原子价壳层电子量子拓扑指数与元素电负性的关系

在基态原子价壳层电子隐核图的基础上, 基于拓扑化学原理以及原子价壳层电子结构特征, 构建了原子价壳层电子量子拓扑指数(AEI), 它对基态原子实现唯一性表征, 结合原子价壳层电子平均化能(∑n_iE_i/∑n_i)等参数, 建立了一套新的元素电负性标度: X_N＝－0.588710AEI₁＋0.761214AEI₂＋0.154982(∑n_iE_i/∑n_i)－0.080929. 该式给出了周期表中氢至镅共95种元素的电负性, 结果表明新电负性标度X_N与Pauling电负性标度颇为一致. 进一步从原子价轨道量子拓扑指数确定了sp, sp², sp³杂化轨道的电负性. 新标度在元素和物质的结构-性质研究中具有一定的适用性.     

The valence isomers of (CH)8 and (SiH)8: An ab initio MO study

The 22 possible valence isomers of the (CH)₈ and (SiH)₈ systems have been studied by ab initio molecular orbital calculations at the MP2/6-31G*//6-31G* + ZPE level. Optimized geometries, relative energies, and, for some selected compounds, vibrational frequencies are reported. The systematic differences between the carbon and silicon compounds are analyzed. © 1994 by John Wiley & Sons, Inc.     

Correspondence on “Core Electron Topologies in Chemical Compounds: Case Study of Carbon versus Silicon”

The conclusions of a recent Communication of Yoshida, Raebiger, Shudo, and Ohno published in this journal, that varying core orbital topologies with minuscule negative tails upon bond formation determine the different chemistries of carbon and silicon and affect ionization energies, excitation energies and bond properties of molecules, are now shown to be based on computational artifacts and oversimplified models. The all‐electron wave function uniquely determines the observables, while its representation by one‐electron orbital products depends on the details of the chosen approximation and therefore need to be considered with great care.     

A theoretical study on the protonation of cycloalkanes CnH2n (n = 3 to 6)

Ab initio self-consistent-field molecular orbital calculations have been carried out for the C_nH_2n (n = 3 to 6) cycloalkanes and various conformers of their protonated forms. The calculated protonation energies for the sequence of conformers of the protonated forms follow the experimentally observed trend. Correlations between optimum C? C? C bond angles at the protonation site and the calculated protonation energies have been observed, and these correlations may be of some use in estimating protonation energy-bond angle relations in other (strained) cyclic compounds when the central carbon atom of a C? C? C moiety is protonated.     

Structure and properties of small silicon and aluminum clusters

Small Si_n and Al_n clusters (n = 3–10) were studied with the semiempirical molecular orbital method (MO) method SINDO1. For each n, various structures were optimized to determine the most stable structure. To obtain good qualitative agreement with available ab initio calculations d orbitals had to be omitted from the basis set. Both silicon and aluminum tend to build three-dimensional structures rather than two- or one-dimensional structures, except for n = 3 or 4. The structure growth was studied by approaching various sites of stable structures with one or more atoms. It was found that silicon and aluminum exhibit different structure growth, and consequently, different most-stable structures. Ionization potentials, HOMO -LUMO energy differences, binding energies per atom, and average atomic valencies are presented.     

A contracted bromine basis set for use in calculation of molecular energies

A contracted [9s6p2d] basis set derived from Dunning's (14s11p5d) primitive Gaussian set for bromine has been used in ab initio molecular orbital calculations of the dissociation energies of HBr, CH₃Br, and Br₂, the ionization potentials of Br and HBr, and the electron affinity of Br. The calculated energies are within 0.1 eV of the experimental values. This is similar to the accuracy obtained in a previous study, also using a contracted [9s6p2d] basis set, of the dissociation and ionization energies of the GeH_n, AsH_n, and SeH_n hydrides.     

