Electronic spectra of organic molecules and their interpretation—V Effect of terminal groups containing multiple bonds on the K-bands of conjugated systems

The investigation of the effect of terminal groups of absorbing (conjugated) systems on the corresponding K-bands is extended to groups containing multiple bonds. A consistent theoretical interpretation of all observations is given based on the consideration of the electronic perturbations in the excited state (due to Heisenberg resonance) and the view that organic molecules are built up of localised bonds and involve inductive electron-shifts only. The empirical comparative analysis of the experimental data shows conclusively that, in contrast to the conclusions arrived at by mathematical methods, the 2000 Å, and not the 1800 Å, band system of benzene corresponds to an excited state of the (approximate) symmetry ¹E_1μ (involving an allowed transition characteristic for all K-bands).     

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 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.     

Aggregation structures of organic conjugated molecules on their optoelectronic properties

The intimate connection between stacking modes and optoelectronic properties of organic conjugated materials has been discussed from the viewpoints of developing microscopic models and further understanding of their functions and potential applications. In particular, three basal dimer configurations (cofacial configuration, staggered configuration, and crossed configuration) and their respective optical (including radiative and non-radiative) and electrical properties are expatiated in detail. Eventually, we put forward the perspective on achieving the promising laser material that features high fluorescence quantum yield and charge mobility.     

Methodology of using expert systems for structure elucidation of organic molecules by their spectra

Major axioms underlying structure elucidation of molecules by their characteristic spectral data are considered. The amount of information obtained by solving the problem on structure elucidation of molecules serves as a basis to evaluate the quality of solution. Conditions for computer-aided solution of problems using expert systems (ES) are analyzed. Reasons for incorrect or zero solutions and for inconsistencies leading to negative results are examined. The role of additional information as a necessary condition for correct solution is discussed. On the basis of this analysis, a general strategy for structure elucidation of molecules by their spectra using ES is suggested. The principal idea is to use the iteration method, gradually increasing the rigidity of constraints. Strategies named ARCHEOLOGIST and SCULPTOR are discussed. Examples are given. Russian Scientific Research Institute of Organic Synthesis. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 3, pp. 548–558, May–June, 1995. Translated by L. Smolina     

Changes in the frequency of theB u-transition in linear conjugated molecules induced by disturbances in the terminal carbon atoms

The theory of the local state spectrum for the Huckel model of heteropolyenes, recently proposed by the authors, is used to examine the possibility that perturbation of the terminal carbon atoms might affect the frequency of theB _u -transition in polyenes of arbitrary length. It is shown for the first time that the position of the Bu band in the spectrum can depend on the extent of the perturbation in a non-monotonic manner. The anomalous behavior of the frequency of the Bu-transition as a function of the heteropolyene parameters is explained by the presence in the -electron spectrum of in-gap local levels. The results obtained permit the prediction of absorption band positions for the lowest dipole-allowed 0–0 electron transitions in neutral and ionic forms of heteropolyenes of the types X- (CH)_2N-X, X+ -(CH)_2N-X^– and X- (CH)_2N- CH₂ and hence to suggest synthetic routes for conjugated molecules and polymeric materials which are optically active in the range required.N. N. Bogolyubov Institute of Theoretical Physics, Ukraine National Academy of Sciences, 14b Metrologichna St., 252143 Kiev, Ukraine. Translated from Teoreticheskiya i Éksperimental'naya Khimiya, Vol. 32, No. 5, 279–286, September–October, 1996. Original article submitted February 20, 1996.     

Deriving the 3D structure of organic molecules from their infrared spectra

The representation of the 3D structure of a molecule by a radial distribution function (RDF) code is described. The use of the RDF code for the simulation of an infrared spectrum by a counterpropagation (CPG) neural network is shown. Furthermore, a CPG network can also be operated in reverse mode: on input of an infrared spectrum an RDF code is obtained for which a 3D structure can be searched in a database. An empirical modelling process is used to refine this 3D structure to obtain a three-dimensional model of the molecular structure that corresponds to the infrared spectrum.     

Effect of conjugated length on the calculated nonlinear second-order optical susceptibilities of some organic molecules

Calculations of the nonlinear second-order optical susceptibilities (β_ijk) for disubstituted conjugated organic molecules are reported. The calculations are performed using the semiempirical INDO /CI method combined with a sum-over-states expression for β_ijk. The calculated results show that the second-order static susceptibility (β₀) varies as the 2.6 power of the length of the conjugated π-electron system. Furthermore, it is found that the amount of substituent charge transfer from ground state to first excited state decreases with inccreasing chain length. A limiting value of β₀ with the length of the conjugated system is discussed.     

Ortho effects in organic molecules on electron impact 13—mechanism of decarboxylation in o-carboxy heteroaromatics

Expulsion of CO₂ from the molecular ions of the 2-(2-carboxyphenyl) derivatives of benzoxazole, benzothiazole and benzimidazole gives rise to the base peaks on electron impact. The proposed mechanism for this fragmentation process, involving the anchimeric assistance of the heteroatom by the initial abstraction of the H˙ from the carboxyl group followed by the expulsion of CO₂ from the molecular ions, is supported by the study of the linked scan spectra.     

