CNDO/2 and MINDO/2 energy band structures of some homopolynucleotides

The all-valence electron band structures of the periodic DNA models polyC, polyT, and polyA have been calculated with the aid of the CNDO /2 and MINDO /2 crystal orbital (CO ) method. According to the obtained results the valence and conduction bands are always of π type. The widths of the bands are usually larger in the CNDO case, than those obtained for the same systems in the PPP CO approximation. On the other hand the MINDO results show similar widths as the PPP ones. The forbidden bandwidths are by 3–4 eV larger in the CNDO /2 case, than in the PPP one, while the MINDO /2 CO method has resulted in similar gaps like the PPP calculation.     

Electronic structures and state densities for faujasite clusters

MINDO/3 and CNDO/2 methods have been applied to clusters simulating the structure of faujasite, with the inclusion of from 6 to 12 aluminum-oxygen and silicon-oxygen tetrahedra. A study has been made of how the integral and orbital electron-state densities vary with the dimensions of the zeolite fragment, the boundary conditions, and the calculation method. The fine structure in the cation state density peaks for aluminum in the valency band is only slightly dependent on the boundary conditions and the cluster dimensions, while the differences between the MINDO/3 and CNDO/2 results amount to a systematic shift in the peaks together with slight changes in the fine structure. On the other hand, the boundary conditions have a considerable effect on the orbital state density profiles for the oxygen ions in the upper valency quasiband for the faujasite clusters.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 210–216, March–April, 1986.     

Treatment of hydrogen bonding within CNDO/2 and MINDO/3: CNDO/2H and MINDO/3H

A formalism has been developed to treat hydrogen-bonded A—H…?B systems within the CNDO /2 and the MINDO /3 methodologies. In this formalism the interactions are divided into three distinct classes; those between (a) two hydrogen-bonded atoms, (b) one hydrogen-bonded and non-hydrogen-bonded atom, and (c) two non-hydrogen-bonded atoms. The last class of interactions is treated solely by the existing CNDO /2 or MINDO /3 method. For A –H…?B systems, the core resonance integrals are individually parametrized depending upon the class of the interaction. Three types of A—H…?B systems have been thus far parametrized. Nine hydrogen-bonded dimers have been studied using the new formalism and the current CNDO /2 and the MINDO /3 methods. MINDO /3 predicts very large interatomic (A –B) distances for the equilibrium geometry, and relatively small stabilization values for the hydrogen-bond energies. CNDO/2 predicts the reverse. The new formalism for both CNDO /2 and MINDO /3 predicts accurate geometries as well as energies for all nine dimers. The new formalisms are called CNDO /2H and MINDO /3H. A general discussion of the nature of hydrogen bonding as exhibited by CNDO /2H and MINDO /3H is presented.     

Study of the electronic energy band structure of polyethylene using MINDO/3

The electronic structure of polyethylene has been calculated using crystal orbital theory and the latest version (MINDO/3) of the MINDO semi-empirical SCF MO method. The results are in better agreement with the observed photoelectron spectrum than those given by an analogous calculation using MINDO/2, which in turn has been shown superior to treatments based on EHT, CNDO, INDO, or ab initio SCF procedures.     

Electronic Structure of Fe+X Alloys (X = Si, Ge, Co)

The electronic structure of Fe–Ge, Fe–Si, and Fe–Co alloys has been investigated by X-ray photoelectron spectroscopy. In Fe–Ge alloys with less than 10 at.% Ge, the Fe–Ge bond is mainly formed by the Fe 4sp- and Ge 4p-electrons. The results obtained for this system are identical to those for the Fe–Si system. The form of the valence band reflects the density distribution of both iron d-electronic states the and p-electronic states of the second component, having more extended density distribution of valence electrons. In FeCo alloys, strong spatial localization of d-electron density takes place in the vicinity of the corresponding atoms, which is stronger on the iron atoms compared to pure iron; the valence band has a two-band structure reflecting the density distribution of the d-states of each component. X-ray photoelectron spectroscopy data are in good agreement with kinetic data for the alloys.     

