Anita Das

Bio: Anita Das is an academic researcher from Indian Institute of Engineering Science and Technology, Shibpur. The author has contributed to research in topics: Diabatic & Vibronic coupling. The author has an hindex of 13, co-authored 33 publications receiving 516 citations. Previous affiliations of Anita Das include University of Calcutta & Texas Tech University.

Polyradical Character of Triangular Non-Kekulé Structures, Zethrenes, p-Quinodimethane-Linked Bisphenalenyl, and the Clar Goblet in Comparison: An Extended Multireference Study.

Abstract: In this work, two different classes of polyaromatic hydrocarbon (PAH) systems have been investigated in order to characterize the amount of polyradical character and to localize the specific regions of chemical reactivity: (a) the non-Kekule triangular structures phenalenyl, triangulene and a π-extended triangulene system with high-spin ground state and (b) PAHs based on zethrenes, p-quinodimethane-linked bisphenalenyl, and the Clar goblet containing varying polyradical character in their singlet ground state. The first class of structures already have open-shell character because of their high-spin ground state, which follows from the bonding pattern, whereas for the second class the open-shell character is generated either because of the competition between the closed-shell quinoid Kekule and the open-shell singlet biradical resonance structures or the topology of the π-electron arrangement of the non-Kekule form. High-level ab initio calculations based on multireference theory have been carried out to ...

Tuning the functional sites in metal–organic frameworks to modulate CO2 heats of adsorption

Abstract: Metal–organic frameworks (MOFs) have been targeted as solid state sorbents for postcombustion carbon dioxide capture due, in part, to the enormous tunability of their structures through the incorporation of different functional sites. The isosteric heat of adsorption (Qst) provides one measure of the interaction of a solid sorbent with guest molecules, and has a bearing on the low pressure (<1 bar) CO2 uptake, selectivity and regenerability of a material. It is a key factor in the design of adsorbents for gas separation; however, it is sometimes overlooked in the evaluation of MOFs for CO2 capture. This highlight article draws together the impact of various functional sites on the CO2 heat of adsorption, and examines the interplay between functional sites and other factors such as competing water adsorption that influence a material's suitability for CO2 capture from industrial streams.

UiO-66@SiO2 core–shell microparticles as stationary phases for the separation of small organic molecules

Abstract: Composite particles containing the Zr-based metal–organic framework (MOF) UiO-66 were prepared using microwave-assisted solvothermal synthesis. Scanning electron microscopy, infrared spectroscopy, powder X-ray diffraction and nitrogen physisorption studies confirmed the deposition of 100–300 nm microporous particles with the UiO-66 topology on the surface of mesoporous 5 μm and non-porous 2.1 μm silica particles. The core–shell particles exhibited a unique flow-dependent separation selectivity (FDSS) effect which allows changes in both the retention and separation selectivity of small molecules by simple variation of the mobile phase flow rate under isocratic conditions. The impact of the loading of UiO-66 as well as the porosity of the underlying silica core (mesoporous and non-porous) on the FDSS effect was evaluated. The prepared adsorbents were also tested for the normal-phase (NP) and reversed-phase (RP) separation of xylene isomers, substituted benzenes and polyaromatic hydrocarbons (PAHs). Efficiencies of up to 32 400 plates per m (styrene, k 1.59) and 37 200 plates per m (anisole, k 2.90) were achieved under NP and RP modes, respectively. The results demonstrate the potential of novel MOF-based stationary phases for the separation of closely related compounds (e.g. positional isomers).

The Generality of the GUGA MRCI Approach in COLUMBUS for Treating Complex Quantum Chemistry

Abstract: The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.

The Chemistry and Applications of Metal-Organic Frameworks

Abstract: Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

Metal-Organic Frameworks for Food Safety.

Abstract: Food safety is a prevalent concern around the world. As such, detection, removal, and control of risks and hazardous substances present from harvest to consumption will always be necessary. Metal-organic frameworks (MOFs), a class of functional materials, possess unique physical and chemical properties, demonstrating promise in food safety applications. In this review, the synthesis and porosity of MOFs are first introduced by some representative examples that pertain to the field of food safety. Following that, the application of MOFs and MOF-based materials in food safety monitoring, food processing, covering preservation, sanitation, and packaging is overviewed. Future perspectives, as well as potential opportunities and challenges faced by MOFs in this field will also be discussed. This review aims to promote the development and progress of MOF chemistry and application research in the field of food safety, potentially leading to novel solutions.

Atomic-scale control of graphene magnetism by using hydrogen atoms

Abstract: Hydrogen atom makes graphene magnetic Graphene has many extraordinary mechanical and electronic properties, but it's not magnetic. To make it so, the simplest strategy is to modify its electronic structure to create unpaired electrons. Researchers can do that by, for example, removing individual carbon atoms or adsorbing hydrogen onto graphene. This has to be done in a very controlled way because of a peculiarity of the graphene's crystal lattice, which consists of two sublattices. Gonzales-Herrero et al. deposited a single hydrogen atom on top of graphene and used scanning tunneling microscopy to detect magnetism on the sublattice lacking the deposited atom (see the Perspective by Hollen and Gupta). Science, this issue p. 437; see also p. 415 Scanning tunneling microscopy shows that a hydrogen atom deposited on graphene makes the complementary sublattice magnetic. [Also see Perspective by Hollen and Gupta] Isolated hydrogen atoms absorbed on graphene are predicted to induce magnetic moments. Here we demonstrate that the adsorption of a single hydrogen atom on graphene induces a magnetic moment characterized by a ~20–millielectron volt spin-split state at the Fermi energy. Our scanning tunneling microscopy (STM) experiments, complemented by first-principles calculations, show that such a spin-polarized state is essentially localized on the carbon sublattice opposite to the one where the hydrogen atom is chemisorbed. This atomically modulated spin texture, which extends several nanometers away from the hydrogen atom, drives the direct coupling between the magnetic moments at unusually long distances. By using the STM tip to manipulate hydrogen atoms with atomic precision, it is possible to tailor the magnetism of selected graphene regions.

Synthesis and characterization of triangulene

Abstract: Triangulene, the smallest triplet-ground-state polybenzenoid (also known as Clar's hydrocarbon), has been an enigmatic molecule ever since its existence was first hypothesized. Despite containing an even number of carbons (22, in six fused benzene rings), it is not possible to draw Kekule-style resonant structures for the whole molecule: any attempt results in two unpaired valence electrons. Synthesis and characterization of unsubstituted triangulene has not been achieved because of its extreme reactivity, although the addition of substituents has allowed the stabilization and synthesis of the triangulene core and verification of the triplet ground state via electron paramagnetic resonance measurements. Here we show the on-surface generation of unsubstituted triangulene that consists of six fused benzene rings. The tip of a combined scanning tunnelling and atomic force microscope (STM/AFM) was used to dehydrogenate precursor molecules. STM measurements in combination with density functional theory (DFT) calculations confirmed that triangulene keeps its free-molecule properties on the surface, whereas AFM measurements resolved its planar, threefold symmetric molecular structure. The unique topology of such non-Kekule hydrocarbons results in open-shell π-conjugated graphene fragments that give rise to high-spin ground states, potentially useful in organic spintronic devices. Our generation method renders manifold experiments possible to investigate triangulene and related open-shell fragments at the single-molecule level.