Change of molecular structure in the excited states: A (CNDO/2) hole-potential study on phosphine

Molecular structure of phosphine in a number of excited electronic states is studied using the method of hole-potential within the basic framework of CNDO/2 theory. Effects of including 3d-functions of phosphorus in the basis set on computed molecular geometries, transition energies and inversion barriers in the excited states have been investigated. An attempt is made to rationalise qualitatively the structural changes in the excitedstate in terms of Walsh-type correlation diagram constructed with the eigenvalues of the Fock operator in theV _N-1 potential model. A simple orbital model for predicting the nature of structural changes in the excited states is proposed.     

Two-photon properties of the La and Lb bands of substituted benzenes computed from CNDO/S

Two-photon tensor invariants proportional to two-photon absorptivities are calculated from CNDO/S CI wavefunctions for a variety of substituted benzenes for transitions to the two lowest singlet states, L_b^? (B_2u^?) and L₃⁺ (B_1u⁺). Providing hexagonal geometry is used, the results demonstrate considerable effectiveness by this method for predicting the relative sensitivity of these bands to vibronic, inductive and conjugative effects, including the intensity ratios for multiple substitutions by aza, methyl and amino. The method fails for fluorobenzene.     

Doubly excited configurations in modified CNDO and INDO methods

The effect of different CI basis sets, including doubly excited configurations, on transition energies calculated by modified CNDO and INDO methods is examined for H₂O, NH₃, CH₄, and H₂CO.     

CNDO/2 calculations and configuration analyses for some hydrogen-bonded systems

The potential energy curves of hydrogen-bonded systems were calculated for the water, methanol, and formic acid dimers and for the hydrogen maleate ion by a modified CNDO/2 method, the core resonance integrals between-electrons being distinguished from those between-electrons, the different bonding parameters being used for -O- and =O, and the core potential integralsV _AB ^c for O-H, C=O, and O...O being determined semi-empirically. Consequently, the following results were obtained: 1) a potential energy curve with a single minimum atr(O-H)=0.95 Å and with a concavity near 1.70 Å for the linear chain dimers of water and methanol; 2) a symmetrical potential energy curve with two minima atr(O-H)=0.95 and 1.78 Å for the cyclic dimer of formic acid; 3) a flat-bottomed symmetrical potential energy curve for the hydrogen maleate ion. The configuration analysis method was applied to the hydrogen-bonded systems; the contributions of the covalent ((O-H-O)^–1) and ionic (O^–H⁺O^–) structures being 54% and 39%, respectively, for the symmetrical hydrogen bonding of the hydrogen maleate ion.

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Zusammenfassung Die Kurven potentieller Energie von Systemen mit H-Brücken wurden im Fall von H₂O-, CH₃OH- und HCOOH-Dimeren und für das Hydrogen-Maleatanion mittels eines modifizierten CNDO-Verfahrens berechnet, wo für Rumpf-- und --Elektronen jeweils verschiedene Resonanzintegrale und verschiedene Bindungsparameter für =O und -O- verwendet und wo die Rumpfpotential-Integrale für O-H, C=O und O...O semiempirisch bestimmt werden.Die Resultate sind 1) eine Potentialkurve mit einem einfachen Minimum für lineare Ketten von H₂O und CH₃OH, 2) eine symmetrische Potentialkurve mit zwei Minima für das cyclische Diniere von HCOOH und 3) eine Potentialkurve mit einem flachen Minimum für das Maleatanion. Konfigurationsanalyse ergab einen Beitrag von 39% für die ionischen Strukturen.

     

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.     

