CO2 reduction using aluminum hydride: Generation of in-situ frustrated Lewis pairs and small molecule activation therein.

CO2 reduction is appealing for the long-term production of high-value fuels and chemicals. Herein, using density functional theory (DFT) based calculations, we study the CO2 reduction pathway to formic acid using aluminum hydride and phosphine derivatives. Our primary focus is on aluminum hydride derivatives, aimed at improving the efficiency of the CO2 reduction process. Substituents with σ-donating properties at the aluminum center are discovered to lower the activation barriers. We demonstrate how di-tert-butylphosphine oxide (LB-O)/di-tert-butylphosphine sulfide (LB-S)/di-tert-butylphosphanimine (LB-N) work together with aluminum hydride to facilitate CO2 reduction process and generate in-situ frustrated Lewis pairs (FLPs), such as FLP-O, FLP-S, and FLP-N. The activation strain model (ASM) analysis reveals the significance of strain energy in determining activation barriers. EDA-NOCV and PIO analyses elucidate the orbital interactions at the corresponding transition states. Furthermore, the study delves into the activation of various small molecules, such as dihydrogen, acetylene, ethylene, carbon dioxide, nitrous oxide, and acetonitrile, using those in-situ generated FLPs. The study highlights the low activation barriers and emphasizes the potential for small molecule activation in this context.

Structure, stability, reactivity and bonding in noble gas compounds.

Noble gases (Ngs) are recognized as the least reactive elements due to their fully filled valence electronic configuration. Their reluctance to engage in chemical bond formation necessitates extreme conditions such as low temperatures, high pressures, and reagents with high reactivity. In this Perspective, we discuss our endeavours in the theoretical prediction of viable Ng complexes, emphasizing the pursuit of synthesizing them under nearly ambient conditions. Our research encompasses various bonding categories of Ng complexes and our primary aim is to comprehend the bonding mechanisms within these complexes, utilizing state-of-the-art theoretical tools such as natural bond orbital, energy decomposition, and electron density analyses. These complex types manifest distinct bonding scenarios. In the non-insertion type, the donor-acceptor interaction strength hinges on the polarizing ability of the binding atom, drawing the electron density of the Ng towards itself. In certain instances, especially with heavier Ng elements, this interaction reaches a magnitude where it can be considered a covalent bond. Conversely, in most insertion cases, the Ng prefers to share electrons to form a covalent bond on one side while interacting electrostatically on the other side. In rare cases, both bonds may be portrayed as electron-shared covalent bonds. Furthermore, a host cage serves as an excellent platform to explore the limits of achieving Ng-Ng bonds (even for helium), under high pressure.

Bonobo Optimizer: A New Tool Toward the Global Optimization of Small Atomic Clusters.

In the realm of structural and bonding investigations within chemical systems, elucidating global minimum energy configurations stands as a paramount goal. As the systems increase in size and complexity, this pursuit becomes progressively challenging. Herein, we introduce Bonobo optimizer (BO), a metaheuristic algorithm inspired by the social and reproductive behaviors of bonobos, to the domain of chemical problem solving. Focusing on small carbon clusters, this study systematically evaluates BO's performance, showcasing its robustness and efficiency. Parametric studies highlight the algorithm's adaptability, consistently converging to global minimum structures. Rigorous statistical validation supports the results, and a comparative analysis against established global optimization algorithms underlines BO's superior efficiency. This exploration extends the applicability of BO to the optimization of atomic clusters, providing a promising avenue for future advancements in computational chemistry.

Electrophilicity index revisited.

This review aims to be a comprehensive, authoritative, critical, and accessible review of general interest to the chemistry community; because the electrophilicity index is a very useful global reactivity descriptor defined within a conceptual density functional theory framework. Our group has also introduced electrophilicity based new global and local reactivity descriptors and also new associated electronic structure principles, which are important indicators of structure, stability, bonding, reactivity, interactions, and dynamics in a wide variety of physico-chemical systems and processes. This index along with its local counterpart augmented by the associated electronic structure principles could properly explain molecular vibrations, internal rotations and various types of chemical reactions. The concept of the electrophilicity index has been extended to dynamical processes, excited states, confined environment, spin-dependent and temperature-dependent situations, biological activity, site selectivity, aromaticity, charge removal and acceptance, presence of external perturbation through solvents, external electric and magnetic fields, and so forth. Although electrophilicity and its local variant can adequately interpret the behavior of a wide variety of systems and different physico-chemical processes involving them, their predictive potential remains to be explored. An exhaustive review on all these aspects will set the tone of the future research in that direction.

