Reliable computations of NMR and EPR spectroscopic parameters for flexible molecules in condensed phases require a proper account of stereo-electronic, dynamic, and environmental effects. In the framework of density functional theory methods, these effects can be introduced by second-order vibrational perturbation theory and polarisable continuum models. Two test studies illustrate potentialities of the integrated computational approach we have developed in the last years for these purposes. Further extensions to mixed discrete-continuum models and stochastic approaches for long-time dynamical effects are also sketched.

We present a new quantum chemical method for the calculation of the equilibrium geometry and the harmonic vibrational frequencies of molecular systems in dense medium at high pressures (of the order of GPa). The new computational method, named PCM-XP, is based on the polarizable continuum model (PCM), amply used for the study of the solvent effects at standard condition of pressure, and it is accompanied by a new method of analysis for the interpretation of the mechanisms underpinning the effects of pressure on the molecular geometries and the harmonic vibrational frequencies. The PCM-XP has been applied at the density functional theory level to diborane as a molecular system under high pressure. The computed harmonic vibrational frequencies as a function of the pressure have shown a satisfactory agreement with the corresponding experimental results, and the parallel application of the method of analysis has reveled that the effects of the pressure on the equilibrium geometry can be interpreted in terms of direct effects on the electronic charge distribution of the molecular solutes, and that the effects on the harmonic vibrational frequencies can be described in terms of two physically distinct effects of the pressure (curvature and relaxation) on the potential energy for the motion of the nuclei.

In this paper, a computational strategy, based on DFT calculations at the M06-2X level, combined with the polarizable continuum model, the Hessian matrix reconstruction method and the Partial hessian vibrational approach is applied to evaluate inter- and intra-layer vibrational couplings between hydrogen bonded and stacked DNA base pairs. The present work demonstrates that this computational scheme can effectively predict and interpret the vibrational couplings of nucleic acids in solution. The effect of the environment described in a cluster or in a continuum manner is necessary in order to improve the agreement with the experimental values.

The effect of the cavity field (CF) on electronic circular dichroism spectra simulated using a polarizable continuum model (PCM) is analyzed. The difference between classical and effective electric field, arising from the charges induced on the surface of the cavity by the radiation field, is shown to have a nonnegligible effect on the rotatory strengths computed within a TDDFT PCM in the case of R-(+)-3-methyl-cyclopentanone, a chiral molecule whose ECD spectrum, both in the gas and in several solvents, has been studied both experimentally and theoretically in recent times. PCM CF factors are on the order of 10% smaller than predicted by the Onsager model for the lowest, Si excited state, in the two conformers known to accommodate almost the whole population at room temperature, in the gas phase and in solution. The percentage increases quite substantially for the second, S2 excited electronic state. In PCM, CF factors are not merely scalar multiplicative factors for the gas-phase rotational strengths. Instead, they show an anisotropy along the axes of the cavity a consequence of the adoption of nonspherical cavities. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 826-838, 2011

In this article, the most advanced extensions of solvation models to chiroptical properties of solvated systems will be reviewed. The main aspects determining the complex phenomenon of solvation will be first discussed in terms of the physical interactions beyond them and successively translated in a computational language introducing the specific models. A particular attention will be devoted to the family of solvation models which couple a quantum-mechanical description of the molecular solute and a continuum description of the solvent. The theoretical analysis will be supported with examples of applications showing the potentialities of the models and their accuracy in capturing the essence of solvent effects on optically active molecules.

We report here a series of works(1-5) devoted to the modelling of the liquid phase for quantum chemistry calculations. The question under consideration is the computation of the electrostatic interaction between charge densities in the presence of a continuum dielectric medium. It consists of solving an elliptic problem of the form -div(epsilon(x)del V) = rho where the field epsilon(x) exhibits a discontinuity on a closed surface. We show how an enhanced use of integral equations methods has recently led to significant progress in this field: reduction of the computational cost in the standard cases, extension of existing methods to sophisticated cases out of reach so far, development of new possibilities. This work has a wide range of applications in chemistry and biology.

