Electronic structure,geometry, and energetics of carbanion and carbocation of [1.1]ferrocenophane: An INDO study

INDO-SCF calculations with constrained geometry optimization have been performed to determine the bridge geometries in [1.1]ferrocenophane and its carbocation and carbanion to address the question of possible C-H-C hydrogen bonding in the carbanion derivative. In the equilibrium geometry of the carbanion, the endo-hydrogen is bonded to one of the bridge carbon atoms and the calculated distance between the two bridge carbons seems too large to accommodate a stable C-H-C hydrogen bond. The results indicate that the observed proton NMR spectrum of carbanion should be interpreted in terms of rapid proton exchange between two bridge carbon atoms rather than a symmetric hydrogen bond. The ground state charge distributions show that the ionic bridges in both carbanion and carbocation are highly conjugated and most of the ionic charge in both molecules is distributed over the ferrocene ring system. The charge on the iron varies only slightly among the three molecules and the formal oxidation state of iron remains +2. The role of the iron seems to be that of a conduit for charge transfer between ferrocene rings upon conjugation.     

Planar Tetracoordinate Silicon in Organic Molecules As Carbenoid-Type Amphoteric Centers: A Computational Study

Designing and synthesizing a stable compound with a planar tetracoordinate silicon (ptSi) center is a challenging goal for chemists. Here, a series of potential aromatic ptSi compounds composed of four conjugated rings shared by a centrally embedded Si atom are theoretically designed and computationally verified. Both Born–Oppenheimer molecular dynamics (BOMD) simulations and potential energy surface scannings verify the high stability and likely existence of these compounds, particularly Si-16-5555 (SiN₄C₈H₈) with 16 π electrons, under standard ambient temperature and pressure. Notably, the Hückel aromaticity rule, which works well for single rings, is inconsistent with the high stability of Si-16-5555 where the 16 p electrons are spread over four five-membered rings fused together. Bonding analyses show that the strong electron donation from the peripheral 12-membered conjugated ring with 16 π electrons to the vacant central atomic orbital Si 3p_z leads to the stabilization for both the ptSi coordination and planar aromaticity. The partial occupation of Si 3p_z results in the peculiar carbenoid-type behaviors for the amphoteric center. By modulating the electron density on the ring with substituent groups, we can regulate the nucleophilic and electrophilic properties of the central Si.     

Pressure-induced cis to trans isomerization of aromatic polyacetylenes prepared using a Rh complex catalyst: A control of π-conjugation length

This study reports that stereospecific polymerization of aromatic acetylenes, e.g., p-methoxyphenylacetylene (pMOPA) and p-ethoxyphenylacetylene (pEOPA)was successfully performed to give polyacetylene selectively bearing cis-transoid forms in high yield when a Rh complex catalyst, [Rh(norbornadiene)Cl]₂ was used in the presence of triethylamine as the polymerization solvent together with a detailed characterization of the resulting polymers, before and after compression. Compression of these polymers induced a cis-trans isomerization at room temperature under vacuum even in the solid state. Based on data collected before and after compression it is estimated that the trans conjugated length, (CC)_n, produced as a result of the compression is n = 26 for PpMOPA and n = 40 for PpEOPA polymers, respectively. We further found that g values in the ESR spectra of the pristine polymer were shifted to higher magnetic field by compression, indicating that unpaired electrons called solitons are stabilized in the trans conjugation length as mobile electrons, although in the pristine polymers the unpaired electrons are stabilized in the less conjugated chain, showing large g value, suggesting a magnetic interaction between oxygen in the alkoxy group of phenyl moiety and unpaired electrons in the cis form. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 217–223, 1998     