Electronic structure and nonlinear optical properties of push–pull conjugated molecules

We present a review of ab initio calculations we recently performed on organic molecules presenting promising quadratic nonlinear optical properties. These molecules constitute so-called push–pull conjugated compounds in which a conjugated segment is capped at one end by an acceptor group and at the other end by a donor group. We foucs on three series of systems: (i) p-amino-p′-nitrodiphenylacetylenes for which “unusual” distorsion patterns have been recently reported; (ii) benzodithiapolyenals, which present among the largest second-order polarizabilities ever measured; and (iii) 2-methylene-2H-pyrrole derivatives. Our results are found to provide a detailed understanding of the avilable experimental data.     

Basis set and electron correlation effects on initial convergence for vibrational nonlinear optical properties of conjugated organic molecules

Using three typical pi-conjugated molecules (1,3,5-hexatriene, 1-formyl-6-hydroxyhexa-1,3,5- triene, and 1,1-diamino-6,6-dinitrohexa-1,3,5-triene) we investigate the level of ab initio theory necessary to produce reliable values for linear and nonlinear optical properties, with emphasis on the vibrational contributions that are known to be important or potentially important. These calculations are made feasible by employing field-induced coordinates in combination with a finite field procedure. For many, but not all, purposes the MP2/6-31+G(d) level is adequate. Based on our results the convergence of the usual perturbation treatment for vibrational anharmonicity was examined. Although this treatment is initially convergent in most circumstances, a problematic situation has been identified.     

Cyclodimerisation reactions of conjugated dienes with aldehydes—I: Formation of tetrahydropyrans and their 300 MHz 1H NMR spectra

The palladium catalysed reaction of butadiene with aldehydes yields 2-substituted 3,6-divinyl-tetrahydropyrans.^1–4 The reaction has been extended to isoprene and myrcene. With formaldehyde only 2,5-substituted pyrans have been isolated. The isomeric tetrahydropyrans obtained were fully characterised by ¹H NMR, and representative 300 MHz spectra are given. Attempts to react aldehydes with 2-methoxybutadiene were unsuccessful.     

查尔酮共轭体系紫外光谱的取代基效应

本文以取代查尔酮为标准系列,提出了一组新的取代基参数αP,并用αP研究了1,5-二取代芳基-1,3-戊二烯-5-酮系列化合物紫外光谱的取代基效应, 并初步探讨了 P与αP之间的关系。     

Photoelectron spectroscopic studies of the adsorption of organic molecules with lone pair orbitals on transition metal surfaces

Ultraviolet and x-ray photoelectron spectroscopy have been employed to investigate the adsorption of methanol, ethanol, diethylether, acetaldehyde, acetone, methyl acetate and methylamine on surfaces of Fe, Ni and Cu. All these molecules adsorb molecularly at low temperatures (≤100 K). Lone pair orbitals of these molecules are stabilized on these metal surfaces (by 0·4–1·0eV) due to molecular chemisorption. The molecules generally undergo transformations as the temperature is raised to 120 K or above. The new species produced seems to depend on the metal surface. Some of the product species identified are methoxy species, formaldehyde and carbon monoxide in the case of methanol and methyl acetate, ethoxy species in the case of ethanol and 2-propanol in the case of acetone. Contribution No. 166 from the Solid State and Structural Chemistry Unit.     

Electronic structure and spectra of fused n/m ring systems with one or two bridgehead nitrogen atoms

Electronic structures and absorption spectra of the quinolizinium cation, all its monoaza-derivatives and the pyridazo(1.2-a)pyridazinium dication, have been computed by using the Pariser-Parr-Pople method, including all singlet mono-excited electronic configurations. The assignment of the observed bands of the quinolizinium cation has been made by comparing its observed transition energies and intensities with those of naphthalene, quinoline and isoquinoline. Discussion is given on reactivity and band interpretation.

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Zusammenfassung Die Elektronenstrukturen und Absorptionsspektren des Quinolizinium Kations und aller seiner Monoazaderivate und des Pyridazo(1,2-a)-pyridazinium Dikations, werden nach der Pariser-Parr-Pople Methode berechnet, wobei alle einzeln angeregten Konfigurationen berücksichtigt werden. Die Banden des Quinolizinium Kations werden durch einen Vergleich der beobachteten Übergangsenergien und Intensitäten mit denen des Naphthalins, Quinolins und Isoquinolins zugeordnet. Reaktivität und Bandeninterpretation werden diskutiert.

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Résumé La structure électronique et les spectres d'absorption du cation quinolizinium, de tous ses monoazadérivés et du dication pyridazo(1,2-a)pyridazinium ont été calculés par la méthode de Pariser-Parr-Pople, avec intéraction de toutes les configurations singulets mono-excitées. L'attribution des bandes d'absorption du cation quinolizinium a été faite en comparant ses énergies de transition et intensités observées avec celles du naphthaléne, quinoléine et isoquinoléine. On a aussi discuté la reactivité et l'interprétation des bandes.