Electronic structure and spectra of organic molecules

The Pariser-Parr-Pople method has been used to calculate the -electronic structures and spectra of 6-fluoro, 6-hydroxy, 6-methoxy, 6-amino, 6-methylamino, 6-dimethylaminopurine and their parent molecule purine. The purine and adenine (6-aminopurine) absorption spectra (singlet-singlet ^* type transitions) in particular are discussed in greater detail. The correlation among the absorption bands of the molecules in question and their ground state properties are also discussed. The theoretical results for singlet-singlet transition energies agree fairly well with the experimental data.

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Zusammenfassung Mit der Methode von Pariser, Parr und Pople werden Struktur und Spektren der -Elektronensysteme von Purin sowie 6-F-, 6-OH-, 6-OCH₃-, 6-NH₂-, 6-NHCH₃- und 6-N(CH₃)₂-Purin berechnet, wobei besonderes Gewicht auf spektrale Übergänge von Purin und Adenin gelegt wurde. Ferner wurde die Korrelation der Purin-Spektren mit den Eigenschaften des Grundzustandes untersucht. Die theoretischen Ergebnisse für Singulettübergänge stimmen ziemlich gut mit den experimentellen Daten überein.

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Résumé A l'aide de la méthode Pariser-Parr-Pople nous avons calculé les structures et les spectres électroniques d'une série de purines (purine, 6-F, 6-OH, 6-OCH₃, 6-NH₂, 6-NHCH₃ et 6-N(CH₃)₂-purine). L'accent a été mis en particulier sur les bandes d'absorption pour la purine et l'adénine. La corrélation entre les spectres d'absorption électronique et l'état fondamental de purines est étudiée. Les résultats théoriques concernant les énergies des transitions singulet-singulet sont en très bon accord avec les données expérimentales.

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Submitted in honour of the seventieth birthday of Professor Aleksander Jabloski.     

Electronic structure and spectra of organic molecules

Calculations of the singlet-singlet ( * type) transitions of a series of 2,6-disubstituted purines are presented. The Pariser-Parr-Pople method is used throughout and the agreement with experiment is good. The effects of the 2-X and 6-X substituents on the purine spectrum are discussed.For Part VIII (SCF MO CI Calculations for Hypoxanthine and Guanine Isomers) see Kwiatkowski, J. S.: Acta physica polon. 36 (1969), in press.     

Electronic structure and spectra of organic molecules

The Pariser-Parr-Pople method is used to predict the singlet-singlet ( ^* type). Electronic transition energies for the anions of hydroxy derivatives of benzene, pyridine and pyrimidine. The core integral for the -O^– substituent was treated as an empirical parameter and its value was chosen to fit results for the phenoxide ion and then applied for the compounds with the -O^– substituents. The results of the calculations for molecules with the -O^– groups are compared with those of the corresponding molecules containing the -OH and -NH₂ substituents as well as with the experimental data. No serious deviations between theoretical and experimental data other than those which could be ascribed to steric hindrances occuring in the molecules were found.

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Zusammenfassung Die Pariser-Parr-Pople-Methode wird benutzt, um die Singlett-Singlett-Übergangsenergien (- ^*-Typ) von Benzol-, Pyridin- und Pyrimidinderivaten zu berechnen, die OH-Gruppen enthalten. Das Rumpfintegral des -O^–-Substituenten wurde wie ein empirischer Parameter behandelt. Sein Wert wurde so gewählt, daß er die Meßergebnisse für das Phenolat-Anion richtig wiedergibt. Die Resultate der Berechnungen für Moleküle mit der -O^–-Gruppe werden mit denjenigen für die entsprechenden Moleküle mit OH- und NH₂-Substituenten als auch mit den experimentellen Daten verglichen. Es werden keine ernsten Abweihungen zwischen theoretischen und experimentellen Daten gefunden, außer solchen, die einer sterischen Hinderung zugeschrieben werden können.

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Résumé La méthode de Pariser, Parr et Pople est utilisée pour prédire les transitions électroniques ( ^*) pour les formes anioniques des derivés hydroxy du benzène, pyridine et pyrimidine. La valeur d'integrale de coeur associée du substituant -O^– a été choisie de façon à reproduire les positions des bandes d'absorption de forme anionique du phenol et après elle a été appliquée pour les composés envisagés. Les resultats des calculs pour l'anions sont mis en comparaison avec les resultats obtenus pour les correspondantes derivés aminés et hydroxy et avec les donnés experimentales. Peu de déviations ont été obtenus, en dehors de ceux que l'on peut attribuer à des encombres steriques.

