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Resonance Theory of Catalytic Action of Transition-Metal Complexes: Isomerization of Quadricyclane to Norbornadiene Catalyzed by Metal Porphyrins
Abstract
The theory of catalytic activity of transition-metal compounds is a fascinating problem especially if a comparison of different catalysts is necessary. The isomerization of quadricyclane (QC) to norbornadiene (NB) catalyzed by transition-metal porphyrins is a challenge and incidentally a suitable benchmark for various theories of catalysis. We analyze this process in detail using a valence bond-like scheme adjusted for the description of reaction centers containing transition-metal atoms. A qualitative explanation of contrasting catalytic behavior of Mn-phthalocyanine and Co-tetraphenylporphyrin is obtained from the analysis of the spectra of local many electron states of free catalysts and their complexes with the reactant/product. This picture is supported by the numerical analysis of potential energy profiles for the QC to NB isomerization in the presence of a catalyst performed in the effective Hamiltonian approximation. This exemplary reaction is put in a more general perspective of theories of catalytic activity of transition-metal complexes and in relation with oxygenation reactions. V C 2013 Wiley Periodicals, Inc.
... Some mechanisms for these catalytic processes promoted by metal complexes have been proposed based on both reactivity data [43,44] and theoretical calculations [24], all highlighting the active participation of the metal centers. According to a mechanism computed for the conversion of a QC to the parent NBD in the presence of a Co(II) phthalocyanines complex, one of the labile C-C bonds of QC gives rise to an oxidative addition to the metal center followed by the formation of a transient carbocation and successive regeneration of the double C=C bonds of NBD [24]. ...
It is urgent yet challenging to develop new environmentally friendly and cost-effective sources of energy. Molecular solar thermal (MOST) systems for energy capture and storage are a promising option. With this in mind, we have prepared a new water-soluble (pH > 6) norbor-nadiene derivative (HNBD1) whose MOST properties are reported here. HNBD1 shows a better matching to the solar spectrum compared to unmodified norbornadiene, with an onset absorbance of λ onset = 364 nm. The corresponding quadricyclane photoisomer (HQC1) is quantitatively generated through the light irradiation of HNBD1. In an alkaline aqueous solution, the MOST system consists of the NBD1 − /QC1 − pair of deprotonated species. QC1 − is very stable toward thermal back-conversion to NBD1 − ; it is absolutely stable at 298 K for three months and shows a marked resistance to temperature increase (half-life t1 /2 = 587 h at 371 K). Yet, it rapidly (t1 /2 = 11 min) releases the stored energy in the presence of the Co(II) porphyrin catalyst Co-TPPC (∆H storage = 65(2) kJ·mol −1). Under the explored conditions, Co-TPPC maintains its catalytic activity for at least 200 turnovers. These results are very promising for the creation of MOST systems that work in water, a very interesting solvent for environmental sustainability, and offer a strong incentive to continue research towards this goal.
... [11,12] These results indicate that a thermally activated pathway is possible, though there are changes in the energy landscape when the molecule is bound in the junction vs. the solution pathway. [30] Finally, we examined the switching process in situ by holding a molecular junction and stepping the bias to 250 mV for 1000 ms (Figure 2 e) at T 0 = 78 K. In this case, the NB-to-QC transition occurred probabilistically with 44.5 % of the junctions switching within the hold time. ...
A single‐molecule memory element is electrically‐controlled using two distinct reaction mechanisms as reported by K. Moth‐Poulsen, J. Hihath, and co‐workers (DOI: 10.1002/anie.202002300). By using separate electrically‐controllable reactions for the forward and reverse reactions the bistable norbornadiene–quadricyclane system can be set in either state. The device can be switched through multiple cycles when a square‐wave voltage signal is applied to the molecule. Each state has a unique conductance value allowing the system to act as a switch or memory device.
... + g state by itself does not imply whatever reactivity since no concrete reaction is specified, as seen from the triplet O 2 ground state. This problem is discussed in our recent publication[52].Downloaded by [Andrei Tchougréeff] at 15:29 16 March 2016 ...
