Solvation Regime Research Articles

How does theory compare to experiment for oscillator strengths in electronic spectra? Proposing range-separated hybrids with reliable accountability.

As an important quantity in atomic and molecular spectroscopy, oscillator strength should be mentioned. Oscillator strength is linked to the transition dipole moment and consequently to the transition probability between two states, where its magnitude is directly connected to the intensity of the peaks in ultraviolet-visible spectra. However, accurately accounting for oscillator strengths still remains one of the greatest challenges in theory and experiment. Given previous efforts in the context of investigations into oscillator strengths, the related theoretical treatments are relatively limited and have proven to be challenging. In this work, the oscillator strengths in the electronic spectra of organic compounds have thoroughly been investigated with the help of optimally tuned range-separated hybrids (OT-RSHs). In particular, variants of the OT-RSHs combined with the polarizable continuum model (PCM), OT-RSHs-PCM, as well as their screened versions accounting for the screening effects by the electron correlation through the dielectric constant, OT-SRSHs-PCM, are proposed for reliable prediction of the oscillator strengths. The role of the involved ingredients in the proposed methods, namely the underlying density functional approximations, short-range and long-range Hartree-Fock (HF) exchange, as well as the range-separation parameter, has been examined in detail. It is shown that any combination of the parameters in the proposed approximations does not render the reliable oscillator strengths, but a particular compromise among them is needed to describe the experimental data well. Perusing all the results of our developed methods, the best ones are found to be the generalized gradient approximation-based OT-RSHs-PCM, coupled with the linear response theory in the non-equilibrium solvation regime, with the correct asymptotic behavior and incorporating no (low) HF exchange contributions in the short-range part. The best proposed approximations also reveal superior performances not only with respect to their standard counterparts with the default parameters but also as compared to earlier range-separated functionals. Finally, the applicability of the best approximation is also put into broader perspective, where it is used for predicting the oscillator strengths in other sets of compounds not included in the process of developing the approximations. Hopefully, our proposed method can function as an affordable alternative to the expensive wave function-based methods for both theoretical modeling and confirming the experimental observations in the field of electronic spectroscopy.

Addressing the Effectiveness and Molecular Mechanism of the Catalytic CO2 Hydration in Aqueous Solutions by Nickel Nanoparticles.

Hydration of carbon dioxide in water solution is the rate limiting step for the CO2 mineralization process, a process which is at the base of many carbon capture and utilization (CCU) technologies aiming to convert carbon dioxide to added-value products and mitigate climate change. Here, we present a combined experimental and computational study to clarify the effectiveness and molecular mechanism by which nickel nanoparticles, NiNPs, may enhance CO2 hydration in aqueous solutions. Contrary to previous literature, our kinetic experiments recording changes of pHs, conductivity, and dissolved carbon dioxide in solution reveal a minimal effect of the NiNPs in catalyzing CO2 hydration. Our atomistic simulations indicate that the Ni metal surface can coordinate only a limited number of water molecules, leaving uncoordinated metal sites for the binding of carbon dioxide or other cations in solution. This deactivates the catalyst and limits the continuous re-formation of a hydroxyl-decorated surface, which was a key chemical step in the previously suggested Ni-catalyzed hydration mechanism of carbon dioxide in aqueous solutions. At our experimental conditions, which expand the investigation of NiNP applicability toward a wider range of scenarios for CCU, NiNPs show a limited catalytic effect on the rate of CO2 hydration. Our study also highlights the importance of the solvation regime: while Ni surfaces may accelerate carbon dioxide hydration in water restricted environments, it may not be the case in fully hydrated conditions.

