Nuclear quantum effects on the non-adiabatic decay mechanism of an excited hydrated electron

We present a kinetic analysis of the non-adiabatic decay mechanism of an excited state hydrated electron to the ground state. The theoretical treatment is based on a quantized, gap dependent golden rule rate constant formula which describes the non-adiabatic transition rate between two quantum states. The rate formula is expressed in terms of quantum time correlation functions of the energy gap, and of the non-adiabatic coupling. These gap dependent quantities are evaluated from three different sets of mixed quantum-classical molecular dynamics simulations of a hydrated electron equilibrated a) in its ground state, b) in its first excited state, and c) on a hypothetical mixed potential energy surface which is the average of the ground and the first excited electronic states. The quantized, gap-dependent rate results are applied in a phenomenological kinetic equation which provides the survival probability function of the excited state electron. Although the lifetime of the equilibrated excited state electron is computed to be very short (well under 100 fs), the survival probability function for the non-equilibrium process in pump-probe experiments yields an effective excited state lifetime of around 300 fs, a value consistent with the findings of several experimental groups and previous theoretical estimates

Nuclear quantum effects in electronically adiabatic quantum time correlation functions : Application to the absorption spectrum of a hydrated electron

A general formalism for introducing nuclear quantum effects in the expression of the quantum time correlation function of an operator in a multi-level electronic system is presented in the adiabatic limit. The final formula includes the nuclear quantum time correlation functions of the operator matrix elements, of the energy gap, and their cross terms. These quantities can be inferred and evaluated from their classical analogs obtained by mixed quantum-classical molecular dynamics simulations. The formalism is applied to the absorption spectrum of a hydrated electron, expressed in terms of the time correlation function of the dipole operator in the ground electronic state. We find that both static and dynamic nuclear quantum effects distinctly influence the shape of the absorption spectrum, especially its high-energy tail related to transitions to delocalized electron states. Their inclusion does improve significantly the agreement between theory and experiment for both the low and high frequency edges of the spectrum. It does not appear sufficient, however, to resolve persistent deviations in the slow Lorentzian-like decay part of the spectrum in the intermediate 2-3 eV region

Fast Computation of Solvation Free Energies with Molecular Density Functional Theory: Thermodynamic-Ensemble Partial Molar Volume Corrections

Molecular Density Functional Theory (MDFT) offers an efficient implicit- solvent method to estimate molecule solvation free-energies whereas conserving a fully molecular representation of the solvent. Even within a second order ap- proximation for the free-energy functional, the so-called homogeneous reference uid approximation, we show that the hydration free-energies computed for a dataset of 500 organic compounds are of similar quality as those obtained from molecular dynamics free-energy perturbation simulations, with a computer cost reduced by two to three orders of magnitude. This requires to introduce the proper partial volume correction to transform the results from the grand canoni- cal to the isobaric-isotherm ensemble that is pertinent to experiments. We show that this correction can be extended to 3D-RISM calculations, giving a sound theoretical justifcation to empirical partial molar volume corrections that have been proposed recently.Comment: Version with correct equation numbers is here: http://compchemmpi.wikispaces.com/file/view/sergiievskyi_et_al.pdf/513575848/sergiievskyi_et_al.pdf Supporting information available online at: http://compchemmpi.wikispaces.com/file/view/SuppInf_sergiievskyi_et_al_07-04-2014.pdf/513576008/SuppInf_sergiievskyi_et_al_07-04-2014.pd

Elektrontranszfer folyamatok egyszerű modelljeinek tanulmányozása kvantum és klasszikus dinamikai módszerekkel = Quantum and classical dynamics investigations of simple models of electron transfer processes

