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In this study, we analyze the photophysics of the N-methyl-6-oxyquinolinium betaine (MQ) solvatochromic dye in aqueous solution, focussing on the important structural rearrangement of its first solvation shell following the electronic excitation, which is characterized by a strong charge transfer. To this aim, we compare the results provided by ground- and excited-state ab-initio molecular dynamics with that of full QM calculations on clusters including a small number of solvent molecules (from 1 to 4), taking into account bulk solvent effects by the Polarizable Continuum Model. The two methods agree in predicting that while in the ground electronic state between three and four water molecules are strongly coordinated to the oxygen atom of MQ, in the excited state two water molecules are present in the first solvation layer of the MQ oxygen. Vertical excitation and emission energies computed on the structures provided by the two approaches allow for estimates of the Stokes shift consistent with the experimental results. On the ground of the present results, some general considerations on the advantages and limitations of the dynamical and static approaches in describing a photoactivated process in solution are proposed. [ABSTRACT FROM AUTHOR]

A new descriptor tool enabling to qualitatively identify excited-state potential energy regions with high decay probability is here disclosed and applied to analyze a photoinduced ring-opening reaction already well characterized from the experimental and theoretical point of view. The analysis based on such descriptor allows one to highlight a high probability of excited-state deactivation within the Franck–Condon region, in agreement with experiments, and to qualitatively indentify the main mechanisms providing efficient pathways of photoreactivity. [ABSTRACT FROM AUTHOR]

In spite of the detailed information provided by advanced time-resolved spectroscopy, the understanding of the excited-state proton transfer (ESPT) reactivity remains difficult to obtain at molecular level. In this work we studied three photoacids showing different strength: the 8-hydroxy-1,3,6-pyrenetrisulfonate weak photoacid, the N-methyl-6-hydroxyquinolinium strong photoacid and the phenol-carboxyether dipicolinium cyanine (QCy9) superphotoacid, focusing on the intermolecular ESPT toward a solvent molecule or a base molecule in aqueous solution. To this aim, the ground and the first singlet excited-state potential energy surfaces of the three systems were characterized by means of the time-dependent density functional theory and a hybrid implicit/explicit model of the solvent. Main structural and photophysical features of the photoacids were assessed and satisfactorily compared with the experimental data. Energy profiles along the PT coordinate were analyzed in both the electronic states. We reproduced many important features of the photoacidity experimentally observed. The results suggest that the relative strength is mainly due to the different extent of charge transfer caused by the electronic transition in proximity of the acid group. Remarkably, we found that even in the case of the strongest photoacid (QCy9), showing a ESPT rate as rapid as to escape the solvent dynamics control, the PT is modulated and supported by the first solvation shell of the proton-accepting molecule. However, a complete understanding of this fascinating field needs the full account for the electronic and the molecular dynamics in play at different timescales. [ABSTRACT FROM AUTHOR]

Most spectroscopic parameters are influenced by nuclear dynamics and by the chemical environment. However, proper inclusion of these effects still represents a challenge in computational spectroscopy studies. In many cases, a route coupling satisfactory accuracy with reasonable computational costs consists in the integration of DFT-based methods to compute spectroscopic parameters, with ab initio molecular dynamics simulations to sample from the classical phase space of the system. Here, we discuss the application of this approach in two case studies of remarkable practical interest, namely the simulation of the absorption spectrum of 9-methyladenine, an adenine nucleoside model; and the prediction of electron spin resonance parameters for nitroxyl radicals, the prototypical spin probes. In both cases, the accuracy of the results increases significantly when the subtle interplay of intra-molecular dynamics and solvent effects is introduced. [ABSTRACT FROM AUTHOR]

In this work, the GLOB model, an effective and reliable computational approach well suited for ab initio and QM/MM molecular dynamics simulations of complex molecular systems in solution, has been applied to study two representative open-shell systems, the cobalt(II) ion and the glycine radical in aqueous solution, with special reference to their structural and magnetic properties. The main structural features of the solvent cage around the cobalt ion and the hydrogen bonding patterns around the neutral and zwitterionic forms of the glycine radical have been investigated in some detail. The general good agreement with experiments supports the use of the present model to investigate more challenging and biological/technological relevant open-shell systems. [ABSTRACT FROM AUTHOR]

In this work, we present an effective and flexible computational approach, which is the result of an on-going development conducted in our group, for simulating complex solute–solvent systems and computing relevant spectroscopic observables. Such an approach is based on QM/MM molecular dynamics techniques using non-periodic conductor boundary conditions and localized basis sets, combined with a posteriori high-level quantum mechanical methods for the calculation of spectroscopic parameters. As illustrative applications, we report structural and spectroscopic analyses of acetone, acrolein and glycine radical in aqueous solutions, where solvent effects on the NMR chemical shifts, UV absorption spectrum and EPR hyperfine coupling constants, respectively, are investigated and favorably compared to experimental measurements. In particular, it will be shown the importance of including dynamical effects in order to reproduce experimental data accurately. Moreover, we present an infrared analysis of formamide in both gas phase and acetonitrile from first-principle molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

A modified A MBER force field has been developed and used to compute UV and NMR spectra of acetone in aqueous solution by an integrated computational tool rooted in the density functional theory, the polarizable continuum model, and classical molecular dynamics. The results show that, provided that classical force fields are carefully reparameterized and validated, they can be part of a robust and effective approach, which can be used also by non-specialists and provides a general and powerful complement to experimental techniques. [ABSTRACT FROM AUTHOR]

The characterization of fundamental vibrational levels from the analysis of ab-initio dynamics is discussed. Results from test calculations are compared with those obtained by a second-order perturbative procedure to include anharmonicity exploiting the same ab-initio potential. The influence of the single molecular modes on vibrationally averaged properties calculated on the fly during the dynamics is also analyzed. Formaldehyde, both in the gas phase and in acetonitrile solution, and the vinyl radical in the gas phase have been chosen as test molecules, while the hyperfine coupling constants of the open shell system is computed as an example of molecular properties. Results allow for a quantitative comparison with experimental data. [ABSTRACT FROM AUTHOR]

This paper describes the extension of our computational strategy for pK predictions of small molecules to large solutes. The basic computational tool results from the coupling of quantum mechanical methods rooted in the density functional theory with the most recent version of the Polarizable Continuum Model. However, a third level is introduced, which includes solute regions far from the reactive center, which are described at a simplified level. This partition, together with the recent implementation of fast cavity generation, powerful iterative solvers, and fast multipole technology, allows us to tackle solutes of the dimension of a small protein. The problems and perspectives of this methodology are analyzed with special reference to the behavior of different Polarizable Continuum Model versions on the challenging playground represented by the pK’s of the different histidine residues occurring in the human prion protein. [ABSTRACT FROM AUTHOR]