16 Марта 2017
Короткий путь в науку
15 Февраля 2017
Опубликован Каталитический бюллетень
The scientific cooperation of the Boreskov Institute of Catalysis with the catalytic communities from various countries is effected in accordance with various forms of cooperation: conducting joint seminars on catalysis, exchanging the information and appropriate materials, exchanging research fellows, visiting scientific centers, and participating in congresses and symposia on theoretical and applied catalysis.
According to research programs, projects and grants, the fundamentals of catalysis are studied jointly with researchers from various universities, institutions, research laboratories and companies. BIC collaborates fruitfully on a commercial basis with the leading companies from more than 20 countries, sells licenses, know-how and performs research projects according to client requests.
Academician Valentin N. Parmon is the Russian representative in the European Federation of Catalytic Societies (EFCATS), Member of the International Association of the Catalysis Societies (IACS)
A QUANTUM CHEMICAL STUDY OF THE MOLECULAR STRUCTURE OF ACTIVE CENTERS AND GROWTH IN ETHYLENE POLYMERIZATION IN THE CATALYTIC SYSTEM LFeCl2/AlMe3 (L = 2,6-BIS-IMINOPYRIDYL)
I.I. Zakharov, V.A. Zakharov
Kinet. Catal., 45(4) (2004) pp. 508-518.
Density functional theory with hybrid exchangecorrelation functional B3P86 is used to calculate the molecular structures of neutral Fe(II) complexes formed in the LFeCl2/AlMe3 system (L=tridentate bis(imine)pyridyl ligand). A simplified model of the LFeCl2complex is used in calculations, where L is replaced by three NH3 ligands. Parameters of geometric and electronic structures of the complexes (NH3)3FeMe(μ -Me)AlMe3 (I) and (NH3)3FeMe(μ -Me)2AlMe2 (IIA and IIB), which are the structures where the Fe–Me and Fe-μ -Me groups are in one or two perpendicular planes, respectively, were determined. Complexes II, which were earlier identified using 1H NMR spectroscopy, are more stable than complex I. Complex IIB is strongly polarized (the distances r(Fe- μ-Me) and r(Al- μ-Me) are 3.70 and 2.08 Å, respectively) and coordinatively unsaturated due to the transfer of the methyl group from (NH3)3FeMe2 onto AlMe3. It has significant electron density deficit in the coordination sphere of the transition metal [(NH3)3FeMe]Q (Q = +0.80 e). The energetic profile of the reaction of ethylene addition to the Fe–Me bond for the complexes (NH3)3FeMe2, IIA and IIB, was calculated. It was shown that, compared to (NH3)3FeMe2, a drastic decrease in the activation energy of ethylene addition is observed in the case of IIB (from 135 to 66 kJ/mol). The reason for the more efficient activation of the complexes LFeMe2 by a weak Lewis acid (AlMe3) and for the increased reactivity of the metal-alkyl bond in complex IIB compared to the zirconocene complex Cp2ZrMe2 is discussed.
A DFT QUANTUM-CHEMICAL STUDY OF THE STRUCTURE OF PRECURSORS AND ACTIVE SITES OF CATALYST BASED ON 2,6-BIS(IMINO)PYRIDYL Fe(II) COMPLEXES
I.I. Zakharov, V.A. Zakharov
Macromol. Theory Simul., 13(7) (2004) pp. 583-591.
A DFT method has been applied for quantumchemical calculations of the molecular structure of charge-neutral complex LFeMe(μMe)2AlMe2 which is formed in system LFeMe2+AlMe3 (L=2,6-bis(imino)pyridyl). Calculations suggested the formation of highly polarized complex LFeMe(μMe)2AlMe2 (II) in system LFeMe2+AlMe3, characterized by r(Fe⎯μMe) = 3.70 Å and r(Al-μMe) = 2.08 Å and deficient electron density on fragment [LFeMe]Q (Q = +0.80 e). Polarization of the complex progresses with the bounding of two AlMe3 molecules (complex LFeMe(μMe)2AlMe2 2AlMe3 (III)) and with replacement of AlMe3 by MeAlCl2 (complex LFeMe(μMe)2AlCl2 (IV)). The activation energy of ethylene insertion into the FeMe bond of these complexes has been calculated. It was found that the heat of π-complex formation increases with increasing of polarization extent in the order II < III < IV. Activation energy of the insertion of coordinated ethylene into Fe-Me bond decreases in the same order: II > III > IV.
