Theoretical calculations of magnetic moments of mono-cyclic conjugated hydrocarbons

The magnetic moments of excited states of mono-cyclic non-alternant conjugated hydrocarbons with 3-9 carbon atoms were theoretically calculated using wavefunctions determined on the basis of the LCAO MO method. The predicted value is in good agreement with experiment for the tropylium cation.     

Asymmetric synthesis of optically active tricarbonyliron complexes of 1,3-dienes

Asymmetric coordination of prochiral 1,3-dienes to form optically active (diene)Fe(CO)₃ complexes has been achieved by direct transfer of the Fe(CO)₃ group from chiral enone complexes.     

Theoretical study of a conjugated aromatic molecular wire

In this study, an organic conjugated molecule, 4,4′-[ethane-1,2-diylidenedi(nitrilo)] dibenzenthiol designed and is proposed as a molecular wire. Structural and electronic responses of this aromatic molecular wire to the static electric field with intensities −1.6 × 10⁻² to +1.6 × 10⁻² a.u., are studied using the DFT-B3LYP/6-31G* level of theory. Natural bond orbital atomic charge analysis shows that the imposition of static external electric field induces polarization—localization of charge on the two ends of molecule, especially on considered terminal contact sulfur atoms. The frontier molecular orbitals (MOs) energy levels including the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) and the HOMO–LUMO gap (HLG) values are modified by the static electric field as well. The electric dipole moment and polarizability of the proposed molecular wire under the studied electric field strengths are considerably increased. The current–voltage characteristic curve is estimated for the proposed molecular wire.     

Mndo study of bond orders in some conjugated BI- and tri-cyclic hydrocarbons

MNDO calculations have been carried out on a number of conjugated bicyclic and tricyclic hydrocarbons. For reference purposes some acenes and monocyclic non-benzenoid hydrocarbons are also mentioned. While the influence of 2 or 6 π-electron systems in stabilising, and of 4 π-electron systems in destabilising, structures is readily evident, it is clear that arrays of 10 or more π-electrons have markedly less effect, and the balance between fixation of bonds and delocalisation in many compounds possessing such systems is subtle.     

Theoretical study of hydrogen-bonded formaldehyde complexes

The hydrogen-bonded complexes involving formaldehyde and a series of proton donors of varying strengths, have been investigated at different levels of ab initio MO theory. The structures of the studied complexes were SCF optimized at the 6-31G basis set level. The binding energy was estimated employing basis set superposition correction, zero-point vibrations and MP2 correlation contribution at the different basis set: STO-3G; 6-31G; MP2/6-31G; 6-31G**; MP2/6-31G**; 6-311G(2d, 2p) and MP2/6-311G(2d, 2p). Linear relationships were found of the calculated binding energy with: the calculated shift in the carbonyl stretching frequency, the changes in carbonyl bond length and the optimum value of hydrogen-bond distance; furthermore the calculations confirm a parallel trend between the proton-donor ability and the strength of the hydrogen bond.     

Valency-bond studies of some conjugated hydrocarbons

A valency bond study of naphthalene has been made in which, assuming the molecular symmetry to be D _2h, successively more canonical structures up to the total of 42 have been included. All four chemically differentiable bonds have been independently varied in length to find the minimum molar energy and thus the equilibrium ground state molecular structure. The result is remarkably close to the observed structure and also agrees well with those deduced by other types of calculation.

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Zusammenfassung Im Rahmen der Methode der Valenzstrukturen wurde eine Untersuchung des Naphthalins durchgeführt, wobei als Symmetriegruppe des Moleküls D _2hangenommen wurde. Die Anzahl der einbezogenen Strukturen wurde schrittweise auf die Gesamtzahl von 42 erhöht. Bei der Minimisierung der molekularen Energie zur Bestimmung der Gleichgewichtsstruktur des Grundzustandes wurden auch die Längen aller vier chemisch unterscheidbaren Bindungen variiert. Diese Gleichgewichtsabstände stimmen sowohl mit den Meßwerten als auch mit den Ergebnissen anderer Rechenverfahren gut überein.

