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Density functional theory based computation predicts the presence of a planar tetracoordinate carbon (ptC) in the global minimum energy structures of CSinGe4−n2+ (n = 1–3) systems.

Density functional theory-based computations predict planar hexacoordinate boron (phB) in global minimum CB6Al0/+. The systems are kinetically stable and show σ- and π-electronic delocalization.

Atomic clusters are unique in many perspectives because of their size and structure features and are continuously being applied for different purposes. To unveil their unconventional properties, in this work, using neutral tetraboron clusters as illustrative examples, we study their exotic behaviors in bonding, aromaticity, and reactivity. We show that both double and triple bonds can be formed, ring current patterns can be totally different, and both electrophilic and nucleophilic reactivities can coexist simultaneously. These features are often in contrast with our conventional chemical wisdom and could enrich the possibility for their potential applications. The methodologies employed in this work can be readily applied to other systems. Our studies should help us better appreciate atomic clusters with many atypical properties and henceforth yield novel applications.

The discovery of fullerenes was a huge milestone in the scientific community, and with it came the urge to discover and analyze various small and large atomic and molecular clusters having a cavity. These cavitands of varied shapes and sizes have wide applications in the encapsulation of rare gas atoms to induce bond formation between them, storage of hydrogen and hydrocarbons to be used as alternative sources of fuel, catalyzation of otherwise slow reactions without using a catalyst, activation of small gas molecules, etc. Various cavitands like fullerenes, [ExBox]4+, cucurbit[n]urils, borospherenes, octa acid, etc. have been used for this purpose. Some clusters including cavitands exhibit fluxional behaviour. Systems in a confined environment often manifest interesting variations in their properties and behaviour, compared to their unconfined counterparts, facilitating the aforementioned applications. In this perspective article, we explore the possibility of making use of this extra degree of freedom, viz., the fluxionality, in changing the catalytic activity of the cavitand.

We investigate the underlying principle behind the occurrence of bond stretch isomerism in Be32-, which has not been revealed yet. Various computational studies of the different isomers are carried out at the complete active space self-consistent field (CASSCF) level of theory in addition to the B3LYP level in conjunction with the 6-311++G(d) basis set. The potential energy surfaces linking the different isomers through transition states and conical intersections are investigated at the CASSCF level, connecting various geometrical isomers of Be32-. The linear intermediate of the Be32- cluster is considered to be of profound importance since its excited state is found to be degenerate and undergoing the Renner-Teller effect, producing two triangular bond stretch isomers. Ab initio molecular dynamics simulations based on the Atom Centered Density Matrix Propagation (ADMP) method also further elucidate the phenomenon of isomerization via the linear intermediate. The variation of the global reactivity descriptors and free energy profile along the bond stretch isomerization path is also investigated. The estimated aromatic stabilization energy also corroborates the stability ordering of the bond stretch isomers.

The role of boron–ligand cooperation in activating molecular hydrogen by a set of six frustrated Lewis pair (FLP) systems is explored using density functional theory. The results obtained from thermochemical calculations show that all the studied FLP systems are capable of activating molecular hydrogen as the activation free energies are not too high (17.6–25.6 kcal mol−1). Sulphur based FLP 6 is the most promising one as it results in the smallest activation barrier among the studied sets. For a given FLP, the introduction of an electron donating –NMe2 group at the para position of the pyridine ring somewhat lowers the barrier and enhances the B–X (X = O, N, S) interaction. The B–X bond present within the FLPs plays a crucial role in facilitating the H2 activation process where it gets changed from the B+–X− type of interaction in the FLP to the B ← X dative bond upon H2 activation as understood from the energy decomposition analysis in combination with the natural orbital for chemical valence theory (EDA-NOCV). This mode of operation is termed as boron–ligand cooperation in analogy with the metal–ligand cooperation in transition metal complexes. The EDA-NOCV results obtained at the TS also support an electron transfer model where simultaneous electron transfer takes place from the Lewis basic center (N) of the FLP to σ*(H2) and from σ(H2) to the Lewis acidic center (B) of the FLP, resulting in a weakened H–H bond. The change in the aromaticity of the pyridine rings during the course of H2 activation is also monitored by nucleus independent chemical shift calculations. Finally, the ability of the studied FLP systems to act as hydrogenation catalysts is elucidated by studying the hydrogenation of CO2 to yield formic acid.

We report a family of systems having a planar pentacoordinate carbon (ppC) based on the next heavier analogue of CAl5+, the ppC system par excellence. Although because of the larger size of Ga, the ppC isomer is not even a local minimum in CGa5+, a single isoelectronic substitution of Ga by smaller sized Be maximizes the bonding in the ppC form. Retaining the 18 valence electron rule, the global minimum structures of CGa4Be, CGa3Be2−, CGa2Be32−, and CGaBe43− clusters and their corresponding lithium salts have a ppC.

