Ab initio SCF and CI study of the electronic spectrum of pyridine N-oxide

Configuration interaction (CI) studies of ground, n *, * * electronically excited states are reported for pyridine N-oxide. The transition energy to the lowest * excited ¹ B ₂ state is calculated at 4.35 eV, compared to the experimental spectrum range of 3.67–4.0 eV. This state lies below the lowest n * excited ¹ A ₂ state calculated at 4.81 eV above the ground state. The only experimentally reported triplet state at 2.92 eV above the ground state is predicted to be the ³ A ₁ (*) state. The calculated energy lies at 3.27 eV. Numerous other high-lying singlet states as well as the triplet states have also been calculated. The intramolecular charge transfer character of the ground and the excited states have been studied in terms of the calculated dipole moment and other physical properties.     

A CNDO CI study of the electronic structure and spectrum of nitrobenzene

The electronic spectrum of nitrobenzene is investigated by means of the modified CNDO method of Del Bene and Jaffé.     

A CNDO CI study of the electronic structure and spectrum of cyclopropane

The electronic states of cyclopropane were investigated using cndo ci calculations. Comparison was made between photoelectron spectra and computed ionization potentials. The structure of the ground state was investigated using localized molecular orbitals.     

A theoretical CNDO CI study of the electronic spectrum and structure of a spiropyran

The electronic structure and spectrum of a model compound of a spiropyran were investigated using an all valence-electrons CNDO CI method. The π → π^* electronic excitations are localized on a given half of the molecule. The photochromic process is discussed on the basis of charge densities and bond orders.     

Ab initio SCF CL study of the electronic spectrum of nitroso-methane

Configuration interaction studies of ground, n_ → π^*, n₊ → π^*, and π → π^* electronically excited states are reported for nitroso-methane in its eclipsed equilibrium geometry. The first (n_ → π^*) and the second (n⁺ → π^*) singlet states are calculated at 2.17 and 7.14 eV. it is shown that a significant delocalization of the nonbonding orbitals on the nitrogen and oxygen is responsible for the large energy gap between these two states. The two lowest triplet states occur at 1.29 and 5.39 eV and are of n_ → π^* and π → π^* origin.     

SCF CI calculation of the electronic spectra of nitroso and nitro benzene

SCF calculation with CI has been found to give a reasonable value for the longest wavelength - ^* transition in nitroso and nitro benzene if a sufficient number of configurations are taken into considerations. The applicability of this method to the case of n- ^* transition is, however, of doubtful validity. For the electron density approximation of chemical reactivity simple Hückel calculation may be good enough if the parameters are carefully chosen.

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Zusammenfassung Eine SCF-CI-Berechnung ergab brauchbare Werte für die langwelligsten - ^*-Übergänge von Nitro- und Nitrosobenzol, wenn genügend Konfigurationen in die Rechnung einbezogen werden. Jedoch ist der Wert der Methode für die Berechnung von n- ^*-Übergängen zweifelhaft. Die Elektronendichte ist nicht wesentlich von der der Hückel-Theorie verschieden, so daß letztere für Fragen der chemischen Reaktivität ausreichend erscheint, wenn die Parameter sorgfältig gewählt werden.

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Résumé Un calcul SCF donne des énergies raisonnables pour les premières transitions - ^* de nitroso- et de nitrobenzène, quand on considère un nombre suffisant de configurations. Mais l'application de ce procédé aux transitions n- ^* est douteuse. Si l'on croisit soigneusement les paramètres, la méthode simple de Hückel semble suffir pour le calcul des densités électroniques dans des problèmes de réactivité chimique.

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Sincere thanks are due to Prof. P. 0. Löwdin, Uppsala, Sweden, for laboratory facilities, where a part of the work was done and to Mr Klaus Appel for Machine Computation on Alwac-III at Uppsala.     

Non-empirical SCF and CI study of the SOH radical

The build-up of triplet—triplet absorption was measured for acridine and phenazine in the gas phase by the picosecond spectroscopy method, and we obt     

SCF MO CI calculations on the electronic spectra of fluorenone and related compounds

The electronic spectra of fluorenone, 9-iminofluorene and 9-ethylidenefluorene are studied by means of the SCF MO CI calculation. It is shown that the lowest singlet-singlet transition of fluorenone located at 380 m. can be assigned to a - ^* transition (¹ B ₂¹ A ₁). The nature and location of the lowest triplet state are also discussed.

