A combinatorial/geometric analysis of parallelogram-like benzenoids

We recently reported an algorithm to count Kekulé (resonance) structures for convex cyclofusenes using a combinatorial/geometric approach. Previously, we presented an algorithm for counting resonance structures for parallelogram-like benzenoids with holes by counting descending paths using rectangular meshes with holes. In this article, we employ a similar combinatorial/geometric approach to determine algorithms that will facilitate counting of the resonance structures in parallelogram-like benzenoids with no holes.     

A combinatorial/geometric analysis of convex cyclofusene

Previously, we presented an algorithm for counting Kekulé structures for parallelogram-like benzenoids with holes by counting descending paths using rectangular meshes with holes. In this article, we describe an algorithm to count Kekulé structures for convex cyclofusenes using a combinatorial/geometric approach.     

On the Properties of Convex Multilayered Cyclofusene

Cyclofusene is a corona-condensed benzenoid whose graph-theoretic representation consists of hexacycles each having exactly two non-adjacent shared edges. Multilayered cyclofusene is a fused hexacylic system which can be partitioned into successive layers of cyclofusene. In such systems, an edge shared by two consecutive hexacycles in the same layer is termed radial. We present upper bounds for the number of radial π-bonds in a convex k-layered cyclofusene.     

Magnetic properties of Cu(II) bischelate complexes with 3-imidazoline nitroxides. 1. Anab initio analysis of spin density delocalization channels

Spin density redistribution between the paramagnetic centers Cu²⁺, N-O, and N’-O’ of bischelate complexes CuL₂ of the copper ion with enaminoketone 3-imidazoline nitroxides (L) are investigated using an ab initio RHF (restricted Hartree-Fock) approach. The most important channels of unpaired electron delocalization over the systems of π- and Σ-bonds in such complexes are revealed. The conformation dependence of spin density localized on paramagnetic centers and delocalized due to a distortion of the structure of the environment of the Cu²⁺ ion from square planar to tetrahedral is analyzed and explained. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 5, pp. 840–849, September-October, 1997.     

Ab initio study on the mechanism of cycloaddition reaction between H<Subscript>2</Subscript>Si=Si: and formaldehyde

The mechanism of the cycloaddition reaction between singlet H₂Si=Si: and formaldehyde has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it could be predicted that the reaction has three competitive dominant reaction pathways. The reaction rules presented is that the 3p unoccupied orbital of the Si: atom in H₂Si=Si: inserts the π orbital of formaldehyde from the oxygen side, resulting in the formation of an intermediate. Isomerization of the intermediate further generates a four-membered ring silylene (the H₂Si–O in the opposite position). In addition, the [2+2] cycloaddition reaction of the two π-bonds in H₂Si=Si: and formaldehyde also generates another four-membered ring silylene (the H₂Si–O in the syn-position). Because of the unsaturated property of the Si: atom in the two four-membered ring silylenes, the two four-membered ring silylenes could further react with formaldehyde, generating two silicic bis-heterocyclic compounds. Simultaneously, the ring strain of the four-membered ring silylene (the H₂Si–O in the syn-position) makes it isomerize to a twisted four-membered ring product.     

Some answers to frequently asked questions about the distortive tendencies of π-electronic system

The paper reviews briefly the various computational strategies, which have been devised by different groups to probe the symmetrizing vs distortive propensities of the π-bonding species of polyenes. All methods point to the same conclusion that the π-bonding components of benzene, allyl, aromatic annulenes and related species have intrinsic distortive tendencies; these species maintain bond-equalized geometries due to the symmetrizing driving force of the corresponding σ frames. Some frequently asked questions, that deal with the compatibility of the π-distortivity scenario with the greater body of experimental data regarding aromatic stability and π-delocalization, are addressed. Many of these questions are immediately answered, once the notion is accepted that delocalized π-systems possess a duality: their π-component is distortive and at the same time resonance stabilized relative to the localized structure with the same geometry. The notion of distortive π-electronic components of polyenes is shown to find a natural place in the wider context of a unified model of electronic delocalization that is valid for both conjugated π- and σ-electronic systems.     

Theoretical study of the valence π → π* excited states of polyacenes: anthracene and naphthacene

The valence π → π ^* excited states of anthracene and naphthacene are studied with multireference perturbation theory with complete active space self-consistent field reference functions. The predicted spectra provide a consistent assignment of all one- and two-photon spectra and T-T spectra of low-lying valence π → π ^* excited states of anthracene and naphthacene. The present theory predicts the valence π → π ^* excitation energies with an accuracy of 0.15 eV for anthracene and of 0.25 eV or better for naphthacene. The excited states of anthracene and naphthacene are compared with those of benzene and naphthalene studied previously. The present calculations predict that, going from anthracene to naphthacene, there is a symmetry reversal of the two lowest singlet state transitions, but not for the triplet, just as indicated by the experimental data. Some general trends of polyacene excited states are discussed based on the calculated results for benzene to naphthacene. Conclusive results obtained for anthracene and naphthacene can be used as a model for understanding the excited states of larger polyacenes. Received: 22 April 1998 / Accepted: 6 July 1998 / Published online: 28 September 1998     

