CO2-Activation and Enhanced Capture by C6Li6: A Density Functional Approach

The capture and storage of CO2, a major component of greenhouse gases, are crucial steps that can positively impact the global carbon balance. The capture of CO2 has been difficult due to its extremely high stability. In this study, we propose a simple and yet effective approach for capture and storage of CO2 by C6Li6. C6Li6 possesses a planar star-like structure, whose ionization energy is lower than that of Li atom and hence, it behaves as a superalkali. Superalkalis are unusual species possessing lower ionization energies than alkali atoms. We have systematically studied the interaction of successive CO2 molecules with C6Li6 using long-range dispersion corrected density functional {\omega}B97xD/6-311+G(d) calculations. We notice that these interactions lead to stable C6Li6-nCO2 complexes (n = 1-6) in which the structure of CO2 moieties is bent appreciably (122-125deg) due to electron transfer from C6Li6, whose planarity is distorted only slightly (less than or equal to 7 deg). This clearly suggests that the CO2 molecules can successfully be activated and captured by C6Li6. We have also analyzed bond-lengths and bond-angle of CO2, their charges and adsorption energy as a function of the number of adsorbed CO2 (n). It has been also noticed that the bond-length of CO2 in C6Li6-nCO2 complexes increases monotonically whereas adsorption energy decreases, ranging 3.18-2.79 eV per CO2 with the increase in n. These findings not only establish the potential of C6Li6 for capture and storage of CO2 molecules but also provide new insights into CO2-activation, capture, and storage by systems having low ionization energies

Organic Superalkalis with Closed-Shell Structure and Aromaticity

Benzene (C6H6) and polycyclic hydrocarbons such as naphthalene (C10H8), anthracene (C14H10) and coronene (C24H12) are well known aromatic organic compounds. We study the substitution of Li replacing all H atoms in these hydrocarbons using density functional method. The vertical ionization energy (VIE) of such lithiated species, i.e., C6Li6, C10Li8, C14Li10 and C24Li12 ranges 4.24-4.50 eV, which is lower than the IE of Li atom. Thus, these species may behave as superalkalis, due to their lower IE than alkali metal. However, these lithiated species possess planar and closed-shell structure, unlike typical superalkalis. Furthermore, all Li-substituted species are aromatic although their degree of aromaticity is reduced as compared to corresponding hydrocarbon analogues. We have further explored the structure of C6Li6 as star-like, unlike its inorganic analogue B3N3Li6, which appears as fan-like structure. We have also demonstrated that the interaction of C6Li6 with a superhalogen (such as BF4) is similar to that of a typical superalkali (such as OLi3). This may further suggest that the proposed lithiated species may form a new class of closed-shell organic superalkalis with aromaticity

Alkalized Borazine: A Simple Recipe to Design Superalkali Species

We propose a simple yet effective route to the design of superalkalis, by successive alkali metal substitution in borazine (B3N3H6). Using Li atoms, our density functional calculations demonstrate that the vertical ionization energy (VIE) of B3N3H6-xLix decreases with the increase in x for x equals 1-6. For x equals 4, the VIE of B3N3H6-xLix becomes lower than that of Li atom, thereby indicating their superalkali nature. More interestingly, all these species are planar such that NICSzz value at the rings center is reduced. These novel superalkalis are expected to stimulate further interests in this field