The story of C_2 continues to fascinate chemists spinning around a possibility of quadruple bonds for p-elements and discussing a wealth of options for the nature of an unconventional fourth bond. This led to lively discussions about the ways of counting or measuring bonds, and the interplay between the bond strength (energy), its length, and rigidity/stiffness (elasticity). Even old concerns about the possibility of theorems in chemistry and thus of the place of chemistry among ‘true’ sciences had been revived. We show that under some mild conditions certain exact statements (lemmas and theorems) about the C_2 molecule can be relatively easily proven which, if not resolves the controversy entirely, at least provides some theoretical reference points to the discussion. Some more general consequences from this experience are discussed as well.
The electronic structure of metal–organic frameworks (MOFs) containing transition metal (TM) ions represents a significant and largely unresolved computational challenge due to limited solutions to the quantitative description of low-energy excitations in open d-shells. These excitations underpin the magnetic and sensing properties of TM MOFs, including the observed remarkable spin-crossover phenomenon. We introduce the effective Hamiltonian of crystal field approach to study the d–d spectrum of MOFs containing TM ions; this is a hybrid QM/QM method based on the separation of crystal structure into d- and s,p-subsystems treated at different levels of theory. We test the method on model frameworks, carbodiimides, and hydrocyanamides and a series of M-MOF-74 (M = Fe, Co, Ni) and compare the computational predictions to experimental data on magnetic properties and Mössbauer spectra.
At the flick of a switch: An electrically controllable reaction at the single‐molecule level is demonstrated. The reversible reaction can be electrically controlled to perform switching and memory functions, where the conductance values of two molecular isomers are encoded as 0 and 1. Multiple switching cycles are observed when a voltage signal is applied, and the absence of stochastic switching suggests application as a memory device.
Abstract
The exponential proliferation of data during the information age has required the continuous exploration of novel storage paradigms, materials, and devices with increasing data density. As a step toward the ultimate limits in data density, the development of an electrically controllable single‐molecule memristive element is reported. In this device, digital information is encoded through switching between two isomer states by applying a voltage signal to the molecular junction, and the information is read out by monitoring the electrical conductance of each isomer. The two states are cycled using an electrically controllable local‐heating mechanism for the forward reaction and catalyzed by a single charge‐transfer process for the reverse switching. This single‐molecule device can be modulated in situ, is fully reversible, and does not display stochastic switching. The I–V curves of this single‐molecule system also exhibit memristive character. These features suggest a new approach for the development of molecular switching systems and storage‐class memories.
The exponential proliferation of data during the Information Age has required the continuous exploration of novel storage paradigms, materials, and devices with increasing data density. As a step toward the ultimate limits in data density, the development of an electrically‐controllable, single‐molecule memristive element is reported. In this device, digital information is encoded through switching between two isomer states by applying a voltage signal to the molecular junction, and the information is read‐out by monitoring the electrical conductance of each isomer. The two states are cycled using an electrically‐controllable local heating mechanism for the forward reaction and catalyzed by a single charge transfer process for the reverse switching. This single‐molecule device can be modulated in situ , is fully reversible, and does not display stochastic switching. The I‐V curves of this single molecule system also exhibits memristive character. These features suggest a new approach for the development of molecular switching systems and storage class memories.
The development of solar energy can potentially meet the growing requirements for a global energy system beyond fossil fuels, but necessitates new scalable technologies for solar energy storage. One approach is the development of energy storage systems based on molecular photoswitches, so-called molecular solar thermal energy storage (MOST). Here we present a novel norbornadiene derivative for this purpose, with a good solar spectral match, high robustness and an energy density of 0.4 MJ kg-1. By the use of heterogeneous catalyst cobalt phthalocyanine on carbon support, we demonstrate a record high macroscopic heat release in a flow system using a fixed bed catalytic reactor, leading to a temperature increase of up to 63.4 °C (83.2 °C absolute temperature). Successful outdoor testing shows proof of concept and illustrates that future implementation is feasible. The mechanism of the catalytic back reaction is modelled using density functional theory (DFT) calculations rationalizing the experimental observations.