The efficacy of B40 borospherene to act as a host for noble gas atoms is explored via density functional theory based computations. Although the Ng@B40 complexes are thermochemically unstable with respect to dissociation into free Ng and B40, it does not rule out their viability as all the systems possess a high activation free energy barrier (84.7-206.3 kcal mol-1). Therefore, once they are formed, it is hard to take out the Ng atom. Two Ng atoms can also be incorporated within B40 for the lighter Ng atoms (He and Ne). In fact, the destabilization offered by the encapsulation of one and two He atoms and one Ne atom inside B40 is significantly less than that in experimentally synthesized He@C20H20, highlighting their greater possibility for synthesis. Although Ar2 and Kr2 encapsulated B40 systems are very much destabilized by the repulsive interaction between Ng2 and B40, an inspection of the bonding situation reveals that the confinement can even induce some degree of covalent interaction between two otherwise non-bonded Ng atoms. Ng atoms transfer electrons towards B40 which is smaller for lighter Ng atoms and gradually increases along He to Rn. Even if the electrostatic interaction between Ng and B40 is the most predominant term in these systems, the extent of the orbital interaction is also considerable. However, the very large Pauli repulsion counterbalances the attractive interaction, eventually turning the interaction repulsive in nature. Ng@B40 also shows dynamical behaviour involving continuous exchange between hexagonal and heptagonal holes, similar to the host cage, as understood from the very little variation in the activation barrier because of the Ng encapsulation. Furthermore, sandwich complexes like [(η5-C5Me5)Fe(η6-B40)]+ and [(η5-C5Me5)Fe(η7-B40)]+ are noted to be viable with the latter being slightly more stable than the former. The encapsulation of Xe slightly improves the dissociation energy associated with the decomposition into Xe@B40 and [Fe(η5-C5Me5)]+ compared to that in the bare one.

The impact of functionalization as well as guest encapsulation on the optoelectronic properties of the ExBox+4 moiety has been investigated with the aid of density functional theory (DFT) based calculations. To this end, three functionalized variants of the ExBox+4 moiety have been modelled in silico, viz. NH2-ExBox+4-F, CN-ExBox+4-NH2 and NO2-ExBox+4-NH2, whereas coronene (Cor), B-doped coronene (BCor), N-doped coronene (NCor), tetrathiafulvalene (TTF), biphenyl (BiPh), tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) molecules have been employed as guests. The results indicate that as a result of functionalization, the ExBox+4 moiety can exhibit tangible non-linear optical (NLO) response properties as vindicated by the first static hyperpolarizability (β). As a result of guest encapsulation, significant variation in the optoelectronic properties of the chosen hosts takes place. BCor/TCNQ/NCor molecules enhance the NLO activity of the hosts while TTF/TCNE suppresses the same. In particular, NCor@host moieties demonstrate very high β values at the static field limit. Time dependent DFT results suggest that in general, the ability of the guest in tuning the transition energy associated with the pivotal electronic transitions of the host plays the dominant role in the observed variation in β. In addition, BCor/NCor@host moieties demonstrate broadband optical absorption capability thereby elucidating the possibility of deriving multi-purpose optoelectronic features such as NLO activity as well as utility in photovoltaic systems. Thermochemical results suggest that all the guest@host systems are stable at 298.15 K. Non-covalent and electrostatic binding forces stabilize the studied systems.

The selectivity of cucurbit[7]uril (CB[7]) towards adsorbing a series of 14 molecules encompassing four hydrocarbons (C2H2, C2H4, C2H6, and CH4), diatomic molecules of halogens (F2 and Cl2), nitrogen oxides (NO2 and NO), carbon oxides (CO2 and CO), SO2, H2S, N2, and H2 is explored via a density functional theory based study. CB[7] is noted to have high selectivity towards adsorbing SO2 over the other considered molecules, highlighting its probable utility to separate SO2 from flue gas or other gas mixtures containing these molecules. The nature of bonding is deciphered via the computations of non-covalent interaction indices and energy decomposition analysis. Although in all cases the dispersion interaction turns out to be the most dominating contributor in stabilizing these complexes, the electrostatic contribution is also considerable. In fact, the combined effect of these two energy terms in SO2@CB[7] is responsible for the obtained selectivity.

Aromaticity in novel cyclic all-pnictogen heterocyclic anions, P2N3(-) and P3N2(-), and in their heavier analogues is studied using quantum mechanical computations. All geometrical parameters from optimized geometry, bonding, electron density analysis from quantum theory of atoms in molecules, nucleus-independent chemical shift, and ring current density plots support their aromaticity. The aromatic nature of these molecules closely resembles that of the prototypical aromatic anion, C5H5(-). These singlet C2v symmetric molecules are comprised of five distinct canonical structures and are stable up to at least 1000 fs without any significant distortion. Mechanistic study revealed a plausible synthetic pathway for P3N2(-) - a click reaction between N2 and P3(-), through a C2v symmetric transition state. Besides this, the possibility of P3N2(-) as a η(5)-ligand in metallocenes is studied and the nature of bonding in metallocenes is discussed through the energy decomposition analysis.