The characteristics of a computational method addressing the description of solvent effects are here presented in a concise way, by selecting for the presentation a topic of large interest, i.e., the electronic, electric, and magnetic properties of molecules. The selection of these properties imposes a choice on the computational methods. They must be of quantum ab initio nature, and possibly treated at a high level of the quantum theory. In such a framework, continuum models are more adequate than the alternative models based on a discrete description of the solvent, because of the high computational cost associated with discrete models coupled to high quantum mechanics levels. (C) 2011 John Wiley & Sons, Ltd. WIREs Comput Mol Sci 2011 1 855-867 DOI: 10.1002/wcms.54

Chapter 4 From Molecular Electrostatic Potentials to Solvation Models and Ending with Biomolecular Photophysical Processes Jacopo Tomasi, Jacopo Tomasi tomasi@dcci.unipi.it Università di Pisa, Dipartimento di Chimica e Chimica Industriale, via Risorgimento 35, I-56126 Pisa, ItalySearch for more papers by this authorChiara Cappelli, Chiara Cappelli chiara@dcci.unipi.it Università di Pisa, Dipartimento di Chimica e Chimica Industriale, via Risorgimento 35, I-56126 Pisa, ItalySearch for more papers by this authorBenedetta Mennucci, Benedetta Mennucci bene@dcci.unipi.it Università di Pisa, Dipartimento di Chimica e Chimica Industriale, via Risorgimento 35, I-56126 Pisa, ItalySearch for more papers by this authorRoberto Cammi, Roberto Cammi roberto.cammi@unipr.it Università di Parma, Dipartimento di Chimica, Viale delle Scienze 17/A, 43100 Parma, ItalySearch for more papers by this author Jacopo Tomasi, Jacopo Tomasi tomasi@dcci.unipi.it Università di Pisa, Dipartimento di Chimica e Chimica Industriale, via Risorgimento 35, I-56126 Pisa, ItalySearch for more papers by this authorChiara Cappelli, Chiara Cappelli chiara@dcci.unipi.it Università di Pisa, Dipartimento di Chimica e Chimica Industriale, via Risorgimento 35, I-56126 Pisa, ItalySearch for more papers by this authorBenedetta Mennucci, Benedetta Mennucci bene@dcci.unipi.it Università di Pisa, Dipartimento di Chimica e Chimica Industriale, via Risorgimento 35, I-56126 Pisa, ItalySearch for more papers by this authorRoberto Cammi, Roberto Cammi roberto.cammi@unipr.it Università di Parma, Dipartimento di Chimica, Viale delle Scienze 17/A, 43100 Parma, ItalySearch for more papers by this author Book Editor(s):Prof. Chérif F. Matta, Prof. Chérif F. Matta Mount Saint Vincent University, Department of Chemistry and Physics, Halifax, Nova Scotia B3M 2J6, Canada Dept. of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3Search for more papers by this author First published: 10 February 2010 https://doi.org/10.1002/9783527629213.ch4Citations: 10 AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onFacebookTwitterLinked InRedditWechat Summary This chapter contains sections titled: Introduction The Molecular Electrostatic Potential and Noncovalent Interactions among Molecules Solvation: the "Continuum Model" Applications of the PCM Method References Citing Literature Quantum Biochemistry RelatedInformation

This paper provides an overview of recent research activities concerning the quantum-mechanical description of structures and properties of electronically excited chromophores in solution. The focus of the paper is on a specific approach to include solvent effects, namely the polarizable continuum model (PCM). Such a method represents an efficient strategy if coupled to proper quantum-mechanical descriptions such as the time-dependent density functional theory (TDDFT). As a result, the description of molecules in the condensed phase can be extended to excited states still maintaining the computational efficiency and the physical reliability of the ground-state calculations. The most important theoretical and computational aspects of the coupling between PCM and TDDFT are presented and discussed together with an example of application to the study of the low-lying electronic excited states of push-pull chromophores in different solvents.

The computational study of excited states of molecular systems in the condensed phase implies additional complications with respect to analogous studies on isolated molecules. Some of them can be faced by a computational modeling based on a continuum (i.e., implicit) description of the solvent. Among this class of methods, the polarizable continuum model (PCM) has widely been used in its basic formulation to study ground state properties of molecular solutes. The consideration of molecular properties of excited states has led to the elaboration of numerous additional features not present in the PCM basic version. Nonequilibrium effects, state-specific versus linear response quantum mechanical description, analytical gradients, and electronic coupling between solvated chromophores are reviewed in the present contribution. The presentation of some selected computational results shows the potentialities of the approach.

