Aurora Muñoz-Losa

Solvatochromic Shifts on Absorption and Fluorescence Bands of N,N-Dimethylaniline.

A theoretical study of the absorption and fluorescence UV/vis spectra of N,N-dimethylaniline in different solvents has been performed, using a method combining quantum mechanics, molecular mechanics, and the mean field approximation. The transitions between the three lowest-lying states have been calculated in vacuum as well as in cyclohexane, tetrahydrofuran, and water. The apparent anomalies experimentally found in water (a blue shift in the absorption bands with respect to the trend in other solvents, and an abnormally high red shift for the fluorescence band) are well reproduced and explained in view of the electronic structure of the solute and the solvent distribution around it. Additional calculations were done with a mixture of cyclohexane and tetrahydrofuran as solvent, which displays a nonlinear solvatochromic shift. Results, although not conclusive, are consistent with experiment and provide a possible explanation for the nonlinear behavior in the solvent mixture.

Excited State Pathways Leading to Formation of Adenine Dimers.

The reaction intermediate in the path leading to UV-induced formation of adenine dimers A═A and AA* is identified for the first time quantum mechanically, using PCM/TD-DFT calculations on (dA)2 (dA: 2deoxyadenosine). In parallel, its fingerprint is detected in the absorption spectra recorded on the millisecond time-scale for the single strand (dA)20 (dA: 2deoxyadenosine).

Solvent Effects on the Structure and Spectroscopy of the Emitting States of 1-Phenylpyrrole.

Theoretical calculations of absorption and fluorescence properties of 1-phenylpyrrole have been performed, at the CASPT2//CASSCF level, in the gas phase and in acetonitrile solution, using in the latter case the ASEP/MD method. In addition to a locally excited state, it was also possible to identify a candidate intramolecular charge transfer state that could explain the second red-shifted fluorescence band that appears in polar solvents. In the gas phase, the charge transfer state is found to lie higher in energy than the locally excited state and the Franck-Condon absorption state, making it unlikely to be reached under these conditions. In acetonitrile solution, the charge transfer state is stabilized and lies much closer to the locally excited state, becoming accessible after absorption. The results indicate that the free-energy surface of the charge transfer state is very flat in solution, and several geometries are possible, ranging from almost planar to twisted and bent. Solvent caging and transition probabilities favor emission from structures with a small twist angle between the rings and without a pyramidal atom.

Correlated ab initio molecular dynamics simulations of the acetone–carbon dioxide complex: implications for solubility in supercritical CO2

Ab initio molecular dynamics simulations of the acetone–CO2 complex (MP2/6-31G(d) level) were performed to investigate the effect of dynamics at finite temperature on the weak electron donor–acceptor intermolecular interactions. In addition, we carried out a study of the free energy of formation of the complex by means of umbrella sampling technique at the MP2 level with a perturbative CCSD(T) correction. The potential of mean force was obtained along a reaction coordinate describing the acetone–CO2 interaction. The results obtained here support some hypothesis that we already explored in past works using static electronic calculations. In particular, when interacting with a molecule having a carbonyl function, carbon dioxide displays both Lewis acid and Lewis base behaviour. This property can be exploited to design molecular systems that are easily solubilised in supercritical CO2.

Dual Fluorescence of Fluorazene in Solution: A Computational Study.

The fluorazene molecule presents dual fluorescence in polar solvents. Its absorption and emission properties in gas phase and in acetonitrile solution have been studied theoretically using the complete active space second-order perturbation//complete active space self-consistent field quantum methodology and average solvent electrostatic potential from molecular dynamics for the solvent effects. In gas phase, two optimized excited-state geometries were obtained, one of them corresponds to a local excitation (LE), and the other is an intramolecular charge transfer (ICT) and lies higher in energy. In acetonitrile solution, a second ICT structure where the molecule remains planar is found, and the energy differences are reduced. Fluorescence energies from LE and the planar ICT have a good agreement with the experimental bands, but emission from the bent ICT has too low an energy.