Computational Cogitation of Cn@Al12 Clusters

A variety of novel C_nAl₁₂ core–shell nanoclusters have been investigated using density functional calculations. A series of C_n cores (n=1–4) have been encapsulated by icosahedral Al₁₂, with characteristic physical properties (energetics and stabilities, ionisation energies, electron affinities) calculated for each cluster. Other isomers, with the C_n moiety bound externally to the Al₁₂ shell, have also been studied. For both series, a peak in stability was found for n(C)=2, a characteristic that appears to be inextricably linked with the relaxation of the constituent parts upon dissociation. Analysis of trends for ionisation energies and electron affinities includes evaluation of contributions from the carbon and aluminium components, which highlights the effects of composition and morphology on cluster properties.     

Some solved problems of the periodic system of chemical elements

Basic questions on the periodic system (PS) of chemical elements are still under discussion. Several common misconceptions will be resolved. The word “chemical element” comprises more than two concepts. The PS can be rationalized on a quantum chemical basis, namely with the aid of four concepts: (1) electron configurations of bonded atoms, (2) realistic sequences of orbital energies, (3) spatial extension of valence and Rydberg orbitals, and (4) energy gaps above the closed 1s² and np⁶ shells (n = 2–6). The PS of known elements cannot be naively extrapolated. The common discussion of the PS in textbooks of general, inorganic, and physical chemistry needs revision. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

New Insights into Factors Influencing B?N Bonding in X:BH3−nFn and X:BH3−nCln for X=N2, HCN,LiCN, H2CNH,NF3, NH3 and n=0–3: The Importance of Deformation

Understanding the bonding in complexes X:BH_3?nF_n and X:BH_3?nCl_n, for X=N₂, HCN, LiCN, H₂CNH, NF₃, NH₃ with n=0–3, is a challenging task. The trends in calculated binding energies cannot be explained in terms of any of the usual indexes, including π donation from the halogen lone pairs to the p(π) empty orbital on B, deformation energies, charge capacities, or LUMO energies, which are normally invoked to explain the higher Lewis acidity of BCl₃ relative to BF₃. The results of the high‐level G3B3 ab initio calculations reported in this study suggest that the interaction energies of these complexes are determined by a combination of at least three factors. These include the decrease in the electron‐accepting ability of B as a result of π donation by the halogen atom, the increase in the electron‐acceptor capacity of B due to deformation of the acid, and the large increase in the deformation energy of the acid with increasing halogen substitution. The dominant effects are those derived from the electronic effects of acid deformation. Deformation not only has direct energetic consequences, which are reflected in the large differences between dissociation (D₀) and interaction (E_int) energies, but also leads to an enhancement of the intrinsic acidities of BH_3?nF_n and BH_3?nCl_n moieties by lowering the LUMO energies to very different extents, consistent with the frontier orbital model of chemical reactivity. Although this lowering depends on both the number and the nature of the halogen substituents, binding energies do not systematically increase or decrease as the number of halogen atoms increases.     

Supertetrahedral aluminum and silicon structures and their hybrid analogues

New structurally stable compounds-superaluminotetrahedrane (Al₁₀₄H₃₂), supersilicotetrahedrane (Si₁₀₄H₃₂) and mixed superaluminosilicotetrahedrane (Al₆₄Si₄₀H₃₂) constructed on the basis of the diamond crystal lattice in which the carbon atoms are replaced by, respectively, Al₄ and Si₄ tetrahedra and which are supramolecular models of corresponding aluminum and silicon crystal structures—have been studied by the B3LYP/6-31G(d,p) density functional theory method. The small difference between the frontier orbital energies indicates that aluminum compounds can have electroconductive properties and silicon compounds can have semiconducting properties.     

Doubly charged ion mass spectra: VI—Amines

Doubly charged ion mass spectra of 22 amines (2–10 carbon atoms) were determined using an Hitachi RMU-7L double focusing mass spectrometer. Molecular ions were not observed in the spectra of aliphatic amines. The most intense product ion peaks in the spectra of lower molecular weight amines resulted from hydrogen elimination from the molecular ion; however, as amine molecular weight increased the largest peaks resulted from both hydrogen and heavy atom elimination from the molecular ion. Dominant ions in the doubly charged ion spectra of lower molecular weight aliphatic amines were from reactions of [C_nH₃N]²⁺ (n:=2, 3, 4) type ions. The spectra of higher molecular weight aliphatic amines spanned a wide mass range. Appearance energies for some of the more prominent ions were measured in the range from 25 to 49 eV. A geometry optimized quantum mechanical self-consistent field molecular orbital treatment was used to compute the energies and structural parameters of prominent ions in the doubly charged ion mass spectra.     

