| Discovering novel species with unusual properties and revealing nature of new intermolecular interactions is always an exciting part of chemistry. In this thesis, theoretical studies were performed on the unusual structures and properies of various representative systems containing special interactions.(1) Superatoms have become basic units in chemistry and have recently attracted more and more attention. Research has shown that superatoms have synthetic utility, and represent potential building blocks for the assembly of novel, nanostructured materials. We proposed a new term"superomolecule"to define a cluster containing two or more superatom subunits connected to each other through chemical bonds (such as ionic bond and covalent bond).Using the CCSD(T)/aug-cc-pVDZ method, we designed and studied the characteristics of structure, aromaticity, superatom, stability, and interactions between subunits of a royal-crown shaped electride superomolecule Li3-N3-Be. This molecule is a charge-separated system and can be denoted as Li32+N33–Be+. As the MP2 energy for Li3++Be2+ is much higher by 181 kcal/mol than that for Li32++Be+, the Li32+ and Be+ are formed in the Li3-N3-Be. Like isolated N33–, the N33– in Li3-N3-Be has triple-fold aromaticity. Isolated Li32+ is nonaromatic because it has only one valence electron dissatisfying the 4n + 2 electron counting rule, whereas the Li32+ subunit of Li3-N3-Be has the aromaticity similar to Li3+, because it could share theπ-electron clouds of the neighboring triple-fold aromatic N33–. Therefore, a possible aromatic ring can be destroyed by the redox interaction between subunits, and a nonaromatic ring can also exhibit aromaticity by the electron sharing interactions. This is new knowledge on aromaticity.The Li3-N3-Be molecule with two aromatic subunits can be considered as a"superomolecule"containing two superatoms (N3 and Li3) and one atom (Be). The superomolecule is defined as a cluster containing two ro more superatom subunits connected to each other through chemical bonds (such as ionic bond and covalent bond), distinguishing from the known supermolecule and supramolecule.Li3-N3-Be is an ionic superomolecule, in which the superatom-superatom bond Li3-N3 and superatom-atom bond N3-Be are both typical ionic bonds with large bond energies of 147 and 88 kcal/mol, respectively. The bonding modes are staggered face to face between superatoms Li3 and N3, and face to point between superatom N3 and atom Be. As Li3-N3-Be has a metal-nonmetal-metal structure, the nonmetal anion in the middle repulses the electron clouds of metal parts and produces an excess electron. This phenomenon of the repulsion results in: (a) the HOMO energy level increased, (b) the electron cloud in HOMO distended, (c) the area of the negative NICS value extended, and (d) the VIE value lowered. So the superomolecule Li3-N3-Be is not only a new metal-nonmetal-metal type sandwich complex but also a new type electride, which comes from the interaction between the alkali superatom (Li3) and the nonmetal superatom (N3).This study on the structure, bonding modes, natures of interactions of the superomolecule enriches the knowledge of inter-superatom chemistry and provides a new means for experimentalists to design new molecular devices and nanomaterials.(2) For the interaction containing triangle-plane subunits, we chose Li3 as the superalkali, and LiF2, BeF3, and BF4 as the superhalogens to assemble a new class of superalkali-superhalogen compound. How will the two types of superatoms bond together? What will the interaction between the two superatoms be? Will there be any unexpected characteristics in these novel compounds? Studying and answering these questions makes good sense for superatom chemistry.Optimized structures, with all real frequencies, of superalkali superhalides (Li3)+(SH)– (SH = LiF2, BeF3, and BF4), are obtained, for the first time at the B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ computational levels. We found superalkali superhalides (Li3)+(SH)– have a variety of structures, based on which we proposed five new inter-superatom bonding modes: edge–face, edge–edge, face–face, face–edge, and staggered face–edge types. We found that Li3–SH bond energy is closely correlated to the superatomic bonding mode type: for isomers with different bonding modes, bond energy order is a > b > c > d > e. In addition, the large superatomic bond energies (121.72 - 170.61 kcal/mol) indicate the strong interaction between superalkali (Li3) and superhalogen (LiF2, BeF3, or BF4) and prove the stability of these Li3–SH species.The HOMO of each Li3–SH species is a doubly occupied, delocalizedσbonding orbital on a Li3 ring, that is, (Li3)+(SH)– compounds are aromatic. The electron clouds in these delocalized HOMOs are pushed out and distended by