Theoretical Studies On The Structures And Properties Of Several Novel Superalkali Clusters

A promising direction in the field of clusters and nanostructures is the synthesis, characterization, and fundamental understanding of materials using clusters as the elementary building blocks. Since the physical and chemical properties of clusters can be tailored by changing their size and composition, cluster assemblies offer an approach for synthesizing designer materials. In a precursory theoretical work, Khanna and Jena proposed that atomic clusters with suitable size and composition could be designed to mimic the chemistry of atoms in the periodic table and that such clusters could be considered as "superatom"One of the exciting examples of superatoms is superalkali including one or more nonmetal atom at the center surrounded by peripheral electronpositive atoms, such as lithium, sodium and potassium. It is well known that ionization potential (IP) of alkali atom is lowest among the elements. Superalkali possesses lower IP than those (5.4-3.9 eV) of alkali-metal atoms. Considering the low IP features of superalkalies, they can be regarded as building blocks and combined with other motifs with low EA to form novel compounds. Consequently, the superalkalies can be used in the synthesis of a new class of charge-transfer salts and play an important role in chemistry.This paper shows systematically theoretical study on several novel superalkali clusters, and the main contributions are as follows: 1) A new class of superalkali is designed by replacing both the central nonmetal atom and the ligand atoms. In this work, FM2+, OM3+, NM4+, CM5+ and BM6+(M=Li, Na) are selected and investigated in detail. The structural characteristics of these cations are strongly related to the electronegativity of the central atom. In addition, all the species studied here can be considered as superalkali cations due to their lower EAvert values than that of Cs. And the EAvert values are determined by both the electronegativity of central atom and the size of ligand atom.2) Our systematic calculations on OM3 (M=Li, Na, K) and their corresponding cations OM3+ have theoretically predicted a new type of superalkalies. The geometries and energetic properties of neutral and positively charged OM3 were obtained computationally. All the OM3 species present planar structure with the stability by ligand-ligand interaction. The ionization potentials (IPs) for the studied OM3 are lower than the IP of Cs atom, hence they can be considered as superalkalies. Due to their low IPs, superalkalies almost always exist as positive ions, usually as the cationic portions of salts. Our results and discussions presented in this work suggest that the'size'of the alkali ligands play a very important role in the IP values. The larger the M ligand is, the lower the IP value is. In addition, most of the electron density distribute in the vicinity of the ligand atom.3) We made the first attempt to investigate the geometries and energetic properties of superalkalies FM2 (M=Li, Na, K and FLi2) and their corresponding cations FM2+ in detail. Actually, the EAvert values for superalkali cations are strictly determined by'size'of ligands. A superalkali is created when the nonmetal atom is surrounded by superalkali and its ionization potential is even lower than that of the constituent superalkali. Our results and discussions presented in this work suggest that novel superalkali can be designed if the peripheral alkali atoms are replaced by superalkali units.4) A new series of binuclear superalkali cations M2Li2k+1+(M=F, O, N, C, B) species.......... The geometries and energetic properties of these cations were obtained theoretically. It is found that the structural characteristics of global minima of binuclear superalkali cations are related to the electronegativities of central atoms. For C2Lig+ and B2Li11+ with relative less electronegativities of central atoms, the lowest energy structures feature two central atoms linked directly to each other. While in the global minima of F2Li3+, O2Li5+, and N2Li7+ cations, the relative more electronegativities of their central F, O and N atoms lead to the preference for central atom-ligand interaction over the central atom-central atom interaction.5) We present our theoretical search for superalkali cations having CO32-, SO32-, SO42-, O42- and O52- as central core. It is found that the stabilities are determined by the structural characteristics of these superalkali cations. For CO3Li3+, SO3Li3+ and SO4Li3+ cations, the structural integrities of central core are found to maintain inside the lowest energy structures. While in the global minima of O4Li3+ and O5Li3+ cations, they resemble cage structures. In addition, EAvert values for these superalkali cations are also related to the structural characteristics of central core. For CO3Li3+, the lowest EAvert value is found for the isomer contains integral CO32- anion. But for the other superalkalies, the lowest EAvert value is found for the isomer in which central atoms are arranged in a chain.Our results presented in this work may add candidates to the research on superalkalies and offer references to future investigation on other superatoms.