AM1/CI, CNDO/S and ZINDO/S computations of absorption bands and their intensities in the UV spectra of some 4(3H)-quinazolinones

A detailed analysis of both frontier MOs and electronic transitions in UV spectra of 16 4-quinazolinone derivatives has been carried out in MO terms, by semiempirical methods AM1/CI, CNDO/S and ZINDO/S. On the basis of experimental and theoretical investigations by the ZINDO/S and CNDO/S methods the long-wavelength bands of 4(3H)-quinazolinone and its derivatives have been assigned to n-->pi(*) transition of the CO fragment and to the transition caused by intramolecular charge transfer from Ph and NCN fragments to CO group. It was shown that theoretically obtained electronic transitions applying method AM1/CI are not in agreement with experimental data observed for the 4(3H)-quinazolinone and 2,4(1H,3H)-quinazolinedione. Good correlation of theoretical and experimental data has been obtained by the method ZINDO/S for the wavelengths and the molar extinction coefficients of the compounds studied. Satisfactory correlation of theoretical and experimental data has also been obtained by the method CNDO/S with singly and doubly excited configurations, for the wavelengths only. Such correlations on experimental and theoretical wavelength and molar absorption coefficients of 4-quinazolinone derivatives are carried out for the first time.     

Electronic structure of transition-metal complexes. I. Parametrization for CNDO/2 method

A new parameterization for the first transition metal has been proposed in the framework of CNDO /2 method. We carried out CNDO /2 calculation of hexamine complexes [M(NH₃)₆]²⁺ and hexa-aquo complexes [M(OH₂)₆]²⁺ in the high spin state where M = Mn, Fe, Co, Ni, and Cu, using new parameters. It is shown that the calculated order of binding energy is Mn? L < Fe? L < Co? L < Ni? L ≈ Cu? L (where L means the ligand), and is in good agreement with experiment. We discussed how the orbital nodes affect the nature of bonding between metal and ligand.     

A CNDO/2 calculation of a rearrangement in cyclopentadienylsilane

The ground state geometry of cyclopentadienylsilane and the transition state geometry for SiH₃ group migration have been calculated by the CNDO technique, a semi-empirical version of the LCAO MO SCF approach.     

Interpretation of the vibrational spectra of methylglyoxal and biacetyl in their first singlet excited electronic states

Laser-induced fluorescence spectra for the first allowed electronic transition (22125 cm^?1) of methylglyoxal (CH₃COCHO) and its perdeutero analog (CD₃COCDO) in a supersonic nozzle beam are quantitatively represented assuming that the potential function governing the CH₃(CD₃) rotation is changed during the transition. In the excited state the potential function is ternary (V₁ = 95 (1 + cos 3θ)cm^?1) as in the fundamental state (V₀ = 134.5 (1 - cos 3θ)cm^?1), but the minima are shifted by an angle of π/3. The spectrum of biacetyl (CH₃COCH₃CO) can be reproduced assuming two uncoupled methyl groups undergoing similar conformational changes during the electronic transition (the estimated potential function is V₁ = 117.5 (1 + cos 3θ) cm^?1 for each methyl group), in perfect agreement with the most recent assignment of the 0-0 transition. These results are consistent with ab initio calculations for the fundamental and first excited singlet states.     

CNDO/2 study on lithium bonding in lithium methoxide—Amine systems

A systematic CNDO/2 study has been carried out on the lithium-bonded model systems, CH₃OLi–NR ₃, formed between lithium methoxide and aliphatic amines. Significant correlations between calculated molecular properties of the complexes and the ionization potentials of the amines have been found, and these are discussed on the basis ofMulliken's charge transfer theory. Similarities and differencies between the lithium bond and the hydrogen bond are discussed.

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CNDO/2 Untersuchungen der Lithium-Bindung in Lithiummethoxid—Amin-Systemen

Zusammenfassung Es wurde für das Modellsystem CH₃OLi–NR ₃ eine systematische CNDO/2 Studie durchgeführt um die Lithium-Bindung zwischen Lithiummethoxid und einer Reihe von aliphatischen Aminen zu untersuchen. Es wurde eine signifikante Korrelation zwischen den berechneten Eigenschaften der Komplexe und den Ionisierungspotentialen der Amine gefunden; das wird auf Basis der Elektronen-Donor-Acceptor Theorie nachMulliken diskutiert. Lithium-und Wasserstoff-Brücken-Bindung werden gegenübergestellt und Analogien bzw. Unterschiede herausgearbeitet.

