The electronic absorption spectra of some acyl azides molecular orbital treatment

The electronic absorption spectra of benzoyl azide and its derivatives: p-methyl, p-methoxy, p-chloro and p-nitrobenzoyl azide were investigated in different solvents. The observed spectra differ basically from the electronic spectra of aryl azides or alkyl azides. Four intense pi-pi* transitions were observed in the accessible UV region of the spectrum of each of the studied compounds. The contribution of charge transfer configurations to the observed transitions is rather weak. Shift of band maximum with solvent polarity is minute. On the other hand, band intensity is highly dependent on the solvent used. The observed transitions are delocalized rather than localized ones as in the case with aryl and alkyl azides. The attachment of the CO group to the azide group in acyl azides has a significant effect on the electronic structure of the molecule. The arrangements as well as energies of the molecular orbitals are different in acyl azides from those in aryl azides. The first electronic transition in phenyl azide is at 276 nm, whereas that of bezoyle azide is at 251 nm. Ab initio molecular orbital calculations using both RHF/6-311G* and B3LYP/6-31+G* levels were carried out on the ground states of the studied compounds. The wave functions of the excited states were calculated using the CIS and the AM1-CI procedures.     

Electronic structure of the peptide linkage. II. A molecular orbital treatment of the electronic spectra of benzohydroxamic acids

The electronic absorption spectra of some benzohydroxamic acids have been analyzed and interpreted using the results of molecular-orbital calculations. Charge density distributions, dipole moments, and the extent of delocalization of the MOS were used to interpret the observed solvent effects. Comparison between the spectra of benzohydroxamic acids and that of simple amides led to conclusions regarding the structural differences between the two classes of compounds.     

The electronic structure of some heterocycles with bridgehead nitrogen: photoelectron spectra and ab initio molecular orbital calculations

He(I) and He(II) photoelectron spectra are reported for the cycl[3,3,3]azine (1), cycl[3,2,2]azine (2), indolizine (6) and imidazo[1,2-a] pyridine (7), as well as He(I) spectra for related compounds (3–5). Ab initio molecular orbital calculations have been used to assign the spectra of 1, 2, 3, 6 and 7, and to give information about the nature of the π-electron energy levels. The first IP for 1 is singularly low (5.86 eV), and this has been interpreted in terms of occupancy of the 1a₁'' orbital which is normally vacant in related compounds. In the cyclazines, the nitrogen lone pair seems to be split into two π-levels.     

A frontier molecular orbital treatment of fulvene cycloadditions : Molecular orbital calculations and photoelectron spectra of substituted fulvenes

MO calculations have been carried out on substituted fulvenes by several semiempirical methods. The results of these calculations are compared with those by other methods, and with photoelectron spectroscopic data obtained here for several substituted fulvenes. Predictions about the periselectivity ([6+4] or [4+2]) of fulvene cycloadditions with dienes, 1,3-dipoles, and ketenes are made and compared with experimental data, where available.     

Non-empirical molecular orbital theory of the electronic structure of molecular crystals

A non-empirical molecular orbital treatment of molecular crystals, based on SCF perturbation theory and matrix partitioning methods is presented.     

A reassignment of molecular orbital configurations of the electronic states of ScF

Ab initio molecular orbital calculations have been performed on low-lying electronic states of ScF. The calculations suggest a reassignment of the molecular orbital configurations of the lowest lying Φ states, and suggest also that some of the low-lying Π states may not be well described by single configuration wavefunctions.     

Trends in the CNDO/2 molecular orbital study of electronic structure in some neurotransmitter drugs

An investigation of electronic structure in some neurotransmitter drugs has been made using the CNDO/2 semi-empirical molecular orbital method.The electronic structure has been conveniently characterized by the electronic parameters nett atomic population (NAP) and bond index (BI). A variation of these electronic parameters with respect to conformation has been studied and has been found unlikely to exceed 0.1 e in most. cases. Further, the useful extent to which the electronic parameters of some commonly occurring functional groups may be regarded as conformationally invariant has been demonstrated. Also presented are (i) a discussion on the intramolecular close-approach of functional groups — the interaction between terminal —COO^? and ?NH₃⁺ groups of α-ω anaino acids is explicitly considered; (ii) an enquiry into the extension of ‘standard’ (idealized) geometry models in the elucidation of electronic structure.The implication of the results and observations presented here are briefly discussed with reference to classical and quantum structure-activity studies of drug molecules.     