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Résumé Une étude du naphtalène a été faite par la méthode de la mésomérie. On a supposé que la symétrie soit D _2het on a compris successivement de plus en plus formules canoniques jusqu'au total de 42. On a fait varier indépendamment les longueurs de toutes les quatre liaisons qu'on peut distinguer chimiquement afin qu'on peut estimer l'énergie moléculaire minimale et ainsi la structure stable de la molécule en état fondamental. Le résultat s'approche remarquablement de la structure observée et s'accorde bien à celles qu'on a déterminées par des autres types de calculs.

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The author gratefully acknowledges helpful discussions with Professor C. A. Coulson (Oxford), Professor D. H. W. Den Boer and Mevrouw P. C. Den Boer (Utrecht) and Dr. E. Clar (Glasgow).     

Theoretical studies of some nonbenzenoid hydrocarbons—IV : Pleiadiene and related compounds

Pleiadiene and other similar compounds have been studied by the semi-empirical SCFMO method of Pariser, Parr and Pople using the core resonance integral value developed by Lo and Whitehead, Dewar et al. and Yamaguchi et al. It has been found that π^*←π transitions predicted by the methods of Lo and Whiteheadand Dewar et al. suitable for the prediction of ground state properties are also in good agreement with experimental results where available and comparable to those predicted by the method of Yamaguchi et al. developed for the prediction of spectral transitions. The resonance stabilization of the molecules 3,4.5,7,8 and 9 have also been studied. It is found that ethylinic linkage across the naphthalene moity in pleiadiene increases the resonance energy of the final compound, in contrast to our previous observation, i.e. ethylinic linkage across the naphthalene moiety reduces the resonance energy of the final compound.     

Theoretical study of main-group metal-borazine sandwich complexes

Using density functional theory within the generalized gradient approximation, we have theoretically studied the formation of neutral metal-aromatic complexes R1-M and R1-M-R2, where M is either neutral lithium, calcium, or gallium and R1 or R2 is benzene or borazine. We first find that calcium atom is an effective mediator for cooperative formation of a sandwich complex with borazine, while others are not. When benzene and borazine are mixed in the presence of calcium, a 1:2:1 mixture of benzene-calcium-benzene, borazine-calcium-benzene, and borazine-calcium-borazine is expected. An "A"-shaped structure is predicted for homo- and heterocomplexes of borazine with partial B-B and B-C bonds, while two rings are planar in the case of homocomplexes of benzene. Our analysis of the electron density distributions in HOMO-1 to LUMO in terms of orbital symmetry in conjunction with analysis of l,m-projected electronic local density of states shows that this correlates with the charge transfer and the interaction of pi states of the rings mediated by empty d-states of Ca, which is ultimately related to the polarity of the B-N bond. We find that there is a large accumulation of electron density on particular atoms upon complex formation, predicting characteristic behavior in electron-transfer reaction and nucleophilic reaction different from those for pure benzene or borazine molecule. The hetero-sandwich complex is of particular interest due to its asymmetrical distribution of excess electrons.     

Alkynylcyclohexadienyl tricarbonyliron complexes: A new directing effect for organoiron-mediated synthesis

The first example of an alkyne-substituted tricarbonyl(η⁵-cyclohexadienyl)iron(1+) complex has been prepared and the ω directing effect of the phenylethynyl substituent has been demonstrated in nucleophile addition reactions. Addition of NC⁻ also occurs at the a position to form an unusual η¹, η³-structure.     

Properties of polythiophene and related conjugated polymers: a density-functional study

Using a parameter-free, density-functional method that has been developed explicitly for the theoretical treatment of infinite, periodic, isolated, helical polymers we study various polymers related to polythiophene. In particular we discuss how the electronic properties of polythiophene are changed when replacing some of the H atoms by CH3 group, by incorporating vinylene bridges into the backbone, or when replacing some or all the CH units of the backbone by N atoms. We observe the weakest effects for the methyl-substitution and the strongest for the N-incorporation. The latter leads to an overall downward shift of all bands, but in contrast to the case for polyacetylene, the unrelaxed compound with N atoms does not have N lone-pair orbitals as the highest occupied ones. Instead these occur at somewhat deeper energies. When comparing the aromatic and quinoid forms we found for the pure compound as well as for the methyl-containing one that the gap closes when passing from the one to the other form which was not found for any of the other materials of the present study. Moreover, the energy of the HOMO was found to depend stronger on the bond-length alternation than the energy of the LUMO, ultimately giving that polarons will induce two asymmetrically placed gap states with the energetically lower one appearing deeper in the gap than the other one.     

