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Out of several pericyclic reactions, Diels-Alder (DA) reaction is one of the most widely used synthetic processes. In the present work, several models and methodologies have been used to determine and to analyze the plausible mechanism of some representative DA cycloaddition reactions. A comparison between the dual descriptor and the bond reactivity indices corresponding to the natural bond orbital of the reagents is included, which provides a complete description of the plausible reaction mechanism. In the next step, two very recent models are used to determine the local electronic density transfer and redistribution between the reactants involved. The description of the local electronic density transfer has been made in two stages; first, the variation in the net charge on the atoms is obtained, and then, the electronic density transfer between the natural bond orbitals is calculated. The values obtained using the two models are correlated with the experimental rate constants of the reactions. Finally, the natural bond orbitals are obtained at several steps along the reaction path and the variation in their partial occupation is compared with the corresponding electron density transfer among these orbitals. Furthermore, frontier molecular orbital (FMO) approach has been employed to understand the more feasible way of interaction between the DA pair. Relative electrophilicity descriptors like net electrophilicity (∆ω±), net reactivity index (NRI, Δ $$ {\omega}_R^{\pm } $$ ), and electrophilicity difference (∆ω) between DA pairs have also been employed to describe the studied reaction mechanisms especially whether they follow non-polar-concerted/polar-stepwise pathway along with their classification in terms of normal or inverse electron demand. Furthermore, adaptive natural density partitioning method (AdNDP) and energy decomposition analyses (EDA) in conjunction with natural orbital for chemical valence (NOCV) have been made use of in order to analyze the actual bonding situation in the transition state (TS).

Literature reports provide ample evidence of the dynamical studies of various fluorophores in different room-temperature ionic liquid (RTIL)-cosolvent mixtures. However, most of the experimental and simulation studies reveal that ∼50% of the spectral relaxation dynamics is fast and cannot be resolved using traditional time correlated single photon counting (TCSPC) measurements. Our group has also investigated the dynamics of a solvatochromic probe coumarin 153 (C153) in a RTIL-cosolvent mixture using a TCSPC setup (S. Sarkar, R. Pramanik, C. Ghatak, P. Setua and N. Sarkar, J. Phys. Chem. B, 2010, 114, 2779-2789). Consequently, a major portion of the solvation dynamics remained undetected and moreover we could not monitor the dynamics beyond 0.4 mole fraction of the cosolvents. Thus in this study, we have rekindled our interest to sufficiently capture the rotational anisotropy and solvation dynamics of C153 beyond 0.4 mole fraction of the cosolvents in the RTIL-cosolvent mixture employing femtosecond fluorescence upconversion measurements. Additionally, we have utilized another RTIL with a higher alkyl chain length and viscosity to obtain a comprehensive and quantitative picture of the role of viscosity on the dynamics of the probe molecule. The most interesting observation of the present work is that the viscosities of different RTIL-cosolvent mixtures can efficiently control the cis-trans isomerization kinetics of the anionic fluorophore merocyanine 540 (MC 540) and the translational diffusion of a hydrophobic probe. The optimization of geometrical structures of [EmimOs]- and [EmimOs]-cosolvent mixtures followed by frequency analyses in both gas and solution phases have been carried out using quantum chemical calculations with the aid of density functional theory (DFT) methods. The computation based on the bond distances, electron densities and non-covalent interactions (NCI) has also been used to investigate the existence of the hydrogen-bond (H-bond). Again to comprehend van der Waals interactions and the conventional hydrogen-bond, the evolution of NCI plots are simulated. Therefore, the detailed experimental and theoretical studies presented in this manuscript lead to the inference that addition of the conventional solvents finely tunes the physicochemical properties of RTILs and broadens their scope of applications in the fields of chemistry and biology.