Ab initio computations are carried out to explore the structure and stability of FNgEF3 and FNgEF (E = Sn, Pb; Ng = Kr-Rn) compounds. They are the first reported systems to possess Ng-Sn and Ng-Pb bonds. Except for FKrEF3, the dissociations of FNgSnF3 and FNgEF, producing Ng and SnF4 or EF2, are only exergonic in nature at room temperature, whereas FNgPbF3 has a thermochemical instability with respect to two two-body dissociation channels. However, they are kinetically stable, having positive activation barriers (ranging from 2.2 to 49.9 kcal mol(-1)) with respect to those dissociations. The kinetic stability gradually improves in moving from the Kr to Rn analogues. The remaining possible dissociation channels for these compounds are found to be endergonic in nature. The nature of the bonding is analyzed by natural bond order, electron density, and energy decomposition analyses. Particularly, the natural population analysis reveals that they are best represented as F(-)(NgEF3)(+) and F(-)(NgEF)(+). All the Xe/Rn-E bonds in FNgEF3 and FNgEF are covalent in nature.

The validity of maximum hardness, minimum electrophilicity and minimum polarizability principles is assessed to explain the phenomenon of torquoselectivity (inward and outward preference) in the conrotatory ring opening reactions of trans-3,4-dimethylcyclobutene into Z,Z- and E,E-butadienes and 3-formylcyclobutene into E- and Z-2,4-pentadienals. The hardness, average polarizability and electrophilicity profiles are computed along the intrinsic reaction coordinate and divided into three relevant stages. The transition states involved in the unfavorable inward conrotation of trans-3,4-dimethylcyclobutene and in the unfavorable outward conrotation of 3-formylcyclobutene are found to be higher in energy, softer, more electrophilic and more polarizable than the transition states corresponding to the torquoselective outward and inward conrotations, respectively. These observations are in conformity with the maximum hardness, minimum electrophilicity and minimum polarizability principles. The sharp changes in the local reactivity descriptors are also observed around the transition states in their respective profiles.

Correction for ‘Scaling properties of information-theoretic quantities in density functional reactivity theory’ by Chunying Rong et al., Phys. Chem. Chem. Phys., 2015, 17, 4977–4988.

Time evolution of various reactivity parameters viz. hardness, electrophilicity, chemical potential, polarizability, etc. in a confined environment has been studied through quantum fluid density functional theory formalism during time dependent processes such as proton-molecule collisions and molecule-field interaction. A Dirichlet type boundary condition has been incorporated to confine the systems. Responses in the reactivity parameters of the diatomic molecules, in the dynamical context, in ground state as well as in excited state, have been reported. Harmonic spectra are generated in the cases of the external laser field interacting with H2 and N2 molecules.

Prompted by the stability of some lithium decorated star-like clusters and super-alkali systems, their hydrogen trapping potential is assessed at the M06/6-311+G(d,p) and the M052X/6-311+G(d) levels, respectively. The effect of an applied electric field is also analyzed. Most of these systems are found to have the potential to become effective hydrogen storage materials with high gravimetric weight percent owing to the charges on the Li centers. The presence of an external electric field improves the situation.

Hydrogen storage capacity of some Li(+)/F(-) doped neutral and charged aromatic/antiaromatic systems is studied at the B3LYP, M05-2X, MPW1K and MP2 levels of theory. Various conceptual density functional theory based global and local reactivity descriptors, nucleus independent chemical shift (NICS), NICS-rate, interaction energy per H(2) molecule, reaction enthalpy and reaction electrophilicity are used for this purpose. It is observed that there is a direct bearing of the hydrogen adsorption capability on the aromaticity and/or the charge of the system (or the charge on a specific center). The latter quantities do also change on gradual hydrogen loading.

The effect of counterions on the bonding, stability and aromaticity of trigonal dianion metal clusters has been analyzed through the behavior of various conceptual density functional theory based reactivity descriptors and the nucleus independent chemical shift calculated at different levels of theory, comprising one-determinant approaches and beyond (QCISD, CASSCF(8,8) and NEVPT2), for a proper benchmarking. Although several important insights into the counter-ion effects are obtained, much needs to be done in order to have a transparent idea therein.

Ayers, Parr, and Pearson recently showed that insight into the hard/soft acid/base (HSAB) principle could be obtained by analyzing the energy of reactions in hard/soft exchange reactions, i.e., reactions in which a soft acid replaces a hard acid or a soft base replaces a hard base [J. Chem. Phys., 2006, 124, 194107]. We show, in accord with the maximum hardness principle, that the hardness increases for favorable hard/soft exchange reactions and decreases when the HSAB principle indicates that hard/soft exchange reactions are unfavorable. This extends the previous work of the authors, which treated only the "double hard/soft exchange" reaction [P. K. Chattaraj and P. W. Ayers, J. Chem. Phys., 2005, 123, 086101]. We also discuss two different approaches to computing the hardness of molecules from the hardness of the composing fragments, and explain how the results differ. In the present context, it seems that the arithmetic mean of fragment softnesses is the preferable definition.