Chemical insight into electron density and wave functions: software developments and applications to crystals, molecular complexes and materials science

This paper overviews the work made by our group during the past 10–15 years on crystalline systems, semiconductor surfaces, molecular complexes and on materials of interest for technological applications, such as the defective silicon or the novel generation thermoelectric materials. Our main aim of extracting chemical insight into the analysis of electron densities and computed wave functions is illustrated through a number of examples. The recently proposed Source Function analysis is reviewed and a few of its more interesting applications are summarized. Software package developments, motivated by the need of a direct comparison with experiment or by the help these packages can provide for interpreting complex experimental outcomes, are described and future directions outlined. A particular emphasis is given to the TOPOND and TOPXD programs, which enable one to analyze theoretical and experimental crystalline densities using the rigorous framework of the Quantum Theory of Atoms in Molecules, due to Bader.     

Two-Phase Problems for Linear Elliptic Operators with Variable Coefficients: Lipschitz Free Boundaries are <Emphasis Type="Italic">C</Emphasis><Superscript><Emphasis Type="Italic">1,γ</Emphasis></Superscript>

We prove C ^1, regularity of Lipschitz free boundaries of two-phase problems for linear elliptic operators with Hölder continuous coefficients.     

Effect of the π Bridge and Acceptor on Intramolecular Charge Transfer in Push–Pull Cationic Chromophores: An Ultrafast Spectroscopic and TD‐DFT Computational Study

Three (donor–π–acceptor)⁺ systems with a methyl pyridinium or quinolinium as the electron‐deficient group, a dimethyl amino as the electron‐donor group, and an ethylene or butadiene group as the spacer have been investigated in a joint spectroscopic and TD‐DFT computational study. A negative solvatochromism has been revealed in the absorption spectra, which implies a solution color change, and interpreted by considering the variation in the permanent dipole moment modulus and orientation upon photoexcitation. The fluorescence efficiency decreases upon increasing solvent polarity, in agreement with the excited‐state optimized geometries (planar in low‐polarity media and twisted in high‐polarity media). Femtosecond transient absorption has revealed the occurrence of a fast photoinduced intramolecular charge transfer (ICT) and the molecular factors that determine an efficient ICT. Considering the crucial role of the ICT in tuning the nonlinear optical (NLO) properties, these compounds can be considered promising NLO materials.     

Solving Internal Habit Formation Models Through Dynamic Programming in Infinite Dimension

In this paper, we study an economic model, where internal habits play a role. Their formation is described by a more general functional form than is usually assumed in the literature, because a finite memory effect is allowed. Indeed, the problem becomes the optimal control of a standard ordinary differential equation, with the past of the control entering both the objective function and an inequality constraint. Therefore, the problem is intrinsically infinite dimensional. To solve this model, we apply the dynamic programming approach and we find an explicit solution for the associated Hamilton–Jacobi–Bellman equation, which lets us write the optimal strategies in feedback form. Therefore, we contribute to the existing literature in two ways. Firstly, we fully develop the dynamic programming approach to a type of problem not studied in previous contributions. Secondly, we use this result to unveil the global dynamics of an economy characterized by generic internal habits.     

Solubility of metal atoms in helium droplets: exploring the effect of the well depth using the coinage metals Cu and Ag

We report a theoretical investigation of the solution properties of Cu and Ag atoms dissolved in He clusters. Employing our recent ab initio ground state pair potential for Me-He (Me = Ag, Cu), we simulated the species Me@He (n) (n = 2-100) by means of diffusion Monte Carlo (DMC) obtaining exact information on their energetics and the structural properties. In particular, we investigated the sensitivity of structural details on the well depth of the two interaction potentials. Whereas Ag structures the first He solvation layer similarly, to some extent, to a positive ion such as Na(+), Cu appears to require the onset of a second solvation shell for a similar dense structure to be formed despite an interaction well of 28.4 μhartree. An additional signature of the different solution behavior between Ag and Cu appears also in the dependence of the energy required to evaporate a single He atom on the size of the MeHe(n) clusters. The absorption spectrum for the (2)P ← (2)S excitation of the metals was also simulated employing the semi-classical Lax approximation to further characterize Me@He(n) (n = 2-100) using novel accurate interaction potentials between He and the lowest (2)P state of Ag and Cu in conjunction with the Diatomic-in-Molecules approach. The results indicated that Ag exciplexes should not form via a direct vertical excitation into an attractive region of the excited manifolds and that there is an interesting dependence of the shape of the Cu excitation bands on the local structure of the first solvation shell.     