Uptake and reactivity of NO2 on the hydroxylated silica surface: A source of reactive oxygen species

We report state-of-the-art first-principles molecular dynamics results on the heterogeneous chemical uptake of NO2, a major anthropogenic pollutant, on the dry and wet hydroxylated surface of α-quartz, which is a significant component of silica-based catalysts and atmospheric dust aerosols. Our investigation spotlights an unexpected chemical pathway by which NO2 (i) can be adsorbed as HONO by deprotonation of interfacial silanols (i.e., –Si–OH group) on silica, (ii) can be barrierless converted to nitric acid, and (iii) can finally dissociated to surface bounded NO and hydroxyl gas phase radicals. This chemical pathway does not invoke any previously experimentally postulated NO2 dimerization, dimerization that is less likely to occur at low NO2 concentrations. Moreover, water significantly catalyzes the HONO formation and the dissociation of nitric acid into surface-bounded NO and OH radicals, while visible light adsorption can further promote these chemical transformations. This work highlights how water-restricted solvation regimes on common mineral substrates are likely to be a source of reactive oxygen species, and it offers a theoretical framework for further and desirable experimental efforts, aiming to better constrain trace gases/mineral interactions at different relative humidity conditions.

Ultrafast Dynamics at the Lipid-Water Interface: DMSO Modulates H-Bond Lifetimes.

Dimethyl sulfoxide (DMSO) is a common cosolvent and cryopreservation agent used to freeze cells and tissues. DMSO alters the H-bond structure of water, but its interactions with biomolecules and, specifically, with biological interfaces remain poorly understood. Here we investigate the effects of DMSO on the H-bond dynamics at the lipid-water interface using a combination of ultrafast two-dimensional infrared (2D IR) spectroscopy and molecular dynamics simulations. Ester carbonyl absorption spectra show that DMSO dehydrates the interface, and simulations show that the area per lipid is decreased. Ultrafast 2D IR spectra measure the time scales of frequency fluctuations at the ester carbonyl positions located precisely between the hydrophobic and hydrophilic regions of the membrane. 2D IR measurements show that low DMSO concentrations (<10 mol %) induce ∼40% faster H-bond dynamics compared with pure water, whereas increased concentrations (>10-20 mol %) once again slow down the dynamics. This slow-fast-slow trend is described in terms of two different solvation regimes. Below 10 mol %, DMSO weakens the interfacial H bond, leading to faster "bulk-like" dynamics, whereas above 10 mol %, water molecules become "relatively immobilized" as the H-bond networks becoming disrupted by the H-bond donor/acceptor imbalance at the interface. These studies are an important step toward characterizing the environments around lipid membranes, which are essential to numerous biological processes.

An open quantum system theory for polarizable continuum models.

The problem of a solute described by Quantum Chemistry within a solvent represented as a polarizable continuum model (PCM) is here reformulated in terms of the open quantum systems (OQS) theory. Using its stochastic Schrödinger equation formulation, we are able to provide a more comprehensive picture of the electronic energies and the coupling between solute and solvent electronic dynamics. In particular, the OQS-PCM proves to be a unifying theoretical framework naturally including polarization and dispersion interactions, the effect of solvent fluctuations, and the non-Markovian solvent response. As such, the OQS-PCM describes the interplay between the solute and the solvent typical electronic dynamical times and yields both the standard PCM and the so-called Born-Oppenheimer solvation regime, where the solvent electronic response is considered faster than any electronic dynamics taking place in the solute. In analyzing the OQS-PCM, we obtained an expression for the solute-solvent dispersion (van der Waals) interactions, which is very transparent in terms of a physical interpretation based on fluctuations and response functions. Finally, we present various numerical tests that support the theoretical findings.

Electronic transitions for a fully polarizable QM/MM approach based on fluctuating charges and fluctuating dipoles: Linear and corrected linear response regimes.

The fully polarizable Quantum Mechanics/Molecular Mechanics (QM/MM) approach based on fluctuating charges and fluctuating dipoles, named QM/FQFμ [T. Giovannini et al., J. Chem. Theory Comput. 15, 2233 (2019)], is extended to the calculation of vertical excitation energies of solvated molecular systems. Excitation energies are defined within two different solvation regimes, i.e., linear response (LR), where the response of the MM portion is adjusted to the QM transition density, and corrected-Linear Response (cLR) in which the MM response is adjusted to the relaxed QM density, thus being able to account for charge equilibration in the excited state. The model, which is specified in terms of three physical parameters (electronegativity, chemical hardness, and polarizability) is applied to vacuo-to-water solvatochromic shifts of aqueous solutions of para-nitroaniline, pyridine, and pyrimidine. The results show a good agreement with their experimental counterparts, thus highlighting the potentialities of this approach.