弱碱柔能克刚:动力学去质子官能化反应

碳氢键的去质子官能化反应是碳碳键构建最常用的方法,是一种重要的碳氢键活化方式.近年来,碱催化碳碳键形成反应在含弱酸性碳氢键化合物作为亲核试剂的底物拓展方面取得了重要进展.强碱性试剂或催化剂是实现这些弱酸性碳氢键官能化反应的关键.根据酸碱平衡理论,相对较强的碱才能够对弱酸性碳氢键发生去质子化反应,形成较大浓度的碳负离子中间体,进而发生亲核反应.相对较弱的碱不足以对弱酸性碳氢键进行去质子化反应,然而尽管碳负离子中间体可能浓度很低,但应该仍然存在于反应体系中.如果可以选择性地进行热力学有利的化学转化,碳负离子中间体的浓度将会下降并引起去质子化平衡的重新构建.结合碳负离子中间体不可逆的转化和去质子平衡的重新构建,弱酸性碳氢键就可以在弱碱条件下实现缓慢却持续不断的去质子官能化反应.为区别于强碱条件下、通过热力学稳定碳负离子中间体的传统碳氢键去质子官能化反应,我们将这种在弱碱条件下、通过热力学不利的碳负离子中间体转化和酸碱平衡重新构建实现的弱酸性碳氢键的官能化反应称为动力学去质子官能化反应.本文总结了碳氢键去质子官能化反应研究现状和本研究团队近年来在弱碱条件下的动力学去质子官能化反应研究进展.     

Towards a miniaturized non-radioactive electron emitter with proximity focusing

Most state of the art gas sensor systems based on atmospheric pressure ionization, such ion mobility spectrometers use radioactive beta-sources (e.g. ³H or ⁶³Ni) to provide free electrons with high kinetic energy to initiate a chemical gas phase ionization of the analytes to be detected. Here, we introduce a non-radioactive electron emitter which generates free electrons at atmospheric pressure. Therefore, electrons are generated in a vacuum by field emission and accelerated towards a 300 nm thin 1 mm² silicon nitride membrane separating the vacuum from atmospheric pressure. Electron currents of about a few hundred microamps can be reached. High energetic electrons of about 10 keV can easily penetrate the membrane without significant loss of kinetic energy. The concept of proximity focusing avoids complex electron lenses to focus the electron beam onto the membrane. The used field emitter tips are made of multi-walled carbon nanotubes. Another benefit of our system is that no insulated power supply operating at high voltage is needed, as necessary for thermo emitters. Here, we show a first prototype of a proximity focused electron gun with field emitting carbon nanotubes. The system is coupled to our drift tube ion mobility spectrometer for validation. Ion mobility spectra similar to those of a ³H ionization source were achieved.     

Expressing kinetic‐energy for vibration–rotation motions in general rotating systems of axes and introducing quasirectilinear vibrational coordinates to simplify Hamiltonian forms

To understand how the internal and rotational motions of a polyatomic system depend on which rotating system of axes is selected, we derived the explicit form of the atomic velocities determined by an observer stationed on the general rotating system of axes. Using the derived velocities, we formulated the kinetic energy expression for vibration–rotation motions with respect to the rotating system of axes. From this expression, we clarified covariant metric tensors under zero angular momentum, which have been confused with an erroneous expression even in the professional literature, and the relationship between the kinetic energy expression and the rotating system of axes. Furthermore, to simplify the Hamiltonian form, we introduced quasirectilinear vibrational coordinates to describe the Hamiltonian. The resulting Hamiltonian form is superior to those of the previous studies in that the kinetic and potential energy expressions are simple and the vibrational frequencies are independent of the original internal coordinates used. In fact, we show that its application for three examples is useful. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 22–29, 2001     

Exact relations between the electron density and external potential for systems of interacting and noninteracting electrons

The differential virial theorem (DVT) is an explicit relation between the electron density ρ( r ), the external potential, kinetic energy density tensor, and (for interacting electrons) the pair function. The time‐dependent generalization of this relation also involves the paramagnetic current density. We present a detailed unified derivation of all known variants of the DVT starting from a modified equation of motion for the current density. To emphasize the practical significance of the theorem for noninteracting electrons, we cast it in a form best suited for recovering the Kohn–Sham effective potential v_s( r ) from a given electron density. The resulting expression contains only ρ( r ), v_s( r ), kinetic energy density, and a new orbital‐dependent ingredient containing only occupied Kohn–Sham orbitals. Other possible applications of the theorem are also briefly discussed. © 2012 Wiley Periodicals, Inc.     