Structure and stability of the sH binary hydrate cavity and host-guest versus guest-guest interactions therein: A DFT approach.

The intrinsic ability of clathrate hydrates to encage gaseous molecules is explored. Encapsulation ability depends on the cavity size and the type of guest gaseous species in addition to the physical parameters, temperature and pressure. Here we have reported the structure, stability and nature of interaction in dissimilar guest occupied sH hydrate cavity. Diatomic gas molecules and small polyatomic hydrocarbons are considered as guests. The irregular icosahedron (512 68 ) cavity of sH hydrate is the host. Different thermodynamic parameters for the guest molecules encapsulation were calculated using three different hybrid DFT functionals, B3LYP, M05-2X, M06, and moreover using dispersion correction (PBE0-D3). With the consideration of large H-bonded systems the 6-31G* and cc-pVTZ basis sets were used for two set of computations. To disclose the nature of interaction between the host-guest systems as well as the interaction between the guest molecules inside the host the non-covalent interaction (NCI) indices and energy decomposition analysis (EDA) were done. Impact of host-guest and guest-guest interactions are discussed.

Frustrated Lewis Pairs: Bonding, Reactivity, and Applications.

The outstanding capability of Frustrated Lewis Pair (FLP) catalysts to activate small molecules has gained significant attention in recent times. Reactivity of FLP is further extended toward the hydrogenation of various unsaturated species. Over the past decade, this unique catalysis concept has been successfully expanded to heterogeneous catalysis as well. The present review article gives a brief survey on several studies on this field. A thorough discussion on quantum chemical studies concerning the activation of H2 is provided. The role of aromaticity and boron-ligand cooperation on the reactivity of FLP is discussed in the Review. How FLP can activate other small molecules by cooperative action of its Lewis centers is also discussed. Further, the discussion is shifted to the hydrogenation of various unsaturated species and the mechanism regarding this process. It also discusses the latest theoretical advancements in the application of FLP in heterogeneous catalysis across various domains, such as two-dimensional materials, functionalized surfaces, and metal oxides. A deeper understanding of the catalytic process may assist in devising new heterogeneous FLP catalysts through experimental design.

On the Nature of the Partial Covalent Bond between Noble Gas Elements and Noble Metal Atoms.

This article provides a discussion on the nature of bonding between noble gases (Ng) and noble metals (M) from a quantum chemical perspective by investigating compounds such as NgMY (Y=CN, O, NO3, SO4, CO3), [NgM-(bipy)]+, NgMCCH, and MCCNgH complexes, where M=Cu, Ag, Au and Ng=Kr-Rn, with some complexes containing the lighter noble gas atoms as well. Despite having very low chemical reactivity, noble gases have been observed to form weak bonds with noble metals such as copper, gold, and silver. In this study, we explore the factors that contribute to this unusual bonding behavior, including the electronic structure of the atoms involved and the geometric configuration of the concerned fragments. We also investigate the metastable nature of the resulting complexes by studying the energetics of their possible dissociation and internal isomerization channels. The noble gas-binding ability of the bare metal cyanides are higher than most of their bromide counterparts, with CuCN and AgCN showing higher affinity than their chloride analogues as well. In contrast, the oxides seem to have lower binding power than their corresponding halides. In the oxide and the bipyridyl complexes, the Ng-binding ability follows the order Au > Cu > Ag. The dissociation energies calculated, considering the zero-point energy correction for possible dissociation channels, increase as we move down the noble gas group. The bond between the noble gases and the noble metals in the complexes are found to have comparable weightage of orbital and electrostatic interactions, suggestive of a partial covalent nature. The same is validated from the topological analysis of electron density.

Impact of Static-Oriented Electric Fields on the Kinetics of Some Representative Suzuki-Miyaura and Metal-Cluster Mediated Reactions.

In order to examine the effect of oriented (static) electric fields (OEF) on the kinetics of some representative Suzuki-Miyaura and metal-cluster mediated reactions at ambient temperatures, density functional theory-based calculations are reported herein. Results indicate that, in general, OEF can facilitate the kinetics of the concerned reactions when applied along the suitable direction (parallel or anti-parallel with respect to the reaction axis). The reverse effect happens if the direction of the OEF is flipped. OEF (when applied along the 'right' direction) helps to polarize the transition states in the desired direction, thereby facilitating favorable bonding interactions. Given the growing need for finding appropriate catalysts among the scientific community, OEF can prove to be a vital route for the same.