Solar energy is abundant all over the world, but to be useful, the energy received must either be transformed to electricity, heat or latent chemical energy. The latter two options have the advantages that the energy can be stored. In molecular solar-thermal energy storage (MOST), solar energy is stored in chemical bonds; this is achieved using compounds undergoing photoinduced isomerisation to metastable isomers. Using a catalyst, the isomer can be recycled to its original form and the stored energy released as heat. This chapter describes the principles of the MOST concept and goes into details about the most studied MOST systems. The last part of the chapter deals with the integration of MOST systems into operational devices.
A study of the scope and limitations of varying the ligand framework around the dinuclear core of FvRu2 in its function as a molecular solar thermal energy storage framework is presented. It includes DFT calculations probing the effect of substituents, other metals, and CO exchange for other ligands on ΔHstorage. Experimentally, the system is shown to be robust in as much as it tolerates a number of variations, except for the identity of the metal and certain substitution patterns. Failures include 1,1′,3,3′-tetra-tert-butyl (4), 1,2,2′,3′-tetraphenyl (9), diiron (28), diosmium (24), mixed iron-ruthenium (27), dimolybdenum (29), and ditungsten (30) derivatives. An extensive screen of potential catalysts for the thermal reversal identified AgNO3–SiO2 as a good candidate, although catalyst decomposition remains a challenge.
Ab initio and density-functional theory (DFT) calculations have been used to investigate the model rearrangements of quadricyclane to norbornadiene catalysed by single CuSO4 and SnCl2 molecules. The isolated reactions with the two molecular catalysts proceed via electron-transfer catalysis in which the hydrocarbon is oxidised, in contrast to systems investigated previously in which the substrate was reduced. The even-electron SnCl2-catalysed reaction shows singlet-triplet two-state reactivity. Solvation by a single methanol molecule changes the mechanism of the rearrangement to a classical Lewis acid-base process.
Ab initio and density functional theory have been used to investigate the title reaction. The principles governing radical electrocyclic reactions are summarized and applied to the quadricyclane to norbornadiene radical cation rearrangement. The simple qualitative picture given by this interpretation is then compared with the detailed results of the calculations. In general, the qualitative theories proposed by Bischof and Haselbach provide an excellent conceptual framework for this and other radical electrocyclic reactions. The performance of the different levels of theory is compared and the density functional methods found to underestimate the energy gained from the pseudo-Jahn-Teller distortion away from a symmetrical transition state. Simple BLYP theory fails completely to reproduce this effect and Becke3LYP gives only a small distortion energy. Our best estimate of the activation energy for the rearrangement is ≤10 kcal mol-1, in agreement with recent calculations by Bach et al. This value is about twice as high as the current experimental estimate.
The authors applied general quantum mechanical ideas in order to establish the form of the many electron wave functions suitable for analysis of catalytic processes. This led us to the conclusion that the relevant wave functions for the electrons of the catalytic complexes must be taken as superpositions of the antisymmetrized products of the wave functions of electrons in excited and ionized states of the catalyst and reactants. With the use of the trial wave function for the electrons of the catalytic complex in such a form, it becomes possible to construct model potential energy surfaces of catalytic reactions as a superposition of the potential energy surfaces of the reactants in different electronic states. The authors formulate the criteria which when satisfied make it possible to implement a catalytic version of a desired chemical transformation. The authors also propose an approach to the explanation of the frequently observed correlations between the catalytic activity and other physical properties of a catalyst. 16 refs., 4 figs.
The method of an effective crystalline field (ECF) suggested previously to assess magnetic and optical properties of transition metal complexes is realized with INDO parametrization of ligand atoms and applied to calculate the ground state and spectrum of low-energy excitations of the central ion in aquo-, amino, and fluoro-complexes of transition metals of the first transition row and of some metalloporphyrins and chlorine-copper complexes.