Organic polymers with indirect magnetic interaction caused by the symmetry of the elementary units

The magnetic properties of a class of 1-D π-systems having a frontier half-filled band of degenerate MOs localized within the elementary units (EU) were studied theoretically. The localization of the MOs is caused by the symmetry of the π-fragments which form the EUs. The localization and non-overlapping of the MOs results in vanishing of the direct (Hund) and the kinetic exchange interaction between the unpaired electrons. The indirect exchange of the unpaired electrons (spin-polarization exchange) occurs via the delocalized π-electrons in the filled energy bands. An approach allowing the calculation of the indirect exchange interaction is applied for a class of polymers having pyrene, azulene, and s-indacene as fragments of the EUs. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 425–434, 1998     

Reaction of (ethoxycarbonylfuryl)methanephosphonates with diethyl carbonate in presence of sodium foil

Reaction of (ethoxycarbonylfuryl)methanephosphonates with diethyl carbonate in presence of sodium foil is studied. It is shown that if the acidifying group is conjugated with the carbanion center of 2-furylmethanephosphonate, addition of the carbanion to the carbonyl group of diethyl carbonate takes place to give 2-furylacetic acid derivatives in high yield. Sodium salts of these CH-acids are synthesized, isolated, and characterized. Their alkylation with alkyl bromoacetates is carried out. If ethoxycarbonyl group is not conjugated with the carbanion center, conversion of starting phosphonate and yield of adduct sharply decreases. Alkyl (2- and 4-ethoxycarbonylfur-3-yl)methanephosphonates immediately after acylation with diethyl carbonate are reduced with sodium-diethyl carbonate system to form alkyl 1-(2- and 4-ethoxycarbonylfur-3-yl)-ethanephosphonates. Formation of intermediate reduction product, the phosphorylated furylacetic aldehyde is also fixed spectroscopically. Simultaneously with the reduction dealkylation of ester group of starting phosphonates and alkyl 1-(3-furyl)ethanephosphonates takes place leading to the carboxylic acid salts. Alkyl (2-methyl-5-ethoxycarbonylfur-3-yl)methanephosphonate does not take part in condensation. It gives only the products of dealkylation under the action of sodium ethylate forming from diethyl carbonate.     

Synthesis and structures of mono(β-diiminato) copper complexes and their catalytic performances for homo- and copolymerizations of methyl acrylate

A series of mono(β-diiminato) copper complexes bearing different coordinating anions and with different fluorine substitution patterns on the β-diiminato ligands were synthesized. All of the complexes were characterized by physicochemical and spectroscopic methods, and in addition, the structures of two of them were confirmed by X-ray diffraction. The crystal structures revealed a four-coordinate mode around the copper center in a slightly distorted square planar geometry, with the β-diimine and the carboxyl groups coordinated to give six- and four-membered chelate rings. The conjugation of C=C and C=N in the β-diiminato ligands and the resonance between C–O and C=O in the carboxyl groups resulted in extensive delocalization of electrons and the formation of conjugated N–C–C–C–N and O–C–O π-bonds, respectively. When activated by modified methylaluminoxane (MMAO), these complexes can effectively catalyze methacrylate (MA) polymerization with activities up to 54.5 kg/mol Cu h, as well as MA/1-hexene copolymerization with the incorporation of MA over 80 %.     

Formation of conjugated polyene and polyyne structure by KrF excimer laser-induced dehydrochlorination on polyvinylidenechloride film

The surface dehydrochlorination of poly(vinyldenchloride) film was performed to produce a conjugated polyyne and polyene structures by photo-irradiation with a KrF excimer laser at 248 nm in a vacuum chamber. The reaction was confirmed by detection of HCl with a mass spectrometer and by measurement of chlorine content on the film with X-ray photoelectron spectroscopy. UV-visible absorption spectroscopy for the laser-treated film showed the formation of new broad absorption bands in the visible and near IR region, indicating that sequential dehydrochlorination induced the formation of conjugated carbon multiple bonds in the polymer chain. Its conjugation number is estimated to be 30 for a triple bond and 10–25 for a double bond from the peak positions on the Raman spectrum of the film. ESR spectra of laser-irradiated PVDC powder also showed long-lived radicals having a narrow band width (ΔH_pp = 0.15 mT), suggesting that the radicals were delocalized on the conjugated bonds. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. A Polym. Chem. 36: 2483–2487, 1998     