CSiGaAl2 -/0 and CGeGaAl2 -/0 having planar tetracoordinate carbon atoms in their global minimum energy structures.

Density functional theory (DFT) is used to explore the structure, stability, and bonding in CSiGaAl2 -/0 and CGeGaAl2 -/0 systems having planar tetracoordinate carbon (ptC). The neutral systems have 17 valence electrons and the mono-anionic systems have 18 valence electrons. The ab initio molecular dynamics simulations for 2000 fs time at two different temperatures (300 and 500 K) supported the kinetic stability of the systems. From the natural bond orbital (NBO) analysis it is shown that there is a strong electron donation from the ligand atoms to the ptC atom. We have used Li+ ion for the neutralization of the mono-anionic systems and more interestingly it does not disrupt the planar structure. The most preferable site for binding of Li+ ion is along the AlAl bond in both of the mono-anionic systems. All the systems in this work have both σ and π aromaticity which is predicted from the computations of nucleus independent chemical shift (NICS). Although the anionic species obey the 18 valence electronic rule, the neutral systems break the rule with 17 valence electrons. However, both sets of systems are stable in the planar form. The bonding analysis of the systems includes molecular orbital, adaptive natural density partitioning (AdNDP), quantum theory of atoms in molecules (QTAIM), electron localization function (ELF) basin, and aromaticity analyses. The energy decomposition analysis (EDA) determines the interaction of Li+ ion with CSiGaAl2 - and CGeGaAl2 - in Li@SiGaAl2 and Li@GeGaAl2 , respectively.

Molecular Electrides: An In Silico Perspective.

Electrides are defined as the ionic compounds where the electron(s) serves as an anion. These electron(s) is (are) not bound to any atoms, bonds, or molecules but are rather localized into the space, crystal voids, or interlayer between two molecular slabs. There are three major categories of electrides, known as organic electrides, inorganic electrides, and molecular electrides. The computational techniques have proven as a great tool to provide emphasis on the electride materials. In this review, we have focused on the computational methodologies and criteria that help to characterize molecular electrides. A detailed account of the computational methods and basis sets applicable for molecular electrides have been discussed along with their limitations in this field. The main criterion for the identification of the electrides has also been discussed thoroughly with proper examples. The molecular electrides presented here have been justified with all the required criteria that support and proved their electride characteristics. We have also presented few systems which have similar properties but are not considered as molecular electrides. Moreover, the applicability of the electrides in catalytic processes have also been presented.

CB6Al0/+: Planar hexacoordinate boron (phB) in the global minimum structure.

Herein, we report for the first time the presence of a planar hexacoordinate boron (phB) atom in the global minimum energy structure of a neutral cluster system. The potential energy surface (PES) has been explored for CB6Al0/+/- systems using density functional theory (DFT). The global minima of CB6Al (1a) and CB6Al+ (1b) contain a phB center. However, the global minimum of CB6Al- (1c) does not have a phB atom. The CCSD(T)/aug-cc-pVTZ level of theory has been applied to compute the relative energies of the low-lying isomers with respect to the 1a and 1b structures of CB6Al and CB6Al+ systems, respectively. The exploration of the PES of CB60/+/- systems indicates that the global minima do not contain a phB atom. However, the incorporation of an aluminium (Al) atom into the CB6 moiety produces structures containing a phB center in the CB6Al0/+ systems. Hence, the Al metal has an important role in attaining a planar geometry having a hexacoordinate boron center. The dynamical stability of CB6Al (1a) and CB6Al+ (1b) was confirmed from the atom-centered density matrix propagation (ADMP) simulation over 20 ps of time at temperatures of 300 K and 400 K. The natural charge computations showed that the charges on the phB are almost zero in both systems. The 1a structure has σ/π-dual aromaticity as predicted from the nucleus independent chemical shift (NICS) values and the gauge-including magnetically induced ring current (GIMIC).

CSinGe4-n2+ (n = 1-3): prospective systems containing planar tetracoordinate carbon (ptC).