     

叠氮化氰的光解离机理

采用多参考态方法,在MRCI+Q//CAS(10,9)/6-311+G(2df)水平上对叠氮化氰(N3CN)的光解离机理进行理论研究.优化得到基态(S0)和低激发态(S1、S2、T1)势能面上的极小点、过渡态、内转换交叉点(IC-S1/S0)和隙间窜跃交叉点(ISC-S1/T1)的结构和能量,构建反应势能面.在MRCI+Q//CAS(10,9)水平上计算N3CN的垂直激发能,并和实验值进行对比.结果表明,在S0、S1、S2和T1态势能面上,N—N键断裂生成N2+NCN是主要解离途径,而C—N键断裂通道是次要通道.实验观测到220 nm处的吸收峰对应分子由S0态到S1态的激发,对应主要光解离产物为NCN[a1△g];而在275 nm处的吸收峰则对应分子被激发到T1态,然后直接生成基态产物NCN[X3Σg-].我们的理论结果与实验测量符合得很好.     

Complete TDA and RPA Calculations on the Electronic Transitions of Fullerene‐C60 in the CNDO/S and INDO/S Approximations*

Complete single‐excitation mixing calculations on the electronic transitions of the icosahedral C₆₀ molecule have been carried out with the Tamm–Dancoff approximation (TDA) and random‐phase approximation (RPA) schemes in the CNDO/S and INDO/S approximations. The complete space of 14,400 (1p–1h) pairs is partitioned into subspaces classified according to the irreducible representations of the Ih group. For this purpose, matrix representations of the group generators are obtained on a fixed set of basis functions and are used to construct the projection operators. Degenerate molecular orbitals in each energy level are symmetry‐adapted to these projection operators. Degenerate (1p–1h) pairs or singly excited configuration wave functions are similarly symmetrized. In addition, the Clebsch–Gordan coefficients are obtained and listed in an Appendix. The TDA and RPA equations are then solved for each irreducible representation separately. Both schemes with the projection operators and with the Clebsch–Gordan coefficients gave the same results as expected, indicating that the calculations were correctly done. The transition energies from the ground state 1¹A_g to low‐lying singlet and triplet excited states and the oscillator strengths for the allowed transitions (n¹T_1u–1¹A_g) are given in tables. A proper way to normalize is discussed for the eigenvectors of the RPA‐type matrix equation. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

The most stable CNDO/2 water dimer

The most stable CNDO/2 water dimer is a planarC _2h species with a partial O-O bond. STO-3G calculations indicate that the stability of this structure is due to an artifact of the CNDO/2 method.     

Calculation of transition metal compounds using an extension of the CNDO formalism

An extended CNDO formalism for the treatment of large transition metal cluster systems is presented. After a detailed discussion of parametrization it is applied to a family of compounds, namely to Co(CO) ₄ ^– , Co₂(CO)₈, Co₄(CO)₁₂, Mn₂(CO)₁₀.The results can be interpreted in the light of simple electron counting rules and additionally allow detailed insight in bonding capabilities of large metal cluster systems.Bridged and unbridged clusters are compared and the results are extrapolated to surface systems. In the case of Co₄(CO)₁₂ two possible stereoisomers of symmetryC _3v andT _d are discussed.     

Supermolecular CNDO/S calculation of parameters for extended hubbard hamiltonian of TCNQ dimers

Based on the CNDO/S-Cl method, the parameters of the extended Hubbard hamiltonian for unpaired electrons on a TCNQ perfect sandwich dimer were determined in wide range of intermolecular separations. Using the exact solution of the Hubbard hamiltonian the low-lying energy states of the isolated (2TCNQ)^2? dimer were calculated. Necessity of inclusion of doubly excited supermolecular states in the CNDO/S-CI energy spectra calculations on (2TCNQ)^2? dimers was emphasized.     