A quantum mechanical investigation on the effects of the solvent and the structure on nonlinear optical activity of a class of merocyanine compounds has been conducted. The interplay of the two effects on the first hyperpolarizability, computed at density functional theory and second-order Møller-Plesset level, has been analyzed in combination with ground state properties and geometries and excited state energies and dipoles. A critical analysis of the simplified two-level model has also been presented.

A short exposition of activities in theoretical chemistry performed in Pisa in the past years is given here. The exposition is based on summaries of critical evaluations I did in the years. These summaries intend to show the influence Salvetti has had on my scientific formation and also testimony on the evolution of quantum and computational chemistry in Pisa and in parallel in the whole discipline. This series of evaluations have always interested Salvetti (perhaps with some influence on his activity as national coordinator of CNR research in Chemistry) and can be used now to define new research fields.

In a recent Account, Cramer and Truhlar presented a comparison between the SM8 method and standard versions of other continuum solvation models implemented in widely available quantum mechanical programs. In that Account, the SM8 model was found to lead to "considerably smaller errors for aqueous and nonaqueous free energies of solvation for neutrals, cations, and anions, with particularly good performance for nonaqueous data". Here, we demonstrate that competing solvation methods are indeed as accurate as the SM8 method, if they are applied with the same rigor.

The spectroscopic behavior of 6-propionyl-2-(N,N-dimethyl)aminonaphthalene (PRODAN) is investigated in different environments, ranging from homogeneous solutions of different polarities to diffuse interfaces mimicking membranes. The variety of experimental data as well as computational results present in the literature still do not clarify the nature of the emission process; in particular, it is not well-established whether fluorescence in such a molecule occurs from a planar or from a twisted intramolecular charge transfer state. The first part of the work is thus devoted to better understand how the electronic transition processes occur in homogeneous solvents. The effect of the medium polarity as well as the hydrogen bond formation are studied. In the second part of the paper, a first attempt to interpret the experimental results of PRODAN in unilamellar vesicles is carried out. The complexity of the still-open questions about the photophysics of PRODAN has prompted us to base the study on quantum-mechanical calculations performed at various levels of theory, namely, DFT, TDDFT, CIS, and SAC-CI, and to include the effects of the environment in a self-consistent way. This is achieved by using the integral equation formalism version of the polarizable continuum model (IEFPCM). IEFPCM is a quite versatile approach, being able to treat equilibrium and nonequilibrium solvation in both homogeneous and heterogeneous media.

An attempt has been made to understand the mechanism of excited-state molecular solvation and its effect on hydrogen bonding in carbonyl compounds in aqueous solution. The correlation between solvation and electronic transitions has been investigated by comparing results obtained either with a supermolecular description in terms of hydrogen-bonded clusters or with a combined method embedding such clusters with a polarizable continuum dielectric mimicking the bulk. water. Popular scalar fields such as molecular electrostatic potential and molecular electron density have been used as useful tools to probe the changes in the hydrogen bonding passing from ground to excited states in the gas as well as solvent phase.

In this work we present a quantum-mechanical study on the structure and electronic spectra of three cationic dyes monomers and dimers: acridine orange (AO), proflavine (PF) and methylene blue (MB). The geometries were obtained from crystallographic data, the electronic properties were calculated with DFT (B3LYP functional) and the theoretical spectra were obtained with ZINDO. The solvation methodology adopted was the Integral Equation Formalism (IEF) version of the Polarizable Continuum Model (PCM). This study shows that the differences, even small, between optimized and crystal geometries are responsible for important spectral characteristics. Also, it indicates possible structures for interacting dimers.

The way that solvent (or host medium) modifies the rate of electronic energy transfer (EET) has eluded researchers for decades. By applying quantum chemical methods that account for the way solvent (in general any host medium including liquid, solid, or protein, etc.) responds to the interaction between transition densities, we quantify the solvent screening. We find that it attains a striking exponential attenuation at separations less than about 20 A, thus interpolating between the limits of no apparent screening and a significant attenuation of the EET rate. That observation reveals a previously unidentified contribution to the distance dependence of the EET rate.