A DFT-based computational study has been performed on the hydrogenation of CO2, catalyzed by a bridged FLP. Formic acid might be formed in two possible pathways as revealed by this study. In one way, the Lewis basic center of the FLP activates H2, and the Lewis acidic center activates CO2 simultaneously in the first step of the reaction. Alternatively, the Lewis basic center of the FLP activates CO2, and the Lewis acidic center activates H2. The simultaneous activation of CO2 and H2 through single TS is confirmed by NBO analysis. Free energy profiles are also generated for both the possible pathways in solvent phase. It appears from these profiles that the first step, i.e., simultaneous activation of CO2 and H2, is the rate determining for both the reaction pathways. A significant amount of barrier height is reduced in comparison to that in the corresponding uncatalyzed reaction as observed in these profiles. The nature of donor-acceptor interactions present in the transition state geometries is further analyzed by energy decomposition analysis (EDA) methods. The EDA analysis shows that the HOMO of the FLP donates electron density to the LUMO of H2, the HOMO of H2 donates electron density to the LUMO of CO2, and several occupied MOs of CO2 donate electron density to the LUMO of FLP at the TS geometry.

Minimization of the energy of a molecule is an important research problem in quantum chemistry. The use of appropriate global optimization algorithms for determining the most stable configuration is a matter of active interest, and various efforts have been made toward achieving the same. Instead of using single method-based techniques, a recent method has been developed for constructing new models where particle swarm optimization can be made use of. In the present study, we propose a convolutional neural network (CNN) model for learning and predicting the energy of a system by training geometries of cluster units containing both metal and non-metal atoms, viz. C5, N42−, N64−, Aun (n = 2 − 8) and AunAgm (2 ≤ n + m ≤ 8) clusters as prototype examples. Initially, several random clusters are generated within a given range in the three-dimensional space and their energies determined using the atom-centered density matrix propagation molecular dynamics simulation (ADMP). A CNN model is constructed from this initial set of clusters, which is later used for generating a huge number of systems required while searching for the stable structure, instead of using time-consuming quantum mechanical calculations in an iterative process. While several global optimization algorithms could have been used, we choose to employ particle swarm optimization (PSO) due to its ease of implementation and efficient rate of convergence.

Confinement causes drastic changes in bonding, reactivity and dynamics. Alkali metal atoms are excellent reducing agents due to their low ionization energies. On the other hand, high electron affinity values allow the halogen atoms to behave as powerful oxidizing agents. In this article, we have shown that those properties get changed in a confined environment created by two fullerenes differing in size.

The mechanism of dihydrogen activation has been theoretically investigated by means of DFT calculation. An experimentally synthesized bridged P/B frustrated Lewis pair (FLP) and two designed FLPs are used for this purpose. The model FLPs 2 and 3 are more efficient than FLP 1 for H2 activation as revealed by the thermochemical and kinetic data. A significant amount of electron density is transferred from H2 molecule to the FLPs at the transition states (TSs) during the process of H2 activation, and this is greater at the corresponding TSs of FLPs 2 and 3 than that of FLP 1. The NICS(0) and NICS(1zz) of the boron heterocycle at the FLPs 2 and 3, and at the corresponding TSs and the product geometries of H2 activation demonstrate that the anti-aromatic character of the rings in the FLPs is remarkably reduced at the TSs and finally at the products and that is most likely responsible for enhanced activity of FLPs 2 and 3 by decreasing the activation barrier.

The crux of quantum chemistry lies in the minimization of energy functional to obtain the optimized geometry of a molecule. This involves formulating the energy minimization task as a global optimization (GO) problem and using an appropriate algorithm for determining the best solution out of a number of feasible solutions. Several new and unconventional algorithms are proposed that augment the efforts toward GO of clusters of atoms using rigorous quantum chemical methods. Among these, swarm intelligence-based nature-inspired metaheuristic algorithms have particularly drawn considerable attention. However, it still has certain drawbacks. In this work, we propose the use of firefly algorithm (FA) in conjunction with density functional theory (DFT)-based calculations for solving the GO problem and establish its superioriority in performance over particle swarm optimization (PSO) through extensive computational studies. Specifically, we show how such a “FA + DFT” approach can be used for GO of Al42− clusters. Each possible structure in the three-dimensional search space is treated as a firefly particle. Starting with an initial pool of particles, newer sets of particles are generated using an evolutionary mechanism, thereby moving toward solutions with particles having “better” structures. This novel approach also enables the possibility of incorporating molecule specific domain knowledge. For example, knowing that Al42− clusters are planar, we can restrict the search space so that only planar structures are explored, thereby achieving faster convergence of the algorithm. As an extension of the current technique, an important correlation between energy stabilization and aromaticity is established.