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Zusammenfassung Die Elektronenspektren von Fluorenon, 9-Iminofluoren und 9-Äthylidenfluoren werden mit Hilfe der SCF MO CI-Methode studiert. Es wird gezeigt, daß der niedrigste Sigulett-Singulett-Übergang in Fluorenon bei 380 m einem ^*-Übergang (¹ B ₂¹ A ₁) zugeordnet werden kann. Die Natur und Lage des niedrigsten Triplett-Übergangs werden ebenfalls studiert.

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Résumé Les spectres électroniques de la fluorenone, du 9 iminofluorène et du 9-éthylidène fluorène sont étudiés à l'aide de calculs SCF-MO-CI. On montre que la plus basse transition singuletsingulet de la fluorénone, située à 380 m, peut être attribuée à une transition ^* (¹ B ₂¹ A ₁ La nature et la position du plus bas état triplet sont aussi étudiées.

     

A non-empirical LCAO MO SCF investigation of electronic relaxation accompanying core ionization in a series of saturated alkanes

Ab initio LCAO MO ΔSCF binding energies and relaxation energies have been computed at the STO-4.31G level for linear and branched alkanes (up to C₆) and are compared with experimental data. The structural dependence of binding energies and relaxation energies is discussed, and it is concluded that binding enegy shifts in these alkanes (compared with methane) arise mainly from relaxation energy changes.     

An ab initio SCF and CI study of ketene imine

The electronic structures of the ground and excited states of ketene imine (_H^HCCNH) have been studied by ab initio SCF and CI calculations. The nucleophilic nature of the β carbon with respect to nitrogen has been discussed using calculated electrostatic potentials and by calculated energy differences between the parent and protonated species. The electronically excited ¹A″ and ³A″ states are found to be almost degenerate.     

AB initio SCF study of the electronic structure and spectrum of CuF2

MC SCF and contracted CI calculations have been performed for the three ligand-field states of CuF₂ and also for two charge-transfer states. With the most extensive basis set the calculated d-d transition energies, including a Davidson correctior for cluster effects, are 4150 cm^?1 (²11g) and 10560 cm^?1 (²Δg). These calculations were made with 98 basis functions, including of orbitals on Cu and d orbitals on F. To check the charge distribution in the molecule, calculations of the ESR g factors were also made at the SCF and CI levels of approximation. Resulting CI values are g_| = 1.93 (1.91) and g₁ = 2.76 (2.60). with corresponding experimental numbers in parentheses.     

SCF CI calculation of the electronic spectra of cis and trans isomers of stilbene and azo-benzene

SCF CI calculation has been made on cis and trans isomers of stilbene and azo-benzene. It has been shown that the electronic spectra of trans isomer should appear at a shorter wave length than the corresponding cis isomers, although the resonance theory reverses their positions. Experimentally azobenzene agrees with the conclusions of SCF theory while stilbene is better represented by the resonance theory.

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Zusammenfassung Für die cis- und trans-Isomeren des Stilbens und Azobenzols wird eine Berechnung nach der SCF-CI-Methode durchgeführt. Es wird gezeigt, daß die Elektronenspectren der Trans-Isomeren bei einer kürzeren Wellenlänge als diejenigen der cis-Isomeren liegen sollten, obwohl nach der Resonanztheorie eine umgekehrte Lage zu erwarten wäre. Die experimentellen Ergebnisse stehen beim Azobenzol in Einklang mit den Resultaten der SCF-Methode während sie beim Stilben besser im Rahmen der Resonanztheorie zu deuten sind.

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Résumé Nous avons calculé les spectres électroniques des isomères cis et trans du stilbéne et de l'azobenzène. Les spectres des isomères trans devraient apparaître à des longueurs d'onde plus courtes que ceux des cis correspondants. La théorie de résonance donne des résultats inverses. Les spectres des azobenzènes s'accordent aux conclusions de la théorie SCF, tandis que les stilbènes se représentent mieux par la théorie de résonance.