the (SH)– anions which results in the following properties: 1) The maximum negative NICS value (about–10 ppm) for Li3 subunit moves out from the center of Li3 ring, 2) The VIE values of these (Li3)+(SH)– compounds are low (4.604 - 6.052 eV), 3) Excess electrons are generated by the repulsion effect of (SH)–, so that these superalkali superhalides exhibit alkalide or electride characteristics.These results on structure, chemical bonding, and interaction between superatoms enriches knowledge on superatom chemistry, and is valuable for creating new research fields of chemistry and material science. (3) For linetype interaction system, using ab initio calculations, we predicted for the first time that the unusual halogen-bonded complex FBrδ+···δ+BrF and hydrogen-bonded complex FBrδ+···δ+HF formed by the directly interactions between two positively charged atoms of different polar molecules have negative interaction energies (respectively -2.73 and -1.36 kcal/mol) and thus can be stable in gas phase (without solvent effects and crystal packing). This discovery enriches new knewledge of intermolecular interaction.That one of the lone pairs of the Br(2) atom points to the positively charged Br(3) atom in FBrδ+···δ+BrF or H(3) atom in FBrδ+···δ+HF causes the formation of novel halogen bond or hydrogen bond between two positively charged atoms of different molecules. Thus, according to the chemically intuitive model, the attraction arising from the special halogen bond or hydrogen bond can exceed the electrostatic repulsion between two contact positively charged atoms, which stabilizes the complex. It is found that the correlation interaction energies are large negative values (-3.86 and -1.36 kcal/mol) representing attractive contributions and are dominant in the interaction energies. Therefore, from the point of view of physics, the dispersion contribution plays an important role in the stabilities of these seemingly repulsive systems. This work may encourage more attention to some unconventional intermolecular interactions because they may be not only used to produce novel structures with particular physical properties but also significant for chemical and physical process and material investigations.(4) Our group has proposed a new idea to design nonlinear optical (NLO) molecules, that is that doping alkali atoms into polar molecules to form loosely bound excess electrons can effectively lower the transitioin energies of crucial excited states and increase the hyperpolarizabilities.In this work, we designed and systematically studiedthe static first hyperpolarizabilities of 18 organic alkalides (M+@n6adz)M′– (M, M′= Li, Na, K; n = 2, 3) formed by the interactions of two alkali-metal atoms with cage adz complexants, for the first time, and obtained the recordβ0 value of NLO molecules.Alkalides (M+@n6adz)M′– with cage adz complexants exhibit large static first hyperpolarizabilities (β0 = 1725 - 318354 au). Especially, all potassides (M+@n6adz)K– have considerably largeβ0 values (1.6×105 - 3.2×105 au) much larger than theβ0 value (3.6×104 au) of the previously designed cuplike alkalide Li+(calix[4]pyrrole)K–. Thus the 26adz and 36adz cage complexants are better than the calix[4]pyrrole cuplike complexant in enhancing the first hyperpolarizability. Furthermore, theβ0 value of 3.2×105 au of (K+@26adz)K– is about 3.5 times larger than that of 8.6×104 au of the well-known organometallic systemRu(trans-4,4'-dibutylaminostyryl-2,2'-bipyridine)32+, and about two times larger than the record value of 1.7×105 au of a long dipolar donor-acceptor conjugated organic molecule. It shows that this type of alkalides could be a new member of the large family of nonlinear optical (NLO) materials with different types.Most (M+@26adz)M′– have largerβ0 values than the corresponding (M+@36adz)M′–, which shows that the smaller 26adz cage is better than the larger 36adz cage. Thus choosing a proper cage size is also an important factor to be considered in the search for alkalides with large NLO responses.The bromide (Li+@26adz)Br– exhibit very smallβ0 value, whereas the alkalides (M+@26adz)M′– and (M+@36adz)M′– with alkali-metal anion M′– have greatβ0 values up to 3.2×105 au. Obviously, the diffused excess electron on the alkali-metal anion plays the crucial role in the large first hyperpolarizability of alkalides (M+@26adz)M′– and (M+@36adz)M′–.For (M+@26adz)M′–, theβ0 value depends not only on the atomic number of the outside M′– but also on the atomic number of the inside M+, while for (M+@36adz)M′–, theβ0 value only depends on the atomic number of the outside M′–.This work exhibits the tunable NLO behavior of these new organic alkalides and provides a new means for experimentalists to design high-performance NLO materials.
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