The effective crystal field method for calculating the magnetic and optical properties of transition metal complexes is extended to take into account the electrons that fill the ligand molecular orbitals whose occupancy changes during chemical transformations. An effective Hamiltonian is constructed for a system of strongly correlating d-shell electrons and electrons on ligand orbitals that intersect each other during a reaction. Taking into account complete configuration interaction for this subsystem makes it possible to determine the structure of the ground and excited electronic states. The suggested calculation procedure is applied to isomerization of quadricyclane to norbornadiene in the coordination sphere of Co-tetraphenylporphyrin.
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Summary NotesExercises and ProblemsReferences
Introduction Two Archetypal Valence Bond Diagrams The Valence Bond State Correlation Diagram Model and Its General Outlook on Reactivity Construction of Valence Bond State Correlation Diagrams for Elementary Processes Barrier Expressions Based on the Valence Bond State Correlation Diagram Model Making Qualitative Reactivity Predictions with the Valence Bond State Correlation Diagram Valence Bond Configuration Mixing Diagrams: General Features Valence Bond Configuration Mixing Diagram with Ionic Intermediate Curves Valence Bond Configuration Mixing Diagram with Intermediates Nascent from “Foreign States” Valence Bond State Correlation Diagram: A General Model for Electronic Delocalization in Clusters Valence Bond State Correlation Diagram: Application to Photochemical Reactivity A Summary References Exercises Answers
Publisher Summary This chapter presents the specific catalytic functions of the transition metal in the various types of transformations, and discusses the associated chemistry. Molecular orbital symmetry conservation constrains all molecular systems to specific paths of transformation. Symmetry conservation principles have proven to be powerful tools for understanding a large body of complex organic chemistry. These concepts further bear on molecular stability. A molecule in one bonding configuration transforms into other configurations primarily through allowed paths. The thermal stability enjoyed by simple olefins to a certain extent rests on orbital symmetry restraints. Both olefin cyclobutanation and double-bond isomerization (through a 1,3-hydrogen shift), involving forbidden passages, are not observed at moderate temperatures. Simple olefins are fixed in their bonding configurations and cannot interconvert through the sterically-preferred paths. The thermal interconversion of olefins is necessarily a high-temperature process involving predominantly the higher energy, allowed transformations incorporating free radical intermediates.
This chapter discusses the fundamental principles of catalytic activity. The field of catalysis represents one of the most intricate branches of chemistry. The typical catalyst is useful not merely because of the average or bulk properties of the material in it, but rather because of the combination of the bulk properties and the very specific form in which the particular catalytic specimen appears. The ultimate development of catalytic theory in quantitative form requires an understanding not only of the average valence characteristics of the atoms in it, but of the unusual arrangements and electronic structures that are possible; for example, of the unusual arrangements of atoms that are possible near the surface in a crystalline catalyst. The discussion of catalytic activity in the chapter can be viewed only as a preliminary attempt to describe the important features of the field in terms of the language that has arisen out of the development of atomic theory.
This chapter discusses the mechanism of the photocatalytic effect and the influence of illumination on the adsorption and catalytic properties of a surface. The chapter focuses on the electronic theory of photocatalytic reactions on semiconductors. A clue to the understanding of the photocatalytic effect is the electronic theory of catalysis on semiconductors. The existence and the basic regularities of the photocatalytic effect follow directly from the electronic theory of catalysis. According to the electronic theory, a particle chemisorbed on the surface of a semiconductor has a definite affinity for a free electron or for a free hole in the lattice. The chemisorbed particle is presented in the energy spectrum of the lattice as an acceptor and in the second as a donor surface local level situated in the forbidden zone between the valency band and the conduction band. In the general, the same particle may possess an affinity both for an electron and a hole.
