide polymorphs as an ensemble of V2O5 chains held together by weaker interchain and interlayer interactions. Similarities and distinctions in the Raman spectra of the polymorphs have been highlighted, and the analysis of the experimental and computational data allows us, for the first time, to put forward spectrum-structure correlations for the four V2O5 structures. These findings are of the utmost importance for an efficient use of Raman spectroscopy to probe the changes at the atomic scale in the V2O5-based materials under electrochemical operation.

A modeling study of methane hydrate decomposition in contact with the external surface of zeolites.

The behavior of methane hydrate (MH) enclosed between the (010) surfaces of the silicalite-1 zeolite was studied by means of molecular dynamics simulations at temperatures of 150 and 250 K. Calculations reveal that the interaction with the hydrophilic surface OH groups destabilizes the clathrate structure of hydrate. While MH mostly conserves the structure in the simulation at the low temperature, thermal motion at the high temperature breaks the fragilized cages of H-bonded water molecules, thus leading to the release of methane. The dissociation proceeds in a layer-by-layer manner starting from the outer parts of the MH slab until complete hydrate decomposition. The released CH4 molecules are absorbed by the microporous solid, whereas water is retained at the surfaces of hydrophobic silicalite and forms a meniscus in the interlayer space. Methane uptake reaches 70% of the silicalite sorption capacity. The energy necessary for the endothermic MH dissociation is supplied by the exothermic methane absorption by the zeolite.

A molecular dynamics study of the interaction of water with the external surface of silicalite-1.

The interaction of water with the hydroxylated (010) surface of silicalite-1 was studied by classical molecular dynamics simulations. Interatomic interactions in the system were described using a set of effective potentials combining well-tested BKS and SPC models. The extended force field is shown to correctly reproduce the structural, energy, and dynamical characteristics of the silica surface OH groups. The interaction of water with the hydrophilic silanols leads to an ordering of H2O molecules in the vicinity of the surface. The ordering is found to be limited to two molecular layers extending to 7 Å above the surface. Despite the hydrophobic nature of the silicalite structure and the presence of hydrophilic surface sites, water molecules are capable of penetrating the porous silicalite system, where they form an H-bonded network blocking further access to the bulk. Water uptake by the zeolite was computed to be small in the time-scale of the simulations. The vibrational dynamics of the surface OH groups and adsorbed water molecules is discussed in detail. In agreement with the results of spectroscopic experiments, water molecules in the ordered surface layer have a spectral signature different from that of molecules more distant from the surface.

Effect of resonance dipole-dipole interaction on spectra of adsorbed SF6 molecules.

Adsorption of SF6 on zinc oxide and on silicalite-1 was investigated by a combination of IR spectroscopy with the calculations of spectra by means of a modernized model, developed previously for liquids. Comparison of the experimental spectra and the results of modeling shows that the complex band shapes in spectra of adsorbed molecules with extremely high absorbance are due to the strong resonance dipole-dipole interaction (RDDI) rather that the surface heterogeneity or the presence of specific surface sites. Perfect agreement between calculated and observed spectra was found for ZnO, while some dissimilarity in band intensities for silicalite-1 was attributed to complicated geometry of molecular arrangement in the channels.

Vibrational dynamics of the salicylideneaniline molecule in the solid phase and the confined state.

The salicylideneaniline (SA) molecule, both in the solid phase and sorbed in silicalite-1 zeolite, was studied by a large palette of vibrational spectroscopic methods (INS, Raman, and infrared) and by computational techniques. The comparison of the experimental and calculated spectra unambiguously indicates that the molecule is present in the cis-enol form in both phases. The results of the study allowed the proposal of a complete assignment of the vibrational spectrum of the SA molecule. The analysis of peak positions in the Raman and INS spectra of the molecule in the solid and sorbed states, and of the corresponding vibrational modes, shows that the confinement by the zeolite mostly affects those modes whose vibrational amplitude is localized on atoms of the phenol ring. This finding suggests that the molecule sits in the zeolite void such that the phenol ring is affected by the sorption to a greater extent than the benzene one. This assumption is corroborated by results of molecular modeling that shows the most energetically preferred position of the molecule in the straight channel of the zeolite framework with the phenol ring lying between two channel intersections, whereas the benzene ring is situated in the intersection.

Structure-dependent interatomic dispersion coefficients in oxides with maximally localized Wannier functions.