A theoretical investigation of the binding ability of different ligands [L = CO, H2O, H2S, N2, NH3, 1,3-dimethylimidazole (DMI), C2H2 and C2H4] with metal isocyanide and cyanide (MNC and MCN; M = Cu, Ag, Au) compounds has been carried out using quantum chemical computations. In order to analyze the thermochemical stability of these complexes, we have calculated the changes in the related dissociation energies and free energies by considering different possible dissociation pathways (four two-body and one three-body) such as (a) LMCN(/NC) = L + MCN(/NC); (b) LMCN = LM + CN; (c) LMCN = L + M + CN; (d) LMCN = LM+ + CN−; and (e) LMCN = L− + MCN+. The possible dissociation processes are endothermic in nature at room temperature suggesting non-spontaneity at 298 K. Our inspection suggests that MNC have higher binding ability than the MCN compounds in all of the L-bonding cases and both of them follow similar trends as Au > Cu > Ag. The natural bond orbital analysis, topological analysis of the electron density from atoms in molecules technique, and energy decomposition analysis have been carried out to characterize the nature of interaction between L and MCN which shows that the L‐M bonds acquire some degree of covalent character. Furthermore, in order to check the validity of the conceptual DFT-based electronic structure principles like maximum hardness and minimum electrophilicity principles, the change in the relevant global reactivity descriptors like chemical hardness (η), chemical potential (μ), and electrophilicity index (ω) is also studied along the isomerization path, LMNC → LMCN.

Chemical reactivity properties such as regioselectivity, electrophilicity and nucleophilicity are important chemical concepts, yet their understanding and quantification are still far from being accomplished. Applying density functional theory (DFT) to appreciate these properties is one route to pursue in the literature. In this work, we present a comparative study to benchmark two approaches in DFT to predict regioselectivity, electrophilicity and nucleophilicity: one with the Hirshfeld charge and the other with the Fukui function. We also examine the impact of 15 different ways to compute atomic charges on the performance of their predictions about these chemical reactivity properties. Our results show that the Hirshfeld charge is able to reliably determine regioselectivity and simultaneously accurately quantify both electrophilicity and nucleophilicity. The Fukui function behaves reasonably well for the prediction of electrophilicity but performs poorly for nucleophilicity. Among all other atomic charges examined in this study, it is only the Voronoi deformation density charge that yields the similar result as the Hirshfeld charge. As the first systematic benchmark study in the literature to compare the two available approaches in DFT about reactivity predictions, this work should fill in the needed knowledge gap and provide an impetus for the future development of chemical reactivity theory using DFT language.

The theoretical calculation at the M05-2X/6-311+G(d,p) level reveals that the B–B bond length in [N4-B2-N4]2− system (1.506 A) is slightly smaller than that of typical B=B bond in B2H2 (1.518 A). These systems interact with each M+ (M = Li, Na, K) ion very strongly with a binding energy of 213.5 (Li), 195.2 (Na) and 180.3 (K) kcal/mol. Additionally, the relief of the Coulomb repulsion due to the presence of counter-ion, M+, the B–B bond contracts to 1.484–1.488 A in [N4-B2-N4]M2. We have further extended our study to [N4-B2-N4-B2-N4]4− and [N4-B2-N4-B2-N4-B2-N4]6− systems. The B–B bond length is found to be 1.496 A in the former case, whereas the same is found to be 1.493 A and 1.508 A, respectively, for the two B–B bonds present in the latter one. The M + counter-ions stabilize such negatively charged systems and thus, create a possibility to design a long 1-D nanowire. Their utilities as probable hydrogen and noble gas (Ng) binding templates are explored taking [N4-B2-N4-B2-N4]Li4 system as a reference. It is found that each Li center binds with three H2 molecules with an average binding energy of 2.1 kcal/mol, whereas each Ng (Ar–Rn) atom interacts with Li center having a binding energy of 1.8–2.1 kcal/mol. The H2 molecules interact with Li centers mainly through equal contribution from orbital and electrostatic interaction, whereas the orbital interaction is found to be major term (ca. 51–58%) in Ng-Li interaction followed by dispersion (ca. 24–27%) and electrostatic interaction (ca. 17–24%).

