How do electron localization functions describe π-electron delocalization?

Scalar fields provide an intuitive picture of chemical bonding. In particular, the electron localization function (ELF) has proven to be highly valuable in interpreting a broad range of bonding patterns. The discrimination between enhanced or reduced electron (de)localization within cyclic π-conjugated systems remains, however, challenging for ELF. In order to clearly distinguish between the local properties of ten highly and weakly π-(de)localized prototype systems, we compare the ELFs of both the canonical wave functions and electron-localized states (diabatic) with those of two closely related scalar fields: the electron localizability indicator (ELI-D) and the localized orbital locator (LOL). The simplest LOL function distinguishes enhanced from weak π-(de)localization in an insightful and reliable manner. LOL offers the finest contrast between annulenes with 4n/4n + 2 π electrons and their inorganic analogues as well as between hyperconjugated cyclopentadiene derivatives. LOL(π) also gives an appealing and intuitive picture of the π-bond. In contrast, the most popular ELF fails to capture subtle contrasting local electronic properties and suffers from the arbitrariness of the σ/π dissection. The orbital separation of the most recent ELI-D is clear-cut but the interpretations sometime less straightforward in the present context.     

The substituent effect of π-electron delocalization in N-methylamino-nitropyridine derivatives: crystal structure and DFT calculations

Structural Chemistry - The crystal and molecular structures of 3-(N-methylamino)-2-nitropyridine, 5-(N-methylamino)-2-nitropyridine and 2-(N-methylamino)-5-nitropyridine have been characterized by...     

Novel Routes to 1,2-Diphosphete and 1,2,4-Triphospholyl π-Complexes

New specific routes to 1,2-diphosphete-, 1,2,4-triphosphol-, and 1,2,4-triphospholyl-π-complexes are reported, which are based on dichloro-1,2-diphosphetene and 1-stannyl-1,2,4-triphosphol as the heterocyclic educts. Details are given for Sn, Mn, Fe, and Co complexes. ESR-data of paramagnetic 1,2,4-triphospholyl sandwich complexes of Co and Mn proof the absence of degenerate SOMOs in these cases.     

Efficient syntheses of π-electron deficient macrocycles

ABSTRACT

Cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) cyclophane represents an ideal π-electron deficient macrocycles, which demonstrates the extraordinary ability of hosting a variety of π-electron-rich guests in its rigid, preorganised cavity. However, this macrocycle undergoes large ring strain and therefore its preparation is often low yielding. In order to increase its yields, high-dilution conditions and template-directed synthesis are employed, which are rather time-consuming, i.e. a few weeks including template removal. Herein, we attempt to solve these problems by either using click reaction, or decreasing the ring strain by using a flexible linker. Three analogues of CBPQT⁴⁺ ring were synthesised without the need of guest template, each of which contains two bipyridinium units. Two of these three macrocycles are able to recognise π-electron rich guests within their cavities. However, the one obtained via click reaction demonstrates low guest recognition ability.     

Clar's aromatic sextet and π-electron distribution in nanographene

Cut into pieces: The π-electron distribution in nanographene fragments isolated between oxidized graphene areas is investigated using scanning probe microscopy (see picture). The edge-shape-dependent localization and migration of the Clar sextet explains the observed π-state distributions and enables investigation of the electronic properties.     

Formation of a defect-free π-electron system in single β-phase polyfluorene chains

Single-molecule spectroscopy can help to uncover the underlying heterogeneity of conjugated polymers used in organic electronics, revealing the most effective molecules in an ensemble in terms of the transport of charge and excitation energy. We demonstrate that β-phase polyfluorene chains can form a near-perfect π-electron system, whereas conventional polymers exhibit chromophoric localization due to perturbation of the conjugation. Broad-band excitation spectroscopy demonstrates that only one absorbing and emitting unit is present on the polymer chain with an average length of ～500 repeat units, illustrating that the material effectively behaves as a molecular quantum wire with strong electronic coupling throughout the entire system.     

Electronic structure and aromaticity of planar singlet π-electron monocyclic π2 carbenes

Induced -electron ring currents and relative diamagnetic susceptibilities of planar singlet -electron monocyclic ² carbenes are calculated in the coupled variant of perturbation theory in the CNO method. It is shown that the parity theorem holds for charged planar ² and ² carbenes formally generated by the detachment of a proton or a hydride ion from alternant hydrocarbons. This leads, in such carbenes, to the same energies of singlet-singlet and singlet-triplet transitions, electric polarizabilities, conjugation energies, -electron ring currents, diamagnetic susceptibilities and current contributions to chemical shifts, whereas the charge contributions to the latter are of opposite signs.Donbas State Academy of Building and Architecture, 1 Derzhavina St., 339023 Makeevka, Donetsk Region, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 157–162, May–June, 1996. Original article submitted December 5, 1994.     

A Simplified version of the Pariser-Parr-Pople treatment of π-electron systems

The semi-empirical configuration interaction treatment due to Pariser and Parr and to Pople has been modified by reducing the set of electronic interaction integrals to ₁₁ and ₁₂ and by limiting the number of configurations to 10. Hückel MOs are used as basis functions. The dependence of the resonance integral _v on the interatomic distance R _v is simulated by making _v a quadratic function of the Hückel bond orders P _v .

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Zusammenfassung Eine Modifikation des semiempirischen CI-Verfahrens von Pariser, Parr und Pople wurde zur Behandlung von -Systemen herangezogen. Als Vereinfachung wurde der Grad der CI-Matrix auf zehn beschränkt und von den Elektronenwechselwirkungsintegralen nur ₁₁ und ₁₂ berücksichtigt. Hückel-MOs wurden als Basisfunktionen verwendet. Die Abhängigkeit des Resonanzintegrals _v vom interatomaren Abstand R _v wurde durch den Ansatz von _v als quadratische Funktion der Hückel-Bindungsordnung P _v angenähert.