QUANTUM–CHEMICAL CALCULATIONS OF THE EFFECT OF CYCLOALIPHATIC GROUPS IN α-DIIMINE AND BIS(IMINO)PYRIDINE ETHYLENE POLYMERIZATION PRECATALYSTS ON THEIR STABILITIES WITH RESPECT TO DEACTIVATION REACTIONS
S.S. Ivanchev, A.V. Yakimansky*, D.G. Rogozin (*Institute of Macromolecular Compounds, St. Petersburg, Russia)
Polymer, 45 (2004) pp. 6453-6459.
For the first time, it is attempted to interpret an experimentally found enhancing effect of cycloaliphatic substituents in aromatic rings of NiII– and PdII-α-diimine and FeII–bis(imino)pyridine ethylene polymerization precatalysts on their catalytic activities at elevated temperatures (60-80°C), using quantum chemical Density Functional Theory calculations of relative stabilities of the complexes with respect to different deactivation processes, including thermal decomposition and one-electron reduction. It was shown that the effect correlates with the calculated higher thermal stabilities of cycloalkylsubstituted FeII-, NiII- and PdII-complexes as compared to the corresponding alkyl-substituted ones. NiII- and PdII-α-diimine complexes with cycloalkyl substituents are shown to be more stable than their alkylsubstituted analogues with respect to both thermal decomposition and one-electron reduction. The averaged difference of the thermal decomposition energies between the complexes with cycloaliphatic substituents on one side and aliphatic ones on the other side is ∼2.3 kcal/mol, corresponding to ∼30 times lower equilibrium constant of the thermal decomposition reaction for the cycloalkyl-containing complexes. For the FeII- and PdII-complexes, the thermal stability correlates with the calculated overlap population of the metal-nitrogen bonds. It was shown that the structure of o-substituents (cycloalkyls vs. alkyls) in the phenyl rings of the ligands does not affect the reaction energies for the transformation reactions of the precatalysts into their corresponding active cationic forms.
COPPER HYDROXIDE CATALYSTS FOR SELECTIVE OXIDATION: QUANTUM CHEMICAL STUDY OF ACTIVE SITES
A.S. Kosheleva, I.V. Yudanov, G.M. Zhidomirov, V.N. Parmon
React. Kinet. Catal. Lett., 82(2) (2004) pp. 347-354.
The Cluster models of the copper hydroxide structure were constructed to investigate the oxidation of organic substrates containing unsaturated C-C bonds in the Cu(OH)2/[Ncy]2[Ocy]2 catalytic system. The B3LYP density functional calculations of the activation barriers for ethylene epoxidation by the mono- and binuclear CuII hydroperoxo complexes corroborated the possibility of non-radical oxygen transfer from the terminal CuOOH hydroperoxo group. The activation barriers presented are relatively high compared to the barriers in the range of 12-15 kcal/mol calculated at the same computational level for TiIV hydroperoxo intermediates and ReVII bisperoxo complexes.
DENSITY FUNCTIONAL STUDY OF Pd NANOPARTICLES WITH SUBSURFACE IMPURITIES OF LIGHT ELEMENT ATOMS
I.V. Yudanov, K.M. Neyman*, N. Rösch* (*Technische Universität Munchen, Germany)
Phys. Chem. Chem. Phys., 6(1) (2004) pp. 116-123.
Atomic H, C, N and O at the surface and in the subsurface region of Pd nanoparticles were studied theoretically using an all-electron scalar relativistic density functional approach. Nanosize metal clusters are modelled by the three-dimensional crystallites Pd79 and Pd116 chosen as octahedral fragments of bulk Pd; these clusters expose (111) and (001) facets. Adsorbed atoms were located at the three-fold hollow sites in the centre of (111) facets. Migration of the atoms from the surface of the cluster Pd79 to the octahedral subsurface (oss) site below was considered. Migration of C from the surface hollow site to the oss position was found to be almost isoenergetic; migration of H is somewhat endothermic (by 0.5 eV). For N and O, a lager endothermicity was calculated. Both H and C species exhibit moderate activation barriers for the diffusion to the oss site. C and O atoms in the tetrahedral subsurface (tss) position of the cluster Pd116 were also studied. For both species, this location is energetically disfavoured, although the endothermic effect of O atom migration to the tss position is ~0.5 eV smaller than to the oss site. Subsurface C impurities were calculated to reduce the adsorption energy of CO molecules at Pd clusters.