The flying ice cube: Velocity rescaling in molecular dynamics leads to violation of energy equipartition

This article describes an unexpected phenomenon encountered during MD simulations: velocity rescaling using standard protocols can systematically change the proportion of total kinetic energy (KE) found in motions associated with the various degrees of freedom. Under these conditions, the simulation violates the principle of equipartition of energy, which requires a mean kinetic energy of RT/2 in each degree of freedom. A particularly pathological form of this problem occurs if one does not periodically remove the net translation of (and rotation about) the center of mass. In this case, almost all of the kinetic energy is converted into these two kinds of motion, producing a system with almost no kinetic energy associated with the internal degrees of freedom. We call this phenomenon “the flying ice cube.” We present a mathematical analysis of a simple diatomic system with two degrees of freedom, to document the origin of the problem. We then present examples from three kinds of MD simulations, one being an in vacuo simulation on a diatomic system, one involving a low resolution model of DNA in vacuo, and the third using a traditional all-atom DNA model with full solvation, periodic boundary conditions, and the particle mesh Ewald method for treating long-range electrostatics. Finally, we discuss methods for avoiding the problem. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 726–740, 1998     

The Hartree–Fock triplet instability: influence of conformation and substitution

The triplet Hartree–Fock (HF) instability is investigated through the examples of substituted ethylenes and the small linear conjugated systems butadiene, hexatriene, octatetraene. A statistical analysis is performed for several conformations of the latters. The second eigenvalue of the instability matrix appears to be able to discriminate the groups of unsaturated compounds. The HF instability is largely influenced by conjugation and mesomeric effects and thus by the geometry. The number of π electrons is also an important factor. The electronic correlation related to the HF instability is not quantified by the energy difference between the post-HF and HF levels.     

Reactions of carbanions with 2,4‐dinitrofluorobenzene leading to stable heptatrienide moieties

Reactions of NCCH₂COOMe 1a and CH₂(CN)₂ 1b with 2,4‐dinitrofluorobenzene 2 at the presence of Et₃ N result in deeply colored crystalline stable salts 4a,4b with anions that contain a system of conjugated bonds. Similar reaction of 2 with phosphorus‐containing zwitterion 6 bearing ethoxy‐ and cyano‐groups at the carbanion center is the first example of the reaction leading to the formation of P‐zwitterion 9 with a negatively charged heptatriene moiety. This reaction proceeds via a new route of decomposition of the intermediate σ‐complex 7 occurring with formation of ethylfluoroformate. © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:108–115, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20267     

Novel π‐Electron Extension System via Chromophores Self‐Polymerization to Enhance the NLO Efficiency

A new strategy for the self‐polymerization of chromophores is investigated to develop a 2,7‐carbazole‐based nonlinear optical (NLO) conjugated polymer with an increasing conjugation length of chromophores. Elongation of the conjugation‐path length in chromophores has established engineering guidelines to enhance optical nonlinearity. Compared with the traditional synthesis of an NLO polymer, the chromophores should be well‐designed at a limited conjugation spacer, and then incorporated into a polymer matrix. In this research, the π‐conjugation spacer of chromophores extended perpendicularly to the dipole of chromophores during the polymerization process. Furthermore, this study marks the first research of integrating the π‐electrons of chromophores and conjugated polymers. These conjugated backbones promote a bulk‐polarization response, leading to large NLO coefficients.