Solvation and Entropic Regimes in Ion-Containing Block Copolymers

The phase behavior of ion-containing block copolymers is studied using a field-theoretic model consisting of fully mobile ions immersed in a heterogeneous dielectric medium. Two distinct thermodynamic regimes are found to emerge, resulting from the competition between ion translational entropy and solvation free energy. In the regime dominated by entropy, the disordered phase is stabilized by the addition of ions. In the regime dominated by solvation free energy, the disordered phase is destabilized, leading to a “chimney”-like channel of stable ordered phases at sufficiently large ion concentrations. The two regimes are demarcated by the parameter λB, which quantifies the variation in solvation energy upon transferring ions between different dielectric domains. Estimates of λB for experimental systems suggest that the majority of block copolymer electrolytes fall within the solvation regime.

Effect of humidity on the interaction of CO2 with alkaline anion exchange membranes probed using the quartz crystal microbalance

The alkaline anion exchange membrane fuel cell (AAEM-FC) is able to deliver a comparable performance to the traditional proton exchange membrane fuel cell (PEM-FC) without the use of precious metal electrocatalysts, making it a more cost-competitive alternative for low-temperature fuel cell applications. However, issues relating to degradation and specifically interaction with CO2 still hinder the technology's commercialisation prospects. With hydration playing a key role in solid polymer electrolyte fuel cell operation, this study examines how membrane hydration affects the AAEM interaction with CO2. The change of membrane conductivity upon exposure to atmospheric CO2 has been compared with the change in viscoelastic properties of a cast thin-film ionomer, both as a function of humidity. The effect of CO2 on the membrane as a function of hydration suggests a link to its solvation and swelling regimes and thus the access of CO2 to the ionic channels within the membrane. The thin-film QCM composite resonator study has suggested that during the solvation (pore opening) regime, there is a linear increase in CO2 uptake as water can further permeate the pore system and the cationic headgroups become increasingly accessible. During the transition to the pore swelling regime, there is a step increase in CO2 uptake as the network is thought to be fully open; as such, subsequent increases in RH do not lead to any significant increase in CO2 uptake.

The hydrophobic effect in a simple isotropic water-like model: Monte Carlo study.

Using Monte Carlo computer simulations, we show that a simple isotropic water-like model with two characteristic lengths can reproduce the hydrophobic effect and the solvation properties of small and large non-polar solutes. Influence of temperature, pressure, and solute size on the thermodynamic properties of apolar solute solvation in a water model was systematically studied, showing two different solvation regimes. Small particles can fit into the cavities around the solvent particles, inducing additional order in the system and lowering the overall entropy. Large particles force the solvent to disrupt their network, increasing the entropy of the system. At low temperatures, the ordering effect of small solutes is very pronounced. Above the cross-over temperature, which strongly depends on the solute size, the entropy change becomes strictly positive. Pressure dependence was also investigated, showing a "cross-over pressure" where the entropy and enthalpy of solvation are the lowest. These results suggest two fundamentally different solvation mechanisms, as observed experimentally in water and computationally in various water-like models.

Electronic excitations in a dielectric continuum solvent with quantum Monte Carlo: acrolein in water.