The interatomic C(6) dispersion coefficients in crystalline and amorphous SiO(2) and ZrO(2) structures were obtained with the approach proposed by Silvestrelli (2008 Phys. Rev. Lett. 100 053002) and based on the use of maximally localized Wannier functions (MLWFs) for partitioning the electron density. Localization of Wannier functions close to the nuclei in oxide systems makes it possible to assign the MLWFs to the atoms in an unambiguous way and then to compute the C(6) coefficients in an atom pairwise manner. A modification of the method is suggested in which the MLWFs are condensed to effective orbitals centred on the atoms and parameters of these effective orbitals are used for computing the interatomic dispersion coefficients. The obtained values of the dispersion coefficients were found to vary not only from one oxide to another, but also between different modifications of the same compound. The oxygen-oxygen coefficient C6(OO) reveals the largest variation and its value in ZrO(2) structures is twice as large as that in SiO(2) ones. Atomic characteristics obtained in the frame of the effective orbital method, such as the self-atom dispersion coefficient, and the oxide ion polarizability were found to correlate with the metal-oxygen bond length and the oxygen coordination number in the systems. This behaviour is attributed to the confinement of electrons by the electrostatic potential. The values of the coefficient and of the polarizability were related to charges of the oxygen atoms. In all studied systems the oxygen atoms having larger absolute values of charge were found to be less polarizable because of a stronger confinement effect. The obtained results can be used in the development of polarizable force fields for the atomistic modelling of oxide materials.

Water behaviour in nanoporous aluminosilicates.

This paper briefly reviews results of molecular dynamics simulation studies of water confined in nanoporous aluminosilicates. The behaviour of confined molecules is shown to be influenced by the nature of the host structure, and the size and the topology of the voids. For some of the systems discussed the ambiguity in results of different modelling studies call for the use of extended potential and structural models. Thus, the use of polarizable force fields was shown to be necessary to take into account the variation of the molecular dipole of confined molecules in different environments.

Electro-Optical Parameters for Computation of Nonresonance Raman Scattering Intensities of Peptides.

A set of electro-optical parameters (EOPs) of cylindrical zero-order bond polarizability model (BPM) for chemical bonds found in peptides was obtained from the results of quantum-chemical computations. The calculation of the polarizability tensors and the Raman scattering activities of four test molecules (N-methylacetamide, N,N-dimethylacetamide, dialanine, and diglycine) has shown that the BPM calculated quantities are in good agreement with the reference data obtained in experiments or derived by quantum-chemical calculations. The mean molecular polarizabilities are reproduced with the maximum relative error of 1.6%. A good agreement was obtained for the Raman activity of stretching vibrations, whereas a limited performance of the EOPs was found for the vibrational modes with a significant contribution of the bending internal coordinates involving H atoms. The origins of the discrepancies are analyzed, and the ways of improvement of the model performance are discussed.

Molecular dynamics study of hydrated imogolite. 2. Structure and dynamics of confined water.

The behaviour of water confined in an imogolite nanotube was studied by means of molecular dynamics simulations. The results of the study show an important difference between the interaction of water molecules with the internal and external surfaces of the nanotube. The analysis of the density profiles of confined molecules, of their spatial organisation, of the size of molecular clusters, of the lifetime of H-bonds in the system and of dynamical characteristics of molecules permits us to qualify the external imogolite surface as hydrophobic, whereas the internal surface reveals a hydrophilic character.

Modelling studies of water in crystalline nanoporous aluminosilicates.

The paper presents a review of molecular modelling studies of hydrated nanoporous aluminosilicates (zeolites and clays) performed during the last decade. A special emphasis is set on the calculation of the dynamical quantities and collective properties of the confined water. Some new results concerning the behaviour of water molecules in the siliceous silicalite and zeolite beta structures are presented.

Electro-optical parameters of bond polarizability model for aluminosilicates.

Electro-optical parameters (EOPs) of bond polarizability model (BPM) for aluminosilicate structures were derived from quantum-chemical DFT calculations of molecular models. The tensor of molecular polarizability and the derivatives of the tensor with respect to the bond length are well reproduced with the BPM, and the EOPs obtained are in a fair agreement with available experimental data. The parameters derived were found to be transferable to larger molecules. This finding suggests that the procedure used can be applied to systems with partially ionic chemical bonds. The transferability of the parameters to periodic systems was tested in molecular dynamics simulation of the polarized Raman spectra of alpha-quartz. It appeared that the molecular Si-O bond EOPs failed to reproduce the intensity of peaks in the spectra. This limitation is due to large values of the longitudinal components of the bond polarizability and its derivative found in the molecular calculations as compared to those obtained from periodic DFT calculations of crystalline silica polymorphs by Umari et al. (Phys. Rev. B 2001, 63, 094305). It is supposed that the electric field of the solid is responsible for the difference of the parameters. Nevertheless, the EOPs obtained can be used as an initial set of parameters for calculations of polarizability related characteristics of relevant systems in the framework of BPM.