Photochemistry and vibrational spectra of matrix-isolated methyl 4-chloro-5-phenylisoxazole-3-carboxylate

Methyl 4-chloro-5-phenylisoxazole-3-carboxylate (MCPIC) has been synthesized, isolated in low temperature argon and xenon matrices, and studied by FTIR spectroscopy. The characterization of the low energy conformers of MCPIC was made by undertaking a systematic investigation of the DFT(B3LYP)/6-311++G(d,p) potential energy surface of the molecule. The theoretical calculations predicted the existence of three low energy conformers. Two of them (I and II) were observed experimentally in the cryogenic matrices. The third one (III) was found to be converted into conformer II during deposition of the matrices, a result that is in agreement with the predicted low III → II energy barrier (<0.3 kJ mol(-1)). In situ UV irradiation (λ > 235 nm) of matrix-isolated MCPIC yielded as final photoproduct the corresponding oxazole (methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate). Identification of the azirine and nitrile-ylide intermediates in the spectra of the irradiated matrices confirmed their mechanistic relevance in the isoxazole → oxazole photoisomerization.     

Steady-state and time-resolved investigations of a crown thioether conjugated with methylacridinium and its complexes with metal ions

The crown thioether 9-[4-(4,7,10,13-tetrathia-1-azacyclopentadecyl]phenyl-N-methylacridinium perchlorate (TCMA) was synthesized and characterized with the aim to verify its ability to interact selectively with metal ions and substantiate the possibility to detect easily the presence of heavy metals in fluid samples. The spectroscopic properties of TCMA, alone and in the presence of metal ions, were therefore studied in polar solvents (MeCN and H(2)O); in particular, steady-state UV-Vis spectrophotometric and fluorimetric techniques were used together with transient absorption spectroscopy with fs time resolution to investigate the spectral and dynamic properties of the lowest excited singlet state of TCMA and of TCMA/metal ion complexes. The absorption in the Vis region is characterized by a charge-transfer nature with the methylacridinium moiety acting as the electron-acceptor and the anilic group as the electron-donor. No emission from the S(1) was detected both in MeCN and H(2)O, while a small S(2)→ S(0) fluorescence emission (λ(max) = 485 nm and ?(F) = 0.0011) was detected in water. Time-resolved measurements with fs resolution of TCMA in MeCN have shown that the relaxed S(1) state is reached ～0.6 ps after the laser pulse, while the S(1)→ S(0) time constant is 3.7 ps. Among the investigated metal ions, only Fe(3+) (in MeCN) and Hg(2+) (in MeCN and H(2)O) were able to form stable complexes (association constant, K(ass) = 1-11 × 10(4) M(-1)) with TCMA. The S(1) state of the TCMA/M(n+) complexes emits with low quantum yield (?(F) = 0.0023-0.014) and decays with time constants much longer than TCMA itself, at least in the case of TCMA/Hg(2+) in MeCN. This study showed that TCMA is a good candidate for colorimetric/fluorimetric sensing of Hg(2+) in aqueous media owing to its high selectivity towards metal ions.     

Lattice and ion dynamics in gel electrolytes based on poly(acrylonitrile) by6Li-7Li NMR

Two composite gel electrolytes prepared from mixtures of poly(acrylonitrile) (PAN), propylene carbonate (PC), ethylene carbonate (EC) and LiClO₄ have been studied with differential scanning calorimetry (DSC) and⁷Li-⁶Li NMR. The data allow estimation of ionic conductivities, local (short range) diffusivities, and lattice dynamics. Furthermore, they point to an unexpected behavior of the Li-rich sample that hints to a complex LiClO₄-PAN phase diagram. Paper presented at the 1st Euroconference on Solid State Ionics, Zakynthos, Greece, September 11–18,1994