The Silicon Carbonyls Revisited: On the Existence of a Planar Si(CO)4

Recently, some works have focused attention on the reactivity of silicon atom with closed-shell molecules. Silicon may form a few relatively stable compounds with CO, i.e. Si(CO), Si(CO)₂, Si[C₂O₂], while the existence of polycarbonyl (n>2) silicon complexes has been rejected by current literature. In this paper, the reaction of silicon with carbonyl has been reinvestigated by density functional calculations. It has been found that the tetracoordinated planar Si(CO)₄ complex is thermodynamically stable. In Si(CO), silicon carbonyl, and Si(CO)₂, silicon dicarbonyl, the CO are datively bonded to Si; Si(CO)₄, silicon tetracarbonyl, may be viewed as a resonance between the extreme configurations (CO)₂Si + 2CO and 2CO + Si(CO)₂; while Si[C₂O₂], c-silicodiketone, is similar to the compounds formed by silicon and ethylene. A detailed orbital analysis has shown that the Si bonding with two CO is consistent with the use of sp ²-hybridized orbitals on silicon, while the Si bonding with four CO is consistent with the use of sp ² d-hybridized orbitals on silicon, giving rise to a planar structure about Si.     

Systematic regularities of molecular SCF energies

New formulae for the approximate computation of total molecular energies are developed based on ab initio calculations of n-alkanes. Their application to various kinds of molecules reveals that good expectation values for total molecular energies can be obtained by considering only the one-electron terms h _i and the nuclear repulsion energy. It is further shown that very good agreement with SCF total energies is obtained by a relationship which connects the total energy with the sum of inner-shell (core) orbital energies. The results turn out to be better than those obtained using Ruedenberg's approximation, which takes both inner-shell and valence-shell orbital energies into account.     

Combined DFT with NBO and QTAIM studies on the hydrogen bonds in (CH<Subscript>3</Subscript>OH)<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> (<Emphasis Type="Italic">n</Emphasis> = 2–8) clusters

The (CH₃OH) _n (n = 2–8) clusters formed via hydrogen bond (H-bonds) interactions have been studied systemically by density functional theory (DFT). The relevant geometries, energies, and IR characteristics of the intermolecular OH···O H-bonds have been investigated. The quantum theory of atoms in molecule (QTAIM) and natural bond orbital (NBO) analysis have also been applied to understand the nature of the hydrogen bonding interactions in clusters. The results show that both the strength of H-bonds and the deformation are important factors for the stability of (CH₃OH) _n clusters. The weakest H-bond was found in the dimer. The strengths of H-bonds in clusters increase from n = 2 to 8, moreover, the strengths of H-bonds in (CH₃OH) _n (n = 4–8) clusters are remarkably stronger than those in (CH₃OH) _n (n = 2, 3) clusters. The small differences of the strengths of H-bonds among (CH₃OH) _n (n = 6–8) clusters indicate that a partial covalent character is attributed to the H-bonds in these clusters. The linear relationships between the electron density of BCP (ρ_b) and the H···O bond length of H-bonds as well as the second-perturbation energies E(2) have also been investigated and used to study the nature of H-bonds, respectively.     

QSAR of genotoxic active benzazoles§

Previously synthesized 2,5-disubstituted benzoxazole and benzimidazole derivatives, were tested for their genotoxic activity in the Bacillus subtilis rec? assay. The results revealed that 5-methyl-2-(p-aminobenzyl)benzoxazole exhibited the highest genotoxic response, which was comparable to 4-nitroquinoline 1-oxide (4-NQO), the reference agent of classical positive mutagen. Among the other tested compounds, four showed a genotoxic activity. A QSAR study revealed that structural parameters IY_C2H4 and IY_CH2O, indicating the bridge elements between the phenyl moiety and the fused ring system at position 2 and the quantum chemical parameter (ΔE?), showing the difference between HOMO and LUMO energies, were found significant for enhancing the genotoxic activity in these compounds. In addition, the substituent effects on positions R and R₁ were found important for the activity as well as holding a substituent possessing a maximum length with a minimum width property on position R₁ like alkyl groups. On the other hand, substituting position R with an electron donating group instead of electron withdrawing group increased the genotoxic activity.     