     

Pyrene‐Containing Twistarene: Twelve Benzene Rings Fused in a Row

Success in obtaining higher‐order twistarenes with precise structures is very important for fundamentally understanding the relationship between the structures and physical properties/optoelectronic applications. In this research, by using the advantages from a retro‐Diels–Alder process (clean reaction) and the cross‐conjugated nature of the pyrene unit, a novel dodeca‐twistarene was prepared for the first time. Its structure, confirmed by single‐crystal XRD analysis, indicates that it possesses a twisted angle (≈30°), and two neighboring molecules in the crystal lattice are perpendicular to each other because of the twisted character and the strong intermolecular CH–π interactions. However, its basic physicochemical properties suggest its instability in air derives from its elevated HOMO energy level, although NICS calculations confirm that the pyrene units contribution poorly to the π conjugation of the overall molecule.     

Theory of the PCP effect and related phenomena

Recent studies of cyclic conjugation in the five-membered ring of acenaphthylene and fluoranthene congeners revealed some peculiar regularities, of which the so-called PCP effect attracted the greatest attention. The PCP effect is a significant increase in the magnitude of cyclic conjugation in the five-membered ring caused by the six-membered rings separated from the five-membered ring by a single carbon–carbon bond. We now present the mathematical formalism of a general theory capable of treating cyclic–conjugation phenomena of this kind, of which the PCP effect is just a special case. Namely we calculate the influence of any cycle present in a polycyclic conjugated molecule on the energy effect of cyclic conjugation of any other cycle.     

Analysis of chemical phenomena by solving constrained hartree-fock equation. I. Method and application to resonance energy in linear polyenes

The authors proposed a general method to analyze chemical phenomena in such static states as resonance and aromaticity, without changing the variational space of the trial wave function. Solution of the Hartree–Fock–Roothaan (HFR ) equation for a molecular system gives the ground-state electronic structure and its energy. One can impose constraint conditions, which have chemical meanings, on the HFR equation (the constrained HF equation). The presented method is to analyze the energy differences between usual and constrained HF solutions. This method has an advantage over conventional methods in that it neither varies the trial wave function nor the geometry. The method was applied to interpretation of resonance energy in unsaturated linear hydrocarbons to show that both vertical and adiabatic resonance energies are produced proportionally to the number of ethylene units and that they originate by the lowering of the kinetic energy of π electrons, namely, resonance releases the kinetic-energy pressure of π electrons. This is consistent with the previous investigations © 1994 John Wiley & Sons, Inc.     

Energy up-conversion in dilute polyfluorene solutions

Poly(9,9-dioctylfluorene) (PFO) shows highly efficient blue emission with photo excitation occurring between 340–400 nm. Here we show that PFO can in dilute solution emit at a wavelength well below that at which it is being exited. This, we propose is related to an energy transfer from conjugated parts of the polymer chain into more localised states which then emit at a lower wavelength. These localised states can be considered as defects in the conjugation of the polymer or as chain ends. These may produce quasi monomer or quasi dimer species within the chain, which will have a HOMO-LUMO gap of higher energy than the conjugated polymer. These then fluoresce at the lower wavelength; essentially causing, by energy transfer, a process of energy up-conversion.      

Effect of Extended π‐Conjugation Structure of Donor–Acceptor Conjugated Copolymers on the Photoelectronic Properties

New donor–acceptor conjugated copolymers based on alkylthienylbenzodithiophene (BDTT) and alkoxynaphthodithiophene (NDT) have been synthesized and compared with their benzo[1,2‐b:4,5‐b′]dithiophene (BDT)‐based analogues to investigate the effect of the extended π conjugation of the polymer main chain on the physicochemical properties of the polymers. A systematic investigation into the optical properties, energy levels, field‐effect transistor characteristics, and photovoltaic characteristics of these polymers was conducted. Both polymers demonstrated enhanced photovoltaic performance and increased hole mobility compared with the BDT‐based analogue. However, the BDTT‐based polymer (with π‐conjugation extension perpendicular to main chain) gave the highest power conversion efficiency of 5.07 % for the single‐junction polymer solar cell, whereas the NDT‐based polymer (with π‐conjugation extension along the main chain) achieved the highest hole mobility of approximately 0.1 cm² V^?1 s^?1 based on the field‐effect transistor; this indicated that extending the π conjugation in different orientations would have a significant influence on the properties of the resulting polymers.