Anik Sen

Effect of steric crowding on ion selectivity for calix-crown hybrid ionophores: experimental, molecular modeling and crystallographic studies

A number of calix[4]arene-azacrowns with variation in ring size and substituents at the upper and lower rims have been synthesized to investigate the effect of steric crowding towards ion selectivity. The structural elucidation of these ionophores has been carried out mainly by 1H NMR and ES-MS in solution and by single crystal X-ray study in the solid state. Interaction of these ionophores with a large number of cations has been investigated by NMR studies. The ionophore with tert-butyl at the upper rim (3) exhibits selectivity towards Na+ only whereas an ionophore of the same size but without tert-butyl at the upper rim (1) shows selectivity towards both Na+ and K+. An ionophore of the same size but with three tosyl substituents at the lower rim (4) exhibits no complexation with any cation. The ionophore with the larger crown ring and without tert-butyl at the upper rim (2) exhibits complexation with K+, Rb+, Ba2+ and weak interaction with Na+. Binding constants with these metal ions have been determined by NMR titration. Molecular modeling studies performed by a molecular mechanics force field (MMFF94) using the Monte Carlo search method and DFT calculations predicted the observed higher selectivity for sterically crowded receptor.

Microwave assisted isomerization of alkenyl aromatics over solid base catalysts: an understanding through theoretical study

Microwave assisted isomerization of estragole to anethole was studied over MgAl and NiAl layered double hydroxides with different M(II)/Al atomic ratios as solid base catalysts. Reactions under microwave overcame the challenges like higher reaction temperature, longer time, larger solvent volume and unproductive recyclability that were encountered with conventional thermal heating for this reaction. MgAl4 (catalyst with Mg/Al atomic ratio of 4.0) gave maximum conversion of 99% with a substrate to catalyst weight ratio 2 : 1 at 140 °C in 90 min using 4 ml DMF. A good correlation was obtained between the activity and Bronsted basicity derived using Hammett studies. Solvent with high polarity and high boiling point assisted the reaction and the catalyst was reusable for up to six cycles without significant loss in activity. Studies extended for different alkenyl aromatics under optimized conditions over MgAl4 revealed a very high conversion (>97%) for estragole and allylbenzene (>99%) while poor conversion for eugenol (19%). The variation in isomerization activity of alkenyl aromatics was rationalized with DFT calculations. The B3LYP/6-31+G* calculated results revealed that the conversion of these alkenyl aromatics was dependent on the substituents attached to the aromatic ring and governed by the pKa of the reactive sites in such systems. Methodology reported here offers an alternate energy efficient and environmentally benign route for the synthesis of alkenyl aromatics, which are extensively used as perfumery chemicals.

First principle study towards the influence of Cd2+ on the morphology of sodium chloride.

The influence of Cd(2+) on the morphology of sodium chloride has been investigated with Density functional methods. The preferential interactions of Cd(2+) ion with the {111} surface of NaCl support the observed octahedron morphology of NaCl. The calculations were performed both in the gas phase and aqueous phase using continuum model (COSMO). We have examined the interaction of Cd(2+) with various surface sites of sodium chloride such as, flat face, steps and kinks. The stabilization of {111} NaCl surface by mixed Cd(2+) ion and explicit water molecules in the ratio of (1:3) is in agreement with the SXRD results (Surf. Sci. 599 (2005) 196 [17]). The Cd(2+) ion prefers to interact with {100} surface of NaCl by surrounding with water molecules, whereas, the mixed layer formation on {111} is not specific in nature. The interaction of CdCl(2) with the surface of sodium chloride is ineffective to induce this phenomenon.