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Based in part on a M. Sci. thesis by M. Berndt, N. Copernikus University, Toru, 1969 (unpublished, in Polish).     

Electronic spectra of metal cluster molecules

The electronic absorption spectra (UV-visible-NIR) of a range of molecular metal cluster compounds, including new spectra of Pt₃₀₉(phen*)₃₆O₂₈ in solution and Au₅₅(PPh₃)₁₂Cl₆ in the solid state, are discussed and compared with the spectra of colloidal particles of the corresponding metals. We consider frontier orbital separations, the development of interband absorptions, the possible appearance of molecular plasma resonances, and charge-transfer in the solid state.     

Electronic spectra and structure of nitroanilines

Vapour spectra and solution spectra in a variety of solvents were recorded for some D,D′A_rA and A,A′A_rD molecules, where DD′NH₂ and AA′NO₂. Ground state dipole moments of D,D′A_rA molecules were determined in dioxane solution.The molecules in molecules (MIM) molecular orbital method was used to calculate the properties associates with the electronic states. It was found that to the first band in the spectra both charge-transfer and locally excited configurations contribute. The dipole moment in the excited state associated with the first intense electronic transition was evaluated from the solvent shift of the band. A general comparison of theoretical results with experiment reveals a good agreement.     

Electronic structure and absorption spectra of indolizines

The electronic structure and spectra of indolizine and a large number of its aza-derivatives have been calculated by the SCFMO-CI method, taking into account all the singlet monoexcited configurations. The results are in good agreement with the experiment. The assignment of the observed bands is also discussed.

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Zusammenfassung Mit dem SCFMO-CI-Verfahren werden Elektronenstruktur und Spektren des Indolizins und mehrerer seiner Derivate berechnet, wobei alle einzeln angeregten Singulett-Konfigurationen berücksichtigt werden. Die Ergebnisse stehen in ausgezeichneter Übereinstimmung mit dem Experiment. Die Zuordnung der beobachteten Banden wird diskutiert.

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Résumé La structure et les spectres électroniques de l'indolizine et de plusieurs ses aza-dérivés ont été calculés par la méthode SCFMO-CI, en considérant toutes les configurations singulets monoexcitées. Les résultats sont en bon accord avec l'expérience. L'attribution des bandes observées est aussi discutée.

     

Electronic and structural properties of borazine and related molecules

Ab initio SCF MO calculations are reported for benzene, s-triazine, borazine and boroxine. The Laplacian of the charge density and the Mulliken population analysis procedure demonstrate that the delocalization of the π electrons decreases and the polarity of the ring bonds increases substantially as the atoms in the ring become more dissimilar. Several other properties, including distortion of the ring angles, puckering of the ring and nuclear quadrupolar coupling constants, emphasize the different chemical properties within the isoelectronic series.     

Electronic spectra and structure of some phenoxazone-type dyes

Quantum-mechanical calculations of the electronic structure and spectra of a number of phenoxazone dyes are given. The distribution analysis of pi-electron density in the ground and excited states makes clear the origin of the colour of these compounds and gives the possibility of direct assessment of organic reagents in terms of colour contrast and sensitivity of analytical reactions. This is illustrated for a series of dihydroxy derivatives of phenoxazone dyes.     

Fine structure of the X-ray spectra of molecules K-absorption spectra of the CO and N2 molecules

Journal of Structural Chemistry -     

Electronic structure and conductance of large molecules and DNA

Using a new technique based on embedding in a local orbital formalism, the electronic structure and electron transmission properties of long biological molecules are calculated, in particular DNA. The electronic structure is found by adding one structural unit at a time to the molecule, and calculating an embedding potential for adding the next structural unit. At present, an extended Hückel scheme is used for the Hamiltonian. The transmission is also calculated within the embedding scheme, taking the molecule–metal contacts into account. The results for transmission depend greatly on the orbitals to which contact is made, and also on energy. The implications of these calculations for conductance are discussed.     

The electronic structure of 6a-thiothiophthen and related molecules: non-empirical calculations and reassignment of the photoelectron spectra

Non-empirical calculations of the electronic structure of the 2,5-dimethyl derivative of the title compound, and the isosteric molecules (1b–4b; 5a, b) formed by replacement of the 1- and/or 6-sulphur atoms by O or NMe are reported. A reassignment of the photoelectron spectra of the molecules in the light of these calculations, and using intensity changes between the unsubstituted and dimethyl derivatives, and substituent effects, shows substantial differences from those recently reported.