Theoretical Design Of Superatom Compounds

This paper reports theoretical design on several types of novel superatom compounds, and systematical study on structures and properties of these superatom compounds. The main contribution is as follows,(1) Our systematic calculations on BF₄-M (M = Li, FLi₂, OLi₃, NLi₄) have theoretically predicted a new type of superhalogen-(super)alkali compounds, at the MP2/6-311+G(3df) level. The preferred orientations between polyhedral BF₄ and Li atom, linear FLi₂, planar OLi₃, and polyhedral NLi₄, are side-point, face-side, face-face, and face-face, respectively. The interaction between superhalogen BF₄ and different shaped (super)alkali M is found to be strong and ionic in nature. Bond energies of these BF₄-M species are in the range of 200.0 226.7 kcal/mol at the CCSD(T)/6-311+G(3df) level, which are much larger than the traditional ionic bond energy of 130.1 kcal/mol of FLi. As a result of high electron affinity of BF₄, different from the alkali halides, the BF₄-M compounds prefer to dissociate into ions rather than neutral fragments. It is found that both the HOMO-LUMO gaps and binding energies per atom of BF₄-M show decreasing tendencies with increasing size of the M subunit. As regards the NLO properties, both polarizabilities and the first hyperpolarizabilities of BF₄-M apparently increase with M becoming larger. However, the introduction of superhalogen leads to decreasedβ0 value, and furthermore, the larger the superhalogen subunit, the less theβ0 value of BF₄-M.(2) Two types of small aluminum clusters-(super)atom compounds, Al₃-X (X=F, LiF2, BeF₃, BF₄) and Al₃-M (M = Li, FLi₂, OLi₃, NLi₄), are theoretically predicted, at the MP2/6-311+(3df) computational level. For Al₃-X species, the preferred orientation between Al₃ and F atom is point-point, while between Al₃ and different shaped superhalogens LiF2, BeF₃, and BF₄ are all side-point. For Al₃-M species, the preferred orientations between Al₃ and Li, FLi₂, OLi₃, and NLi₄ are face-point, face-side, side-face and side-face, respectively. Large bond energies and the NBO and AIM analyses show that there are strong interactions between Al₃ and (super)halogen and between Al₃ and (super)alkalie. However, as a result of the shell closure of Al₃+ cation, the bond energies of Al₃-X are larger than those of the Al₃-M compounds. Furthermore, we found that the bond energy is closely correlated to the bonding mode type; for isomers of Al₃-superhalogen with different bonding modes, the bond energies order is ff type > ss type > ps type, and for Al₃-superalkalies the bond energies order is sf type > fs type > ss type, respectively.The HOMO orbital of each Al₃-X and Al₃-M species is a doubly occupied, delocalizedσbonding orÏ€bonding orbital on a Al₃ ring, consequently, Al₃-X and Al₃-M compounds are aromatic, and their aromaticity can be confirmed by the negative NICS values. We find that when the geometrical deformation of Al₃ is very large, the aromaticities of Al₃-X and Al₃-M molecules may disappear, but in the case of maintaining aromaticity, the Al₃- ring of Al₃-M may have greater deformation than the Al₃+ ring of Al₃-X.We hope that the principles obtained from this study may, in the future, provide a useful guidance for superatom researches and new materials designing.