Structure of carbon fluorochlorides CF n Cl m (n,m ≤ 3) and their singly charged negative ions

The electronic and geometrical structures of carbon fluorochlorides with low coordination numbers (n 3) and their singly charged anions are calculated using the functional density method. The results of the calculations are used to evaluate the electron affinities (EA) of the neutral compounds and the first ionization potentials of the anions as well as the energies of fragmentation through different decay channels of both series. The adiabatic EA of carbon fluorochloride CF _k Cl _3–k is shown to be determined mainly by the presence of a CX₂ unit in these compounds. There are no monotonic changes in stability of either the neutral compounds withn = 3 or the anions withn = 2 or 3 upon successive substitution of one halogen by another.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1044–1049, June, 1993.     

Electronic charge density and mean kinetic energy of lithium orbitals in LiH,LiH+, Li2, Li2+, LiH2+, and Li2H+

The electron density near the lithium nucleus in the species LiH, LiH⁺, Li₂, Li₂⁺, LiH₂⁺, and Li₂H⁺ was analyzed by transforming the SCF molecular orbitals into a sum of atomic contribnutions, for both core and valence orbitals. These “hybrid-atomic” orbitals were used to compare: electron densities, orbital polarizations, and orbital mean kinetic energies with the corresponding lithium atom quantities. Core-orbital electron densities at the lithium nucleus were observed to increase by up to 0.5% relative to the lithium atom 1s orbital. Lithium cores also exhibited polarization but, surprisingly, in the direction away from the internuclear region. Similar dramatic changes were seen in the electron densities of the valence orbitals of lithium: The electron density at the nucleus for these orbitals increased two-fold for homonuclear species and twenty-fold for heteronuclear triatomic species relative to the electron density at the nucleus in lithium atom. The polarization of the valence orbital electronic charge, in the vicinity of the lithium nucleus, was also away from the internuclear region. The mean “hybrid-atomic” orbital kinetic energies associated with the lithium atom in the molecules also showed changes relative to the free lithium atom. Such changes, accompanying bond formation, were relatively small for the lithium core orbitals (within 0.2% of the value for lithium atom). The orbital kinetic energies for the lithium valence electrons, however, increased considerably relative to the lithium atom: By a factor of about 2 in homonuclear diatomics, by a factor of 7 in heteronuclear diatomics, and by a factor of 11 in the triatomic species. In summary, the total electronic density (core plus valence) at the lithium nucleus remained remarkably constant for all of the species studied, regardless of the effective charge on lithium. Thus, the drastic changes noted in the individual lithium orbitals occurred in a cooperative fashion so as to preserve a constant total electron density in the vicinity of the lithium nucleus. In all cases, bond formation was accompanied by an increase in the orbital kinetic energy of the lithium valence orbital. We suggest that these two observations represent important and significant features of chemical bonding which have not previously been emphasized.     

Anab initio molecular orbital study of substituted carbonyl compounds

Ab initio SCF calculations with a minimal STO-3G basis set have been performed to determine the equilibrium geometries of two series of carbonyl compounds, RCHO and R₂ CO. For the mono-substituted compounds, R may be CH₃, NH₂, OH, F, CHO, and C₂ H₃. In the disubstituted compounds, R has been restricted to the isoelectronic saturated groups. The computed equilibrium geometries of these compounds are in satisfactory agreement with the experimentally-determined geometries, with an average difference of 0.021 Å between computed and experimental bond lengths, and 1.9 ° in corresponding bond angles. An analysis of the effect of the substituent on the electronic structure of the carbonyl group has also been made. The saturated groups are found to be electron-withdrawing groups relative to H, with the electron withdrawing ability increasing in the order CH₃ 2 2H₂ are also electron withdrawing relative to H, and comparable to CH₃ in acetaldehyde. Vertical ionization potentials andn* transition energies have also been calculated for these molecules at their optimized geometries, experimental geometries, and geometries given by a standard model. The effect of changes in molecular geometry on these computed properties has been analyzed.