Density functional theory (DFT) based calculations have been carried out to explore the potential energy surface (PES) of CSinGe4-n2+/+/0 (n = 1-3) systems. The global minimum structures in the di-cationic states (1a, 1b, and 1c) contain a planar tetracoordinate carbon (ptC). For the CSi2Ge22+ system, the second stable isomer (2b) also contains a ptC with 0.67 kcal mol-1 higher energy than that of the 1b ptC isomer. The global minima of the neutral and mono-cationic states of the designed systems are not planar. The 1a, 1b, and 1c structures follow the 18 valence electron rule. The relative energies of the low-lying isomers of CSiGe32+, CSi2Ge22+, and CSi3Ge2+ systems with respect to the global minima were calculated using the CCSD(T)/aug-cc-pVTZ method. Ab initio molecular dynamics simulations for 50 ps time indicate that all the global minimum structures (1a, 1b, and 1c) are kinetically stable at 300 K and 500 K temperatures. The natural bond orbital (NBO) analysis suggests strong σ-acceptance of the ptC from the four surrounding atoms and simultaneously π-donation occurs from the ptC center. The nucleus independent chemical shift (NICS) showed σ/π-dual aromaticity. We hope that the designed di-cationic systems may be viable in the gas phase.

DFT exchange: sharing perspectives on the workhorse of quantum chemistry and materials science.

In this paper, the history, present status, and future of density-functional theory (DFT) is informally reviewed and discussed by 70 workers in the field, including molecular scientists, materials scientists, method developers and practitioners. The format of the paper is that of a roundtable discussion, in which the participants express and exchange views on DFT in the form of 302 individual contributions, formulated as responses to a preset list of 26 questions. Supported by a bibliography of 777 entries, the paper represents a broad snapshot of DFT, anno 2022.

On the Periodicity of the Information Theory and Conceptual DFT-Based Reactivity Descriptors.

The periodic trends in conceptual density functional and information theory-based reactivity descriptors are reported for the atoms H to Ba (Z = 1 to 56). Ionization potential, electron affinity, electronegativity, and hardness show periodic behavior following the Aufbau principle and popular electronic structure principles. They are in agreement with those reported in standard chemistry textbooks. The trend in the electrophilicity index, however, shows an interesting behavior, where it contradicts earlier reports. Our calculation reveals that the noble gas elements correspond to minimum ω values in each period which obey the minimum electrophilicity principle as well as reflect their low reactivity. Periodic trends in electroaccepting and electrodonating powers, along with that of net electrophilicity, are as expected. The behavior of information theory-based Shannon and GBP entropies, along with the Shannon entropy of shape function are also explored across the periodic table.

Controlling Tunneling Oscillation and Quantum Localization in an Asymmetric Double-Well Potential: A Bohmian Perspective.

The roles of spatial symmetry and strength of external time-dependent perturbation on the dynamics of a quantum particle, initially localized in one of the wells of an asymmetric double-well potential are studied using the recently developed techniques incorporating quantum theory of motion and time-dependent Fourier grid Hamiltonian methods. The model used here includes a mimic of the related experimental situations which is considered as a perturbation to the static double-well potential. Analysis of localized and delocalized phase space structures and corresponding time-profile of tunneling probability reveal the recipe toward controlling the tunneling oscillations by modulating the parameters of applied perturbation. A study on a stochastic pulsating potential also reveals the root to the quantum localization, even in moderate field strength.

Can the Fluxionality in Borospherene Influence the Confinement-Induced Bonding between Two Noble Gas Atoms?

A density functional theory study is performed to determine the stability and bonding in the neon dimer inside the B30N30 fullerene cage, the fluxional B40 cage, and within non-fluxional cages such as B12N12 and C60. The nature of bonding in the Ne2 encapsulated B40 is compared with the that in other cages in an attempt to determine whether any possible alterations are brought about by the dynamical nature of the host cage apart from the associated confinement effects. The bonding analysis includes the natural bond order (NBO), Bader's Atoms-in-Molecules electron density analysis (AIM), and energy decomposition analysis (EDA), revealing the non-covalent nature of the interactions between the Ne atoms and that between the Ne and the cage atoms. The formation of all the Ne2@cage systems is thermochemically unfavourable, the least being that for the B30N30 cage, which can easily be made favourable at lower temperatures. The Ne-Ne distance is lowest in the smallest cage and increases as the cage size increase due to steric relaxation experienced by the dimer. The dynamical picture of the systems is investigated by performing ab initio molecular dynamics simulations using the atom-centred density matrix propagation (ADMP) technique, which shows the nature of the movement of the dimer inside the cages, and by the fact that since it moves as a single entity, a weak bonding force holds them together, apart from their proven kinetic stability.