All-valence-electron band structures of infinite stacked poly(TCNQ) and poly(TTF) chains

We have computed the energy band structures of the inifite poly(TCNQ), poly(TCNQ^2?), poly(TTF) and poly(TTF²⁺) chains using the CNDO/2 and where possible the MINDO/2 crystal orbital approximation schemes. The results show a broad conduction band for poly(TCNQ) and a broad valence band for poly(TTF). The bandwidths within the MINDO/2 CO scheme are found to be smaller by roughly a factor of $\text{1}\text{2}$ as compared with those found within the CNDO/2 CO scheme. Our findings are in agreement with a bandwidth of 0.4–0.5 eV for the conduction band of TCNQ—TTF crystals as found by experiments. A brief discussion is given of the k-dependence of the physically interesting bands which is not always simple.     

Potassium-intercalated H(2)Pc films: Alkali-induced electronic and geometrical modifications

X-ray spectroscopy studies of potassium intercalated metal-free phthalocyanine multilayers adsorbed on Al(110) have been undertaken. Photoelectron spectroscopy measurements show the presence of several charge states of the molecules upon K intercalation, due to a charge transfer from the alkali. In addition, the comparison of valence band photoemission spectra with the density functional theory calculations of the density of states of the H(2)Pc(-) anion indicates a filling of the formerly lowest unoccupied molecular orbital by charge transfer from the alkali. This is further confirmed by x-ray absorption spectroscopy (XAS) studies, which show a decreased density of unoccupied states. XAS measurements in different experimental geometries reveal that the molecules in the pristine film are standing upright on the surface or are only slightly tilted away from the surface normal but upon K intercalation, the molecular orientation is changed in that the tilt angle of the molecules increases.     

A chemically useful definition of electron difference densities

Molecular difference densities (DD ) are conventionally constructed using spherically averaged atomic densities at the appropriate positions. For atoms in degenerate ground states, this is an unphysical choice, and artifacts dominate the DD . We suggest the extraction of both the position and the orientation of an atom with an open valence shell from x-ray scattering or molecular density data. Subtracting the oriented atoms yields a uniquely defined, as well as chemically meaningful, DD. Covalent bonds to electronegative atoms such as O are no longer exceptional but show bond charges of normal magnitude. Lone pairs are characterized by a dipolar density shift from the bond to the back side of the atomic core.     

A comparison of the HAM/3 semiempirical method with the MINDO/3 and CNDO/2 semiempirical methods and the GAUSSIAN 70 ab initio method; a comparison of ionization potential calculations and experimental photoelectron spectra of some imines and diimines

Molecular orbital calculations have been performed on eight molecules (containing 16–80 electrons) using the GAUSSIAN 70, MINDO/3, CNDO/2 and HAM/3 programs. The molecules contain nitrogen—nitrogen (diimines) or carbon—nitrogen (imines) double bonds. A comparison of the results of each method with experimental photoelectron data and an analysis of the cost effectiveness indicates that the HAM/3 method will prove a useful tool for photoelectron spectroscopists.     

A chemically useful definition of electron difference densities

Molecular difference densities (DD ) are conventionally constructed using spherically averaged atomic densities at the appropriate positions. For atoms in degenerate ground states, this is an unphysical choice, and artifacts dominate the DD . We suggest the extraction of both the position and the orientation of an atom with an open valence shell from x-ray scattering or molecular density data. Subtracting the oriented atoms yields a uniquely defined, as well as chemically meaningful, DD . Covalent bonds to electronegative atoms such as O are no longer exceptional but show bond charges of normal magnitude. Lone pairs are characterized by a dipolar density shift from the bond to the back side of the atomic core.     