DFT CLUSTER MODELING OF MOLECULAR AND DISSOCIATIVE HYDROGEN ADSORPTION ON Zn2+ IONS WITH DISTANT PLACING OF ALUMINUM IN THE FRAMEWORK OF HIGH-SILICA ZEOLITES
A.A. Shubin, G.M. Zhidomirov, V.B. Kazansky*, R.A. van Santen** (*Zelinsky Institute of Organic Chemistry, Moscow, Russia; **Schuit Institute of Catalysis, Eindhoven University of Technology, Eindhoven, The Netherlands)
Catal. Lett., 90(3-4) (2003) pp. 137-142.
The problem of various cationic positions in zeolites with high Si/Al ratio in the framework is discussed. The statistical distribution of Al in the lattice of pentasils makes probable appearance of the structures with distant placing of two Al atoms. Cations, localized at such sites, should be very strong Lewis acids that are highly active in different chemical reactions. An example of such site is considered for two Zn2+ ions stabilized in the zeolite fragment represented by two adjacent 5-membered rings sharing the common edge. DFT calculations of molecular and dissociative hydrogen adsorption by such sites are in agreement with experimental results. Adsorption of dihydrogen by zinc ion at such sites results in unusually large low frequency shift of H-H stretching vibrations indicating essential activation of adsorbed H2 molecule. The calculated path of heterolytic dissociative adsorption of dihydrogen and of the proton migration to the distantly placed basic oxygen of such acid-base pair are in agreement with the previously published DRIFT experimental data.
MODELS OF FINELY DISPERSED MgO AND V2O5 ON SILICA. THEORETICAL ANALYSIS OF OPTICAL PROPERTIES BY TD DFT METHOD
V.I. Avdeev, G.M. Zhidomirov
Res. Chem. Intermed., 30(1) (2004) pp. 41-64.
Calculations of the optical properties of vanadium (V) complexes VOCl3, [VOCl4]1-, [VOCl5]2-, and [VO4]3- ion have been performed in the frames of the density functional theory (DFT). Full spectrum of excited states in the region of 25000-60000 cm-1 has been analyzed by the time-dependent DFT method (TDDFT). Spectroscopy of structure defects – low-coordinated (LC) oxygen ions, and surface point defects – F+ and F sites in MgO has been studied in the cluster approximation. The charge transfer spectra and frequencies of normal vibrations for a number of active site models of finely dispersed oxides MgO and V2O5 on silica have been calculated. Comparison of obtained results with experimental electronic diffuse reflectance spectra and fundamental frequencies confirms a hypothesis about the structure of active centers of finely dispersed oxide V2O5 on silica as monomeric forms, (O=V-On).
MOLECULAR MECHANISM OF ETHYLENE EPOXIDATION ON SILVER: STATE OF THE PROBLEM AND THEORETICAL APPROACHES
G.M. Zhidomirov, V.I. Avdeev, A.I. Boronin
In “Computer Material Science”, Eds. C.R.A. Catlow, E. Kotomin, NATO Sci. Ser. III, IOS Press, Amsterdam, Vol. 187, 2003, pp. 334-358.
A new concept of the oxygen species epoxidizing ethylene on silver is presented. The epoxidizing oxygen is formed on the defects of the partially oxidized metal silver surface. Oxygen saturates the bulk of silver at high temperature (T>500 K) and pressure of the reaction medium, and the whole subsurface layer becomes highly defective. A cluster model of the defect structure surface ASV, including a silver atom vacancy and the subsurface oxygen atoms is considered. Calculations were performed in the framework of DFT approach. It was shown that the subsurface oxygen atoms tend towards self-association and formation of quasi-molecular oxygen structures inside of the vacancy space. Adsorption of the oxygen atom on site ASV also provides stabilization of the surface quasi-molecular (“ozonide”) form, ASV + O → ASd-O. The experimental XPS, UPS, IR, and Raman spectroscopy providing evidence in favor of stabilization of the quasi-molecular oxygen forms on the reactive silver surface is discussed. A theoretical interpretation of the experimental data is based on the proposed model of associative oxygen forms.