The chemistry of complexes becomes interesting due to their structural diversity in different environments like in aqueous phase, in gas-phase or in the interior of a host. In the last few decades, powerful tools for the determination of gas-phase have been developed. In this context, the microsolvation approach of Li7P7 helix, where the passage from the bare double-strand helix to the hydrated denatured helix, has been addressed through successive attachment of water molecules using density functional theory. The stability of helical structure of the small clusters has been analyzed on the basis of polar bonding interaction between oxygen end of water molecule and Li centers of the Li7P7 helix. The Li7P7 helix is favored when associated with zero to eight water molecules, but the binding of the ninth water molecule brings a drastic change in the structure. Our results suggest that the natural charges on some sites in Li7P7 are large enough to induce partial and eventually total dissociation of water molecules. We shed light on the bonding situation through natural bond orbital, quantum theory of atoms in molecules and energy decomposition analyses which suggest dominant electrostatic interaction between Li centers of Li7P7 and O centers of water molecules (accounting for 60–64% of total bonding attraction). Nevertheless, 31–36% of total attraction is also originated from the orbital interaction. Variation in reactivity on microhydration is also analyzed. In order to check the site selectivity, we have computed conceptual density functional theory-based local reactivity descriptors such as dual descriptor based on the Fukui function, Δf(r), and multiphilic descriptor based on the philicity, Δω(r).

A density functional theory based study is performed to investigate the noble gas (Ng = Ar-Rn) binding ability of nitrates, sulfates and carbonates of noble metal (M). Their ability to bind Ng atoms is assessed through bond dissociation energy and thermochemical parameters like dissociation enthalpy and dissociation free energy change corresponding to the dissociation of Ng bound compound producing Ng and the respective salt. The zero-point energy corrected dissociation energy values per Ng atom for the dissociation process producing Ng atom(s) and the corresponding salts range within 6.0–13.1 kcal/mol in NgCuNO3, 3.1–9.8 kcal/mol in NgAgNO3, 6.0–13.2 kcal/mol in NgCuSO4, 3.2–10.1 kcal/mol in NgAgSO4, 5.1–11.7 kcal/mol in Ng2Cu2SO4, 2.5–8.6 kcal/mol in Ng2Ag2SO4, 8.1–19.9 kcal/mol in Ng2Au2SO4, 5.7–12.4 kcal/mol in NgCuCO3, 2.3–8.0 kcal/mol in Ng2Ag2CO3 and 7.3–18.2 kcal/mol in Ng2Au2CO3, with a gradual increase in moving from Ar to Rn. For a given type of system, the stability of Ng bound analogues follows the order as Au > Cu > Ag. All dissociation processes are endothermic in nature whereas they become endergonic as well in most of the cases of Kr-Rn bound analogues at 298 K. Natural population analysis along with the computation of Wiberg bond indices, and electron density analyses provide insights into the nature of the Ng-M bonds. The Ng-M bonds can be represented as partial covalent bonds as supported by the different electron density descriptors.

The impact of the size of the confining regime on the thermodynamic and kinetic outcome of a representative Diels-Alder reaction between ethylene and 1,3 butadiene has been investigated in silico. To this end, two organic hosts namely cucurbit[6]uril (CB[6]) and cucurbit[8]uril (CB[8]) have been considered in order to impose confinement on the reactants/transition state/product of the concerned reaction. The obtained results have been compared with the recently reported (Chakraborty et al. ChemPhysChem 18:2162-2170, 2017) corresponding case of the same reaction happening inside cucurbit[7]uril (CB[7]). Results indicate that as compared to the reaction of ethylene and 1,3 butadiene inside CB[7], both CB[6] and CB[8] cavitands slow down the same reaction at 298.15 K and 1 atm. It appears that the size of the cavitand plays a crucial role in affecting the kinetic outcome of the considered reaction. While CB[7] can enforce productive alignment of the reactants inside its cavity thereby facilitating the reaction, neither CB[6] nor CB[8] can perform the same task as effectively. This situation bears qualitative resemblance with the cases of enzyme catalyzed reactions.

