1,721,105 research outputs found
Solvent effects on linear and nonlinear optical properties of Donor-Acceptor polyenes: investigation of electronic and vibrational components in terms of structure and charge distribution changes
No full textWe investigate the influence of solvation media upon the relationship among structure, spatial distribution of electron density, and linear and nonlinear electric properties for two series of push-pull π-conjugated molecules. The analysis is performed on both electronic and vibrational components of static polarizability and first hyperpolarizability, and the effects solvent induce on them are analyzed singularly within the framework of the polarization continuum model. Solvent is found to affect the extent of charge separationinducedinthegroundstateofthesemolecules. Thischargeseparationleadstoageometricdistortion, measured by the bond-length-alternation (BLA) parameter, which shows a solvent-induced evolution of the molecular geometry from a neutral, bond-alternated polyene-like structure, to a partially ionic cyanine-like structure, and ultimately to an ionic bond-alternated structure. As a consequence large changes in the linear andnonlinearresponsepropertiesarefound,inboththeirelectronicandvibrationalcontributions. Regarding the latter, we recall that studies on vibrational (hyper)polarizabilities for molecular systems in solution are presented here for the first time
Quantum-mechanical continuum solvation study of the polarizability of halides at the water/air interface
No full textIn this work, a theoretical study of static polarizabilities of halides (fluoride, chloride, bromide and iodide) at the water/air interface is presented. The study has been carried out employing a new development of the polarizable continuum model (PCM) to treat model interfaces between two different media. Within this framework the water/air interface is modeled as a water/vacuum interface having bulk properties (e.g., density or permittivity) varying smoothly from the bulk water to the vacuum in the interfacial region. The model allows the inclusion of electrostatic and repulsion effects arising from this inhomogeneous environment into the quantum chemical calculation for each of the selected ionic species. Static polarizabilities are then calculated as a function of the ion-interface distance. The results on polarizabilities together with those obtained for other properties and with free energy profiles are finally discussed in relation to the recent simulation results showing a preference of heavier halides for the interface than for bulk water
Excited States and Solvatochromic Shifts within a Nonequilibrium Solvation Approach: a New Formulation of the Integral Equation Method (IEF) at the SCF, CI and MCSCF level
No full textThe effects of the solvation on excited states are studied in the framework of a nonequilibrium regime between solute and solvent charge distributions. The approach, which exploits a separation of the polarization into slow and fast components, is inserted in a new formulation of the recently developed continuum solvation model known as integral equation formalism. This new version, implying a large computational gain both in time consuming and memory occupation, is here implemented at the Hartree–Fock level as well as at the multiconfiguration self-consistent field and configuration interaction levels. Examples of application of the method to solvatochromic shifts for low-lying excitation energies of formaldehyde, acetaldehyde, and acetone in water are shown
Fast evaluation of geometries and properties of excited molecules in solution: A Tamm-Dancoff model with application to 4-dimehylaminobenzene
No full textWe present a method to evaluate ab initio energy, wave function, and gradient of a solvated molecule in an electronically excited state. In particular, this paper extends the Polarizable Continuum Model (PCM) to a specific level of theory for the interpretation of the electronic spectra: configuration interaction (CI) among all singly substituted determinants using a Hartree-Fock reference state. This method, in very wide use, allows investigations of both structures and properties of electronically excited molecules through the evaluation of analytical energy derivatives with respect to various parameters. The most relevant formal aspects on the extension of the theory to solvated systems are discussed, and numerical applications to the study of geometries and one-electron properties of the low-lying excited states of 4-dimethylamino benzonitrile (DMABN) in acetonitrile solution are presented
Electronic excitation energies of molecules in solution within continuum solvation models: Investigating the discrepancy between state specific and linear response methods
No full textIn a recent article (R. Cammi, S. Corni, B. Mennucci, and J. Tomasi, J. Chem. Phys. 122, 104513,2005), we demonstrated that the state-specific (SS) and the linear-response (LR) approaches, two different ways to calculate solute excitation energies in the framework of quantum-mechanical continuum models of solvation, give different excitation energy expressions. In particular, they differ in the terms related to the electronic response of the solvent. In the present work, we further investigate this difference by comparing the excitation energy expressions of SS and LR with those obtained through a simple model for solute-solvent systems that bypasses one of the basic assumptions of continuum solvation models, i.e., the use of a single Hartree product of a solute and a solvent wave function to describe the total solute-solvent wave function. In particular, we consider the total solute-solvent wave function as a linear combination of the four products of two solute states and two solvent electronic states. To maximize the comparability with quantum-mechanical continuum model the resulting excitation energy expression is recast in terms of response functions of the solvent and quantities proper for the solvated molecule. The comparison of the presented expressions with the LR and SS ones enlightens the physical meaning of the terms included or neglected by these approaches and shows that SS agrees with the results of the four-level model, while LR includes a term classified as dispersion in previous treatments and neglects another related to electrostatic. A discussion on the possible origin of the LR flaw is finally given
Excitonic splitting in polymeric materials: a quantum-mechanical model including interchain interactions and dielectric effects
No full textWe present a quantum mechanical model for the calculation of the excitonic splitting of conjugated molecu- lar materials; both short- and long-range interchain effects are explicitly included. The model is based on the time-dependent (TD) density functional approach and it introduces the effects of the proximate molecular systems in a perturbative framework. The new important aspect of our model is that both the single chain properties and the interchain effects are evaluated in the presence of an embedding environment which is modeled to mimic the dielectric interactions of the distant chains. This environment is here approximated with a continuum anisotropic dielectric. Such anisotropy is introduced to take into account the different dielectric properties of crystals (or films) of conjugated molecular systems along and perpendicular to the direction of the chains. In the model the dielectric environment is directly introduced in the quantum-mechanical equations through proper operators to be added to the Hamiltonian. An application to oligomers of polyacetylene quan- tifies the relative importance of the adjacent chains as well as of the dielectric medium showing the funda- mental role played by the latter toward a direct comparison with experimental data
Medium Effects on the Properties of Chemical Systems: Electric and Magnetic Response of donor-acceptor systems within the Polarizable Continuum Model
No full textAs a companion study to the overview of the methods recently implemented in the polarizable continuum model ŽPCM. for the calculation of molecular properties in condensed media, we present a homogeneous set of results regarding electric and vibrational components of dipole Žhyper.polarizabilities, nuclear magnetic shieldings, and vibrational circular dichroism ŽVCD. spectra. The attention is mainly focused on some molecules, all of formula C5H7NO, which contain the same donor and acceptor groups but have different structures. The analysis of medium effects on these and some other related molecules is here centered on water as solvent. Different computational approaches are exploited: CPHF and SOS for Žhyper.polarizabilities, GIAO and LORG for magnetic shieldings, and GIAO also for VCD spectra
Analytical Free Energy Second Derivatives with respect to Nuclear Coordinates: a Complete Formulation for Electrostatic Continuum Solvation Models
No full textWe present the theory and the implementation of analytical free energy second derivatives with respect to nuclear displacements for a molecular solute described within the framework of the polarizable continuum model. The formulation applies to a cavity with an accurately modeled molecular shape and it permits a complete consideration of all aspects of the solvation model. In particular, the implementation uses the recently proposed method known as the integral equation formalism IEF, and it can be applied to Hartree–Fock and to density functional calculations. The analysis of both formal and technical features is reported as well as some numerical applications to solvatochromic shifts in IR vibrational frequencies and to transition state searches for reactions in solutions
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