THE FRONTIER ORBITALS OF BUCKMINSTERFULLERENE C_(60)

The frontier orbitals HOMO and LUMO of the fullerene C_(60) hasbeen given in a system of 60 motion-coordinates, of which each origin is ateach C atom of fullerene C_(60) and each z-axis is in the directionperpendicular to C_(60)'S spherical surface. In this motion-coordinate systemthe HOMO and LUMO of fullerene C_(60) obviously display characteristics ofspherical n-molecular orbitals π_n. The Fenske-Hall quantum-chemical methodis used in calculation for the electronic structure of the fullerene C_(60).     

Ab initio study of the transition-metal carbene cations

The geometries and bonding characteristics of the first-row transition-metal carbene cations MCH_2~ were investigated by ab initio molecular orbital theory (HF/LANL2DZ). All of MCH_2~ are coplanar. In the closed shell structures the C bonds to M with double bonds; while in the open shell structures the partial double bonds are formed, because one of the σ and π orbitals is singly occupied. It is mainly the π-type overlap between the 2p_x orbital of C and 4p_x, 3d_(xz), orbitals of M~ that forms the π orbitals. The dissociation energies of C—M bond appear in periodic trend from Sc to Cu. Most of the calculated bond dissociation energies are close to the experimental ones.     

Effects of Anions and Cations on π-Complexation Between Olefin and Metal Halide

An ab initio molecular orbital study was performed to determine the effects of anions and cations on the π-complexation of C2H4 on MX(M=Ag, Cu; X=F, Cl). The calculated results show the following order of adsorption strength: F-＞Cl- for anions; Cu ＞Ag for cations. The results can be explained by the detailed analysis of atomic charge, orbital energy and orbital population by using the natural bond orbital(NBO) theory: (1) anions with stronger electronegativity can attract more electrons from the s orbital of M, while at the same time it does not obviously weaken the d orbital occupation of M, thus the nearly vacant s orbital and the sufficiently filled d orbitals of M help with forming σ-donation and d-π* backdonation with the π orbital and the π* orbital of olefin, respectively; (2) a smaller energy gap of symmetry-adapted orbitals between olefin and a cation can favor the electron transfer, that is why Cu forms stronger adsorption with olefin than Ag does.     

LOCALIZED INDO STUDY OF THE ELECTRONIC STRUCTURE OF Sc(Sc_6Cl_(12)Co)

Spin-unrestricted localized INDO method was used to calculate the electronic structure ofrare earth cluster Sc(Sc_6Cl_(12)Co).Based on the analysis of the composition of the molecularorbitals and bond orders,it was pointed out that the interstitial transition metal atom Co in theoctahedral Sc skeleton forms strong covalent bond with six Sc atoms and the bonding of Sc-Cl is mainly ionic in character.There are nine valence molecular orbitals in the cluster.     

A NEW REACTIVITY INDEX——PSEUDO ORBITAL INDEX

The concept of pseudo orbital has been proposed, which includes the interactions of all the orbitals and extends the applicable limit of Fukui's frontier orbitals. An example shows that the pseudo orbital reactivity indexes are probably able to explain more complicated phenomena in chemical reactions.     

SCF-CNDO/2-CO STUDY ON NITROGEN-CONTAINING ANALOGS OF POLYACETYLENE

The electronic structures of polymethineimine (PMI), polyazine (PAZ) and polyazoethene(PAE) are studied on the basis of the tight- binding LCAO -8CF-CO calculations under the CNDO/2 approximations. Our result indicates that they are all poor candidates for new electric conducting materials as alternatives to polyacetylene(PA), and this is discussed in terms of the electronic structures.PMI end PAZ have been synthesized successfully, and some of their properties have been investigated. In this article we intend to explain why and how degree of conjugation effects on their intrinsic electric conductivities by studying their electronic structures on the basis of the tight-binding LCAO-SCF-CO calculations including all valence atomic orbitals(AO'S) with complete neglect of differential overlap(CNDO/2) approximations.     