Mass spectrometry of heterocyclic compounds. XV—The mechanism of the fragmentation under electron impact of 3-p-NH2-phenyl-5-phenyl-1,2,4-oxadiazole. A CNDO/2 treatment

Molecular orbital calculations using the semi-empirical CNDO/2 method were carried out on the molecular ion of 3-p-NH₂-phenyl-5-phenyl-1,2,4-oxadiazole, whose structure had been determined by X-ray diffraction. The calculated diatomic interaction energy values are consistent with the mechanistic proposals made previously in terms of the quasi-equilibrium theory concerning the cleavage of the heterocycle ring after electron impact.     

Une extension de la méthode CNDO/2 á l'étude des complexes d'éléments de transition

An extension of the CNDO/2 formalism, similar to the method of Clack, Hush and Yandle but with a different parametrisation, is proposed for investigating the electronic structure of transition metal complexes. The results for Ni(CO)₄ and Ni(PF₃)₄ agree well with those ofab initio calculations and with some experimental data. Such a semi-empirical approach may be performed, usingab initio results as reference data, in order to interpret the physical and chemical behaviour of largeseries of complexes in their ground state.

     

Vibrational energy storage in high pressure mixtures of diatomic molecules

CO/N₂, CO/Ar/O₂, and CO/N₂/O₂ gas mixtures are optically pumped using a continuous wave CO laser. Carbon monoxide molecules absorb the laser radiation and transfer energy to nitrogen and oxygen by vibration–vibration energy exchange. Infrared emission and spontaneous Raman spectroscopy are used for diagnostics of optically pumped gases. The experiments demonstrate that strong vibrational disequilibrium can be sustained in diatomic gas mixtures at pressures up to 1 atm, with only a few Watts laser power available. At these conditions, measured first level vibrational temperatures of diatomic species are in the range T_V=1900–2300 K for N₂, T_V=2600–3800 K for CO, and T_V=2200–2800 K for O₂. The translational–rotational temperature of the gases does not exceed T=700 K. Line-of-sight averaged CO vibrational level populations up to v=40 are inferred from infrared emission spectra. Vibrational level populations of CO (v=0–8), N₂ (v=0–4), and O₂ (v=0–8) near the axis of the focused CO laser beam are inferred from the Raman spectra of these species. The results demonstrate a possibility of sustaining stable nonequilibrium plasmas in atmospheric pressure air seeded with a few percent of carbon monoxide. The obtained experimental data are compared with modeling calculations that incorporate both major processes of molecular energy transfer and diffusion of vibrationally excited species across the spatially nonuniform excitation region, showing reasonably good agreement.     

Calcul théorique des énergies de divers arrangements géométriques de la molécule (η5-C5H5)2TiCl2

The molecular energy of the complex (η⁵-C₅H₅)₂TiCl₂ has been calculated for experimental geometry and for several hypothetical forms by a semi-empirical CNDO/2 approach. The energy difference between experimental (quasi-tetrahedral) geometry and a planar geometry is ca. 20 kcal mol^?1. This difference is sufficiently high to explain the difficulty of the inversion process.     

Quantenchemische modellrechnungen zur ionischen polymerisation mit hilfe der EHT- und CNDO/2-Methode

Both the EHT and the CNDO/2 methods were applied to the calculation of simple models of the active centres in ionic polymerization. For the models of active anionic centres CH₃^?Mt^? and CH₃O^?Mt⁺ (Mt = Li, Na, MgX), only the results obtained by CNDO/2 permitted conclusions on comparative reactivities; the EHT method failed.Both methods gave results of good agreement for the models of active cationic centres of the type R⁺MtX_{n + 3}^? (R = H, CH₃, OH₃; Mt = B, Al; X = F, Cl). The counterion remains in its tetrahedral structure during the interaction with the cation.