523 research outputs found
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
How Solvent Controls Electronic Energy Transfer and Light Harvesting: Toward a Quantum-Mechanical Description of Reaction Field and Screening Effects
No full textThis paper presents a quantum-mechanical study of electronic energy transfer (EET) coupling
on over 100 pairs of chromophores taken from photosynthetic light-harvesting antenna proteins.
Solvation effects due to the protein, intrinsic waters, and surrounding medium are analyzed in terms of
screening and reaction field contributions using a model developed recently that combines a linear
response approach with the polarizable continuum model (PCM). We find that the screening of EET
interactions is quite insensitive to the quantum-mechanical treatment adopted. In contrast, it is greatly
dependent on the geometrical details (distance, shape, and orientation) of the chromophore pair
considered. We demonstrate that implicit (reaction field) as well as screening effects are dictated
mainly by the optical dielectric properties of the host medium, while the effect of the static properties is
substantially less important. The empirical distance-dependent screening function we proposed in a
recent letter (Scholes, G. D.; Curutchet, C.; Mennucci, B.; Cammi, R.; Tomasi, J. J. Phys. Chem. B
2007, 111, 6978-6982) is analyzed and compared to other commonly used screening factors. In
addition, we show that implicit medium effects on the coupling, resulting from changes in the transition
densities upon solvation, are strongly dependent on the particular system considered, thus preventing
the possibility of defining a general empirical expression for such an effect
Toward a Quantum-Mechanical Description of 2D-IR Spectra of Solvated Systems: The Vibrational Mode Coupling within A Polarizable Continuum Model
No full textThe extension of the polarizable continuum model (PCM) to evaluate solvent effects on vibrational coupling
is reported for both the transition dipole coupling and the Hessian matrix reconstruction (HMR) methods. A
comparative analysis of the two approaches is reported for a model system, i.e., formaldehyde dimers in
different spatial arrangements, with the aim of dissecting solvent effects in their two main contributions, the
modification of the transition dipole moments and the screening of their interaction. The HMR-PCM formalism
is finally applied to the evaluation of the vibrational coupling for (s)-N-methyl acetylproline amide in aqueous
and dichloromethane solutions. In the latter case, a comparison with experimental findings is presented and
used to gain a better understanding of the conformational state
Modelling the Solvation of Peptides. The case of (s)-N-Acetylproline Amide in Liquid Water
No full textThe structure and properties of (s)-N-acetylproline amide (NAP) in aqueous solution are studied by exploiting
a continuum solvation model. The conformational preference of NAP as a function of the environment is
discussed as well as data for a number of chiral and non-chiral spectroscopic and response properties (IR/
VCD, Raman/VROA, UV/CD, ORD, NMR), whose calculation with the accounting of solvent effects is now
possible due to recent developments introduced in the PCM approach. When available, calculated results are
compared with experimental data, so as to evaluate the quality of the continuum approach to the solvation of
this system
Toward a General Formulation of Dispersion Effects for Solvation Continuum Models
No full textWe revised the quantum model of Amovilli and Mennucci (J. Phys. Chem. B 1997, 101, 1051) to include the dispersion contribution to the solvation free energy within the framework of continuum models. Our revised formulation makes use of a single adjustable solvent dependent
parameter, and it can be readily generalized to different quantum mechanical descriptions. In particular, we made use of DFT and applied the model to investigate dispersion effects on vertical excitation energies within a time-dependent DFT framework. Our findings show that dispersion effects constitute a significant component of the absolute solvent effect but when relative solvent-solvent shifts are considered a cancellation effect is observed
Glycine and alanine: a theoretical study of solvent effects upon energetics and molecular response properties
No full textIn the present work, we show that polarisable continuum model is able to reproduce the stability of zwitterionic forms of amino acids alanine and glycine in water solution at B3-LYP/6-31G (d) level of theory. The model is then extended to the calculation of vibrational frequencies, Vibrational circular dichroism spectra and nuclear magnetic resonance chemical shifts. The agreement with experimental data is good, except in the case of vibrations where specific hydrogen bond interactions are involved. In the latter case, a supermolecular approach may help in the predictions of some vibrational frequencies of the groups which form hydrogen bonds. (C) 2000 Elsevier Science B.V. All rights reserved
Ab-initio model to predict NMR shielding tensors for solutes in liquid crystals
No full textWe present a model for the ab initio calculation of the nuclear magnetic shielding tensors for molecules in liquid crystalline solvents. The electrostatic interactions between the solute and the anisotropic solvent are described within the integral equation formalism (IEF) framework and the shielding tensors are calculated at the density functional theory linked to the gauge invariant atomic orbital (GIAO) approach. The IEF–GIAO approach is tested on the calculation of the shielding tensors for the different nuclei of CH3CN and C6H5NO2 in positive and negative anisotropic solvents
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