Theoretical study of the conformational equilibrium of 1,4-dioxane in gas phase, neat liquid, and dilute aqueous solutions

AbstractnThe conformational equilibrium of 1,4-dioxane in the gas phase, in the pure liquid, and in aqueous solution has been studied by means of the Average Solvent Electrostatic Potential from Molecular Dynamics (ASEP/MD) method and the Integral Equation Formalism for the Polarizable Continuum Model (IEF-PCM). The dioxane molecule was described at the DFT(B3LYP)/aug-cc-pVTZ level. In the three phases, the equilibrium is almost completely shifted toward the chair conformer, with populations of the twist-boat conformers lower than 0.01xa0%. The equilibrium is dominated by the internal energy of the molecule, as the solute–solvent interaction free energies are very similar in the three conformers considered (chair, 1,4 twist-boat, and 2,5 twist-boat). In the pure liquid, where the dioxane–dioxane interaction is dominated by the Lennard-Jones term, the structure is characteristic of a van der Waals liquid. However, the decrease in the C–H distance from gas phase to solution, the increase in the C–H vibrational frequencies, and the presence of a shoulder in the O–Haxial pair radial distribution function point to the presence of a weak C–H–O hydrogen bond. The analysis of the occupancy maps of water oxygen and hydrogen atoms around the 1,4-dioxane molecule confirms this conclusion. Contrary to what is found in small water-dioxane clusters, in the liquid, there is a preference for oxygen atoms to interact with axial hydrogen atoms to form C–H–O hydrogen bonds. Comparison of ASEP/MD and IEF-PCM results indicates that including specific interactions is very important for an adequate description of the solute–solvent interaction; however, the influence of these interactions does not translate in changes in the relative stability of the conformers because it cancels out when energy differences are calculated.

The optical properties of adenine cation in different oligonucleotides: a PCM/TD-DFT study

The absorption spectra of several systems containing an adenine (A) cation are computed in water by TD-DFT calculations, using three different functionals and including solvent effects by a mixed discrete–continuum approach. Our calculations well reproduce the experimental absorption spectrum of deoxyadenosine nucleoside cation (dA+), which provides an intense peak at 350xa0nm and a weaker one at 600xa0nm, allowing its assignment. M052X and CAM-B3LYP predict that the hole is essentially localized on a single A base also in the cationic forms of dApdA dinucleotide, both in single and in double strand, and of (dApdT) duplex, exhibiting absorption spectra similar to that of dA+. For all these compounds, B3LYP predicts instead the delocalization of the hole over two A bases. On this ground, it is possible to propose which would be the spectral signature of partial hole delocalization. Vertical and adiabatic ionization potentials for the compounds under investigation are also computed, providing values in good agreement with the available experimental results.

How Methylation Modifies the Photophysics of the Native All-Trans Retinal Protonated Schiff Base: A CASPT2/MD Study in Gas Phase and in Methanol

A comparison between the free-energy surfaces of the all- trans-retinal protonated Schiff base (RPSB) and its 10-methylated derivative in gas phase and methanol solution is performed at CASSCF//CASSCF and CASPT2//CASSCF levels. Solvent effects were included using the average solvent electrostatic potential from molecular dynamics method. This is a QM/MM (quantum mechanics/molecular mechanics) method that makes use of the mean field approximation. It is found that the methyl group bonded to C10 produces noticeable changes in the solution free-energy profile of the S1 excited state, mainly in the relative stability of the minimum energy conical intersections (MECIs) with respect to the Franck-Condon (FC) point. The conical intersections yielding the 9- cis and 11- cis isomers are stabilized while that yielding the 13- cis isomer is destabilized; in fact, it becomes inaccessible by excitation to S1. Furthermore, the planar S1 minimum is not present in the methylated compound. The solvent notably stabilizes the S2 excited state at the FC geometry. Therefore, if the S2 state has an effect on the photoisomerization dynamics, it must be because it permits the RPSB population to branch around the FC point. All these changes combine to speed up the photoisomerization in the 10-methylated compound with respect to the native compound.

Accelerating QM/MM Calculations by Using the Mean Field Approximation

It is well known that solvents can modify the frequency and intensity of the solute spectral bands, the thermodynamics and kinetics of chemical reactions, the strength of molecular interactions or the fate of solute excited states. The theoretical study of solvent effects is quite complicated since the presence of the solvent introduces additional difficulties with respect to the study of analogous problems in gas phase. The mean field approximation (MFA) is used for many of the most employed solvent effect theories as it permits to reduce the computational cost associated to the study of processes in solution. In this chapter we revise the performance of ASEP/MD, a quantum mechanics/molecular mechanics method developed in our laboratory that makes use of this approximation. It permits to combine state of the art calculations of the solute electron distribution with a detailed, microscopic, description of the solvent. As examples of application of the method we study solvent effects on the absorption spectra of some molecules involved in photoisomerization processes of biological systems.