The feasibility of having MF3 (where M = Rh, Ir, Pd, Pt, Ag, Au) supported AlN nanoflakes (AlNF) was investigated through density functional theory based calculations. The thermodynamic analysis reveals that the superhalogen MF3 molecules can bind with the host AlNF in a thermodynamically favorable way. The nature of interaction in between the metal centers and the host is of partly covalent type whereas the F centers bind with the host in a non-covalent fashion as vindicated by natural bond orbital and atoms-in a-molecule analyses. An ab initio molecular dynamics study carried out at 298 K temperature confirms the stability of the MF3@AlNF moieties in a dynamical context. The MF3 guests can reduce the HOMO-LUMO gaps of the host nanoflakes. In general, the MF3@AlNF complexes can sequestrate polar adsorbates such as CO, NO, and H2O in a thermodynamically favorable way in most of the cases. An ab initio molecular dynamics calculation illustrates that the MF3@AlNF can adsorb the chosen representative polar molecules in a more favorable way as compared to the corresponding adsorption scenario in the case of pristine AlNF.

The potential for gas storage (C2H2, C2H4, C2H6, CO2, CO, H2, N2, NO2, NO) molecules and rare gas (Rg) atoms (He n –Xe n , where n = 1, 2) within the recently synthesized octa acid (OA) moiety is assessed through density functional theory-based computations. It is shown that C2H2, C2H4, C2H6, N2, Kr, and Xe atoms/molecules bind with octa acid in a thermodynamically favorable way. Wiberg bond indices, non-covalent interaction indices, and energy decomposition analyses are used to explore the nature of the interaction between guest atoms and octa acid. The nature of the interaction in between either two guest atoms (in the cases of Rg atoms) or guest and cage atoms is mostly of non-covalent type in nature. An ab initio molecular dynamics simulation carried out at 50 and 298 K temperatures reveal that many of the studied systems particularly concerning polar and π electron cloud containing guest molecules show good dynamical stability at both temperature regimes. Except for the case of Ne-encapsulated octa acid, all other rare gases tend to get liberated from the host at room temperature although they remain inside the host at low temperature, thereby showing good dynamical stability of the Rg-encapsulated octa acid complexes up to 500 fs. In order to reaffirm the dynamical stability, Ne2@OA and CO@OA are studied at 50 and 298 K up to 600 fs as test cases.

A molecular dynamics simulation is carried out to explore the possibility of using sI clathrate hydrate as hydrogen storage material. Metastable hydrogen hydrate structures are generated using the LAMMPS software. Different binding energies and radial distribution functions provide important insights into the behavior of the various types of hydrogen and oxygen atoms present in the system. Clathrate hydrate cages become more stable in the presence of guest molecules like hydrogen.

Hydrogen trapping ability of various metal–ethylene complexes has been studied at the B3LYP and MP2 levels of theory using the 6-311+G(d,p) basis set. Different global and local reactivity descriptors and the associated electronic structure principles provide important insights into the associated interactions. There exist two distinct classes of bonding patterns, viz., a Kubas type interaction between the metal and the H2 molecule behaving as a η2-ligand and an electrostatic interaction between the metal and the atomic hydrogens.

Several sandwich-like metal clusters have been studied at the B3LYP/6-311 + G* level of theory. Bonding and reactivity have been analysed through various geometrical parameters and conceptual density functional theory based global reactivity descriptors. Aromaticity patterns have been understood in terms of the associated nucleus independent chemical shift values. Possibility of bond-stretch isomerism in some doped clusters is explored. Preferable sites for electrophilic and nucleophilic attacks have been identified using different local reactivity descriptors.

The atomic number (Z) and electrophilicity index (ω) have been utilized to explain the toxicity of various alkali and transition-metal ions as well as to predict that of the arsenic ions. The toxicity of two different training sets of arsenic derivatives is described using the global electrophilicity (ω) and number of nonhydrogenic atoms (N NH) along with the local philicity $${(\omega _{\rm As}^+)}$$ and the atomic charge (Q As) on the arsenic atom. Applying the regression models from the training sets, toxicity of some unknown arsenic derivatives is predicted.