We investigate here the vertical n → π(*) and π → π(*) transitions of s-trans-acrolein in aqueous solution by means of a polarizable continuum model (PCM) we have developed for the treatment of the solute at the quantum Monte Carlo (QMC) level of the theory. We employ the QMC approach which allows us to work with highly correlated electronic wave functions for both the solute ground and excited states and, to study the vertical transitions in the solvent, adopt the commonly used scheme of considering fast and slow dielectric polarization. To perform calculations in a non-equilibrium solvation regime for the solute excited state, we add a correction to the global dielectric polarization charge density, obtained self consistently with the solute ground-state wave function by assuming a linear-response scheme. For the solvent polarization in the field of the solute in the ground state, we use the static dielectric constant while, for the electronic dielectric polarization, we employ the solvent refractive index evaluated at the same frequency of the photon absorbed by the solute for the transition. This choice is shown to be better than adopting the most commonly used value of refractive index measured in the region of visible radiation. Our QMC calculations show that, for standard cavities, the solvatochromic shifts obtained with the PCM are underestimated, even though of the correct sign, for both transitions of acrolein in water. Only by reducing the size of the cavity to values where more than one electron is escaped to the solvent region, we regain the experimental shift for the n → π(*) case and also improve considerably the shift for the π → π(*) transition.

A robust and effective time-independent route to the calculation of Resonance Raman spectra of large molecules in condensed phases with the inclusion of Duschinsky, Herzberg-Teller, anharmonic, and environmental effects.

We present an effective time-independent implementation to model vibrational resonance Raman (RR) spectra of medium-large molecular systems with the inclusion of Franck-Condon (FC) and Herzberg-Teller (HT) effects and a full account of the possible differences between the harmonic potential energy surfaces of the ground and resonant electronic states. Thanks to a number of algorithmic improvements and very effective parallelization, the full computations of fundamentals, overtones, and combination bands can be routinely performed for large systems possibly involving more than two electronic states. In order to improve the accuracy of the results, an effective inclusion of the leading anharmonic effects is also possible, together with environmental contributions under different solvation regimes. Reduced-dimensionality approaches can further enlarge the range of applications of this new tool. Applications to imidazole, pyrene, and chlorophyll a1 in solution are reported, as well as comparisons with available experimental data.

Assessing the non-ideality of the CO2-CS2 system at molecular level: A Raman scattering study

The dense phase of CO2-CS2 mixtures has been analysed by Raman spectroscopy as a function of the CO2 concentration (0.02-0.95 mole fractions) by varying the pressure (0.5 MPa up to 7.7 MPa) at constant temperature (313 K). The polarised and depolarised spectra of the induced (ν2, ν3) modes of CS2 and of the ν1-2ν2 Fermi resonance dyad of both CO2 and CS2 have been measured. Upon dilution with CO2, the evolution of the spectroscopic observables of all these modes displays a "plateau-like" region in the CO2 mole fraction 0.3-0.7 never previously observed in CO2-organic liquids mixtures. The bandshape and intensity of the induced modes of CS2 are similar to those of pure CS2 up to equimolar concentration, after which variations occur. The preservation of the local ordering from pure CS2 to equimolar concentration together with the non-linear evolution of the spectroscopic observables allows inferring that two solvation regimes exist with a transition occurring in the plateau domain. In the first regime, corresponding to CS2 concentrated mixtures, the liquid phase is segregated with dominant CS2 clusters, whereas, in the second one, CO2 monomers and dimers and CO2-CS2 hetero-dimers coexist dynamically on a picosecond time-scale. It is demonstrated that the subtle interplay between attractive and repulsive interactions which provides a molecular interpretation of the non-ideality of the CO2-CS2 mixture allows rationalizing the volume expansion and the existence of the plateau-like region observed in the pressure-composition diagram previously ascribed to the proximity of an upper critical solution temperature at lower temperatures.

Influence of Temperature on CO2 Absorption Rate and Capacity in Ionic Liquids

Ionic liquids (ILs) present a new alternative for postcombustion CO2 capture because their properties can be tuned by means of different cation–anion combinations in order to obtain the most suitable properties for a specific task. This work is a novel study that investigates the absorption kinetics and evolution of the chemical transformations produced by CO2 absorption in ILs at different temperatures. A large range of pure ILs was tested with a screening process based on the pKa anion for efficient and reversible CO2 capture. CO2 uptake by selected ILs was determined for a wide range of pressures between atmospheric pressure and 2000 kPa at room temperature. Results show that CO2 absorption capacities of [bmim][Ac] and [bmim][Phen] at 100 kPa are close to those obtained at higher pressures, suggesting the existence of two solvation regimes. This was confirmed by IR analysis. The kinetics were significantly affected by the temperature and were shown to increase sharply with it. The optimum temperature for CO2 capture in [bmim][Ac] and [bmim][Phen] was found to be 323.15 and 348.15 K, respectively.

