A quantum chemical study of the structure of fluorinated silver acetate(I) monomers and dimers

Quantum chemical calculations of the equilibrium structure, potential energy surface cross-sections along the nonrigid degrees of freedom of (CF₃COOAg)₂, (CHF₂COOAg)₂, (CH₂FCOOAg)₂, and (CH₃COOAg)₂ dimers and the corresponding monomers are presented. The B3LYP method with the augmented cc-pVTZ basis set for C, O, F atoms and the Stuttgart 1997 RSC basis set together with the relativistic effective potential for Ag atoms is used. It is shown that in all dimers the eight-membered ring is a rather rigid planar fragment, and the Ag atoms can be bonded with the bond order of 0.2 in dimers. Almost free rotation of acyclic groups around the C-C bond in (CH₃COOAg)₂ and (CF₃COOAg)₂ dimers transforms into a hindered one in the (CHF₂COOAg)₂ dimer, and further into the existence of syn- and anti-structures divided by a high rotation barrier in the (CH₂FCOOAg)₂ dimer. In monomers, the ratio of the internal rotation barriers is similar. With increasing number of fluorine substitution for hydrogen atoms in dimers the Ag-Ag bond length is found to increase (2.79 Å, 2.81 Å, 2.83 Å, 2.84 Å) and the ring rigidity to simultaneously decrease in the acetate-triflouroacetate series.     

Ab initio calculation of the electronic structure of TCNQ and its ions

Ab initio SCF-LCAO-MO calculations have been performed for TCNQ and its positive and negative ions in various electronic states. A basis set consisting of 412 primitive Gaussian type orbitals contracted to 180 was used in the investigation. The electron density distribution in TCNQ and the negative ions, and the redistribution upon ionization has been illustrated by plotting difference density contour maps. The quinone structure of the neutral TCNQ system undergoes a transformation to a benzenoid structure when electrons are added. Electronic transitions, ionization potentials and electron affinities have also been calculated.     

Hydrogen-bonded charge-transfer complexes of TTF containing a uracil moiety: crystal structures and electronic properties of the hydrogen cyananilate and TCNQ complexes

[structure: see text] A novel TTF-based donor with a uracil moiety, TTF-(1-n-butyluracil-5-yl) (TnbU), was synthesized. Crystal structures of both TnbU and the charge-transfer complex of TnbU-hydrogen cyananilate possess complementary double hydrogen bonds through uracil moieties and pi-stacking dimer structures between TTF skeletons. Furthermore, the TnbU-TCNQ charge-transfer complex shows a high electrical conductivity underlying the partial charge-transfer accompanied by a hydrogen-bonding interaction, which was substantiated in terms of the measurements of the IR, electronic spectra, and conductivity.     

On the electronic structure of the hydrogen-bond: Formamide and its dimers

The CNDO/2 method is applied to hydrogen-bonded systems treated as a single entity. The stabilization energies obtained are reasonable. The electron-displacements along the H-bond show -gain and -Ioss by the proton-donor atom and -loss with -gain by the proton acceptor. A slight overall transfer of charges is observed from one unit to the next.This work was supported by grant n 67-00-532 of the Délégation Générale á la Recherche Scientifique et Technique (Comité de Biologie Moléculaire).     

Electronic spectra of dimers of TMPD,CA and TCNQ

A semi-empirical SCF MO CI calculation in the PPP approximation was performed on the ionic and neutral dimers of TMPD, CA and TCNQ. The dimers were treated as single conjugated systems of π electrons (supermolecule) in a perfect sandwich structure. For the three compounds, the existence of a low lying charge transfer band has been confirmed for ionic but not for neutral dimers.     

First fused perpendicular hybrid tetrathiafulvalene (TTF) dimers: a new strategy in pi-extended and rigidified TTF

[structure: see text] The synthesis, theoretical calculations, and crystallographic and electrochemical properties of fused perpendicular tetrathiafulvalene (TTF) dimers incorporating both a TTF unit and a quinonoid pi-extended TTF are described as a new strategy for obtaining pi-extended, rigidified, and sulfur-rich analogues of TTF.     

Theoretical investigations of the perylene electronic structure: Monomer,dimers, and excimers

The structural and electronic properties of perylene molecule, dimers, and excimers have been computationally studied. The present work represents the first systematic study of perylene molecule and dimer forms by means of long‐range corrected time‐dependent density functional theory (TDDFT) approaches. Initially, the study explores the photophysical properties of the molecular species. Vertical transitions to many excited singlet states have been computed and rationalized with different exchange‐correlation functionals. Differences between excitation energies are discussed and compared to the absorption spectrum of perylene in gas phase and diluted solution. De‐excitation energy from the relaxed geometry of the lowest excited singlet is in good agreement with the experimental fluorescence emission. Optimization of several coplanar forms of the perylene pair prove that, contrary to generalized gradient approximation (GGA) and hybrid exchange‐correlation functionals, corrected TDDFT is able to bind the perylene dimer in the ground state. Excitation energies from different dimer conformers point to dimer formation prior to photoexcitation. The fully relaxed excimer geometry belongs to the perfectly eclipsed conformation with D_2h symmetry. The excimer equilibrium intermolecular distance is shorter than the separation found for the ground state, which is an indication of stronger interchromophore interaction in the excimer state. Excimer de‐excitation energy is in rather good agreement with the excimer band of perylene in concentrated solution. The study also scans the energy profiles of the ground and lowest excited states along several geometrical distortions. The nature of the interactions responsible for the excimer stabilization is explored in terms of excitonic and charge resonance contributions. © 2015 Wiley Periodicals, Inc.     