A transition from regular to chaotic behaviour in the dynamics of a classical Henon-Heiles oscillator in the presence of an external field is shown to have a similar quantum signature when studied using the pertaining phase portraits and the associated Kolmogorov-Sinai-Lyapunov entropies obtained through the corresponding Bohmian trajectories.

Quantitative-structure-toxicity-relationship (QSTR) models are developed for predicting the toxicity (pIGC50) of 252 aliphatic compounds on Tetrahymena pyriformis. The single parameter models with a simple molecular descriptor, the number of atoms in the molecule, provide reasonable results. Better QSTR models with two parameters result when global electrophilicity is used as the second descriptor. In order to tackle both charge-and frontier-controlled reactions the importance of the local electro (nucleo) philicities and atomic charges is also analysed.

The toxicological structure-activity relationships are investigated using conceptual DFT based descriptors like global and local electrophilicities. In the present work the usefulness of electrophilicity in predicting toxicity of several polyaromatic hydrocarbons (PAH) is assessed. The toxicity is expressed through biological activity data (pIC50) defined as molar concentration of those chemicals necessary to displace 50% of radiolabeled tetrachlorodibenzo-p-dioxin (TCDD) from the arylhydrocarbon (Ah) receptor. The experimental toxicity values (pIC50) for the electron acceptor toxin like polychlorinated dibenzofurans (PCDF) are taken as dependent variables and the DFT based global descriptor electrophilicity index (omega) is taken as independent variable in the training set. The same model is then tested on a test set of polychlorinated biphenyls (PCB). A good correlation is obtained which vindicates the importance of these descriptors in the QSAR studies on toxins. These toxins act as electron acceptors in the presence of biomolecules whereas aliphatic amines behave as electron donors some of which are also taken into account for the present work. The toxicity values of the aliphatic amines in terms of the 50% inhibitory growth concentration (IGC50) towards ciliate fresh-water protozoa Tetrahymena pyriformis are considered. Since there is no global nucleophilicity we apply local nucleophilicity (omegamax+) as the descriptor in this case of training set. The same regression model is then applied to a test set of amino alcohols. Although the correlation is very good the statistical analysis reflects some cross validation problem. As a further check the amines and amino alcohols are used together to form both the training and the test sets to provide good correlation. It is demonstrated that the toxicity of several toxins (both electron donors and acceptors) in the gas and solution phases can be adequately explained in terms of global and local electrophilicities. Amount of charge transfer between the toxin and the biosystem, simulated as nucleic acid bases and DNA base pairs, indicates the importance of charge transfer in the observed toxicity. The major strength of the present analysis vis-à-vis the existing ones rests on the fact that it requires only one descriptor having a direct relationship with toxicity to provide a better correlation. Importance of using the information from both the toxin and the biosystem is also analyzed.

Local and global reactivity descriptors defined within {N, NS, υ(r)} and {Nα, Nβ, υ(r)} representations provide a remedy to the problem of inadequacy of hitherto-known reactivity descriptors in {N, υ (r)} representation in the analyses of situations where spin multiplicity changes are present. The tailor-made nature of the spin-dependent representations for specific processes is highlighted and a discussion on the convenience to use each representation is included. The connection between both representations is presented as a linear transformation. Generalized Fukui functions associated with processes where the number of electrons and/or the multiplicity change are calculated for closed-shell (NH3, H2O, HCOOH) and open-shell molecules (CH2) with BLYP/aug-cc-pVTZ level of theory, using both the finite differences and the frozen core approximations. Chemical processes involving spin transfer require the explicitly spin-dependent reactivity descriptors whose definitions and domain of applicability are analyzed. The method of calculation of these quantities, using finite differences and frozen core approximations, highlights that these two techniques provide similar trends, however for cases where orbital relaxation is important, the finite differences approximation should be used.