Possible effects of fluxionality of a cavitand on its catalytic activity through confinement.

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.

Substituent Effects on Electride Characteristics of Mg2(η5-C5H5)2: A Theoretical Study.

An ab initio study has been carried out on the substituted binuclear sandwich complexes of Mg2(η5-C5H5)2. We have checked whether the substitution destroys the electride properties of a complex, as it needs to satisfy several stringent criteria to obtain the status of an electride. The thermochemical results show that the complexes are stable at room temperature and 1 atm pressure. From the analysis of the various electron density descriptors and the natural bond orbital (NBO) for all the complexes, it is confirmed that the Mg-Mg bonds are covalent and the metal-ligand bonds are ionic in nature. The charges on each Mg atom in the studied complexes are +1. Analysis of the electron density descriptors shows the presence of a non-nuclear attractor (NNA) at the middle of the bond formed by the two Mg atoms when attached to the ligands. The electride characteristics are exhibited by all of the designed complexes. We also report the aromaticity behavior and reactivity descriptors of these complexes. The electride characteristics of Mg2(η5-C5H5)2 complex get affected on substitution, as both the NNA population and the nonlinear optical properties (NLO) of the complexes are changed.

Reactivity Dynamics.

The chemical reactivity of a molecule as a whole or of an atom in a molecule varies during a chemical reaction. A variation of global and local reactivity descriptors in the course of a physicochemical process was studied within a quantum fluid density functional theory framework. Effects of a physical confinement and the electronic excitation therein were studied. In this Perspective, we also highlight the direction of a spontaneous chemical reaction in the light of the dynamical variants of the conceptual density functional theory-based electronic structure principles. An exhaustive state-of-the-art dynamical study is warranted in order to understand a chemical reaction from a reactivity perspective augmenting the associated molecular reaction dynamics analysis.

XNgNSi (X = HCC, F; Ng = Kr, Xe, Rn): A New Class of Metastable Insertion Compounds Containing Ng-C/F and Ng-N Bonds and Possible Isomerization therein.

Recently, astronomically important silaisocyanoacetylene (HCCNSi) possessing a large dipole moment has been detected for the first time with the help of crossed molecular beam experiments. Quantum chemical computations at higher levels of theory have also been performed to characterize the transient species. In this study, we have analyzed the equilibrium geometry, stability, reactivity, and energetics as well as the nature of bonding in the noble gas (Ng) inserted HCCNSi compound. We have also considered its F analogue to understand the influence of the most electronegative atom in the compound. Metastable behavior of the XNgNSi compounds (X = HCC, F; Ng = Kr-Rn) is examined by calculating thermochemical parameters like free energy change (ΔG) and zero-point-energy-corrected dissociation energy (D0) at 298 K for all possible two-body (2B) and three-body (3B) (both neutral as well as ionic) dissociation channels using coupled-cluster theory [CCSD(T)] in addition to density functional theory (DFT) as well as second order Møller-Plesset perturbation theory (MP2). The set of predicted compounds is found to be endergonic in nature, having high positive free energy change suggesting the thermochemical stability of the compounds except for the 2B Ng-release paths. Though thermodynamically feasible, they are kinetically protected with very high activation free energy barriers. Interestingly, the release of Ng from the parent moiety XNgNSi produces the XSiN isomer, by 180° flipping of the NSi moiety. This can also be seen in the dynamical simulation carried out with the help of atom-centered density matrix propagation (ADMP) technique at 2000K for 1 ps. The bonding in Ng-C, Ng-F, and Ng-N bonds of the studied compounds is analyzed and described with the aid of natural bond orbital (NBO), topological parameters computed using atoms-in-molecules theory (AIM), energy decomposition analysis (EDA), and adaptive natural density partitioning (AdNDP) methods. The natural charge distribution on the constituent atoms suggests that the compounds can be partitioned into both ways of representations, viz., neutral radical as well as ionic fragments. Lastly, the reactivity of the compounds is scrutinized using certain reactivity descriptors calculated within the domain of conceptual density functional theory (CDFT).