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Sincere thanks are due to Prof. P. O. Löwdin for providing financial assistance and laboratory facilities at Uppsala where a part of the work was done and to Dr. Klaus Appel for machine computation on Alwac III.     

All-valence-electron CI treatment of the electronic spectrum of trans-butadiene

An all-valence-electron CI treatment is reported for the low-lying valence and Rydberg states of butadiene. All singly- and doubly-excited configurations relative to a series of the leading terms in a given CI expansion are taken into account, with resulting secular equation orders of as high as 150 000. The agreement between calculated and experimental transition energies is invariably better than 0.2 eV where comparison is possible, with all low-lying valence triplet and Rydberg singlet excited states being unambiguously assigned. The valence-shell excitation to the 2 ¹A_g species is concluded to correspond to the 7.06 eV band system, while the forbidden singlet—singlet transition reported by McDiarmid is assigned as x₂ → 3s. The possibility of an avoided crossing between Rydberg valence ¹B_u excited states having a determining influence on the appearance of the broad intense V₁—N absorption is also discussed.     

The electronic structure and one-electron properties of BH computed from SCF and CI wave functions

The SCF and CI wave functions for BH, obtained in calculations described in detail elsewhere [2], are compared through their electron distributions and electron moments.Taken in part from a Ph.D. thesis submitted to the University of Toronto in 1971.     

SCF and CI Studies of Hund's rules for the effects of electronic correlation and delocalization. I

In connection with the reinterpretation of Hund's multiplicity rules for molecules, a detailed study has been made of the energy differences in the total energy and its components for the triplet and singlet Π_u states of the hydrogen molecule and the analogous states of the four- and six-membered hydrogen atom rings. For the hydrogen molecule, both SCF and CI studies indicated that the outer electron is considerably more contracted in the triplet than in the singlet state. In both approximations, the energy difference is dominated for all bond distances of chemical and physical significance by the electron-nuclear attraction component and not by the electron repulsion component as predicted by simple first-order perturbation theory. Although the correlation energy for each of the states is of the same magnitude as the energy differences considered here, the difference of the correlation energies is much smaller. It had little effect on the qualitative differences between these states of the hydrogen molecule. For the four- and six-membered rings, SCF studies were made on the lowest singlet and triplet states where one electron was promoted from the σ_g to a Π_u orbital. Even though the coupled electrons were more delocalized in these cases, the electron repulsion became relatively more important. However in all cases, the lower state had the highest electron repulsion energy and lower electron-nuclear attraction. The triplet state continued to have the more contracted outer open-shell orbital.     

A CI non-empirical pseudopotential calculation of the vertical valence excited states of the iodine molecule

The non-empirical atomic pseudopotential proposed by Durand and Barthelat has been used, together with the CIPSI algorithm for large scale CI, to calculate the vertical transition energies of the iodine molecule, in a valence extended (double-zeta + d) basis set. All the valence excited states were considered. The mixing of configurations is very important especially for the Σ⁺_g, Π_g and Π_u symmetries. The experimentally known transition energies are calculated within a 1 eV error, despite the lack of diffuse orbitals and spin-orbit interaction. Some qualitative Mulliken's estimates are discussed. A new ³Σ⁺_g state from the 10 σ_u → 11 σ_u single excitation is predicted in the 9 eV region.     

Limited GSMO CI and ESMO CI study of electronic transitions in FNO

Limited CI calculations of vertical excitation energies and oscillator strengths have been performed in the ground state molecular orbital basis set (GSMO) and the excited state ones (ESMO). The absorption and emission spectrum of FNO is rediscussed on the basis of these calculations. The relaxation energy of excited states and the convergence of the CI expansion in the GSMO and ESMO basis sets is discussed.     

A small gaussian basis set for non-empirical all-electron SCF calculations on iodine compounds

A small gaussian lobe basis is given for MO SCF calculations on iodine compounds within reasonable computing times. The reliability of the calculations is tested by a comparison of the calculated with the experimental X-ray fluorescence spectrum of iodine. The agreement of both, as well as the correspondence of calculated and experimental physical properties of diatomic compounds, indicate the basis to be suitable for a quantum chemical treatment of larger molecules containing this element.