Localized molecular orbital studies of certain compounds with [A_3X_3] six-membered rings

The localized molecular orbitals (LMOs) of certain quasi-aromatic organic and inor-ganic molecules with six-membered rings have been calculated by virtue of the ab initio methodusing STO-3G and 4-31G basis sets as well as the CNDO/2 method. It is shown that there existsextensively-delocalized p-pπ bonding in these quasi-aromatic systems. The localized pictures ofthe π-type LMOs for the heterocyclic and homocyclic systems from the σ-π localization schemeare discussed. The Generator Orbital approach is utilized to account for the bonding patterns.     

Ab initio Studies on the Fe-S Protein Model Complexes

In this paper, ab initio calculations of the iron-sulfur protein model complexes have been completed on Fe_2S_2(SH)_2~(n-) and Fe_4S_4(SH)_4~(n-)(n=2, 3). The results indicate that the occupied terminal sulfur characteristic orbitals are found in the front orbital site and the energy levels of the occupied Fe 3d-like orbitals appear internally below the S-H bonding orbitals in the valence band. Although the energy sequence is different from what was reported in literature, our results are in agreement with the relevant experimental facts. We have discussed the reason that variations of the active sites are produced by various oxidation levels. The action mechanism of the Fe-S proteins as electron carriers in the biological processes is also explored preliminarily.     

The mechanism of hydrogen abstraction by high valence transition metal oxo compounds

We present here a systematic theoretical study to explore the underlying mechanisms of the H abstraction reaction from methane. Various abstracting agents have been modeled, using oxygen radicals and a set of high valence metal oxo compounds. Our calculations demonstrate that although H abstraction from CH3–H by metal oxoes can be satisfactorily fitted into the Polanyi correlation on the basis of oxygen radicals, the mechanisms behind are significantly different. The frontier orbital analyses show that there are three electrons and three active orbitals(3e,3o) involved in H abstraction by oxygen radicals; whereas an additional orbital of π*M – Ois involved in H abstraction by M=O, resulting in a(4e,4o) interaction. In terms of valence bond state correlation diagram, we find that H abstraction by a metal oxo may benefit from the contribution of ionic resonance structures, which could compensate the penalty of opening the M–O π bond. We believe that these findings can help to design more effective catalysts for the activation of light alkanes.     

Electronic structure and property studies of the first crowned [60] fulleropyrrolidine

The computations of the electronic structures and properties of a novel crowned [60] fulleropyrrolidine (CFP) were performed by means of AM1 methods. It has been indicated that CFP has four isomers in which the dihedral angle between phenyl group and pyrrolidine ring is around ±90°. The study of electronic structures showed that the energy levels of frontier orbitals are determined mainly by C_(60). C_(60) acted as an electronic acceptor, whereas crown ether acted as an electronic donor, which implies that there exists intramolecular charge transfer effect in this molecule. The study of nonlinear optical properties implied that the hyperpolarizability of CFP can match that of p-nitroaniline. In the meantime, the hyperpolarizability properties of CFP could be influenced by the orientation of crown ether moiety.     

THEORETICAL STUDIES ON THE STRUCTURES AND REACTIVITY OF SILYLENOIDS (Ⅰ)——STRUCTURES AND ISOMERIZATION OF SILYLENOID H_2SiLiCl

The structures of silylenoid H_2SiLiCl have been studied by RHF/3-21G gradient method. Three equilibrium states and two isomerization transition states are located. The electronic energy, Mulliken populations, dipole moments and frontier molecular orbitals (FMO) of every structure and the isomeric barriers are also given and analyzed. Further single point calculations on the 3-21G optimum geometries were performed using 6-21G basis set. The calculations show that three-membered ring structure of H_2SiLiCl is the most stable and detectable, and the "classical" tetrahedral form is not the local minimum on the potential energy surface.     

LOCALIZED MOLECULAR ORBITAL STUDY ON ELECTRONIC STRUCTURES OF BOWL-SHAPED AROMATIC HYDROCARBONS

The localized molecular orbitals for typical bowl-shaped circulenes are obtained by the use of INDO-LMO method. It is found that these bowl-shaped circulenes with strained π-electron systems are still aromatic and the rim π-bonds with larger localization form reactive regions for formation of buckminsterfullerene.     