Designing New Electrolytes for Lithium Ion Batteries Using Superhalogen Anions

The electrolytes used in Lithium Ion Batteries (LIBs) such as LiBF4, LiPF6 etc. are Li-salts of some complex anions, BF4-, PF6- etc. The investigation shows that the vertical detachment energy (VDE) of these anions exceeds to that of halogen, and therefore they behave as superhalogen anions. Consequently, it might be possible to design new electrolytic salts using other superhalogen anions. We have explored this possibility using Li-salts of various superhalogen anions such as BO2-, AlH4-, TiH5- and VH6- as well as hyperhalogen anions, BH4-y(BH4)y-(y = 1 to 4). Our density functional calculations show that Li-salts of these complex anions possess similar characteristics as those of electrolytic salts in LIBs. Note that they all are halogen free and hence, non-toxic and safer than LiBF4, LiPF6 etc. In particular, LiB4H13 and LiB5H16 are two potential candidates for electrolytic salt due to their smaller Li-dissociation energy ({\Delta}E) than those of LiBF4, LiPF6 etc. We have also noticed that {\Delta}E of LiBH4-y(BH4)y varies inversely with the VDE of BH4-y(BH4)y- anions, which increases with the increase in y. These findings may guide experimentalists and future researchers to design and synthesize more efficient and environment friendly electrolytic salts for LIBs

Reduction of Nitrogen Oxides (NOx) by Superalkalis

NOx are major air pollutants, having negative impact on environment and consequently, human health. We propose here the single-electron reduction of NOx (x = 1, 2) using superalkalis. We study the interaction of NOx with FLi2, OLi3 and NLi4 superalkalis using density functional calculations, which lead to stable superalkali-NOx ionic complexes with negatively charged NOx. This clearly reveals that the NOx can successfully be reduced to NOx- anion due to electron transfer from superalkalis. It has been also noticed that the size of superalkalis plays a crucial in the single-electron reduction of NOx

Design of NnH3n+1+ Series of Non-Metallic Superalkali Cations

The species with lower ionization energy than alkali atoms are referred to as superalkalis. Typical superalkalis include a central electronegative core with excess metal ligands. We propose a new series of non-metallic NnH3n+1+ superalkali cations using MP2/6-311++G(d,p) level. These cations are designed by successive replacement of H-ligands of ammonium cation (NH4+) by ammonium (NH4) moieties. The resulting NnH3n+1+ cations, which can be expressed in the form of [NH4 (n-1)NH3]+ complexes, possess a number of unusual N-H N type of partially covalent H-bonds, with the interacting energy in the range 7.8-24.3 kcal/mol. These cations are stable against loss of a proton (NnH3n+H+) and loss of ammonia [(n-1)NH3+NH4+]. The vertical electron affinities (EAv) of NnH3n+1+ decreases monotonically from 4.39 eV from n = 1 to 2.39 eV for n = 5, which suggest their superalkali nature. This can be explained on the basis of electron localization on core (central) N-atom (Qc), as EAv correlates linearly with Qc. We have also demonstrated that this series may be continued to obtain new superalkali cations with even lower EAv, by exemplifying N9H28+ with the EAv of 1.84 eV. N9H28+ is stabilized by four partially covalent H-bonds (8.5 kcal/mol each) and four electrostatic H-bonds (0.4 kcal/mol each). This led to an exponential relation between EAv and n, which may provide an approximate EAv of for any value of n in NnH3n+1+ series

OxH2x+1+ Clusters: A New Series of Non-Metallic Superalkali Cations by Trapping H3O+ into Water

The term superalkali refers to the clusters with lower ionization energy than alkali atoms. Typical superalkali cations include a central electronegative core with excess metal ligands, OLi3+, for instance, which mimic the properties of alkali metal ions. We report a new series of non-metallic superalkali cations, OxH2x+1+ (x = 1-5) using ab initio MP2/6-311++G(d,p) level. These cations are designed by successive replacement of H-ligands of hydronium cation (OH3+) by ammonium (OH3) moieties followed by their geometry optimization. The resulting OxH2x+1 + clusters, which can be expressed in the form of OH3 + (x-1)H2O complexes, possess a number of electrostatic as well as partially covalent H-bonds, with the interacting energy in the range 5.2-29.3 kcal/mol as revealed by quantum theory of atoms in molecules analyses. These cations are found to be stable against deprotonation as well as dehydration pathways, and their stability increases with the increase in x. Interestingly, the vertical electron affinities (EAv) of OxH2x+1 + clusters decreases rapidly from 5.16 eV for x = 1 to 2.67 eV for x = 5, which suggest their superalkali nature. It is also possible to continue this series of non-metallic superalkali cations for x > 5 with even lower EAv, down to an approximated limit of 1.85 eV, which is obtained for OH3 + trapped into water cavity implicitly using polarizable continuum model. The findings of this study will not only provide new insights into structure and interactions of OxH2x+1 + clusters but also reveal their novel properties, which can be exploited their interesting applications

