Swastik Mondal

A New Half-Condensed Schiff Base Compound: Highly Selective and Sensitive pH-Responsive Fluorescent Sensor

A new probe, 3-[(3-benzyloxypyridin-2-ylimino)methyl]-2-hydroxy-5-methylbenzaldehyde (1-H) behaves as a highly selective fluorescent pH sensor in a Britton-Robinson buffer at 25 °C. The pH titrations show a 250-fold increase in fluorescence intensity within the pH range of 4.2 to 8.3 with a pK(a) value of 6.63 which is valuable for studying many of the biological organelles.

Experimental dynamic electron densities of multipole models at different temperatures.

It is shown that the dynamic electron density corresponding to a structure model can be computed by inverse Fourier transform of accurately calculated structure factors, employing the method of fast Fourier transform. Maps free of series-termination effects are obtained for resolutions better than 0.04 Å in direct space, corresponding to resolutions larger than 6 Å(-1) in reciprocal space. Multipole (MP) models of α-glycine and D,L-serine at different temperatures have been determined by refinement against X-ray diffraction data obtained from the scientific literature. The successful construction of dynamic electron densities is demonstrated by their topological properties, which indicate local maxima and bond-critical points (BCPs) at positions expected on the basis of the corresponding static electron densities, while non-atomic maxima have not been found. Density values near atomic maxima are much smaller in dynamic than in static electron densities. Static and low-temperature (∼20 K) dynamic electron-density maps are found to be surprisingly similar in the low-density regions. Especially at BCPs, values of the ∼20 K dynamic density maps are only slightly smaller than values of the corresponding static density maps. The major effect of these zero-point vibrations is a modification of the second derivatives of the density, which is most pronounced for values at the BCPs of polar C-O bonds. Nevertheless, dynamic MP electron densities provide an estimate of reasonable accuracy for the topological properties at BCPs of the corresponding static electron densities. The difference between static and dynamic electron densities increases with increasing temperature. These differences might provide information on temperature-dependent molecular or solid-state properties like chemical stability and reactivity. In regions of still lower densities, like in hydrogen bonds, static and dynamic electron densities have similar appearances within the complete range of temperatures that have been considered (20-298 K), providing similar values of both the density and its Laplacian at BCPs in static and dynamic electron densities at all temperatures.

Disorder and defects are not intrinsic to boron carbide

A unique combination of useful properties in boron-carbide, such as extreme hardness, excellent fracture toughness, a low density, a high melting point, thermoelectricity, semi-conducting behavior, catalytic activity and a remarkably good chemical stability, makes it an ideal material for a wide range of technological applications. Explaining these properties in terms of chemical bonding has remained a major challenge in boron chemistry. Here we report the synthesis of fully ordered, stoichiometric boron-carbide B13C2 by high-pressure–high-temperature techniques. Our experimental electron-density study using high-resolution single-crystal synchrotron X-ray diffraction data conclusively demonstrates that disorder and defects are not intrinsic to boron carbide, contrary to what was hitherto supposed. A detailed analysis of the electron density distribution reveals charge transfer between structural units in B13C2 and a new type of electron-deficient bond with formally unpaired electrons on the C–B–C group in B13C2. Unprecedented bonding features contribute to the fundamental chemistry and materials science of boron compounds that is of great interest for understanding structure-property relationships and development of novel functional materials.

Structural study of three o-hydroxyacetophenone derivatives using X-ray powder diffraction: interplay of weak intermolecular interactions

A family of three o-hydroxyacetophenone derivatives, 2-hydroxy-5-methoxy-4-methylacetophenone (1), 5-methoxy-4-methyl-2-(phenylmethoxy)acetophenone (2) and 2-O-(α-methyl carboethoxy)-5-methoxy-4-methyl-acetophenone (3) were synthesized and their crystal structures were solved using laboratory X-ray powder diffraction data along with a detailed analysis of the Hirshfeld surfaces and 2D-fingerprint plots, facilitating a comparison of intermolecular interactions. The DFT optimized molecular geometries in 1 and 3 agree closely with those obtained from the crystallographic studies. In 2, however, the relative orientation of the two planar phenyl rings as established by the X-ray analysis and quantum mechanical calculations differs by 34.9°. The crystal packing in the compounds is stabilized by an interplay of weak hydrogen bonds, such as C–H⋯O, C–H⋯π and π⋯π interactions, forming supramolecular assemblies. The intermolecular C–H⋯O hydrogen bonds in 1–3 generate C11(6) chains and Rmn(X) rings, which are further connected through C–H⋯π hydrogen bonds and π⋯π interactions to produce parallel molecular sheets built by fused R22(24) rings in 1, a three-dimensional architecture with C22(16)[R22(6)][R22(14)] synthons in 2, and a two-dimensional columnar framework in 3.

