Carlotto, Silvia (2008) Modeling of dynamic solvation effects. [Tesi di dottorato]
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Reactivity of molecular and supramolecular systems is greatly modified by the surrounding environment, often a fluid medium, and an active area of research is nowadays the study of the influence of a solvent structure on the static and dynamic properties of photo-active and paramagnetic probes, varying solvent properties, sample geometry and external perturbations.
Standard continuum solvent theories are based on crude representations of the probe. Solvation processes depend in a specific way upon the structure of the solute, and in particular on molecular features as shape, flexibility, distribution of charges and anisotropy of the polarizability. Augmented solvent continuum approaches have been developed to interpret chromophore dynamics to account for persistent solvent local structures. Description of collective solvent modes is also necessary to understand relaxation processes affecting dynamics at longer times, in complex fluid environments: phase transitions in supercooled liquids, rheological properties of emulsions and colloids, confinement effects and finally micro and nano-probes dynamics. The inclusion of solvent effects is of great importance, in order to understand the physical mechanisms responsible of the tuning of the optical properties and therefore to the ultimate possibility to design nanomaterials with specific optical response. Theoretical methodologies based on stochastic and hydrodynamic modeling have proven over the years to be a powerful approach, especially when coupled with advanced quantum mechanical treatments, to describe effectively the dynamical aspects of solvation. The relationship between spectroscopic measurements and molecular properties can be gathered only indirectly, that is, structural and dynamic molecular characteristics can be inferred by the systematic application of modelling and numerical simulations to interpret experimental observables. A straightforward way to achieve this goal is the employment of spectroscopic evidence as the "target" of a fitting procedure of molecular, mesoscopic and macroscopic parameters entering the model. A more refined methodology is based on the combination of quantum mechanical calculations of structural parameters possibly including environmental and fast vibrational and librational averaging, and direct feeding of calculated molecular parameters into dynamic models based on molecular dynamics, coarse grain dynamics, and above all stochastic modeling or a combination of the three.
Our main objective in this PhD work has been to discuss the degree of progress of advanced theoretical models are explored, aimed at clarifying the influence of solvent-driven relaxation processes on optic, magnetic and rheological observables. In particular we have developed integrated computational approaches to the interpretation of fluorescence emission of organic molecules in solvated environments, CW-ESR spectroscopy and rheological properties of ordered systems via combination of advanced quantum mechanical approaches, stochastic modeling of relaxation processes, and, in the last case, macroscopic models.
In the first period we have shown that the model proposed is able to reproduce the spectral position and shape of the emission spectra. In particular the model reproduces the red shift expected for TICT excited states when the dielectric constant of the solvent increases. We developed a stochastic approach to the interpretation of the emission fluorescence of 4-(N,N-dimethylamino) benzonitrile (DMABN). Than we proceed by extending the modeling approach, in which internal degrees of freedom are coupled with an effective solvent relaxation variable. The extension of the model is applied to the simulation of the emission spectra of DMABN-Crown5, a DMABN derivative. Evaluation of potential energy surfaces using advanced QM approach and estimates of dissipative parameters based on hydrodynamic arguments are discussed. Emission fluorescence is calculated by solving a diffusion/sink/source equation for the stationary population of excited state, and compared to experimentally measured emission fluorescence of DMABN and DMABN-Crown5. Next we developed the complete a priori simulation of the ESR spectra of complex systems in solution. The usefulness and reliability of the method are demonstrated on the very demanding playground represented by the tuning of the equilibrium between 310- and ?-helices of polypeptides by different solvents. The starting point is good agreement between computed and X-ray diffraction structures for the 310-helix adopted by the double spin-labelled heptapeptide Fmoc-(Aib-Aib-TOAC)2-Aib-OMe. Next, density functional computations, including dispersion interactions and bulk solvent effects, suggest another energy minimum corresponding to an ?-helix in polar solvents, which, eventually, becomes the most stable structure. Computation of magnetic and diffusion tensors provides the basic ingredients for the building of complete spectra by methods rooted in the Stochastic Liouville Equation (SLE). The remarkable agreement between computed and experimental spectra at different temperatures allowed us to identify helical structures in the various solvents. The generality of the computational strategy and its implementation in effective and user-friendly computer codes pave the route toward systematic applications in the field of biomolecules and other complex systems. Finally, the purpose of the last part of the PhD period has been to analyze the dynamical behavior of a low viscosity nematic liquid crystals in presence of micro-size probe. We present a study of the translational friction coefficients of spherical and ellipsoidal probes moving in nematic liquid crystalline fluids, by solving numerically the constitutive hydrodynamic equations of nematic. The evaluation of the translational friction coefficients is based on a numerical solution of Leslie-Ericksen constitutive equations for the case of incompressible nematic fluids. The nematic medium is described by a vector field which specifies the director orientation in each point and by the velocity vector field. Simulation of director dynamics surrounding the moving probe are presented, and the dependence of translational diffusion upon liquid crystal viscoelastic parameters is discussed. The time evolution of director field, described by Leslie-Ericksen equations, is studied in the presence of an orienting magnetic field in two characteristic situations: director of motion parallel and perpendicular to field.
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