QUANTUM-CHEMICAL STUDY OF CnF2n+2 CONFORMERS. STRUCTURE AND IR SPECTRA
L.N. Ignatieva*, A.Yu. Beloliptsev*, S.G. Kozlova**, V.M. Buznik (*Institute of Chemistry, Vladivostok, Russia; **Nikolaev Institute of Inorganic Chemistry, Novosibirsk, Russia)
J. Struct. Chem., 45(4) (2004) pp. 599-609.
Quantum-chemical calculations of the geometrical structure and vibrational spectra of CnF2n+2 oligomers (n = 5–8) in the chain and branched conformations are reported. The lengthening of the chain of CnF2n+2 does not substantially affect the geometrical parameters of the oligomers. In all cases under study, the most optimal structure of the molecule is a zigzag chain with bond lengths R(C-C) = 1.53–1.54 Å and R(C-F) = 1.36–1.34 Å; the chain is rolled into a helix, which makes an angle of 17° with the plane. The IR spectra are sensitive to the structural deficiency of oligomers CnF2n+2 associated with the lateral trifluoromethyl groups formed in the chain; the spectra can be used for revealing defects of this type in the structure of polytetrafluoroethylene (PTFE). The possibility of defects associated with the lateral CF3 groups in the structure of PTFE and its lowtemperature modifications is explained based on the calculated total energies of CnF2n+2.
STRUCTURE OF MODIFIED POLYTETRAFLUOROETHYLENE ACCORDING TO DFT CALCULATIONS AND 19F NMR SPECTROSCOPY
V.M. Buznik, S.P. Gabuda*, S.G. Kozlova*, A.K. Tsvetnikov** (*Nikolaev Institute of Inorganic Chemistry, Novosibirsk, Russia; **Institute of Chemistry, Vladivostok, Russia)
J. Struct. Chem., 44(6) (2003) pp. 1152-1155.
The molecular structure of n-C7F16 and the 19F nuclear magnetic screening tensors are calculated by density functional theory (DFT) methods. The results of calculations are compared with 19F NMR data, and it is shown that fine polytetrafluoroethylene (PTFE) contains the terminal CF3 groups in its structure.
MODELING THE GAS-PHASE REDUCTION OF NITROBENZENE TO NITROSOBENZENE BY IRON MONOXIDE: A DENSITY FUNCTIONAL THEORY STUDY
I.L. Zilberberg, M. Ilchenko*, O. Isayev**, L. Gorb**, J. Leszczynski** (*Institute of Cell Biology and Genetic Engineering, Kiev, Ukraine; **Computational Center for Molecular Structure and Interaction, Department of Chemistry, Jackson State University, Jackson, Mississippi)
J. Phys. Chem. A, 108(22) (2004) pp. 4878-4886.T
he gas-phase selective reduction of nitrobenzene (NB) to nitrosobenzene (NSB) by iron monoxide has been for the first time studied by means of density functional theory (DFT) using both the hybrid and pure exchange-correlation functionals. As shown at both DFT levels, when interacting with NB, the iron center donates an electron into the nitro group to form the NB- anion radical strongly coupled by FeO+. This electron-transfer characteristic of the NB--FeO+ intermediate reveals itself in the S2 operator expectation value that exceeds its eigenvalue of S(S+1) by almost 1.0. Further reaction steps necessary to obtain nitrosobenzene from this intermediate are discussed. One of the possible steps based on the abstraction of oxygen from the nitro group by a ferrous center is considered in detail. This reaction appears to be favorable at the pure DFT level, whereas the hybrid theory predicts small endothermicity for the process.
POSSIBLE MOLECULAR STRUCTURE OF PROMOTED LEWIS ACIDITY SITES IN ZnZSM-5
G.M. Zhidomirov, A.A. Shubin, V.B. Kazansky*, R.A. van Santen** (*Zelinsky Institute of Organic Chemistry, Moscow, Russia; **Schuit Institute of Catalysis, Eindhoven University of Technology, Eindhoven, The Netherlands)
Int. J. Quant. Chem., 100(4) 2004) pp. 489-494.