Electron densities by the maximum entropy method (MEM) for various types of prior densities : a case study on three amino acids and a tripeptide

Dynamic model densities according to Mondal et al. [(2012), Acta Cryst. A68, 568-581] are presented for independent atom models (IAM), IAMs after high-order refinements (IAM-HO), invariom (INV) models and multipole (MP) models of α-glycine, DL-serine, L-alanine and Ala-Tyr-Ala at T ≃ 20 K. Each dynamic model density is used as prior in the calculation of electron density according to the maximum entropy method (MEM). We show that at the bond-critical points (BCPs) of covalent C-C and C-N bonds the IAM-HO and INV priors produce reliable MEM density maps, including reliable values for the density and its Laplacian. The agreement between these MEM density maps and dynamic MP density maps is less good for polar C-O bonds, which is explained by the large spread of values of topological descriptors of C-O bonds in static MP densities. The density and Laplacian at BCPs of hydrogen bonds have similar values in MEM density maps obtained with all four kinds of prior densities. This feature is related to the smaller spatial variation of the densities in these regions, as expressed by small magnitudes of the Laplacians and the densities. It is concluded that the use of the IAM-HO prior instead of the IAM prior leads to improved MEM density maps. This observation shows interesting parallels to MP refinements, where the use of the IAM-HO as an initial model is the accepted procedure for solving MP parameters. A deconvolution of thermal motion and static density that is better than the deconvolution of the IAM appears to be necessary in order to arrive at the best MP models as well as at the best MEM densities.

Modulated anharmonic ADPs are intrinsic to aperiodic crystals: a case study on incommensurate Rb2ZnCl4

The superspace maximum entropy method (MEM) density in combination with structure refinements has been used to uncover the modulation in incommensurate Rb2ZnCl4 close to the lock-in transition. Modulated atomic displacement parameters (ADPs) and modulated anharmonic ADPs are found to form an intrinsic part of the modulation. Refined values for the displacement modulation function depend on the presence or absence of modulated ADPs in the model.

Charge density distribution of 3‐(1‐aminoethylidene)‐2‐methoxy‐2‐oxo‐2,3‐dihydro‐2λ5‐benzo[e][1,2]oxaphosphinin‐4‐one

A combined experimental and theoretical study of one oxaphosphinane derivative was made on the basis of a topological analysis of its electron density distributions. The electron density was determined from a high-resolution X-ray diffraction data set measured with synchrotron radiation at 100 K, whereas theoretical calculations were performed using density functional theory (DFT) methods at the B3LYP\6-311++G(3df,3pd) level of approximation. The charge-density distribution and analysis of topological properties revealed that the P-O bond is of the transit closed-shell type. The crystal structure possesses one intra- and several intermolecular hydrogen bonds. They were characterized quantitatively by topological properties using Baders Atoms in Molecules theory. All hydrogen bonds were classified as weak. Further analysis of the experimental electron density by the source function allowed the intramolecular hydrogen bond to be characterized as an isolated hydrogen bond, in contrast to the resonance-assisted hydrogen bond in related molecules, such as chromone derivatives.

Resonance-stabilized partial proton transfer in hydrogen bonds of incommensurate phenazine–chloranilic acid

Correlated variations of chemical bonds demonstrate stabilization by the resonance of the chloranilic acid anion. Proton transfer in some of the intermolecular hydrogen bonds is responsible for the ferroelectic properties.

An efficient and reusable polymer-supported palladium catalyst for the Suzuki cross-coupling reactions of aryl halides

A reusable, air-stable polymer-anchored palladium (II) Schiff base complex catalyst, P-[{(NCH)C6H4}Pd(OAc)]2 was prepared and was found to be highly active in Suzuki cross-coupling reactions of aryl halides with phenyl boronic acid in an aqueous medium to give biaryl products in high yields.

New insights into the bonding mechanism of boron carbide

Boron carbide has a mysterious power to accommodate a large variation of atomic proportions (with a general formula of B12+xC3−x , 0.06 > x > 1.7) within the same general rhombohedral crystal structure [1]. The structure of stoichiometric boron carbide (B13C2 or B12CBC) consists of the 12-atom icosahedral B12 clusters and the 3-atom linear CBC chains. The mystery of chemical bonding in this structure is that the icosahedral B12 unit is deficient by two electrons and the CBC chain with a divalent boron atom can provide only one electron; thus there seems to be a net deficiency of one electron in B13C2 composition. One solution to this problem is replacing any boron atom in B13C2 by a carbon atom, which will result in a stoichiometry B12C3 or B4C. However, the so-called electron precise B12C3 could never been experimentally isolated [1]. This puzzle of chemical bonding in boron carbide has remained unsolved since many decades. A high-resolution aspherical electron density study has been undertaken in order to get more insights into the enigma of chemical bonding in boron carbide. The study was performed on the basis of a multipole model of electron density distribution in B13C2 constructed using low-temperature, high-resolution, single-crystal synchrotron X-ray diffraction data [2]. Electron densities have been analyzed using the Baders quantum theory of atoms in molecules [3]. The study reveals existence of an unprecedented electron-deficient bond and a charge transfer between structural units of boron carbide. The bonding model successfully explains the origin of a range of physical and chemical properties of boron carbide. A Comparison of electron density distributions in boron-rich materials related to boron carbide reveals a generalized bonding mechanism of B12 icosahedra that explains why boron carbide and related materials can preserve the same crystal structure over a range of compositions. [1] Balakrishnarajan, M. M. et al. (2007). New. J. Chem. 31, 473-485. [2] Mondal, S., et al. (2016). Sci. Rep. 6, 19330. [3] Coppens, P. (1997). X-ray Charge Densities and Chemical Bonding. Oxford University Press, New York, USA.