The applicability of DFT -based descriptors for the development of toxicological structure - activity relationships is assessed. Emphasis in the present study is on the quality of DFT -based descrip- tors for the development of toxicological QSARs and, more specifically, on the potential of the electrophili - city concept in predicting toxicity of benzidine derivatives and the series of polyaromatic hydrocarbons (PAH) expressed in terms of their biological activity data ( pIC50). First, two benzidine deriv atives, which act as electron -donating agents in their interactions with biomolecules are considered. Overall toxicity in general and the most probable site of reactivity in particular are effectively described by the global and local electrophilicity parameters re spectively. Interaction of two benzidine derivatives with nucleic acid (NA) bases/selected base pairs is determined using Parr's charge transfer formula. The experimental bi o- logical activity data ( pIC50) for the family of PAH, namely polychlorinated dibenz ofurans (PCDF), poly- halogenated dibenzo-p-dioxins (PHDD) and polychlorinated biphenyls (PCB) are taken as dependent variables and the HF energy (E), along with DFT-based global and local descriptors, viz., electrophilicity index (ω) and local electrophilic power (ω + ) respectively are taken as independent var iables. Fairly good correlation is obtained showing the significance of the selected descriptors in the QSAR on toxins that act as electron acceptors in the presence of biomolecules. Effects of populatio n analysis schemes in the cal- culation of Fukui functions as well as that of solvation are probed. Similarly, some electron -donor ali- phatic amines are studied in the present work. We see that global and local ele ctrophilicities along with the HF energy are adequate in explaining the toxicity of several substances, both electron donors or a c- ceptors when they interact with biosystems, in gas as well as solution phases.

Adsorption of small molecules like CH4, CO and NH3 into the acid sites of zeolites is analysed as an interaction between an electrophile and a nucleophile. Global reactivity descriptors like softness and electrophilicity, and local reactivity descriptors like the Fukui function, local softness and local philicity are calculated within density functional as well as Hartree-Fock frameworks using both Mulliken and Hirshfeld population analysis schemes. The HSAB principle and the best electrophile-nucleophile combination suggest that the reaction between the NH3 and Bronsted acid site of the zeolite is the strongest. Interaction between the zeolite and a small probe molecule takes place through the most electrophilic atom of one with the most nucleophilic atom of the other. This result is in conformity with those provided by the frontier orbital theory and the local HSAB principle.

The interrelationships between global reactivity descriptors such as chemical hardness, chemical potential, polarizability and electrophilicity and associated electronic structure principles were investigated in detail by considering distortion along the normal coordinates from the equilibrium structure and internal rotation. The necessary conditions on the extremum of electrophilicity were probed along with other electronic structure principles associated with the global reactivity descriptors. It was observed that an extremum in electrophilicity is obtained where both chemical potential and chemical hardness attain their respective exiremal values in course of the molecular vibrations as well as internal rotations.

Inter-relationships between the electrophilicity index (Ω), Hammett constant (op@#@) and nucleusindependent chemical shift (NICS (1) — NICS value one angstrom above the ring centre) have been investigated for a series of meta- and para-substituted benzoic acids. Good linear relationships between Hammett constant vs electrophilicity and Hammett constant vs NICS (1) values have been observed. However, the variation of NICS (1) against CO shows only a low correlation coefficient.

Attempts are made to gain insights into the effect of confinement of noble gas atoms on their various reactivity indices. Systems become harder, less polarizable and difficult to excite as the compression increases. Ionization also causes similar effects. A quantum fluid density functional technique is adopted in order to study the dynamics of reactivity parameters during a collision between protons and He atoms in different electronic states for various projectile velocities and impact parameters. Dynamical variants of the principles of maximum hardness, minimum polarizability and maximum entropy are found to be operative.

Dynamic profiles of various chemical reactivity indices like chemical potential, chemical hardness, electrophilicity, susceptibility, etc., within a confined environment during the interaction of atoms with strong oscillating time dependent magnetic fields have been studied. In the present study hydrogen and helium atoms in ground state (n = 1), as well as in excited state (n = 20) are considered. Time-dependent Schrodinger equations are solved for the ground and excited states of hydrogen atom and the Rydberg state of the helium atom while a generalized nonlinear Schrodinger equation is solved for the ground state of the helium atom. Dirichlet type boundary condition has been used to implement confinement to the systems. With an increase in the degree of confinement the system gets harder and hence becomes more stable. Keeping the confinement radius fixed, systems get more stabilized in strong field compared to weak field.