It is suggested that the abnormal methane IR frequency shift observed in ZnZSM-5 zeolites is caused by adsorptive interaction of CH4 with particularly strong Zn2+ Lewis sites. Various molecular structures of zinc ion species in high-silica zeolites are discussed and it is concluded that increased Lewis acidity of a sites is associated with their decreased structural stability. It is shown that Zn2+ in zeolite cationic position with distantly placed Al ions can produce observed shift of vibrational frequencies for adsorbed methane molecule.
COMPUTATIONAL STUDY OF BENZENE TO PHENOL OXIDATION CATALYZED BY N2O ON IRON EXCHANGED FERRIERITE
N.A. Kachurovskaya*, G.M. Zhidomirov, R.A. van Santen* (*Schuit Institute of Catalysis, Eindhoven University of Technology, Eindhoven, The Netherlands)
J. Phys. Chem. B, 108(19) (2004) pp. 5944-5950.
An Fe(II) ion at an α-cation exchange position of ferrierite, (Fe)α was taken as a model for the active site in nitrous oxide decomposition and in the selective oxidation of phenol with nitrous oxide. The oxygen deposited by decomposition of N2O is commonly referred to as α-oxygen, (O)α. Comparison with the results of cluster model calculations was performed for reaction of benzene to phenol oxidation. Periodic calculations predict the same reaction path for benzene oxidation like cluster model study. Differences in the adsorption modes for both types of calculations were analyzed and discussed.
COMPARATIVE ENERGIES OF Zn(II) CATION LOCALIZATION AS A FUNCTION OF THE DISTANCE BETWEEN TWO FORMING CATION POSITION ALUMINIUM IONS IN HIGH-SILICA ZEOLITES
N.A. Kachurovskaya*, G.M. Zhidomirov, R.A. van Santen* (*Schuit Institute of Catalysis, Eindhoven University of Technology, Eindhoven, The Netherlands)
Res. Chem. Intermed., 30(1) (2004) 99-103.
Periodical calculations of Zn(II) metal cation stabilization in cationic positions with distantly placed aluminium ions has been performed for high-silica ferrierite. It was found that decrease of the stabilization energy at large distances between Al ions (more than 10 Å ) is about of 2 eV in comparison with nearest possible position of two Al ions in the zeolite lattice and weekly depended on following increase of the Al---Al distance. Main changes in stabilization energy occurred within 3 Å interval of these distances. Only for the localizations of both Al ions in one zeolite ring zinc cationic form is more stable than hydrogen form.
CLOSED AND OPEN-SHELL ATOMIC OXYGEN ON SILVER: TWO DISTINCT PATTERNS OF THE O1S BINDING ENERGY AND X-RAY ABSORPTION O K-EDGE SPECTRA AS REVEALED BY DENSITY FUNCTIONAL THEORY
S.Ph. Ruzankin, I.L. Zilberberg, G.M. Zhidomirov
Res. Chem. Intermed., 30(1) (2004) pp. 75–85.
The electronic structure of atomic oxygen adsorbed species is studied by means of the density functional theory in the context of the ethylene epoxidation on the silver surface. The adsorbed oxygen species are modeled by the Ag2O molecule either in its closed (1A1) or open-shell states (3B1 and 1B1 ). In both open-shell states the 1s level appears to be lower than that in 1A1 by about 2 eV. This is apparently a sequence of the separation of electron pair, occupying the π*-type highest occupied molecular orbital (HOMO), decreasing the electron density at the oxygen center. Such variation of the O1s level for closed and open-shell Ag2O states seems to explain the X-ray photoelectron spectroscopy (XPS) data concerning two distinct atomic oxygen species on silver surface having the O1s binding energy of about 528 and 530 eV, called nucleophilic and electrophilic oxygen, respectively. The X-ray absorption O K-edge spectra (XANES) calculated for two types of the Ag2O states by means of multiple-scattered-Xα-based approach appears to be in a qualitative agreement with those experimentally recorded for nucleophilic and electrophilic oxygen.
EXPANSION OF THE UNRESTRICTED DETERMINANT IN THE BASIS OF PAIRED ORBITALS
I.L. Zilberberg, S.Ph. Ruzankin
Chem. Phys. Lett., 394(1-3) (2004) pp. 165-170.