An e.s.r. study of copper(II) complexes of N-hydroxyalkylsalicylideneimines

X-band e.s.r. spectra of copper(II) complexes of empirical formula Cu(Sal.NCH2CH2O)X (where X= H2O, py or Cl) and [Cu(Sal.NCH2CH2CH2O)]2 (Sal.N= OC6H4CHN) in solid and solution states at room temperature and at 77K are reported. In frozen solution at 77K the spectra show axial symmetry with a dx2−y2 ground state. The in-plane π-bonds show a moderate degree of covalence, while the out-of-plane π-bonds possess ionic character. The orbital populations of these complexes are 70.9, 81.3, 93.3 and 82.4% respectively, indicating a dx2−y2 ground state. This revised version was published online in June 2006 with corrections to the Cover Date.     

Physical factors determining the activation energy of alkyl radical addition to unsaturated compounds

A parabolic model of the transition state is used for the analysis of experimental data (rate constants and activation energies) for reactions of addition of alkyl and phenyl radicals to multiple bonds of unsaturated compounds. The parameters describing the activation energy as a function of the enthalpy of the reactions were calculated from the experimental data. The activation energy depends also on the strength of the forming C−C bond, the presence of π-bonds in the α-position near the attacked C=C bond and the presence of polar groups in the monomer and radical. The empirical dependence of the activation energy of a thermoneutral addition reactionE _e0 on the dissociation energyD _e of the forming C−C bond was obtained:E _e0=(5.95±0.06)·10⁻⁴ D _e ² kJ mol⁻¹, indicating the important role of triplet repulsion in the formation of the transition state of radical addition. The contribution of the polar interaction to the activation energy of addition of polar radicals to polar monomers was calculated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 445–450, March, 1999.     

A quantitative prediction of the electronic spectra of thiocarbonyl chromophores: TD-DFT versus SAC-CI

In this contribution, we set up a SAC-CI methodology to evaluate the n → π* and π → π* vertical transition energies of a series of thiocarbonyl derivatives. We show that Frozen-Core SAC-CI provides accurate vertical excitations energies. Nevertheless, in order to obtain converged results, the R2S2 unliked integrals have to be taken into account in L3-SAC-CI calculations. In addition, we present the comparative performances of three computational procedures, INDO/S, TD-DFT, and SAC-CI, for the calculation of valence excited states energies and it turns out that: (1) no tuning of the exact exchange (α) included in TD-DFT allows to consistently reproduce the SAC-CI results; (2) SAC-CI and TD-B3LYP both evaluate the n → π*, as well as the π → π* transition energies, with a similar accuracy. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Synthesis and photophysical properties of luminescent mononuclear and dinuclear gold(I) complexes with ethynyl-substituted phenanthrolines

A novel asymmetric dinuclear gold(I) complex with 3,6-diethynylphenanthroline, 3,6-bis{(PPh₃)–Au–C≡C}₂-phen, has been synthesized from Au(PPh₃)Cl (PPh₃ = triphenylphosphine) and 3,6-diethynyl-1,10-phenanthroline. The asymmetrical dinuclear gold(I) complex, 3,6-bis{(PPh₃)–Au–C≡C}₂-phen, demonstrated a weak phosphorescence assignable to the metal-perturbed ³ π–π* transition in the long wavelength region compared to an intense emission of the symmetrical dinuclear complex with 3,8-diethynylphenanthroline, 3,8-bis{(PPh₃)–Au–C≡C}₂-phen. A similar tendency of phosphorescent bands for the mononuclear gold(I) complexes with 5-ethynylphenanthroline, 5-{(PPh₃)–Au–C≡C}-phen, and 3-ethynylphenanthroline, 3-{(PPh₃)–Au–C≡C}-phen was observed. The absorption bands assignable to the π–π*(C≡Cphen) transition and phosphorescent emission assignable to the metal-perturbed ³ π–π* transition for these four gold(I) complexes were reasonably consistent with the results calculated by DFT and TD-DFT.     

Sandwich complexes of Si, Ge, Sn, and Pb with cyclopentadienyl type derivatives of corannulene and fullerene C60: stability estimates and molecular and electronic structure prediction by the MNDO/PM3 method

The ability of cyclopentadienyl type derivatives of corannulene C₂₀H₁₀ and fullereneI _h -C₆₀ to form η⁵-π-complexes and the problem of their existence is discussed. MNDO/PM3 calculations of half-sandwich complexes η⁵-π-MC₂₀H₁₅, η⁵-π-MC₂₀H ₁₅ ⁺ , η⁵-π-MC₆₀H₅, η⁵-π-MC₆₀H₅ and sandwich complexes 2η⁵-π-M(C₂₀H₁₅)₂, 2η⁵-π-M(C₂₀H₁₅)₂, 2η⁵-π-M(C₆₀H₅)₂ (M=Si, Ge, Sn, Pb) were performed. For all systems studied, local minima were found on corresponding potential energy surfaces and the heats of formation, geometric parameters, and distributions of effective atomic charges were calculated. Sandwich complexes are most likely to exist with M=Si and Ge. The energy and geometric characteristics of η⁵-π-complexes of corannulene were compared with those of η⁵-π-complexes of fullereneI _h -C₆₀. It was concluded that the results of quantum-chemical calculations of sandwich type corannulene derivatives can be used for simulation of the geometry and electronic structure of analogous complexes of fullereneI _h -C₆₀. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1649–1656, September, 1999.     