σ-bond conjugation in polysilanes,a pes scaled free-electron approach for the interpretation of skeletal cleavage reactions

The σ-orbital energies calculated by the simple free-electron model with a parametrization procedure proposed recently by Von Szentplay correlate very closely with the σ-band positions of the corresponding photoelectron spectra. For the σ-orbitals of three series of molecules: H(CH₂)_nH (n = 2–4); H(SiH₂)_nH (n = 2–5); Me(SiMe₂)_nMe (n = 2–4) the FEMO model yields a standard error SE) of 0.060 eV. Compared with HMO results (LCGO, LCBO and Sandorfy C) the correlation is significantly improved. The free-electron results are more accurate than those obtained using the PPP, CNDO/2, MINDO/3, SAMO and ab initio methods.The free-electron frontier orbital densities were successfully used to account for features of the skeletal cleavage reactions of polysilanes.     

The electronic structure of periodic protein models

The energy band structures of a two-dimensional polyformamide network have been calculated with the aid of the CNDO/2 and MINDO/2 crystal orbital method in the first neighbour's interactions approximation. For comparison also one-dimensional polyformamide chains have been computed with the same methods. The features of the obtained band structures are discussed.

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Zusammenfassung Es wurden die Energiebandenstrukturen eines zweidimensionalen Polyformamid-Netzwerkes in einer Näherung, die die Wechselwirkung der ersten Nachbarn berücksichtigt, mit Hilfe der CNDO/2 und MINDO/2 Kristallorbital-Methoden berechnet. Zum Vergleich wurden auch eindimensionale Polyformamid-Ketten unter Verwendung derselben Methoden berechnet. Die Eigenschaften der erhaltenen Bandenstrukturen werden diskutiert.

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Résumé Calcul de la structure des bandes d'énergie d'un réseau bidimensionnel de polyformamide à l'aide des méthodes d'orbitales cristallines CNDO/2 et MINDO/2 dans l'approximation d'interaction des premiers voisins. A titre de comparaison les chaînes unidimensionnelles de polyformamide ont été calculées avec les mêmes méthodes. Discussion des caractéristiques des structures de bande obtenues.

     

Generalization of electronegativity concept

A generalization of the original method introduced by Hinze and Jaffé for calculating the orbital electronegativities is proposed. This generalization is based on a new energy partitioning scheme within the framework of CNDO approximation and permits the orbital electronegativities to be calculated for atoms in actual valence states in which they occur in real molecules.     

Molecular orbital studies on some 4-substituted pyridine N-oxides and 4-nitroquinoline N-oxide

The electronic structures of a series of 4-substituted pyridine N-oxides and 4-nitroquinoline N-oxide are investigated using the simple Pariser-Parr-Pople (PPP), a modified PPP, IEH and MINDO/2 methods. The electronic absorption band maxima and dipole moments are calculated and compared with experimental values. The photoelectron spectra of these compounds are assigned. The nature of the N-oxide group is characterized using the orbital population distributions. The antifungal activity exhibited by some of these compounds is discussed in terms of the nucleophilic frontier electron densities, superdelocalizabilities and electron acceptor properties. The effect of the electron releasing as well as the electron withdrawing substituents on the physico-chemical properties is explained.     

哌嗪二酮的气相Hel光电子能谱及其量化计算

给出了哌嗪二酮的气相HeI紫外光电子能谱(UPS), 并进行了化合物分子的HAM/3, MNDO, MINDO/3, INDO, CNDO/2和EHMO等量子化学计算研究. UPS谱低电离能(<11.00 eV)区的四重峰被指认为分子体系中氧-氧, 氮-氮原子孤对轨道间的通过键相互作用导致的分裂峰. 表明HAM/3和MNDO计算法是预指该化合物实验电离能正确次序、轨道对称性类型以及通过键相互作用导致分裂大小的较好方法.     