Electronic absorption spectra of some nicotinamides and nicotinic acids. Molecular orbital treatment

The electronic absorption spectra of the position isomers nicotinamide and isonicotinamide, nicotinic acid, and isonicotinic acid were investigated, together with the spectra of thionicotinamide, N-methyl nicotinamide and nicotinic acid N oxide. Apparent differences in the spectra of the position isomers were interpreted in terms of the torsion angle between the planes of the molecule, the height of the barrier to internal rotation, and the results of molecular orbital (MO) calculations. The largest perturbation effect was observed in the case of thionicotinamide whereas the smallest effect was observed in the case of nicotinic acid N oxide. MO calculations have indicated the existence of overlapping transitions. The observed transitions proved to be π-π* transitions, none of the n-π* was observed. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64 : 689–701, 1997     

A semiempirical molecular orbital calculation of borazine and some B-trifluoroborazines

A charge self-consistent semiempirical calculation has been performed on borazine and four fluoroborazines in which all the valence electrons of the molecules studied have been included. The results of the Mulliken population analyses are discussed in terms of the effect of fluorine bonded to boron and/or nitrogen. The correspondence between experiment and calculation for the orbital energies is also discussed.

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Zusammenfassung An Hand der Resultate ladungskonsistenter semi-empirischer Rechnungen für Borazin und vier Fluoroborazine unter Einbeziehung aller Valenzelektronen wird mittels Populationsanalyse die Frage diskutiert, wie sich die Bindung von Fluor an Bor und/oder Stickstoff auswirkt. Außerdem wird auf die Korrespondenz zwischen experimentellen Daten und Orbitalenergien eingegangen.

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Résumé Un calcul semi-empirique avec autocohérence des charges a été effectué sur le borazole et quatre fluoroborazoles avec introduction de tous les électrons de valence. Discussion des résultats de l'analyse de population de Mulliken en termes d'effet du fluor lié au bore et (ou) à l'azote. Discussion de la correspondance entre expérience et théorie pour les énergies orbitales.

     

STOP: A slater-type orbital package for molecular electronic structure determination

A general ab initio package using Slater-type atomic orbitals is presented. This package, called STOP, uses the one-center two-range expansion method to evaluate the multicenter electronic integrals. Thoroughly optimized numerical techniques, in particular, convergence accelerators and suitable Gauss quadratures, are used in the algorithms which provide accurate numerical values for all these integrals. STOP thus provides wavefunctions for general molecular structures at the self-consistent field level for the first time over a Slater-type orbital basis. © 1996 John Wiley & Sons, Inc.     

Predicting the UV-vis spectra of Tetraarylcyclopentadienones: Using DFT molecular orbital energies to model electronic transitions of organic materials

Tetraphenylcyclopentadienone, due to its intrinsically low HOMO-LUMO gap, has been suggested as a valuable repeat unit in conducting polymers for nanoscale electronics. The HOMO and LUMO of tetraphenylcyclopentadienone appear to be associated with the relevant pi orbitals of unsubstituted cyclopentadienone. Using previously developed carbonylative coupling reactions, a series of tetraarylcyclopentadienones was synthesized, accessing a range of substituents not previously available. The UV-vis spectra of these molecules were compared to their calculated wave functions and predicted transitions. A quantitative structure-activity relationship was discovered that may greatly simplify prediction of band gaps for oligomers and polymers built from these tetraarylcyclopentadienones.     

Measurement of the electronic spectra of physisorbed molecular layers

A technique for measuring the electronic spectra of physisorbed molecular layers in the presence of an ambient gas is described. To illustrate the technique, measurements of the UV spectra of dimethylcadmium physisorbed layers have been made for different values of gas pressure and substrate temperatures.     

Molecular orbital theory of the electronic structures of one-dimensional molecular crystals

A non-empirical tight-binding LCAO SCF MO treatment of one-dimensional molecular crystals based on the SCF perturbation theory is presented. The simpler version of this method at the level of the CNDO/2 approximation is also given.     

Simple molecular orbital calculations on the electronic structure of iron-porphyrin complexes

The simple Hückel method was first applied to the electronic structure of the iron-porphyrin complexes by Pullman et al. [12]. In this paper, their work is extended to include (a) the effect of a dipole or a point charge placed at the sixth coordination position, and (b) the effect of a nitrogen atom placed at the fifth coordination position. A set of new parameter values is used, whose estimation is made by directing special attention to their dependence on the charge distribution among the atoms.The resulting charge distribution for ferro-porphyrin seems to be reasonable. The fact that the position of the Soret peak is insensitive to the sixth ligand can be understood from the resulting orbital energy levels.The difficulty of finding a reasonable charge distribution for ferri-porphyrin is discussed.