MC6Li6 (M = Li, Na and K): A New Series of Aromatic Superalkalis

Organic superalkalis are carbon-based species possessing lower ionization energy than alkali atom. In the quest for new organic superalkalis, we study the MC6Li6 (M = Li, Na, and K) complexes and their cations by decorating hexalithiobenzene with an alkali atom using density functional theory. All MC6Li6 complexes are planar and stable against dissociation into M + C6Li6 fragments, irrespective of their charge. These complexes are stabilized by charge transfer from M to C6Li6, although the back-donation of charges tends to destabilize neutral species. Furthermore, their degree of aromaticity increases monotonically from M = Li to K, unlike MC6Li6+ cations, which are not aromatic as suggested by their NICS values. The adiabatic ionization energies of MC6Li6 (3.08-3.22 eV) and vertical electron affinities of MC6Li6+ (3.04-3.15 eV) suggest that MC6Li6 species form a new series of aromatic superalkalis. The variation of the ionization energy of MC6Li6 is found to be in accordance with the NICS values of MC6Li6+. The superalkali nature of MC6Li6 and its relation with NICS values are explained on the basis of the positive charge delocalization. We believe that these species will not only enrich the aromatic superalkalis but also their possible applications will be explored

Application of Superhalogens in the Design of Organic Superconductors

Bechgaard salts, (TMTSF)2X (TMTSF = tetramethyl tetraselenafulvalene and X = complex anion), form the most popular series of organic superconductors. In these salts, TMTSF molecules act as super-electron donor and X as acceptor. We computationally examine the electronic structure and properties of X in commonly used (TMTSF)2X (X = NO3, BF4, ClO4, PF6) superconductors and notice that they belong to the class of superhalogens due to their higher vertical detachment energy than halogen anions. This prompted us to choose other superhalogens such as X = BO2, BH4, B2F7, AuF6 and study their (TMTSF)2X complexes. Our findings suggest that these complexes behave more or less similar to those of established (TMTSF)2X superconductors, particularly for X = BO2 and B2F7. We, therefore, believe that the concept of superhalogen can be successfully applied in the design of novel organic superconductors

Superhalogens as Building Blocks of Complex Hydrides for Hydrogen Storage

Superhalogens are species whose electron affinity (EA) or vertical detachment energy (VDE) exceed to those of halogen. These species typically consist of a central electropositive atom with electronegative ligands. The EA or VDE of species can be further increased by using superhalogen as ligands, which are termed as hyperhalogen. Having established BH4- as a superhalogen, we have studied BH4-x(BH4)x- (x = 1 to 4) hyperhalogen anions and their Li-complexes, LiBH4-x(BH4)x using density functional theory. The VDE of these anions is larger than that of BH4-, which increases with the increase in the number of peripheral BH4 moieties (x). The hydrogen storage capacity of LiBH4-x(BH4)x complexes is higher but binding energy is smaller than that of LiBH4, a typical complex hydride. The linear correlation between dehydrogenation energy of LiBH4-x(BH4)x complexes and VDE of BH4-x(BH4)x- anions is established. These complexes are found to be thermodynamically stable against dissociation into LiBH4 and borane. This study not only demonstrates the role of superhalogen in designing new materials for hydrogen storage, but also motivates experimentalists to synthesize LiBH4-x(BH4)x (x = 1 to 4) complexes