Absorption and Emission Spectra of Solvated Molecules with the EOM-CCSD-PCM Method.

The accurate calculation of transition energies and properties of isolated molecules is not enough for realistic simulations of their absorption and emission spectra in solution. In fact, the solvent influences the solute geometry, electronic structure, and response to external fields, and a proper description of the solvent effect is fundamental. However, the computational cost of including explicit solvent molecules around the solute becomes rather onerous when an accurate method such as the equation of motion coupled cluster singles and doubles (EOM-CCSD) is employed. The polarizable continuum model of solvation (PCM) may provide an efficient alternative to explicit models, since the sampling of solvent configurations is implicit and the solute-solvent mutual polarization is naturally accounted for. In this contribution, the absorption and emission spectra of molecules in solution are modeled through the EOM-CCSD-PCM method. The equilibrium solvation regime is employed for the geometry optimization of the solute molecule in the ground and excited states, while the nonequilibrium solvation regime is employed for vertical transitions. The theory, implementation, and prototypical applications of the method are presented. The numerical tests involve solvents that are particularly challenging for PCM: low-polar and protic polar solvents. Nonetheless, the experimental trends are well reproduced, and the overall agreement with the measured data is remarkable.

Solvent Vapor Annealing of Single Conjugated Polymer Chains: Building Organic Optoelectronic Materials from the Bottom Up.

Optoelectronic devices based on organic materials show a strong relationship between the morphological structure of the material and the function of the device. One of the grand challenges in improving the efficiencies of these devices is hence achieving morphological control throughout the entire course of processing. One of the most important postprocessing methods is solvent vapor annealing, which has repeatedly demonstrated its utility in improving the efficiency of organic-material-based devices by changing bulk-film morphology. This Perspective discusses the recent impact of single-molecule spectroscopy techniques in unraveling morphological changes and molecular dynamics and presents solvent vapor annealing as a tool to build organic optoelectronic materials from the bottom up. In particular, we discuss examples of how solvent vapor annealing at the single-chain level can be split into two different regimes, (i) the solvation regime, in which intrachain interactions and molecular dynamics during solvent vapor annealing can be probed, and (ii) the aggregation regime, in which the influence of interchain interactions can be probed. Finally, it will be shown that solvent vapor annealing in the aggregation regime can be used to build highly ordered mesoscopic objects with distinct properties such as long-range energy transfer.

Carbon Dioxide in 1-Butyl-3-methylimidazolium Acetate. I. Unusual Solubility Investigated by Raman Spectroscopy and DFT Calculations

The unusual solubility of carbon dioxide in 1-butyl-3-methylimidazolium acetate (Bmim Ac) has been studied by Raman spectroscopy and DFT calculations. It is shown that the solubility results from the existence of two distinct solvation regimes. In the first one (CO(2) mole fraction ≤ 0.35), the usual Fermi dyad is not observed, a fact never reported before for binary mixtures with organic liquids or ionic liquids (IL). Strong experimental evidence complemented by effective DFT modeling shows that this regime is dominated by a chemical reaction leading to the carboxylation of the imidazolium ring accompanied by acetic acid formation. The reactive scheme proposed involves two concerted mechanisms, which are a proton exchange process between the imidazolium cation and the acetate anion and the carboxylation process itself initiated from the formation of "transient" CO(2)-1-butyl-3-methylimidazole 2-ylidene carbene species. In that sense, CO(2) triggers the carboxylation reaction. Moreover, this dynamic picture circumvents consideration of a long-lived carbene formation in dense phase. The second regime is characterized by the detection of the CO(2) Fermi dyad showing that the carboxylation reaction has been strongly moderated. This finding has been interpreted as due to the interaction of the acetic acid molecules with the COO group of acetate anions involved in monodentate forms with the cation. The observation of the Fermi doublet allows us to infer that CO(2) essentially preserves its linear geometry and that the nature and strength of the interactions with its environment should be comparable to those existing in organic liquids and other IL as well. These results have been supported by DFT calculations showing that the CO(2) molecule interacts with energetically equivalent coexisting structures and that its geometry departs only slightly from the linearity. Finally, we find that the CO(2) solvation in Bmim Ac and 1-butyl-3-methylimidazolium trifluoroacetate (Bmim TFA) cannot be straightforwardly compared neither in the first regime due to the existence of a chemical reaction nor in the second regime because CO(2) interacts with a variety of environments not only consisting of ions pairs like in Bmim TFA but also with carboxylate and acetic acid molecule.