This chapter reviews the current state of the electron theory of catalysis on semiconductors. The electron theory of catalysis has a two-fold aim. First, it seeks to disclose the elementary mechanism of the catalytic act. Second, the electron theory seeks to elucidate the relation between the catalytic and electronic properties of a semiconductor. In its present stage of development, the electron theory of catalysis deals with catalysts that, by their electrical properties, belong to the class of semiconductors. Catalysis on semiconductors is extremely widespread. This is because of the circumstance that in most cases, a metal is enclosed in a semiconducting coat and the processes that apparently take place on the surface of the metal actually take place on the surface of this semiconducting coat, whereas the underlying metal frequently takes practically no part in the process. The results of the electron theory as developed for semiconductors are fully applicable to dielectrics. However, they cannot be automatically applied to metals.
We applied general quantum mechanical ideas in order to establish the form of the many-electron wave functions suitable for analysis of catalytic processes. This led us to the conclusion that the relevant wave functions for the electrons of the catalytic complexes must be taken as superpositions of the antisymmetrized products of the wave functions of electrons in excited and ionized states of the catalyst and reactants. With use of the trial wave function for the electrons of the catalytic complex in such a form, it becomes possible to construct model potential energy surfaces of catalytic reactions as a superposition of the potential energy surfaces of the reactants in different electronic states. We formulate the criteria which when satisfied make it possible to implement a catalytic version of a desired chemical transformation. We also propose an approach to the explanation of the frequently observed correlations between the catalytic activity and other physical properties of a catalyst.
Orbital symmetry controls in an easily discernible manner the feasibility and stereochemical consequences of every concerted reaction.
The photochemistry of quadricyclane (Q) was explored by single-photon excitation to high vibrational levels. Spectra of the v = 4-7 carbon-hydrogen overtones were recorded by using intracavity absorption and photoacoustic detection. These spectra were compared to the infrared fundamental spectrum and assigned. Excitation of the v = 5 and v = 6 bands of cyclopropanoid and methylenic hydrogens leads to reaction. At least one intermediate, probably a vibrationally excited form of norbornadiene (N), must be involved because partitioning among various reactions channels is pressure dependent. Apparent rate constants were measured and correlated with variations in pressure according to the Stern-Volmer relationship. Experimental values were compared with rate constants calculated by RRKM theory. Although there is a modest amount of excess reaction observed for the lowest excitation energies above threshold, overall evaluation provides no significant evidence for concentration of energy in localized modes for times that are long compared with reaction times.
Density functional calculations with the hybrid B3LYP functional have been used to study the ground state of norbornadiene bound to the photosensitizer [Cu(8-oxoquinolinato)]. The main bonding interaction between the ligand and the metal is {pi} back-donation, metal to ligand, which reduces the {pi}-{pi}{sup *} band gap in norbornadiene. CIS calculations performed on free norbornadiene and on the system where norbornadiene interacts with the photosensitizer have shown that the photosensitizer reduces the excitation energy to promote the system to the first excited singlet state. In terms of wavelength, this difference implies that light with {lambda} almost 100 nm longer can be used to photoactivate the norbornadiene with respect to free norbornadiene.
The fundamentals of molecular quantum mechanics have changed remarkably little since the 1930’s: the most commonly used methods of constructing molecular wave functions are still dominated by expansions in terms of Slater determinants, in which individual electrons are described by spin-orbitals; the spin-orbitals (one-electron wavefunctions) still usually refer to an independent-particle model (IPM) in which each electron ‘sees’ an average field due to the nuclei and to all its companion electrons — in the spirit of Hartree and Fock; and simple first approximations, in which the wavefunction is built up from a few configurations of ‘occupied’ orbitals, are still systematically refined by adding more and more configurational functions (CFs) in a typical configuration interaction (CI) calculation. It is true that the techniques have changed: Fock space, for example, is now more familiar to quantum chemists than it used to be; the simple self-consistent field (SCF) of Hartree and Fock is nowadays often replaced by its multi-configuration counterpart; and CI may be admitted by diagrammatic perturbation theory or by unitary group methods. But, beautiful as some of these techniques may be, they are simply sophisticated and clever ways of doing the same old things!