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Résumé Le procédé semi-empirique de l'interaction de configuration selon Pariser, Parr et Pople a été modifié en réduisant l'ensemble des intégrales d'interaction électronique à ₁₁ et limitant le nombre des configurations à 10. Les fonctions de base sont des orbitales moléculaires selon Hueckel. La dépendance entre l'intégrale de résonance _v et la distance interatomique R _v est simulée en posant que _v est une fonction quadratique des ordres de liaisons P _v selon Hückel.

     

Pairing theorem for π-electron carbenes

The pairing theorem is proved for planar carbenes formally generated by removing a proton or hydride ion from alternant hydrocarbons, and having identical structure and multiplicity, opposite charges, and different states (² or ²) of the carbene carbon atom. According to this theorem, the elements of the bond-order/residual-charge matrix between different alternant sets coincide with each other, but those between identical sets have opposite signs; for elements of the spin-density matrix, the relationship is the reverse. For such carbenes, this leads to coincidence of the singlet-singlet and singlet-triplet transition energies, electric polarizabilities, conjugation energies, -electron ring currents, diamagnetic susceptibilities, and current contributions to chemical shifts, whereas the charge contributions to the chemical shifts have opposite signs.Donbass State Academy of Construction and Architecture, 1 Derzhavin Street, Makeevka 339023, Donetsk Oblast, Ukraine. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 32, No. 3, pp. 147–152, May–June, 1996. Original article submitted December 16, 1994.     

On the aromatic character of some ten π-electron systems

The electronic structure of 1,4,5-triaza-cyclohepta-2,6-diene has been studied. The treatment accounts for the failure of detecting, by experiment, a cyclic conjugation in this and related seven membered ring systems.

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Zusammenfassung Die Elektronenstruktur des 1,4,5-triaza-cycloheptadien-2,6 wurde theoretisch untersucht. Das Ergebnis erklärt das Mißlingen von Versuchen, in diesem und verwandten Siebenringsystemen zyklische Konjugation aufzufinden.

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Résumé La structure électronique du 1,4,5-triaza-cycloheptadiène-2,6 a été étudiée. Le résultat explique pourquoi une conjugaison cyclique n'a pas été trouvée expérimentalement ni dans ce composé ni dans d'autres systèmes cycliques semblables à sept membres.

     

Correlation effects and interaction energy of π-electron shells in cumulene and acetylene molecules

By means of Hartree—Fock energy variance calculations it has been shown that the specific electroncorrelation energy in extended cumulene molecules is approximately 0.3 eV/electron and the specific energy of interaction between -electron shells is 0.1 eV/electron.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 6, pp. 514–517, November–December, 1993.     

Remark on the moment expansion of total π-electron energy

Summary The truncated expansion of the function |x| was frequently used to express the total Hückel -electron energy (E) in terms of moments. We now present an identity which connectsE with an infinite series of moments. This series is convergent. Lower and upper bounds forE are obtained, based on the same infinite moment expansion.     

A quantum chemical study of conformational and tautomeric preferences, intramolecular hydrogen bonding and π-electron delocalization on dinitrosomethane; in gas phase and water solution

HF, B3LYP, and MP2 methods with the standard basis set, 6-311++G(d,p), were employed to study various aspects of dinitrosomethane (DNM). These results are compared with the outcomes of G2, G2MP2, G3, and CBS-QB3 methods. In the present study, we first characterized the equilibrium conformations, especially global minima. In general, the nitroso-oxime (NO) tautomers of DNM are stabler than the dioxime and dinitroso ones. Furthermore, it was found that the stablest form of NO tautomer is global minima among the known local minima. Surprisingly, the chelated form of NO tautomer, with O–H···O intramolecular hydrogen bond (IMHB), is less stable than the global minimum. In spite of this instability, we comprehensively studied various aspects of IMHB to evaluate the effect of heteroatom’s (N). The results of open–close and related rotamer models predict that the heteroatoms weaken the hydrogen bond, whereas, the geometric, topologic, and natural bond orbital parameters emphasize on opposite conclusion. The HOMA of aromaticity aromaticity index clearly predicts that the π-electron delocalization of chelated form of NO tautomer is greater than the malonaldehyde. Finally, the solvent effects on the properties of DNM tautomers have been estimated by continuum (PCM, IPCM, and SCIPCM), discrete, and mixed models. Theoretical results clearly show that the potential energy surface of DNM, especially global minima, is strongly affected by the solvent.     

Sharp upper bounds for total π-electron energy of alternant hydrocarbons

The energy E(G) of a graph G is defined as the sum of the absolute values of all the eigenvalues of the adjacency matrix of the graph G. This quantity is used in chemistry to approximate the total π-electron energy of molecules and in particular, in case G is bipartite, alternant hydrocarbons. In this paper, we show that if G = (V ₁, V ₂; E) is a bipartite graph with edges and , then

Construction of oriented π-electron arrays based on two-dimensional supramolecular organizates

Two-dimensional supramolecular organizates are shown to be powerful tools for constructing oriented π-electron arrays, which are applicable to molecular photonics devices and artificial photosynthesis. Complementary hydrogen bonding at the interface of an amphiphilic nucleobase monolayer can form two-dimensional stacking of base-pairs, which is expected to be an electron medium for photoinduced electron transfer as well as double-strand DNAs. DNAs are also immobilized as a monomolecular film of the polyion-complex with an amphiphilic cationic intercalator. A new type of monolayer-forming stilbene amphiphile having a trinitrofluorenylidene group is prepared for designing spatially separated electron donor and acceptor layers.