Determination of the interface band alignment of Mg2Si/4H-SiC heterojuction for potential photodetector application

The Mg₂Si/4H-SiC heterojunction was prepared by radio frequency (RF) magnetron sputtering technique. The binding energies of Mg 2p, Si 2p, and C 1s core levels and the maxima of valence band were measured by X-ray photoelectron spectroscopy (XPS). Using the optical bandgap of Mg₂Si (0.78 eV) and 4H-SiC (3.25 eV), the band offsets of valence band (VBO) and conduction band (CBO) at Mg₂Si/4H-SiC interface were identified as 1.47 and 1.00 eV, respectively. The band alignment was evaluated to be type-I band alignment. The Mg₂Si/4H-SiC heterojunction could be a promising candidate for the infrared (IR) photodetector.     

x-RAY PHOTOELECTRON VALENCE BAND STUDIES ON FIREFLY D-(-)-LUCIFERIN

Abstract— The valence band spectrum of firefly D-(-)-luciferin (Ln) was measured for the first time by x-ray photoelectron spectroscopy (XPS). Since the conversion of Ln to decarboxydehydroluciferin (dcrL) by x-ray irradiation occurred even within ∼30 min, the effect of the irradiation on the valence band spectrum of Ln was investigated quantitatively. We found that the XPS valence band spectrum of Ln during irradiation for 3 h corresponds to the state density of Ln calculated by the LCAO-ASMO-SCF-CI method for π-electrons. Moreover, the weak coupling model for π-electron transfer between Ln and oxygen was proposed for the primary electronic process in Ln chemiluminescence, assuming that the calculated π-electron state densities of Ln in acidic and basic solutions reflect the characteristics of their XPS valence band spectra.     

Electronic structure of crystalline FeSi and FeGe

X-Ray photoelectron valence band spectra were recorded for stoichiometric compounds FeSi and FeGe; their electronic structure was calculated by the ab initio full-potential linear muffintin orbital method. For nonmagnetic FeSi, good agreement was obtained between the calculated densities of states and the X-ray photoelectron spectrum in both peak positions and forbidden gap. For FeGe, which is an antiferromagnet, the nonmagnetic calculation yields worse agreement with experiment and explicitly indicates that the paramagnetic phase is unstable. In both compounds, the calculation gives a high degree of d—p hybridization and covalence, which is estimated quantitatively. In FeGe, the degree of covalence of the Fe-Fe bond is higher than that of the Fe-Ge bond. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 6, pp. 1098–1102, November–December, 1998.     

Quantum-chemical investigation of α-Al2O3 in the molecular-cluster approximation

Clusters of corundum including from 4 to 14 AlO₆ octahedrons have been calculated by the MINDO/3 and CNDO/2 methods. The influence of various boundary conditions on the electronic structure of fragments of the oxide has been investigated. The quasi-band structure of the clusters has been examined on the basis of calculations of the integral, orbital, and local densities of their electronic states. The electronic structure of clusters with point defects, viz., anion and cation vacancies, has been studied, and the dependence of the combined density of states, which determines the imaginary part of the dielectric constant, on the nature of the defects has been demonstrated. Small fragments of aluminum oxide have been calculated by a nonempirical method in the STO-3G basis. The general tendencies in the variation of the quasi-band structure of clusters with different boundaries for the semiempirical and nonempirical methods have been demonstrated.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 651–659, November–December, 1985.     

Semi-empirical molecular orbital calculations of the properties of 2:1 hydrogen bonded complexes of water with some ketones and ethers

The properties (molecular geometries and hydrogen bond energies) of a number of 2:1 complexes formed between water and some ketones and ethers have been calculated using both the CNDO/2 and the MINDO/3 semi-empirical molecular orbital methods. The CNDO/2 results have been compared with those obtained using the MINDO/3 method, and have been correlated with the i.r. spectroscopic properties of the complexes. The results for the 2:1 complexes have also been compared with those of 1:1 complexes formed with the same solvents.