Ranjita Das

Theoretical investigation of hydrogen adsorption in all-metal aromatic clusters

Molecular hydrogen adsorption in binary all-metal aromatic systems has been explored using ab initio quantum chemical calculations. For this study, we have considered different classes of bimetallic clusters, viz. Be3M2, Mg3M2 and Al4M2 (M = Li, Na and K). In all these bimetallic clusters, the interaction energies of the alkali metal ion with the base metal surface are quite high and the alkali metal sites are found to carry partial positive charges which enhance the adsorption enthalpies of molecular hydrogen on them. Among the three classes considered here, Mg3M2 are found to have poor hydrogen adsorption enthalpies as compared to the other two classes due to less charge on the alkali metals. Although the charge developed on K is more than that developed on Li and Na, the hydrogen adsorption in Be3K2 and Al4K2 is found to be weak in comparison to their Li and Na doped counterparts. In the case of Be3Li2 and Be3Na2, the hydrogen adsorption energies are found to be quite comparable to the optimum adsorption energy proposed for ambient temperature hydrogen storage and the gravimetric density of hydrogen is found to be 22.64 and 14.12 wt% respectively, with three H2 molecules adsorbed per alkali metal atom. In the case of Al4M2, the positive charges on the alkali metal atoms as well as the hydrogen adsorption energies are found to be higher as compared to those in Be3M2 clusters. The gravimetric densities of hydrogen in hydrogenated Al4Li2 and Al4Na2 are found to be respectively 11.59 and 9.4 wt% with four H2 molecules adsorbed per alkali metal atom.

A (T-P) phase diagram of hydrogen storage on (N4C3H)6Li6.

Temperature-pressure phase diagrams are generated through the study of hydrogen adsorption on the (N(4)C(3)H)(6)Li(6) cluster at the B3LYP/6-31+G(d) level of theory. The possibility of hydrogen storage in an associated 3D functional material is also explored. Electronic structure calculations are performed to generate temperature-pressure phase diagrams so that the temperature-pressure zones are identified where the Gibbs free energy change associated with the hydrogen adsorption process on (N(4)C(3)H)(6)Li(6) cluster becomes negative and hence thermodynamically favorable. Both adsorption and desorption processes are likely to be kinetically feasible as well.

A one-pot Garratt–Braverman cyclization and Scholl oxidation route to acene–helicene hybrids

We report a one-pot Garratt–Braverman cyclization and Scholl oxidation route to polyaromatic compounds, some having a band gap below 3 eV, starting from bis-propargyl sulfones, ethers and protected amines. The method has the advantage of constructing 3 C–C bonds in one-pot in good yields.

Redox and Lewis acid–base activities through an electronegativity-hardness landscape diagram

Chemistry is the science of bond making and bond breaking which requires redistribution of electron density among the reactant partners. Accordingly acid–base and redox reactions form cardinal components in all branches of chemistry, e.g., inorganic, organic, physical or biochemistry. That is the reason it forms an integral part of the undergraduate curriculum all throughout the globe. In an electronegativity (χ)- hardness (η) landscape diagram the diagonal χ = η line separates reducing agents from oxidizing agents as well as Lewis acids from Lewis bases. While electronegativity is related to the degree of electron transfer between two reactants, hardness is related to the resistance to that process. Accordingly the electronegativities of oxidizing agents/Lewis acids are generally greater than the corresponding hardness values and the reverse is true for reducing agents/Lewis bases. Electrophiles and nucleophiles are also expected to follow similar trends.

Structure-stability diagrams and stability-reactivity landscapes: a conceptual DFT study

Electrophilicity and hardness have been shown to be adequate in constructing structure-stability diagrams. Maximum hardness principle and minimum electrophilicity principle provide a rough guide toward locating the domains of stability and reactivity in a fitness landscape. Bonding in solids, aromaticity, magic alkali clusters, bond—stretch isomers, multivalent superatoms, etc. have been analyzed within this purview.

Host–Guest Interactions in ExBox4+

The host-guest interaction between poly aromatic hydrocarbon/azine and the newly synthesized ExBox(4+) complex is studied with the help of density functional theory. The solvent-phase interaction energy is found to decrease with gradual substitution of methine groups (=CH-) from the six-membered ring of guest molecules with N atoms in the resultant azine@ExBox(4+) complex. The nature of the binding interaction is studied with the help of newly developed noncovalent interaction (NCI) plot program package along with energy decomposition analysis and charge decomposition analysis. The interactions are mostly π-type van der Waals interactions.

Aromaticity in Polyacenes and Their Structural Analogues

The successful synthesis of different polyacenes including theoretical assessment on the stability of larger acenes are dis- cussed. The existence of favorable aromaticity criterion in polyacenes is understood in terms of different aromaticity indicators like nu- cleus independent chemical shift (NICS), harmonic oscillator model of aromaticity (HOMA), bond resonance energy (BRE). Clars � - sextet rule is also very much effective in explaining their aromaticity. By virtue of low HOMO-LUMO gap, the probable application of polyacenes in the field of organic electronics is also highlighted. The polyacene analogues of inorganic ring compounds, viz., BN-acenes, CN-acenes, BO-acenes, BS-acenes, AlN-acenes and of alkali ring compounds, viz., Na-acenes and K-acenes also have polyacene-like aromaticity although in few cases the origin of aromaticity and qualitative nature of aromaticity differ significantly.

Some novel molecular frameworks involving representative elements

Several new molecular frameworks with interesting structures, based on clusters of main group elements have been studied at different levels of theory with various basis sets. Conceptual density functional theory based reactivity descriptors and nucleus independent chemical shift provide important insights into their bonding, reactivity, stability and aromaticity.

Aromaticity and conceptual density functional theory

Aromaticity is one of the most fascinating popular qualitative chemical concepts in chemistry1–4. Michael Faraday5 isolated benzene by distillation in 1825. He noticed that although benzene is an unsaturated compound with H : C :: 1 : 1 it is much less reactive than the related unsaturated aliphatic...