The infra-red spectra of five olefin complexes with platinous chloride show that the olefin exists, with its double bond, in the complex. The C=C stretching frequency is lowered by some 143 wave-numbers, about twice the lowering observed for silver-ion complexes. These facts and all data relating to olefin complexes of PdII, PtII, CuI, Ag I, and HgII in the literature are consistent with a structure schematically shown in the Figures, essentially Dewar's structure (Bull. Soc. chim., 1951, 18, C 79), and with no other structure yet proposed. From the dipole moments of compounds of the type C2H 4,am,PtC12 (am = amine) we estimate that the C 2H4 → Pt bond moment is about 2 D less (or greater) than that of the N → Pt bond. If we assume that this difference is due only to double-bond formation as required by the above structure, it indicates not more than about 1/3 double-bond character in the C2H4-Pt bond. Hel'man's evidence for the quadrivalency of platinum in olefin complexes of platinous chloride is shown to be consistent with bivalency. The chlorination of (C2H4PtC12)2 yields ultimately ethylene dichloride, but not in the first stages of the chlorination. A much improved preparation of (C2H4PtC12)2 has been found. Attempts to prepare platinous chloride complexes of but-2-yne and of diphenylacetylene were unsuccessful.
A methodology for conceptualization and construction of potential energy curves for a variety of organic reactions is presented. We suggest that in all reactions which involve covalent bond-making and bond-breaking steps, the ground potential energy curve arises from an intersection of two curves - one which is initially a ground configuration and the other an excited configuration which contains the VB "image" of the product. The curve crossing turns out to be a heuristic electronic promotion which prepares the closed shell reactants for bond reorganization. It may also be described as a switchover of two VB structures, one reactant-like, the other product-like. This behavior is shown to be general whenever at least one of the reactants is closed shell. Examples are given for nucleophilic, electrophilic, and symmetry-forbidden reactions.
Basic Concepts. General Density Matrix Theory. Coupled Systems. Irreducible Components of the Density Matrix. Radiation from Polarized Atoms: Quantum Beats. Some Applications. The Role of Orientation and Alignment in Molecular Processes. Quantum Theory of Relaxation. Appendix A: The Direct Product. Appendix B: State Multipoles for Coupled Systems. Appendix C: Formulas from Angular Momentum Theory. Appendix D: The Efficiency of a Measuring Device. Appendix E: The Scattering and Transition Operators. Index.
Aus den nach bekannten Verfahren hergestellten Quadricyclanen (I), (III), (V) und (VII) entstehen bei der Behandlung mit Rhodium(I)-Komplexen in einer thermodynamisch kontrollierten Reaktion die Norbornadien-Derivate (II), (IV), (VI) bzw. (VIII), während aus dem Oxa-quadricyclan (IX) in einer kinetisch kontrollierten Reaktion das thermodynamisch stabilere Hydroxyfulven (X) erhalten wird.
We present investigations aimed at using the norbornadiene/quadricyclane system as a framework for a switchable cation carrier. According to molecular modelling studies a suitably substituted system should perform a pincer-like motion upon isomerisation and reisomerisation, while complexing and decomplexing a host molecule. To this end, we investigated the complexing abilities of a 1,5,6,7-tetrasubstituted quadricyclane polyether and its corresponding norbornadiene isomer. The picrate extraction method was applied to determine the extraction constants Kex and the association constants Kass of these polyethers with alkali metal cations in chloroform. As expected, the complexes of the quadricyclane polyether with Li+, Na+, K+ and Cs+ are more stable than those of the norbornadiene isomer. This difference reaches a maximum for potassium. The interconversion between the norbornadiene polyether and the corresponding quadricyclane was also examined. The norbornadiene can be converted photochemically to the quadricyclane with 50% conversion while the reverse reaction is catalysed by neutral aluminium oxide in 71% yield. No by-product was detected in either of the isomerization reactions.