An analysis of spin mixture contained in the unrestricted determinant using Löwdin–Amos-Hall paired orbitals (POs) is developed. The space of POs is divided into subspaces of completely, partially and non-overlapping orbitals. Spin contamination is shown to be associated with the ‘active’ space of 2k partially overlapping POs with k being in practice fairly small number. In the basis of POs the unrestricted determinant is expanded in a linear combination of restricted determinants describing ‘basis’ configurations contained 0,1, . . . , k pairs of spatially separated α and β spins.
UNRESTRICTED DENSITY FUNCTIONAL THEORY OF THE BONDING BETWEEN NO RADICAL AND FERROUS ION
I.L. Zilberberg, S.Ph. Ruzankin, S.E. Malykhin, G.M. Zhidomirov
Chem. Phys. Lett., 394(4-6) (2004) pp. 392–396.
Bonding between NO and the Fe(II) center to form so-called FeNO7 unit has been studied using simplistic Fe(NO)(OH)2 system within the unrestricted density functional theory using hybrid and pure exchange-correlation functionals. To analyze in details the obtained unrestricted solution for ground state with Sz = 3/2 corresponding Kohn–Sham determinant is expanded in the basis of Löwdin–Amos–Hall paired orbitals. This expansion allowed to assign the electron structure of FeNO7 unit as a mixture of charge-transfer configurations Fe+ ··· NO+ and Fe3+ ··· NO-, and the covalent configuration Fe2+ = NO0.
A COMPARATIVE, TWO-DIMENSIONAL 14N ESEEM CHARACTERIZATION OF REDUCED [2Fe-2S] CLUSTERS IN HYPERTHERMOPHILIC ARCHAEAL HIGHAND LOW-POTENTIAL RIESKE-TYPE PROTEINS
S.A. Dikanov*, A.A. Shubin, A. Kounosu**, T. Iwasaki**, R.I. Samoilova*** (*Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, USA; **Department of Biochemistry and Molecular Biology, Nippon Medical School, Tokyo, Japan; ***Institute of Chemical Kinetics and Combustion, Novosibirsk, Russia)
J. Biol. Inorg. Chem., 9(6) (2004) pp. 753-767.
It is reported here the comparative orientationselected ESEEM and HYSCORE studies of the reduced clusters from two hyperthermophilic Riesketype proteins; a high-potential, archaeal Rieske protein called sulredoxin (SDX) from Sulfolobus tokodaii with weak homology to the cytochrome bc-associated Rieske proteins, and a low-potential, archaeal homolog of an oxygenase-associated Rieske-type ferredoxin (ARF) from Sulfolobus solfataricus. 14N ESEEM and HYSCORE spectra of SDX and ARF show well-defined variations, which are primarily determined by changes of quadrupole couplings (up to 50% depending on the selected orientation) of the two coordinated nitrogens. These are due to variations in coordination geometry of the histidine imidazole ligands rather than to variations of hyperfine couplings of these nitrogens, which do not exceed 8–10%. The measured quadrupole couplings and their differences in the two proteins are consistent with those calculated using the reported crystal structures of high- and low-potential Rieske proteins. These results suggest that exploration of quadrupole tensors might provide a more accurate method for characterization of the histidine coordination in different proteins and mutants than hyperfine tensors, and might have potential applications in a wider range of biological systems.
THE INFLUENCE OF TEMPERATURE AND PRESSURE ON HYDROGEN ATOM TRANSFER IN TUNNEL SOLID-PHASE REACTIONS
L.I. Trakhtenberg*, A.A. Fokeev*, V.L. Klochikhin*, S.P. Dolin**, B.N. Plakhutin (*Karpov Institute of Physical Chemistry, Moscow, Russia; **Kurnakov Institute of General and Inorganic Chemistry, Moscow, Russia)
Chem. Phys., 23(6) (2004) pp. 56-68.
Dependencies of the rate constant of a solid-phase tunnel reaction on temperature and pressure are considered. The T-dependence is determined by intermolecular and mild intramolecular vibrations in the reactants that cause oscillations in the permeability of the potential barrier for the tunneling atom. The influence of the external pressure is accounted for by a static decrease in the width and height of the potential barrier. The role of various promoting vibrations and T- and P-dependencies in the “acridine-fluorene” system are estimated using quantum chemical calculations of parameters of the reaction system. Anomalous values are discovered before and after annihilation of the first trace constituent of the UHF function in the “acridine-fluorene” system.