Toward an Accurate Modeling of Optical Rotation for Solvated Systems: Anharmonic Vibrational Contributions Coupled to the Polarizable Continuum Model.

We present a newly implemented methodology to evaluate vibrational contributions (harmonic and anharmonic) to the optical rotation of solvated systems described by means of the polarizable continuum model (PCM). Proper account of an incomplete solvation regime in the treatment of both the electronic property and the molecular vibrations is considered, as well as the inclusion of cavity field effects. In order to assess the quality of our approach, test calculations on (R)-methyloxirane in various solvents and (S)-N-acetylproline amide in cyclohexane and aqueous solution are presented. The comparison with experimental findings is also shown.

Measurement of Two Solvation Regimes in Water-Ethanol Mixtures Using X-Ray Compton Scattering

Water-ethanol mixtures exhibit interesting anomalies in their macroscopic properties. Despite a lot of research, the origin of the anomalies and the microscopic structure itself is still far from completely known. We have utilized the synchrotron x-ray Compton scattering technique to elucidate the structure of aqueous ethanol from a new experimental perspective. The technique is uniquely sensitive to the local molecular geometries at the angstrom and subangstrom scales. The experiments reveal two distinct mixing regimes in terms of geometry: the dilute 5 mol % and the concentrated >15 mol % regimes. By comparing with pure liquids, the former regime is characterized by an intramolecular and the latter by an intermolecular change. The findings bring new light to evaluating the hypothesis of formation of clathratelike structures at the dilute concentrations.

Towards an accurate description of anharmonic infrared spectra in solution within the polarizable continuum model: Reaction field, cavity field and nonequilibrium effects

We present a newly developed and implemented methodology to perturbatively evaluate anharmonic vibrational frequencies and infrared (IR) intensities of solvated systems described by means of the polarizable continuum model (PCM). The essential aspects of the theoretical model and of the implementation are described and some numerical tests are shown, with special emphasis towards the evaluation of IR intensities, for which the quality of the present method is compared to other methodologies widely used in the literature. Proper account of an incomplete solvation regime in the treatment of the molecular vibration is also considered, as well as inclusion of the coupling between the solvent and the probing field (cavity field effects). In order to assess the quality of our approach, comparison with experimental findings is reported for selected cases.

Sodium Diffusion through Aluminum-Doped Zeolite BEA System: Effect of Water Solvation

To investigate the effect of hydration on the diffusion of sodium ions through the aluminum-doped zeolite BEA system (Si/Al = 30), we used the grand canonical Monte Carlo (GCMC) method to predict the water absorption into aluminosilicate zeolite structure under various conditions of vapor pressure and temperature, followed by molecular dynamics (MD) simulations to investigate how the sodium diffusion depends on the concentration of water molecules. The predicted absorption isotherm shows first-order-like transition, which is commonly observed in hydrophobic porous systems. The MD trajectories indicate that the sodium ions diffuse through zeolite porous structures via hopping mechanism, as previously discussed for similar solid electrolyte systems. These results show that above 15 wt % hydration (good solvation regime) the formation of the solvation cage dramatically increases sodium diffusion by reducing the hopping energy barrier by 25% from the value of 3.8 kcal/mol observed in the poor solvation regime.