Theoretical Investigations on the Structures and Properties of the Side-on Complexes B_2(N_2)_2 and Monocyclic B_n(N_2)_n~m (n=3～6,m=-1～+2)

The structures and energies of the side-on complexes B2(N2)2 and monocyclic Bn(N2)nm (n = 3～6,m = -1～+2) between N2 (1∑+g) and B (2P) have been investigated by the DFT-B3LYP and MP2 methods at the 6-311+G(2d) and aug-cc-pVTZ levels. The analyses of NICS (Nucleus Independent Chemical Shifts),NBO (nature bond orbital),AIM (atoms in molecules) and frontal orbitals have been used to reveal the origin of coordination bond between the π-electron donor N2 group and B atom,accompanied by the comparison with the end-on complexes. The results have indicated that the side-on coordination complexes can be formed due to the relative strong fluidity of the π-electrons,and the nature of the coordination bond has been exposed to be that the N2 group offers 1πu electron to the 2p orbital of boron. The coordinate energies of the side-on complexes are less than those of the end-on complexes. Furthermore,the aromaticity of side-on coordination complex is weaker than that of the corresponding end-on coordination complex.     

CHROMATOGRAPHIC SEPARATION ON GRAPHITE OF FULLERENE-C_(60) AND-C_(70)

The mixture of fullerene—C_(60) and —C_(70) was successfully separated in larger amount by liquid chromatogra-phy on graphite column.The column was first eluted with a mixture of hexane and toluene,and then withtoluene.The eluates were monitored by UV/visible absorption spectroscopy.Graphite column shows high perfor-mance for separation.and much larger quantities of pure C_(60) can be easily obtained.The synthesis of fullerenes(C_(60) and C_(70))in macroscopic quantities has stimulated scientists to study on themethod for separating C_(60) and C_(70).Because of the poor solubility of fullerenes C_(60)/C_(70) in most organic solvents,theseparation of C_(60) and C_(70) proved to be a challenging task.Most attempts at the separation have used silica and alu-mina as the stationary phase and a mixture of hexane and benzene(or toluene)as the mobile phase.Fullerenes,however,have only limited solubility in hexane,these separations are usually only achieved by limit-ing the amount of sample to a few milligrams.The hexane-silica technique using phenylglycine based HPLCcolumns appears to offer good separation,but only for submilligram amounts. The use of graphite powder as the stationary phase in liquid chromatography is an advanced separation tech-nique for fullerenes C_(60)/C_(70).The technique is based on that the fullerene-C_(60) is different from C_(70) in the de-gree of electron delocalisation,and C_(60) is much less diamagnetic than C_(70).It is expected that the dissimilarity willcause the C_(60)/C_(70) molecules to exhibit different interactions with graphite,and can be separated from each other onthis basis.     

ELECTRONIC STRUCTURE AND CHEMICAL BONDING OF TRISINDENYL SAMARIUM

The electronic structure and chemical bonding of Sm(Ind)3 were studied by using INDO calculation method.The results showed that the 6s 6P and 5d orbitals of Sm mixed with the atomic orbitals of ligands to some extent while the 4f orbital of Sm was strongly localized,participating in bonding less than 2%.The HOMO and the LUMO were of π-type.The chemical bond of Sm(Ind)3 is considerably covalent in character.The net charge distribution on the carbon atoms of the indenyl group is unequal.The Some bonds in Sm(Ind)3 THF is discussed.     

INDO STUDIES ON THE ELECTRONIC STRUCTURE OF[Ln(CCl_3COO)_3·phen·C_2H_5OH]_2(Ln=Pr,Sm)

The electronic structure and chemical bonding of the title comp-lexes have been studied by an unrestricted INDO program made applicable forthe lanthanoid compounds.The results indicated:(1)In coordinated bonds O-Lnand N-Ln,5d orbitals of Ln have large contribution in all valence orbitalsof Ln and 4f orbitals have very small contribution.(2)The covalent chara-cter and ionic character are almost equal in the chemical bond which iscomparatively weak between phen,C_2H_5OH and Ln are mainly ionic with somecovalent character.     