Fluorescence spectra and structure of the difluoro(dibenzoylmethanato)boron monomers and dimers absorbed on silica gel

It has been shown that difluoro(dibenzoylmethanato)boron ((dbm)BF₂) can be absorbed on silica gel in the form of fluorescent monomers and dimers with the emission properties that change in the presence of vapors of volatile organic compounds, such as ethanol, acetone, toluene, and meta-xylene. Fluorescence quenching was observed for the (dbm)BF₂ monomers and dimers in the case of ethanol and acetone, whereas the formation of fluorescent exciplexes with monomers and enhancement of the dimer fluorescence were observed in the case of toluene and meta-xylene. Results of the quantum-chemical calculations of the structure of the (dbm)BF₂ monomer complex with the matrix and toluene and (dbm)BF₂ dimers with matrix are presented.     

A first-principles density-functional calculation of the electronic and vibrational structure of the key melanin monomers

We report first-principles density-functional calculations for hydroquinone (HQ), indolequinone (IQ), and semiquinone (SQ). These molecules are believed to be the basic building blocks of the eumelanins, a class of biomacromolecules with important biological functions (including photoprotection) and with the potential for certain bioengineering applications. We have used the difference of self-consistent fields method to study the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, Delta(HL). We show that Delta(HL) is similar in IQ and SQ, but approximately twice as large in HQ. This may have important implications for our understanding of the observed broadband optical absorption of the eumelanins. The possibility of using this difference in Delta(HL) to molecularly engineer the electronic properties of eumelanins is discussed. We calculate the infrared and Raman spectra of the three redox forms from first principles. Each of the molecules have significantly different infrared and Raman signatures, and so these spectra could be used in situ to nondestructively identify the monomeric content of macromolecules. It is hoped that this may be a helpful analytical tool in determining the structure of eumelanin macromolecules and hence in helping to determine the structure-property-function relationships that control the behavior of the eumelanins.     

Investigation of electronic structure in chlorine-substituted methanes

The electronic structure, dipole moments, and nuclear magnetic resonance and nuclear quadrupole resonance constants of the CH₄, CCl₄, CHCl₃, CH₂Cl₂, and CH₃Cl molecules were calculated by the Parr-Pariser-Pople (PPP) and Wolfsberg-Helmholz (WH) methods, with self-consistency of the charges on the atoms. Conclusions were reached on the applicability limits of these methods. The calculated values were compared with experimental data.     

Quadricyclanes. Part I: Photoelectron spectra and electronic structure

The photoelectron spectra of quadricyclane ( 3 ) and 3-methylidene quadricyclane ( 4 ) have been measured. The results served as a basis for the elucidation of the electronic structure of these compounds, which agreed with theoretical calculations. It is found that the symmetry of the HOMO of 3 is different when compared to that of its valence isomer norbornadiene ( 1 ). The results also indicate that the structure of the highest occupied orbitals cannot be derived by only considering the Walsh-orbitals of the two three-membered rings. In addition one of the Walsh-components of the four-memebered ring has to be taken into account.     

Investigation of the electronic structure of Ticl4

The electronic absorption spectrum of gaseous TiCl₄ over the range 4000–1100 Å is presented and discussed in light of extensive CNDO-MO calculations. From calculated transition energies and the variation of these energies with respect to small changes in Ti-Cl bond length a tentative assignment of the six-band spectrum is offered.

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Zusammenfassung Das Absorptionsspektrum von gasförmigen TiCl₄ wurde im Bereich 4000–11000 Å untersucht. Die mit der CNDO-MO-Methode berechneten Übergangsenergien und die Variation dieser Energien mit kleinen Änderungen der Ti-Cl-Bindungslänge erlauben eine Zuordnung der sechs Banden des Spektrums.

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Résumé Le spectre d'absorption électronique de TiCl₄ gazeux dans la zone 4000–1100 Å a été obtenu et discuté à la lumière de calculs CNDO MO. A partir des énergies de transition calculées et des variations de ces énergies par rapport à de faibles modifications dans la longueur de la liaison Ti-Cl, on propose une attribution possible des six bandes du spectre.

