Quantum Chemical Studies On Metal Dithiolene And Fullerenes In Their Structure, Aromaticity, And Optical Properties

Aromaticity is a virtual quality, rather than physical observable as described by Scheleyer et al. The aromaticity is not an exclusive concept as the development of science. Since Boldyrev and Wang found the first aromatic all-metallic cluster of Al₄^2-, a new research field merged. The classification of aromaticity goes far beyond the conventional benzenoid hydrocarbons and their related heteroarenes. The criteria of aromaticity have some new contents. Especially the nucleus-independent chemical shifts (NICS) proposed by Scheleyer in 1996 has been applied extensively in most study referred to aromaticity.Metal dithiolene ring is a unique hetero-pentagon containing a transition metal atom. Using one empty d orbital of the metal atoms, the ring forms a conjugated system with 6Ï€electrons. There are two groups of metal dithiolene complexes: one includes the complexes with ligands other than dithiolate (mixed ligand metal dithiolene) and the other includes the complexes with only dithiolate as ligands. Therefore, the representational complexes of metal dithiolen: CpCoS₂C₂R₂ (Cp, cyclopentadienyl) and Ni(S₂C₂R₂)₂ are studied on their aromaticity in this paper.1. The optimized geometries in theoretical calculation agree well with in experiment. There are two reasons for the weaker aromticity of hetero-pentagon in CpCoS₂C₂R₂ complexes with respect to that of the isolated CoS₂C₂⁺¹. The better equalization of bond lengths in the CoS₂C₂⁺¹ is the first reason. The other reason is the contribution to the NICS from the cobalt atom is much larger in the CoS₂C₂⁺¹. The planar character of Cp is destroyed slightly in the complexes. At the same time, the size of Cp becomes bigger than the isolated Cp^-1 and this is caused by the cobalt atom in the hetero-pentagon. The p-electron delocalization causes stronger aromaticity of the Cp in the complexes than that of the isolated Cp^-1.2. The theoretical study on Ni(S₂C₂R₂)₂ predicted that all atoms of the complexes are on the same plane strictly. Both analysis of the canonical molecular orbitals and natural local molecular orbitals (NLMO) predicted that theσbonds between Ni and S reduce aromaticity of the NiS₂C₂ hetero-pentagon in Ni(S₂C₂H₂) and Ni(bdt)₂ complexes. The analysis of NLMO predicted the three d atomic orbitals with lone pair electrons of Ni atom and the delocalizedÏ€bond among S-C-C-S have large contribution to NICS(0) and NICS(1) in Ni(S₂C₂H₂) and Ni(bdt)2. The C-CÏ€bond has large contribution to NICS(n) (n=0 or 1) in Ni(S₂C₂H₂)2, while the delocalized S-CÏ€bond has large contribution to NICS(n)iso (n=0 or 1) in Ni(bdt)₂.Since the discovery of C₆₀, along with its macroscopic scale synthesis, fullerene study as an interdisciplinary field has attracted extensive attention. The observed fullerenes belong to the subset of classical isomers with isolated pentagons satisfying the isolated-pentagon rule (IPR), which is only possible for Cn cages with n=60 or n=70+2k for k>=0. In those cases (e.g. C₆₂-C₆₈) it is impossible to have all pentagons isolated in fullerene cage, and the most stable isomer corresponds to the situation with the smallest value of e55: the number of pentagon-pentagon fusions, known as the pentagon adjacency penalty rule (PAPR). Fullerenes could confine atoms in their interior because of their closed-cage structure. In most endohedral metallofullerenes, introduction of metal atoms into fullerene cages leads to increase in the electron affinity relative to the corresponding empty-cage. Without modifying the structural features of the outer fullerene shell, the optical and electronic properties change with the encapsulated metal cluster. Enhancement of third-order nonlinear optical susceptibility observed in endohedral metallofullerenes further establishes them as potential candidates for nonlinear optical devices. In this paper, the systemic studies on C₆₂, C₆₄, and C₆₆ are performed.3. The 2385 classical isomers and four non-classical isomers based on C₆₀ of C₆₂ have been studied. Cs:7mbr, with a chain of four adjacent pentagons surrounding a heptagon, is predicted to be the most stable isomer, followed by C_2v:4mbr which is 3.15 kcal/mol higher in energy. C₂:0032 with e₅₅=3 is the most stable isomer in the classical frameworks. Concentration analyses reveal that Cs:7mbr prevails in a wide temperature range. The simulated IR spectra show important differences in positions and intensities of the vibrational modes for different isomers. The analysis of electronic spectra and second-order hyperpolarizabilities predicted that the second-order hyperpolarizabilities of the five most stable isomers of C₆₂ larger than that of C₆₀. In addition, The NICS and the density of states of the three most stable isomers show that the square in C_2v:4mbr and the adjacent pentagons in C_s:7mbr and C₂:0032 possess high chemical reactivity.4. The calculation of the 3465 classical isomers of C₆₄ predicted the three most stable isomers [C₆₄(D2:0003), C₆₄(Cs:0077), and C₆₄(C₂:0103)] has the lowest value of e₅₅ (e₅₅=2). The analysis of relative concentration predicted that the relative stability of the five most stable isomers have no change among the wide temperature range. The relative stabilities of C₆₄ isomers change with charging in ionic states. Doping also affects the relative stabilities of fullerene isomers as demonstrated by Sc₂@C₆₄(D₂:0003) and Sc₂@C₆₄(C_s:0077). The bonding of Sc atoms with C₆₄ elongates the C-C bond of two adjacent pentagons and enhances the local aromaticity of the fullerene cages. Charging, doping, and derativization can be utilized to isolate C₆₄ isomers through differentiating the electronic and steric effects. All the hexagons in the isomers with e₅₅=2 display local aromaticity.5. Among all 4478 classical isomers of C₆₆, C₆₆(C_s:0060) with the lowest e₅₅=2 (e₅₅=2) was predicted to be the most stable isomer, followed by isomers C₆₆(C_2v:0011) and C₆₆(C₂:0083). The IR spectra and aromaticity of the most isomers were predicted and compared among the isomers. The relative stabilities of fullerene isomers change with charges and doping of metals. The structures and relative stabilities of the most stable metallofullerene were delineated and compared with in experiment. Sc₂@C₆₆(C₂:0083) is the most stable metallofullerene in our predictions, although Sc₂@C₆₆(C_2v:0011) has been observed. Charge-transfer between Sc₂ and the fused pentagons and the bonding between these two moieties significantly decrease the strain energies caused by the pair of fused pentagons and thus stabilize the fullerene cage.