Hydration structure of Na+ and K+ from ab initio molecular dynamics based on modern density functional theory

Molecular dynamics (Born–Oppenheimer) simulations based on density functional theory have been carried out to investigate the solvation structure of monovalent Na⁺ and K⁺ cations in water under ambient conditions. Four recently proposed van der Waals (vdW) density functionals (LMKLL, DRSLL, DRSLL-PBE, DRSLL-optB88), the semiempirical vdW method of Grimme (BLYP-D3) and conventional gradient-corrected (GGA-BLYP) density functionals are applied in order to evaluate their accuracy in describing the hydration structure of alkali metal ions. Theoretical results are compared to available experimental data. Our results indicate that addition of corrections accounting for dispersion forces significantly improves the agreement between predicted and measured coordination numbers for both Na⁺ and K⁺ cations. Analysis of radial distribution functions brings further support to the notion that the choice of the generalised gradient approximation density functional impacts crucially on the computed structural properties. DRSLL-optB88 and BLYP-D3 provide the best agreement with experiment.     

Tuning the photophysics of a bio-active molecular probe ‘3-pyrazolyl-2-pyrazoline’ derivative in different solvents: Dual effect of polarity and hydrogen bonding

The solvatochromic behavior of 3-pyrazolyl 2-pyrazoline derivative (PYZ), a newly synthesized molecular probe having pharmaceutical importance, has been studied in various solvents of different polarity. The Kamlet and Taft solvatochromic comparison method was utilized to rationalize the solute-solvent interactions from absorption and emission measurements. Spectroscopic studies reveal that the solvatochromic behavior of the dye depends not only on the polarity of the medium but also on the hydrogen-bonding properties of the solvents. The non-radiative relaxation process is facilitated by an increase in the polarity of the media. The photophysical response of PYZ in different solvents has been explained considering solute-solvent interactions.     

Platinum- and gold-catalyzed hydrative carbocyclization of oxo diynes for one-pot synthesis of benzopyrones and bicyclic spiro ketones

We report a one-pot synthesis of benzopyrones and tricyclic spiroketones from hydrative carbocyclization of oxodiyne substrates catalyzed by PtCl2 and PPh3AuCl/AgOTf, respectively. The distinct carbocyclizations with Pt and Au catalysts stem from their altered regioselectivity in the oxo-assisted hydration of the neighboring alkyne carbons.     

Moments of escape times of random walk

Extending an idea of Spitzer [2], a way to compute the moments of the time of escape from (−N,L) by a symmetric simple random walk is exhibited. It is shown that all these moments depend polynomially onL andN. The research of this author was supported by the National Board of Higher Mathematics, Bombay, India     

Nanozipper formation in the solid state from a self-assembling tripeptide with a single tryptophan residue

The self-assembly of a terminally protected tripeptide Boc-γ-Abu(1)-Ala(2)-Trp(3)-OMe (γ-Abu = γ-aminobutyric acid) 1 results in the formation of a nanostructured supramolecular zipper through various non-covalent interactions in the crystal in which the indole side-chain of the Trp(3) residue plays a key role via N-H?π interactions.     

Fluorescence Blinking Beyond Nanoconfinement: Spatially Synchronous Intermittency of Entire Perovskite Microcrystals

Abrupt fluorescence intermittency or blinking is long recognized to be characteristic of single nano‐emitters. Extended quantum‐confined nanostructures also undergo spatially heterogeneous blinking; however, there is no such precedent in dimensionally unconfined (bulk) materials. Herein, we report multi‐level blinking of entire individual organo–lead bromide perovskite microcrystals (volume=0.1–3 μm³) under ambient conditions. Extremely high spatiotemporal correlation (>0.9) in intracrystal emission intensity fluctuations signifies effective communication amongst photogenerated carriers at distal locations (up to ca. 4 μm) within each crystal. Fused polycrystalline grains also exhibit this intriguing phenomenon, which is rationalized by correlated and efficient migration of carriers to a few transient nonradiative traps, the nature and population of which determine blinking propensity. Observation of spatiotemporally correlated emission intermittency in bulk semiconductor crystals opens the possibility of designing novel devices involving long‐range (mesoscopic) electronic communication.     

Continuum theory of polymer crystallization

We present a kinetic model of crystal growth of polymers of finite molecular weight. Experiments help to classify polymer crystallization broadly into two kinetic regimes. One is observed in melts or in high molar mass polymer solutions and is dominated by nucleation control with G approximately exp(1/TDeltaT), where G is the growth rate and DeltaT is the supercooling. The other is observed in low molar mass solutions (as well as for small molecules) and is diffusion controlled with G approximately DeltaT, for small DeltaT. Our model unifies these two regimes in a single formalism. The model accounts for the accumulation of polymer chains near the growth front and invokes an entropic barrier theory to recover both limits of nucleation and diffusion control. The basic theory applies to both melts and solutions, and we numerically calculate the growth details of a single crystal in a dilute solution. The effects of molecular weight and concentration are also determined considering conventional polymer dynamics. Our theory shows that entropic considerations, in addition to the traditional energetic arguments, can capture general trends of a vast range of phenomenology. Unifying ideas on crystallization from small molecules and from flexible polymer chains emerge from our theory.     

Density functional theory treatment of electron correlation in the nuclear-electronic orbital approach

This paper presents the nuclear-electronic orbital density functional theory [NEO-DFT(ee)] method for including electron-electron correlation and nuclear quantum effects self-consistently in quantum chemical calculations. The NEO approach is designed to treat a relatively small number of nuclei quantum mechanically, while the remaining nuclei are treated classically. In the NEO-DFT(ee) approach, the correlated electron density is used to obtain the nuclear molecular orbitals, and the resulting nuclear density is used to obtain the correlated electron density during an iterative procedure that continues until convergence of both the nuclear and electronic densities. This approach includes feedback between the correlated electron density and the nuclear wavefunction. The application of this approach to bihalides and acetylene indicates that the nuclear quantum effects do not significantly impact the electron correlation energy, but the quantum nuclear energy is enhanced in the NEO-DFT(ee) B3LYP method. The excellent agreement of the NEO-DFT(ee)-optimized bihalide structures with the vibrationally averaged geometries from grid-based quantum dynamical methods provides validation for the NEO-DFT(ee) approach. Electron-proton correlation could be included by the development of an electron-nucleus correlation functional. Alternatively, explicit electron-proton correlation could be included directly into the NEO self-consistent-field framework with Gaussian-type geminal functions.