Probing the influence of pH dependent citric acid towards the morphology of rock salt: a computational study

The influence of citric acid under different pH conditions towards the morphology of sodium chloride was investigated employing DFT calculations. Prior to any analysis, conformational search of citric acid and its three dissociated forms was performed with molecular mechanics MM2* and DFT (B3LYP/6-31+G*) methods in the gas and aqueous phase. The low energy conformers generated in the aqueous phase were found to be in good agreement with the available crystal structures. The calculated higher composition of citric acid at natural pH = 0.75 i.e., α0 = 0.99 is not effective for the habit modification of sodium chloride. Quantum chemically derived molecular electrostatic potential (MESP) showed the possible sites of citric acid and its dissociated forms with the estimated deepest Vmin values and their magnitudes provide an insight for their interaction with the crystal surfaces of sodium chloride. The composition analysis and MESP derived data suggest that the dissociated forms of citric acid at higher pH values are responsible for the change in habit of sodium chloride from cubes to octahedrons. Further, slab model calculations performed to mimic the surfaces of sodium chloride with DFT methods using a conductor-like screening model in the aqueous phase (COSMO) for the interactions of citric acid and dihydrogen citrate validates that there is no preference for the {111} plane of NaCl with citric acid. However, a significant preference was seen with dihydrogen citrate, which quantifies the MESP analysis and corroborates the dependence of morphology on the pH of the solution.

Is Dual Morphology of Rock-Salt Crystals Possible with a Single Additive? The Answer Is Yes, with Barbituric Acid**

Crystal face lift: barbituric acid is shown to be a new crystal-habit modifier for sodium chloride crystals. Two morphologies of salt crystals can be prepared separately with this new additive. It is of the few additives able to induce rhombic dodecahedron crystals for NaCl, and is required only a trace of amount, unlike other additives, such as glycine.

What is the minimum number of water molecules required to dissolve a potassium chloride molecule

This work answers an unsolved question that consists of determining the least number of water molecules necessary to separate a potassium chloride molecule. The answer based on accurate quantum chemical calculations suggests that tetramers are the smallest clusters necessary to dissociate KCl molecules. The study was made with Møller‐Plesset second‐order perturbation theory modified with the cluster theory having single, double, and perturbative triple excitations. With this extensive study, the dissociation of KCl molecule in different water clusters was evaluated. The calculated results show that four water molecules stabilize a solvent separated K+/Cl− ion‐pair in prismatic structure and with six water molecules further dissociation was observed. Attenuated total reflection infrared spectroscopy of KCl dissolved in water establishes that clusters are made of closely bound ions with a mean of five water molecules per ion‐pair [K+(H2O)5Cl−]. (Max and Chapados, Appl Spectrosc 1999, 53, 1601; Max and Chapados, J Chem Phys 2001, 115, 2664.) The calculated results tend to support that five water molecules leads toward the formation of contact ion‐pair. The structures, energies, and infrared spectra of KCl molecules in different water clusters are also discussed.

In silico studies toward understanding the interactions of DNA base pairs with protonated linear/cyclic diamines.

Protonated amino groups are ubiquitous in nature and important in the fields of chemistry and biology. In search of efficient polyamine analogues, we have performed DFT calculations on the interactions of some simple cyclic and constrained protonated diamines with the DNA base pairs and compared the results with those obtained for the corresponding interactions involving linear diamines, which mimic biogenic polyamines such as spermine. The interactions are mainly governed by the strong hydrogen bonding between the ligand and the DNA base pairs. The DFT calculations suggest that the major-groove N7 interaction (GC base pair) with linear diamine is energetically more favored than other possible interactions, as reported with spermine. The cyclic diamines exhibited better interactions with the N7 site of the AT and GC base pairs of DNA than the linear diamines. The net atomic charges calculated for the protonated amine hydrogens were higher for the cyclic systems than for the linear diamines, inducing better binding affinity with the DNA base pairs. The stable conformers of cyclic diamines were predicted using the MP2/aug-cc-pVDZ level of theory. The positions of the protonated diamine groups in these cyclic systems are crucial for effective binding with the DNA base pairs. The DFT-calculated results show that diequatorial (ee) 1,2-cyclohexadiamine (CHDA) is a promising candidate as a polyamine analogue for biogenic polyamines. Molecular dynamics simulations were performed using explicit water molecules for the interaction of representative ligands with the DNA base pairs to examine the influence of solvent molecules on such interactions.