Theoretical study of a conjugated reaction-center model of anionic acetylene polymerization

The energy of the ground state, charge distribution, and bond orders were calculated by a MO method in the SCF-CNDO/2 approximation for various configurations of the monomer, dimer, and trimer model of both the associated and dissociated reaction center of an anionic acetylene polymerization. According to the results of these calculations, cooperation of π and σ bonds leads to a charge delocalization which increases with growing chain length and is the reason for kinetic deactivation of the center. Propagation is stopped, probably by a cooperative effect of the kinetic and thermodynamic factors.     

Inter- and intramolecular carbene reactions of diazoketones tethered to tricarbonyliron coordinated acyclic dienes. New tricarbonyliron complexes of cyclohexa-2,4-dienone and cyclopent-2-enone

A series of diazoketones tethered to tricarbonyliron coordinated dienes at the terminal position have been synthesized, and their decomposition reactions as carbene precursors studied. Intermolecular reactions with nucleophilic olefins were observed with Cu(acac)₂ as catalyst, the tricarbonyliron playing the role of an efficient protecting group of the diene fragment. With rhodium^II acetate, intramolecular reactions predominate, with formation of five-membered rings, in the side chain, or less commonly, as a cyclopentenone joined side by side to a rearranged diene ligand. Unusually, due to cationic stabilization by the iron, six-membered rings in the side chain are also formed here. A still different reaction was observed during the thermal decomposition of a diazoketone tethered to a coordinated diene at position 2: the formation of a stable complex of a cyclohexa-2-4-dienone (iron assisted carbene reaction and multiple hydrogen migrations).     

Theoretical study of the thermodynamics and kinetics of hydrogen abstractions from hydrocarbons

Thermochemical and kinetic data were calculated at four cost-effective levels of theory for a set consisting of five hydrogen abstraction reactions between hydrocarbons for which experimental data are available. The selection of a reliable, yet cost-effective method to study this type of reactions for a broad range of applications was done on the basis of comparison with experimental data or with results obtained from computationally demanding high level of theory calculations. For this benchmark study two composite methods (CBS-QB3 and G3B3) and two density functional theory (DFT) methods, MPW1PW91/6-311G(2d,d,p) and BMK/6-311G(2d,d,p), were selected. All four methods succeeded well in describing the thermochemical properties of the five studied hydrogen abstraction reactions. High-level Weizmann-1 (W1) calculations indicated that CBS-QB3 succeeds in predicting the most accurate reaction barrier for the hydrogen abstraction of methane by methyl but tends to underestimate the reaction barriers for reactions where spin contamination is observed in the transition state. Experimental rate coefficients were most accurately predicted with CBS-QB3. Therefore, CBS-QB3 was selected to investigate the influence of both the 1D hindered internal rotor treatment about the forming bond (1D-HR) and tunneling on the rate coefficients for a set of 21 hydrogen abstraction reactions. Three zero curvature tunneling (ZCT) methods were evaluated (Wigner, Skodje & Truhlar, Eckart). As the computationally more demanding centrifugal dominant small curvature semiclassical (CD-SCS) tunneling method did not yield significantly better agreement with experiment compared to the ZCT methods, CD-SCS tunneling contributions were only assessed for the hydrogen abstractions by methyl from methane and ethane. The best agreement with experimental rate coefficients was found when Eckart tunneling and 1D-HR corrections were applied. A mean deviation of a factor 6 on the rate coefficients is found for the complete set of 21 reactions at temperatures ranging from 298 to 1000 K. Tunneling corrections play a critical role in obtaining accurate rate coefficients, especially at lower temperatures, whereas the hindered rotor treatment only improves the agreement with experiment in the high-temperature range.     

