1,720,980 research outputs found
Calorimetric investigation of the aggregation of lithium perfluorooctanoate on poly (ethyleneglycol) oligomers in water
No full textThe aim of this work is to investigate the effect of the nature of the counterion on the thermodynamics of aggregation of an ionic perfluorosurfactant on poly(ethyleneglycol) (PEG) of varying molecular weight in aqueous solution. The physicochemical properties of micellization and aggregation on PEG of lithium perfluorooctanoate (LiPFO) were investigated at 298.15 K by microcalorimetry, electrical conductivity, and viscosity. They were then compared with the same properties measured for the CsPFO/PEG system. As in the latter system, when wrapping around LiPFO micellar clusters the PEG polymers undergo a conformational change and form a compact complex. This complex reaches a constant thermodynamic behaviour for molecular weights larger than ∼2400 Da, which nicely matches the value ∼2600 Da found for CsPFO-PEG aggregates. The electrostatic screen provided by the polymer chain practically cancels the differences in the thermodynamic properties of the free micelles. This leads to thermodynamic parameters of aggregation of the two salts on the polymers, which are practically independent of the nature of the counterion
On the Metric of Charge Transfer Molecular Excitations: A Simple Chemical Descriptor
No full textA new index is defined with the aim of further exploring the
metric of excited electronic states in the framework of the time-dependent
density functional theory. This descriptor, called Δr, is based on the charge
centroids of the orbitals involved in the excitations and can be interpreted in
term of the hole−electron distance. The tests carried out on a set of
molecules characterized by a significant number of charge-transfer excitations
well illustrate its ability in discriminating between short (Δr ≤ 1.5 Å) and
long-range (Δr ≥ 2.0 Å) excitations. On the basis of the well-known pitfalls
of TD-DFT, its values can be then associated to the functional performances
in reproducing different type of transitions and allow for the definition of a
“trust radius” for GGA and hybrid functionals. The study of other systems, including some well-known difficult cases for other metric descriptors, gives further evidence of the high discrimination power of the proposed index. The combined use with other density or orbital-based descriptors is finally suggested to have a reliable diagnostic test of TD-DFT transitions
Communication: One third: A new recipe for the PBE0 paradigm
No full textWe analyze the performances of the parameter-free hybrid density functional PBE0-1/3 obtained combining the PBE generalized-gradient functional with a predefined amount of exact exchange of 1/3, as recently discussed by Cortona [J. Chem. Phys.136, 086101 (Year: 2012)10.1063/1.3690462]. The numerical results that we have obtained for various properties, such as atomization energies (G2-148 dataset), weak interactions (NCB31 dataset), hydrogen-bond length optimizations, and dissociation energies (HB10 dataset), and vertical excitation energies, show an increased performance of PBE0-1/3 with respect to the widely used PBE0. We therefore propose to use one third as the mixing coefficient for the PBE-based hybrid functional
Control of Coherences and Optical Responses of Pigment-Protein Complexes by Plasmonic Nanoantennae
No full textThe key for light-harvesting in pigment-protein complexes are molecular excitons, delocalized excited states comprising a superposition of excitations at different molecular sites. There is experimental evidence that the optical response due to such excitons can be largely affected by plasmonic nanoantennae. Here we employ a multiscale approach combining time-dependent density functional theory and polarizable classical models to study the optical behavior of the LH2 complex present in bacteria when interacting with a gold nanorod. The simulation not only reproduces the experiments but also explains their molecular origin. By tuning the chromophoric unit and selectively switching on/off the excitonic interactions, as well as by exploring different setups, we clearly show that the dramatic enhancement in the optical response, unexpectedly, is not accompanied by changes in the coherences. Instead polarization effects are dominant. These results can be used to design an optimal control of the light-harvesting process through plasmonic nanoantennae
Benchmarking Time-Dependent Density Functional Theory for Excited State Geometries of Organic Molecules in Gas-Phase and in Solution