Quasi-classical trajectory studies of the stereodynamics of the reaction O + HCl → ClO + H

The stereodynamics of the O + HCl → ClO + H reaction are investigated by quasi-classical trajectory (QCT) method. The calculations are carried out on the ground 1 ¹ A′ potential energy surface (PES). The orientation and alignments of the product rotational angular momentum for the title reaction are reported. The influence of collision energy on the product vector properties is also studied in the present work. Four (2π/σ)(dσ₀₀/dω _t ), (2π/σ)(dσ₂₀/dω _t ), (2π/σ)(dσ₂₂₊/dω _t ), and (2π / σ)(dσ₂₁₋/dω _t ), and have been calculated in the center of mass frame.     

Heptaphyrins: Expanded porphyrins with seven heterocyclic rings

Expanded porphyrins containing seven pyrrole/heterocyclic rings linked in a cyclic fashion are termed heptaphyrins. The number ofπ-electrons in heptaphyrins depends on the number ofmeso carbon bridges used to link the heterocyclic rings, accordingly heptaphyrins with 28π-electrons and 30π-electrons are reported to date. Both condensation reactions of the appropriate precursors and acid-catalysed oxidative coupling reactions have been utilized to synthesise the heptaphyrins. The 30π heptaphyrins exhibit rich structural diversity where some of the heterocyclic rings in the macrocycle undergo a 180° ring flipping. An overview of the synthetic methods employed for the synthesis of heptaphyrins, their spectroscopic properties, structural behaviour and aromatic properties are highlighted in this paper. Dedicated to Professor C N R Rao on his 70th birthday     

Synthesis, cyclic voltammetry, and UV–Vis studies of ferrocene-dithiafulvalenes as anticipated electron-donor materials

Abstract  

New ferrocenyl ketones were obtained as precursors of novel π-conjugated ferrocene-dithiafulvalene (Fc-DTF) and π-extended-ferrocenedithia-fulvalenes (π-exFc-DTF) as electron-donor conducting materials from ferrocene by a direct aroylation process using the Friedel–Crafts reaction. Novel Fc-DTF conjugates were synthesized using the Wittig–Horner reaction and their structures were determined. The redox behavior of the ferrocenyl carbonyl compounds Fc-DTF and Fc-π-exDTF was investigated in comparison to the parent ferrocene by means of cyclic voltammetry. A one-electron redox behavior was observed for carbonylferrocenes as one wave potential, while a two-electron process was observed as two oxidation waves for the conjugates. Introduction of electron-withdrawing groups led to increasing E _pa values and decreasing ΔE _p values. The UV–Vis spectra of some compounds were studied in comparison with ferrocene. The absorption spectra showed a red-shift with a slight increase in the absorption intensities.     

Structure of aluminum(III (μ-OXO)bis [phthalocyaninate]): A quantum chemical study

The quantum chemical (U)B3LYP/6-31G(d ₅,p) method is used to determine the structural parameters and ionization potentials of aluminum(III) (μ-oxo)bis[phthalocyaninate] and monophthalocyaninate PcAlCl. Two (PcAl)₂O structures with similar energies are obtained: an eclipsed high-symmetry (D _4h ) structure with a rectangular AlOAl group and a retarded low-symmetry (C _s ) structure with the AlOAl bond angle of 167.5°. The AlO bonds include a substantial π component. The energy needed for the detachment of two electrons from a free (PcAl)₂O molecule is 13.46 eV.     

A theoretical study of the reactivity of carbon monoxide with cyclobutadiene

GRINDOL molecular orbital calculations have been performed for the cyclobutadiene-carbonmonoxide (CB-CO) complex. Starting from the rectangular structure of CB with the CO molecule along the C₂-axis perpendicular to the ring, geometry optimization leads to a stable C_4u-structure with the square geometry of CB moiety as the distance (CB-CO) decreases. A hill top in the potential energy curve corresponds to transition of CB from rectangular to square geometry. Bader’s population analysis carried out for different optimized geometries clearly reveals that the stability of the CB-CO complex arises due to ππ* back donation from CB to the CO molecule. The cause of the ‘hill top’ in the potential energy curve is also explained in the light of Bader’s population analysis. Results of trapping of products arising from photochemical reactions with CB is also discussed.