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Zusammenfassung Die einfache Hückelsche Methode ist auf die Elektronenstruktur der Eisen-Porphyrin-Komplexe zuerst von Pullman et al. [12] angewandt worden. In der folgenden Arbeit wird ihr Verfahren auf (a) die Wirkung eines Dipols oder einer Punktladung an der sechsten Koordinationsstelle und (b) die eines Stickstoffatoms an der fünften erweitert. Ein Satz neuer Parameterwerte wird verwandt, bei deren Bestimmung besonders auf ihre Abhängigkeit von der Ladungsverteilung geachtet wird.Die erhaltene Ladungsverteilung für Ferroporphyrin erscheint vernünftig. Die Unempfindlichkeit der Lage der Soret-Bande gegen den sechsten Liganden ist aus den erhaltenen Energieniveaus zu verstehen.Die Schwierigkeit, eine vernünftige Ladungsverteilung für Ferriporphyrin zu finden, wird diskutiert.

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Résumé La simple méthode de Hückel a été appliquée à la structure électronique des complexes fer-porphyrine pour la première fois par Pullman et al. [12]. Dans l'article suivant, leur travail est étendu afin d'inclure (a) l'effet d'un dipôle ou d'une charge ponctuelle sur la sixième position coordinative, et (b) l'effet d'un atome de nitrogène sur la cinquième position. Un jeu de nouvelles valeurs des paramètres est usé qu'on détermine en tenant compte spécialement de leur dépendance de la distribution des charges atomiques.La distribution de charge obtenue pour la ferroporphyrine semble être raisonnable. Le fait que la position de la bande Soret est insensitive contre le sixième ligand, peut être compris à l'aide des énergies des orbitales calculées.La difficulté de trouver une distribution de charge raisonnable pour la ferriporphyrine est discutée.

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The research reported in this paper was sponsored in part by the King Gustaf VI Adolf's 70-Years Fund for Swedish Culture, Knut and Alice Wallenberg's Foundation, the Swedish Natural Science Research Council, and in part by the Aeronautical Research Laboratory, OAR, through the European Office, Aerospace Research, United States Air Force.

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On leave from Department of Physics, Faculty of Science, University of Tokyo, Tokyo, Japan.     

A theoretical study of electronic spectra of radical cations of some dihydroxynaphthalenes

Electronic transition energies of radical cations of 1.2-, 1.3-, 1.6-, and 1.7?dihydroxynaphthalenes are calculated using an open-shell SCF method with configuration interaction. The results are critically analyzed and a correlation diagram is given that shows the energy-shift and intensity variation in the electronic transitions when moving from one system to another, thus revealing the characteristic behavior of the transitions depending on the positions of the hydroxyl substituents. An interesting relation connecting the electronic spectroscopy with the UV photoelectron spectroscopy is suggested on the basis of which the first ionization potentials (IPS ) of the substituted aromatic systems can be inferred from the calculated energy of the A-type (HOMO → LUMO ) transitions for their radical cations. Furthermore, the predictability of the IP s is found to be considerably increased with the incorporation of “molecular size” in the regression.     

Studies on the electronic absorption spectra of some monoazo derivatives

The electronic absorption spectra of a series of azo dye compounds containing -Cl, -SO3H and -OH groups were recorded in twenty one solvents with different polarities. The solvents were selected to cover a wide range of parameters (refractive index, dielectric constant and hydrogen bonding ability). Solvent-induced effect on the maximum absorption band frequencies of azo dye compounds are described in terms of solute-solvent interactions. Multiple linear regression equation for νmax was performed using polarizability-polarity parameters and Kamlet-Taft solvatochromic parameters. The results help to assign the solvent-solute interactions and the solvatochromic potential of the investigated compounds. It is concluded that the electronic of the substituent and hydrogen bonding acceptor ability of the solvents are the major factors to the interaction in solutions.     

A sequential molecular mechanics/quantum mechanics study of the electronic spectra of amides

We report gas-phase electronic spectra of formamide, N-methyformamide, acetamide, and N-methylacetamide at 300 K calculated using a combination of classical molecular dynamics and time-dependent density functional theory (TDDFT). In comparison to excitation energies computed using the global minima structures, the valence npi* and pi(nb)pi* states show a significant red-shift of 0.1-0.35 eV, while smaller shifts are found for the n3s and pi(nb)3s Rydberg states. In this work, we have identified the physical origin of these shifts arising from variations of the molecular structure. We present simple relationships between key geometrical parameters and spectral shifts. Consequently, electronic spectra can be generated directly from ground-state structures, without additional quantum chemical calculations. The electronic spectrum of formamide in aqueous solution is computed using TDDFT using an explicit solvent model. This provides a quantitative determination of the condensed-phase spectrum. In general, this study shows that temperature effects can change the predicted excitation energies significantly and demonstrates how electronic spectra at elevated temperatures can be computed in a computationally efficient way.