No full textWe analyze potentials and limits of the Time- Dependent Density Functional Theory (TD-DFT) approach for the determination of excited-state geometries of organic molecules in gas-phase and in solution. Three very popular DFT exchange- correlation functionals, two hybrids (B3LYP and PBE0) and one long-range corrected (CAM-B3LYP), are here investigated, and the results are compared to the correlated RI-CC2 wave function approach. Solvent effects are further analyzed by means of a polarizable continuum model. A total of 15 organic chromophores (including both small molecules and larger push−pull systems) are considered as prototypes of n → π* and π → π* singlet excitations. Our analysis allows to point out specific correlations between the accuracy of the various functionals and the type of excitation and/or the type of chemical bonds involved. We find that while the best ground-state geometries are obtained with PBE0 and B3LYP, CAM- B3LYP yields the most accurate description of electronic and geometrical characteristics of excited states, both in gas-phase and in solution
On the TD-DFT Accuracy in Determining Single and Double Bonds in Excited-State Structures of Organic Molecules
No full textWe present an analysis on the behavior of the TD-DFT approach in the determination of excited-state structures with particular attention to single and double bonds. The analysis is based on a direct comparison with the highly correlated CASPT2 ab initio approach. Six DFT exchange-correlation functionals differing in the
Hartree-Fock exchange percentage and the type of correlation functional are considered and applied to
the study of seven prototype organic molecules characterized by two families of excitations (acrolein, acetone, diazomethane, and propanoic acid anion for n-π* and cis-1,3-butadiene, trans-1,3-butadiene, and pyrrole for π-π*), and three protonated Schiff bases, used as model chromophores for 11-cis retinal. Our analysis allows pinpointing specific correlations between accuracy of the various functionals and category of excitation and/or type of chemical bond involved in the corresponding geometry relaxation. We confirm the role of the long-range correction of the potential to obtain a balanced description of excitation energies and excited-state structures, but we also point out that, for a small system, B3LYP and PBE0 also give results close to CASPT2
Planar versus twisted intramolecular charge transfer mechanism in Nile Red: new hints from theory
No full textUsing a time-dependent density functional theory approach and taking into account bulk solvent effects, we investigate the absorption and fluorescence spectra of Nile Red. In particular, we have assessed both the planar and twisted intramolecular charge transfer mechanism by using a panel of exchange correlation functionals including both global and range-separated hybrids, refined solvent models and the simulation of vibronic couplings. It turned out that the appropriate choice of the functional is of prime importance to obtain, not only quantitatively accurate values, but also qualitatively correct evolution of the spectral features with respect to the dihedral angles of the amino group. At the light of this study, the interpretation of the experimental data is critically re-examined and compared to typical dual-fluorescent molecules
Structures and Properties of Electronically Excited Chromophores in Solution from the Polarizable Continuum Model Coupled to the Time-Dependent Density Functional Theory
No full textThis 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
Negative solvatochromism of push–pull biphenyl compounds: a theoretical study
No full textWe have investigated the negative solvatochromism observed for 4-(phenyloxido)-N-methylpyridinium in solvents of varying polarity using a hierarchy of solvation models (continuum, discrete, and mixed) combined with a (TD)DFT description. Overall, the significant hypsochromic shift measured in the experiments is reproduced qualitatively and quantitatively through calculations. More in detail, the role played by the solvent in tuning the spectral properties of POMP has been correlated with geometrical and electronic effects, and the π–π aggregation effects of POMP have been shown to be important in solvents of low and medium polarity
The Fate of a Zwitterion in Water from Ab-Initio Molecular Dynamics: Monoethanolamine (MEA)-CO2
No full textUnderstanding the fundamental reactions accompanying the capture of carbon dioxide in amine solutions is critical for the design of high-performance solvents and requires an accurate modeling of the solute–solvent interaction. As a first step toward this goal, using ab initio molecular dynamics (Car–Parrinello) simulations, we investigate a zwitterionic carbamate, a species long proposed as intermediate in the formation of a stable carbamate, in a dilute aqueous solution. CO2 release and deprotonation are competitive routes for its dissociation and are both characterized by free-energy barriers of 6–8 kcal/mol. Water molecules play a crucial role in both pathways, resulting in large entropic effects. This is especially true in the case of CO2 release, which is accompanied by a strong reorganization of the solvent beyond the first coordination shell, leading to the formation of a water cage entrapping the solute (hydrophobic effect). Our results contrast with the assumptions of implicit solvent mod
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