Jia-Yuan Liu

On the Potential Application of Superalkali Clusters in Designing Novel Alkalides with Large Nonlinear Optical Properties

A new series of superalkali-based alkalides, i.e., Li3(+)(calix[4]pyrrole)M(-), Li3O(+)(calix[4]pyrrole)M(-), and M3O(+)(calix[4]pyrrole)K(-) (M = Li, Na, and K), have been theoretically designed and investigated by means of density functional theory computations. These species have diverse structural isomers, in which the embedded superalkali units maintain their identities and prefer the horizontal orientation over the vertical one. All the proposed alkalides exhibit considerable first hyperpolarizabilities (β0) up to 34,718 au. Especially, a prominent M(-) atomic number dependence of (hyper)polarizabilities is observed for the Li3(+)(calix[4]pyrrole)M(-) and Li3O(+)(calix[4]pyrrole)M(-) compounds. Besides, the dependence of the nonlinear optical response of such alkalides on the species of involved superalkalis is also investigated. We hope that this work will promote further application of superalkalis and, on the other hand, attract more research interest and efforts in exploring new, unconventional alkalides.

Migrations of pentagon-heptagon defects in hexagonal boron nitride monolayer: the first-principles study.

The first-principles calculations are employed to study the migrations of pentagon-heptagon (5-7) defects in hexagonal boron nitride monolayer (h-BN). A type of grain boundaries, consisted of 5-7 defects, is constructed on the basis of experimental observations. With the absorption of a pair of atoms, one 5-7 defect in the grain boundary migrates apart by one unit cell and afterward migrates again through the bond rotation. It is also found that the two migrations could be replaced by one single step when the pair of absorbed atoms is located at another specific site in the same heptagon. Energy barriers and reaction paths for the migrations of 5-7 defects in h-BN by the bond rotation are theoretically investigated by the standard nudged elastic band method and the generalized solid-state nudged elastic band method. To elucidate the difference between the bond rotation process of the 5-7 defects with N-N bonds and those with B-B bonds, a couple of typical 21.7° grain boundaries with either N-N or B-B bonds are investigated. It is shown that the energy barrier of the migration of defects with N-N bonds is lower than that with B-B bonds in this type of grain boundaries.

Insight into structural and π–magnesium bonding characteristics of the X2Mg⋯Y (X = H, F; Y = C2H2, C2H4 and C6H6) complexes

Quantum chemical calculations have been performed to study the nature of interaction of complexes formed by MgX2 (X = H, F) molecules with acetylene, ethylene, and benzene, respectively. Results indicate that nonbonded interactions, namely π–magnesium bonds, contribute to the stability of the resulting dimers. An intriguing structural evolution has been found for the complexes with different π electron donors. Upon complexation, both the Mg–X and C–C bonds lengthened, accompanied by redshifted X–Mg–X and C–C stretching vibrations. NBO analysis reveals that the main charge-transfer from the conjugated molecules to MgX2 is from the πCC bonding orbital to the empty lone pair orbital of Mg. Energy decomposition analysis indicates that the stability of the topic complexes mainly comes from the attractive electrostatic interaction and polarization, similar to the case of π–beryllium bonds. When compared with other nonbonded interactions, it is found that π–magnesium bonds are stronger than π–lithium and π–sodium bonds. Especially, they are comparable in strength to π–beryllium bonds with MgF2 playing the role of Lewis acid.

Does Alkaline-Earth-Metal-Based Superalkali Exist?

A series of MkF2k-1+ (M = Mg, Ca; k = 2, 3) cations have been theoretically investigated to make a new attempt to design superalkali species. As expected, most of these cations were identified as pseudoalkali or even superalkali cations in view of their low electron affinities (EAs). The stability of these cationic clusters is indicated by considerable HOMO-LUMO gaps and positive dissociation energies. More intriguingly, these alkaline-earth-metal-based cations have advantages over alkali-metal-based superalkalis in two aspects: (1) they possess much larger binding energy values; (2) they can keep the chemical stability along with the increasing cluster size. Therefore, it is proposed here that the alkaline-earth-metal atoms could partner with halogens to construct stable cations of low EA value, which may add new candidates to the superalkali family.

On the feasibility of designing hyperalkali cations using superalkali clusters as ligands.

The possibility of using superalkali clusters instead of alkali atoms as ligands to design a class of cationic compounds, referred to as hyperalkali cations, has been examined by using gradient-corrected density functional theory. By taking typical superalkalis (FLi2, OLi3, and NLi4) as examples, a series of hyperalkali cations ML2+ [M = (super)halogen; L = superalkali] have been constructed and investigated. Calculational results show that all the superalkali moieties preserve their geometric and electronic integrity in these proposed cations. The stability of these studied cations is guaranteed by the strong ionic bonds between superalkali ligand and (super)halogen core, as well as their large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps and positive dissociation energies. In particular, all these proposed cations possess lower vertical electron affinities (2.36-3.56 eV) than those of their corresponding cationic superalkali ligands, verifying their hyperalkali nature. We, therefore, hope that this study will provide an approach to obtain new species with excellent reducing capability by utilizing various superalkalis as building blocks.

Characterisation of superalkaline-earth-metal halides, hydroxide and chalcogenides

ABSTRACT A new series of superalkaline-earth-metal compounds Al14X (X = F2, Cl2, Br2, I2, (OH)2, O, S) has been investigated at the TPSS/Def2-TZVPP level. It is found that both interaction position and mode between Al14 and X ligands influence isomer stability of the resulting superatom compounds. On the one hand, the top and bottom sites of Al14 have higher reactivity relative to middle ones. On the other hand, different ligands favour different binding patterns when combining with Al14. For example, the halogen ligands prefer to occupy on-top sites of Al14. In contrast, the end-on bound configurations are the least favourable ones for Al14 oxide. Natural population analysis indicates that the Al14 cluster donates electrons to X ligands. Atoms in molecules analysis also suggests ionic bond character of the connection(s) between Al14 and X moieties. Thus, the Al14X compounds are much like divalent salts where superatom Al14 behaves just like an alkaline-earth-metal atom. Furthermore, considerable dissociation energies and large highest occupied molecular orbital and lowest unoccupied molecular orbital gaps confirm stability of these superatom compounds.

Hyperhalogen properties of early-transition-metal borates

The equilibrium structures, stability and magnetic properties of Sc(BO2)n−/0 (n = 1–4) clusters were investigated on the basis of density functional theory calculations. The BO2 ligands prefer to stretch out in the most stable Sc(BO2)n− anions but tend to come together in the lowest-lying Sc(BO2)4 structure. According to the MP2 results, the Sc(BO2)4− species could be classified as hyperhalogen anions since they have larger vertical electron detachment energies (VDEs, 5.44–8.85 eV) than that of the superhalogen anion BO2−. With titanium and vanadium playing the role of central atom, the Ti(BO2)n−/0 (n = 1–5) and V(BO2)n−/0 (n = 1–6) clusters were studied in a similar manner. In these cases, the central transition metal atoms are inclined to keep their intrinsic spin. In addition, the hyperhalogen identity of the Ti(BO2)n− (n = 4, 5) and V(BO2)n− (n = 3–6) species were also confirmed by the calculated VDE values.