Gradual migration of Na+ from Mg3 2− brings about fascinating change in aromatic and magnetic behavior of inorganic Mg3Na2 cluster, which is addressed at the B3LYP and QCISD levels. During this process, Na+ takes away the electron density from Mg3 2− causing a net decrease in aromaticity. A tug-of-war between the Pauli repulsion and the aromaticity is shown to be responsible for the observed stability and aromaticity trends in singlet and triplet states. Implications of a spin crossover vis-a-vis a possible superexchange are also explored.

Dynamics of a wavepacket on a model fractal surface has been studied by solving the pertinent time dependent Schrodinger equation. Spatial and temporal behaviour of charge and current densities and a local chemical potential for two different fractal lattices have been considered. Important insight into the dynamics has been obtained through time dependence of various quantities like macroscopic kinetic energy, global current, Shanon entropy, density correlation and global chemical potential. This study would be helpful in simulating adsorption and catalysis.

Temporal evolution of electronegativity and hardness associated with a collision process between a Be atom and a proton has been studied within a quantum fluid density functional framework. In the presence of a third collisional partner to take away excess energy, this collision may lead to a chemical reaction producing a BeH+ molecule. For comparisonab initio SCF level calculation (with 6–31G** basis set) on BeH+ molecule with different geometries have been performed. Electronegativity equalization and maximum hardness principles are analyzed.

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.

Temporal evolution of local and global hardness during an ion-atom collision process has been studied within a quantum fluid density functional framework. A dynamical variant of the maximum hardness principle has been found to be operative. Entropy maximises in the encounter regime. Time dependence of density and its laplacian provides important insights into the collision processvis-a-vis the hardness maximisation.

It is shown that molecular hardness can be expressed as the geometric mean of the hardness values for constituent atoms. Using this principle the hardness values for several molecules have been calculated from the pertinent atomic data obtained through a five-point finite difference formula. Finally, gradation of several acids and bases into hard, border-line and soft categories has been made on the basis of their calculated hardness values.

Threesimulated expressions are suggested regarding the Z-dependence of groundstate energies of neutral atoms at the Hartree-Fock, exact nonrelativistic and relativistic levels. The Hartree-Fock-level Z−1/3-expansion contains three previously derivedplus two newly derived terms, the latter signifying exchange corrections from the ‘inner core’ and the ‘core mantle’ of the Lieb atom. This leads to better agreement than any previous expression, with every coefficient being physically transparent. The Z-dependence of the correlation energy is obtained from a semiempirical Wigner-type correlation potential while the relativistic correction is taken to be in between the values suggested separately by Scott and Schwinger. Agreement with reference values is again better than before. It appears, however, that the atomic Z−1/3-expansion should not proceed beyond terms O(Z).

Density functional theory (DFT) has not been applied on a large scale to time-dependent problems and problems involving excited states. Atomic and molecular collisions involving both these types of phenomena remain outside the purview of DFT. An amalgamation of quantum fluid dynamics (QFD) with DFT considerably broadens the range of applicability of traditional (ground-state) DFT. Ion-atom collisions have been studied by jointly using DFT and QFD.

Using the double-zeta wavefunctions of Snyder and Basch [5], the Weizsacker correction \(T_w = \tfrac{1}{8}\int d \vec r(\nabla \rho \cdot \nabla \rho )\rho ^{ - 1} \) has been evaluated for the molecules H2, HF, BF, N2 and CO. There was no linear correlation between Tw and the binding energy for these molecules.

Gas-surface scattering is speculated as a meaningful problem for understanding the physics of fractals. Fractal behaviour can be associated with a self-similar geometry on a solid surface. The interaction potential for a gas atom or molecule approaching the lattice depends primarily on local factors but a parametric dependence of the cross-section data on the fractal dimension can be conceived. Such a dependence on the self-similar character of a multi-centred target is more explicit when multiple scattering is included. Application of approximation schemes like the previously developed average wavefunction method to this problem is suggested.