Kutatásunk során egy felesleg elektron fizikai tulajdonságait vizsgáltuk vizes és metanolos közegben kevert kvantumos-klasszikus molekuladinamikai szimulációk alkalmazásával. Modelljeink magukba foglaltak véges méretű víz és metanol molekulafürtöket anionokat, víz-levegő határfelületen stabilizálódó felesleg elektront és tömbfázisú hidratált elektront. Megállapítottuk, hogy az elektron két alapvető módon stabilizálódhat: a vizsgált rendszer határfelületén, vagy annak belsejében. Az állapotok stabilitását a kísérletek fizikai körülményei határozzák meg, s ezektől függően elektrontranszfer folyamat mehet végbe a két lokalizációs mód között. A vizsgált rendszerek fizikai jellemzését elvégeztük, a kísérleti észlelésekre egy új, konzisztens magyarázattal szolgáltunk. A vizsgált modellekkel kapcsolatban az elektronállapotok közötti sugárzásmentes átmenetek sebességének számítására kidolgozott, a Fermi aranyszabályon alapuló kvantummechanikai formalizmusunk alkalmazásával megbecsültük a hidratált és metanolban szolvatált elektron gerjesztett állapotának élettartamát. Az eljárás további alkalmazásaként a hidratált elektron klasszikus módon számított optikai abszorpciós spektrumát is korrigáltuk. Eredményeink jó összhangban vannak a kísérleti tapasztalatokkal. A hidratált elektron rendszert, mint hasznos és egyszerű modellt alkalmaztuk a biomolekulákat körülvevő oldószer molekulák és ellenionok eloszlását modellező klasszikus sűrűségfunkcionál módszer kidolgozása során is. | We examined the physical properties of excess electrons in water and methanol using mixed quantum-classical molecular dynamics simulations. The investigated systems included finite size water and methanol cluster anions, excess electrons stabilized on water/air interfaces, and the bulk hydrated electron. We found that excess electrons can stabilize in two localization modes in polar media: on the interface or in the interior of the examined system. The relative stability of the localization modes is determined by the experimental conditions. Depending on the conditions electron transfer can take place between the two localization modes. We performed the physical characterization of the examined systems, and gave a new and consistent interpretation of the experimental observations. In connection with the examined models, we developed a new quantum mechanical formalism for the description of the radiationless electronic transitions based on the Fermi golden rule. We applied the formalism to compute the excited state lifetime of an excess electron in water and methanol and, as a somewhat different application, the optical absorption spectrum of the hydrated electron. Our results are in good agreement with the experimental findings. We also applied the hydrated electron system, as a useful and simple model, in the development of the classical density functional theory modeling the counterion and solvent distribution of important biomolecules

Hydration of Clays at the Molecular Scale: The Promising Perspective of Classical Density Functional Theory

We report here how the hydration of complex surfaces can be efficiently studied thanks to recent advances in classical molecular density functional theory. This is illustrated on the example of the pyrophylite clay. After presenting the most recent advances, we show that the strength of this implicit method is that (i) it is in quantitative or semi-quantitative agreement with reference all-atoms simulations (molecular dynamics here) for both the solvation structure and energetics, and that (ii) the computational cost is two to three orders of magnitude less than in explicit methods. The method remains imperfect, in that it locally overestimates the polarization of water close to hydrophylic sites of the clay. The high numerical efficiency of the method is illustrated and exploited to carry a systematic study of the electrostatic and van der Waals components of the surface-solvant interactions within the most popular force field for clays, CLAYFF. Hydration structure and energetics are found to weakly depend upon the electrostatics. We conclude on the consequences of such findings in future force-field development.Comment: 24 pages, 8 figures. Molecular Physics (2014

Electronic Excited State Lifetimes of Anionic Water Clusters: Dependence on Charge Solvation Motif

An ongoing controversy about water cluster anions concerns the electron-binding motif, whether the charge center is localized at the surface or within the cluster interior. Here, mixed quantum-classical dynamics simulations have been carried out for a wide range of cluster sizes (n ≤ 1000) for [(H2O)n]- and [(D2O)n]- , based on a non-equilibrium first-order rate constant approach. The computed data are in good general agreement with time-resolved photoelectron imaging results (n ≤ 200). The analysis reveals that, for surface state electrons, the cluster size dependence of the excited state electronic energy gap and the magnitude of the non-adiabatic couplings have compensating influences on the excited state lifetimes: the excited state lifetime for surface states reaches a minimum for n ~ 150 and then increases for larger clusters. It is concluded that the electron resides in a surface-localized motif in all of these measured clusters, dominating at least up to n = 200

Molecular Density Functional Theory for water with liquid-gas coexistence and correct pressure

The solvation of hydrophobic solutes in water is special because liquid and gas are almost at coexistence. In the common hypernetted chain approximation to integral equations, or equivalently in the homogenous reference fluid of molecular density functional theory, coexistence is not taken into account. Hydration structures and energies of nanometer-scale hydrophobic solutes are thus incorrect. In this article, we propose a bridge functional that corrects this thermodynamic inconsistency by introducing a metastable gas phase for the homogeneous solvent. We show how this can be done by a third order expansion of the functional around the bulk liquid density that imposes the right pressure and the correct second order derivatives. Although this theory is not limited to water, we apply it to study hydrophobic solvation in water at room temperature and pressure and compare the results to all-atom simulations. With this correction, molecular density functional theory gives, at a modest computational cost, quantitative hydration free energies and structures of small molecular solutes like n-alkanes, and of hard sphere solutes whose radii range from angstroms to nanometers. The macroscopic liquid-gas surface tension predicted by the theory is comparable to experiments. This theory gives an alternative to the empirical hard sphere bridge correction used so far by several authors.Comment: 18 pages, 6 figure