MICROSCOPIC MECHANISM OF STRENGTH REDUCTION OF ROCKS

Uniaxial compression tests have been conducted on Jinan gabbro at six different axial strain rates ranging from 10~(-8)/s to 10~(-3)/s. Results show that the strength decreases by about 23% with decreasing strain rate. The stress level C_0~' /or the onset of dilatancy is constant for six strain rates. When σ_1>C_o~' ε_1 and D increase with decreasing strain rate (?) under a specific stress level.Microscopic observations on the stressed rock samples reveal that the density of microcracks for the samples loaded at low strain rates is higher than that at high strain rates; in addition there are also cracks nearly perpendicular to it, which is responsible for the increase of D value.The creep test under the stress σ_(σ_1>C_0~') confirms that the growth of microcracks in the rock has been time-dependent effects, and the steady-state creep with (?)=constant developed until the rock fails.Summing up the above reslts shows that when σ>C_0~', the constitution law of the rockswill be time-dependent, and the     

The role of 3d orbitals in the bonding of phosphorus compounds

Ab initio LCAO-MO-SCF calculations for several typical molecules containing phosphorushave been undertaken to study the role of phosphorus 3d orbitals in the bonding.It is emphasizedthat the discussion about the 3d orbital participation in bonding should be based on a reasonable choiceof basis sets and it seems suitable to choose the atomic orbitals in proper molecular environment asthe basis set.As an approximation,the optimized minimal STO-NG basis sets have been adoptedin the present paper.The results obtained well exhibit the model of 3d orbital participation in bonding.It is shown that under the influence of highly electronegative ligands the phosphorus 3d orbitals con-tract greatly,their energy levels drop considerably,and thus they can effectively participate in bond-ing.The presence of highly electronegative ligands seems necessary.The contribution of 3d orbitalsto bonding is achieved mainly through the concertedformation of σ bonds and p-d backbonds,thoughthe contribution to σ bonding is minor.The three-center,four-electron bond modelis only approxi-mately correct.The results of the present paper demonstrate that the model of 3d orbital participationin bonding favoured by experimental chemists is reasonable and possesses sound ground.     

Electronic property and molecule design for luminescent metal complexes of tris(8-hydroxyquinoline) gallium

By means of ab initio HF and DFT B3LYP methods, the structure of Gaq3 (q = 8-hydroxyquinoline) was optimized. The frontier molecular orbital characteristics and energy levels of Gaq3 have been analyzed systematically in order to study the electronic transition mechanism in Gaq3. Three derivatives of Gaq3 and their polymers were designed and the possibilities that they were employed as luminescent materials were discussed. The regularities and characteristic of energy bands of Gaq3 and its derivatives were also investigated. The results show that the electronic π-π* transitions in Gaq3 are localized on the quinolate ligands. The emission of Gaq3 is due to the electron transitions from a phenoxide donor to a pyridyl acceptor. Two possible electron transfer pathways are presented, one by carbon atoms, and the other via metal cation Ga3 . The derivatives of Gaq3 may possess high luminescence efficiency.     

Theoretical Studies on Geometry, Stability and Electronic Spectra of DATB and TATB

The geometries of DATB( 1,3-diamino-2,4 ,6-trinitrobenzene) and TATB( 1,3,5-tri-amino-2,4,6-trinitrobenzene) have been optimized by M1NDO/3 method. It is found that the total energy of the molecule is rather low at the plane configuration. It is also found that the orbitals between the amino-N and its adjacent nitro-N as well as the amino-N and its adjacent nitro-N on the ring are rather close and matched in symmetry and have high orbital interaction through space, which are evidently seen from the computed results of the atomic bond index. The electronic spectra of DATB have been computed by using INDO/2-CI method, and the results are well conformed with the experimental facts. For instance, the transition from 'A' (ground state" ) to 'A' (the 7th exciled state) corresponds to the computed spectrum line(2. 48×104 cm-1) which is close to the experimental value (2. 43×104 cm-1). From a similar computation, we may predict that the major spectrum line of TATB will be 2. 75, 2. 77, 3. 28 and 3. 32×104 cm