Theoretical study of water-pyridine complexes using intermolecular potentials

Intermolecular potential functions have been used to determine the equilibrium structures of the water-pyridine complexes. The dimer and symmetrical 2:1 water pyridine systems have been studied. Three water models, ST2, TIPS2, and EMPWI have been combined with two different Lennard Jones nonbonded parameters and various charge distributions for the pyridine molecule to describe the systems. For the dimer, results show two distinguishable classes of preferential hydration sites, which are specific sites corresponding to hydrogen-bonded dimer and nonspecific sites located near the hydrophobic regions. Calculations performed on hydrogen-bonded symmetrical complexes show that the planar complex is generally less stable than the complex with water molecules perpendicular to the pyridine plane. For these complexes, the major factor that influences the hydrogen-bonded configurations is the choice of the water model. The importance of atomic charge distributions for the solute over the choice of potential parameters is pointed out. Finally, the effective lone pair representation on the aromatic nitrogen atom is shown to improve the hydrogen bond geometry and the stability of the complexes.     

Theoretical study on the structure and property relationship of the cationic conjugated polyelectrolytes

A theoretical study using density functional theory was performed to understand the structure/property relationship of the cationic conjugated polyelectrolytes, poly[9,9-bis-(6′-N,N,N-trimethylammonium) hexyl] fluorene-alt-4,7-(2,1,3-benzothiadiazole)] (PFBT-X, where X = Br). The torsion angle between the fluorene and benzothiadiazole units in the PFBT monomer was found to substantially affect the structural and electronic properties of the cationic PFBT monomer. The changes of geometrical parameter, HOMO and LUMO energy levels, and band gap, as well as the absorption maximum are discussed in terms of the torsion in the PFBT monomer structure. For comparison, its neutral analogue, the monomer of poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) was also studied. The length of conjugation backbone was also examined.     

Gas chromatographic study of donor-acceptor complexes I. Theoretical study

Summary New general equations have been derived to calculate, by gas-liquid chromatography, the equilibrium constants for complexing reactions between a volatile solute and a non-volatile reagent dissolved in a stationary liquid phase. These equations, which are more general than the relationships usually accepted, take into account the differences in molecular sizes between the inert solvent and the complexing reagent and the activity coefficients of the various species in solution. An equation relating the reaction enthalpy, the dissolution enthalpy of the solute in the liquid stationary phase and the composition of this phase, is also given.

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Gas-chromatographische Untersuchung von Donator-Akzeptor-Komplexen. I. Theoretische Untersuchung

Zusammenfassung Es wurden neue Gleichungen entwickelt, mit deren Hilfe durch Daten der Gas-Flüssig-Chromatographie die Gleichgewichtskonstanten bei der Komplexbildung zwischen einem flüchtigen Gelösten und einem nicht flüchtigen Reagens, die in einer flüssigen stationären Phase gelöst sind, berechnet werden können. Diese Gleichungen sind allgemeiner als die üblicheweise verwendeten Beziehungen und berücksichtigen die Unterschiede in den Molekülgrößen zwischen dem inerten Lösungsmittel und dem Komplexbildner-Reagens, sowie die Aktivitäts-Koeffizienten der verschiedenen Spezies in Lösung. Es wird ebenfalls eine Gleichung angegeben, die die Reaktionsenthalpie, die Lösungsenthalphie des Gelösten in der flüssigen stationären Phase und die Zusammensetzung dieser Phase in Beziehung setzt.

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Etude par chromatographie en phase gazeuse des complexes donneur-accepteur. I-Etude théorique

Résumé De nouvelles équations ont été développées pour calculer, par chromatographie gaz-liquide, les constantes d'équilibres de formation de complexes entre un soluté volatil et un réactif non volatil dissous dans la phase fixe. Ces équations sont plus générales que celles, plus simples, habituellement utilisées. Elles tiennent compte des différences de taille des molécules de solvant et d'agent complexant et des coefficients d'activité des espèces en solution. Une relation entre l'enthalpie de la réaction, l'enthalpie de dissolution du soluté dans la phase stationnaire liquide et la composition de cette phase est aussi donnée.