     

Oligosaccharide Analogues of Polysaccharides. Part 2. Regioselective deprotection of monosaccharide-derived monomers and dimers

The Me₃Si? C(1) bond of the bis-(trimethylsilyl)ethynylated anhydroalditol 2 is selectively cleaved with BuLi to yield 3 / 4 , while AgNO₂/KCN in MeOH cleaves the Me₃Si? C(2′) bond, leading to 5 (Scheme 1). Both Me₃Si groups are removed with NaOH in MeOH (→ 7 ), the (i-Pr)₃Si group is selectively cleaved with HCl in aq. MeOH ( → 6 ); all silyl substituents are removed with Bu₄NF ( → 8 ). Acetolysis transformed 9 into 13 , which was desilylated to 14 , while thiolysis of 9 led to a mixture 11 / 12 . The tetraacetate 14 has also been obtained from 9 via 10 . Oxidative dimerisation of either 3 or 5 , or of a mixture 3 / 5 yields only the homodimers 15 and 16 (Scheme 2); treatment of 16 with AgNO₂/KCN yielded 17 , deprotection proceeding much more slowly than the cleavage of the Me₃Si? C(2′) group of 2 . The iodoalkyne 20 , required for the cross-coupling with 5 according to Cadiot-Chodkiewicz, was prepared by deprotection of 3 / 4 to 18 , methoxymethylation (→ 19 ), and iodination. Cross-coupling yielded mostly 21 , besides the homodimer 22 . Similarly, cross-coupling of 20 and 23 (obtained from 5 ) led to 24 and 22 . The structure of 24 was established by X-ray analysis (Fig.), showing a C(6)–C(5′) distance of 5.2 Å. The conditions for deprotecting 2 were applied to 21 , and led to 25 (AgNO₂/KCN), 26 (aq. NaOH), 27 (Bu₄NF), and 29 (HCl/MeOH; Scheme 3). Attempted deprotection of the propargylic-ether moiety with BuLi, however, failed. The dimer 27 was further deprotected to 28 . Acetolytic (Ac₂O/Me₃SiOTf) debenzylation of the dimer 30 , obtained from 10 , gave 31 (83%) which was deacetylated to 32 (Scheme 4). Cross-coupling of 5 and the bromoalkyne 33 , obtained from 10 , yielded 34 ; again, acetolysis proceeded well, leading to 35 . The cellobiose derivative 38 was prepared from the lactone 36 via 37 . The glycosidic linkage of 38 proved resistant to the conditions of acetolysis, leading to 39 . Acetolysis of the benzylated thiophene 40 (from 30 with Na₂S) yielded the octaacetate 41 , but proceeded in substantially lower yields (50%).     

All-valence-electron band structures of infinite stacked poly(TCNQ) and poly(TTF) chains

We have computed the energy band structures of the inifite poly(TCNQ), poly(TCNQ^2?), poly(TTF) and poly(TTF²⁺) chains using the CNDO/2 and where possible the MINDO/2 crystal orbital approximation schemes. The results show a broad conduction band for poly(TCNQ) and a broad valence band for poly(TTF). The bandwidths within the MINDO/2 CO scheme are found to be smaller by roughly a factor of $\text{1}\text{2}$ as compared with those found within the CNDO/2 CO scheme. Our findings are in agreement with a bandwidth of 0.4–0.5 eV for the conduction band of TCNQ—TTF crystals as found by experiments. A brief discussion is given of the k-dependence of the physically interesting bands which is not always simple.     

Versatility of the electronic structure of compound I in catalase-peroxidases

Catalase-peroxidases (KatGs) are bifunctional heme proteins, belonging to the family of class I peroxidases, that are able to catalyze both catalatic and peroxidatic reactions within a peroxidase-like structure. We investigated the electronic structure of reaction intermediates of the catalytic cycle of KatGs by means of density functional theory (DFT) QM/MM calculations. The outcome was that the ionization state of the KatG-specific covalent adduct (Met264-Tyr238-Trp111) affects the radical character of compound I (Cpd I). Specifically, in the optimized structures, substantial radical character is observed on the proximal Trp330 when Tyr238 is protonated, whereas when Tyr238 is deprotonated the radical localizes on the Met+-Tyr(O-)-Trp adduct. These findings are not affected by protein thermal fluctuations, although details of the spin density distribution are affected by the geometry of the active site. Calculations provide structures in good agreement with the crystal structure of BpKatG Cpd I. They also provide an explanation for the experimental findings of the mobile and catalatic-specific residue Arg426 being 100% in conformation R in the X-ray structure of BpKatG treated with organic peroxides. The role of different Cpd I forms in the catalase and peroxidase reaction pathways is discussed.     

Adsorption of TTF,TCNQ and TTF-TCNQ on Au(111):An in situ ECSTM study

The adsorption and adlayer structures of tetrathiofulvalene (TTF), tetracyanoquinodimethane (TCNQ) and TTF-TCNQ on Au(111) have been systematically investigated by in situ electrochemical scanning tunneling microscopy (ECSTM) and cyclic voltammetry in 0.1 mol?L?1 HClO4. All the three molecules were found to form well-ordered adlayers in the double-layer potential region of Au(111). For TTF and TCNQ adlayers, (6×3) and (4×7) structures have been observed, respectively. A structural transition was observed on...     

Polymerization of complexed monomers. I. Alternating copolymers from cyclopentene and complexed polar monomers

Alternating equimolar copolymers of cyclopentene with acrylonitrile and methyl acrylate were prepared in the presence of ethylaluminum sesquichloride. Varying conditions of monomer ratio, temperature, light, and reaction time were studied. The structures of the polymers and mechanistic implications are discussed.