Starting with Z-2, 3-diamino-2-butenedinitrile as diamine component the synthesis of the low molecular N,N,O,O-chelate 3.Co and its ligand 3 is optimized. An easy synthesis of the chelate 8.Co covalently bound on macroreticular resins is presented. The chelates 8.Co are the most active heterogenous catalysts for the valence isomerization of quadricyclane to norbornadiene today.
The mechanism of the catalytic activity of transition metal complexes is discussed in terms of an effective Hamiltonian of a reactant-catalyst system. The potential energy surface has been constructed for the reaction of catalytic isomerization of quadricyclane to nonbornadiene. The properties of the catalyst are found to affect strongly the activation energy of the reaction.
A general scheme for theoretical treatment of organometallic reactivity is proposed. It is based upon the notion that the reactivity of a molecule is strongly affected by its coordination to metal-containing fragments. Based upon this idea we describe the large-scale organometallic reactions as reactions of the ligands in the coordination spheres of transition metal complexes. We propose here a quantum mechanical framework for analysis of effects of coordination on the reactivity and give several examples of qualitative energy profiles for reactions in the ligand spheres of transition metal complexes. © 1996 John Wiley & Sons, Inc.
The partitioning technique for solving secular equations is briefly reviewed. It is then reformulated in terms of an operator language in order to permit a discussion of the various methods of solving the Schrödinger equation. The total space is divided into two parts by means of a self‐adjoint projection operator O. Introducing the symbolic inverse T = (1—O)/(E—H), one can show that there exists an operator Ω = O + THO, which is an indempotent eigenoperator to H and satisfies the relations HΩ = EΩ and Ω2 = Ω. This operator is not normal but has a form which directly corresponds to infinite‐order perturbation theory. Both the Brillouin‐ and Schrödinger‐type formulas may be derived by power series expansion of T, even if other forms are perhaps more natural. The concept of the reaction operator is discussed, and upper and lower bounds for the true eigenvalues are finally derived.
New photoresponsive polymers bearing norbornadiene (NBD) moieties were synthesized in high yields without any insoluble gel products by the selective cationic polymerization of 2-[[(3-phenyl-2,5-norbornadienyl)-2-carbonyl]oxy]ethyl vinyl ether (PNVE) and also by the copolymerizations with 2-phenoxyethyl vinyl ether, 2-chloroethyl vinyl ether, or isobutyl vinyl ether using BF3.O(C2H5)2 as a catalyst in dichloromethane solution. The photochemical properties of the polymers thus obtained were evaluated in the film state or in dichloromethane solution. The rate of photochemical reaction of the pendant NBD moiety to the quadricyclane (QC) group in poly[2-[[(3-phenyl-2,5-norbornadienyl)-2-carbonyl]oxy]ethyl vinyl ether] [P(PNVE)] was higher than those of pendant NBD moieties in the copolymers in the film state, although the rate of the reaction of the NBD moiety of P(PNVE) was nearly the same as those of the NBD moieties in the copolymers in dichloromethane solution. Furthermore, it was found that the rate of the Photoisomerization of the pendant NBD moiety and the photochemical reversion of the resulting QC groups were strongly affected by the wavelength of the irradiating light. The rate of thermal reversion of the produced QC groups in polymer films with low T(g) was higher than that of the QC groups in the polymer film with high T(g), and the rate of catalytic reversion of the QC group in the polymer increased with increasing catalyst concentration in the solution.
In this paper, we report on the bond coupled electron transfer (BCET) isomerization reaction of norbornadiene (NB) to quadricyclane (Q). We have used several triplet sensitizers to examine electron and energy quenching by NB. In the case of acetophenone, energy transfer generates the detectable 3NB, which rearranges to give 3Q which relaxes to Q. Using chloranil, benzoquinone, and 2,5-dichlorobenzoquinone as triplet sensitizers, the triplet ion pairs are generated, but cannot access either 3NB or 3Q, and so the ion pair undergoes return electron transfer to the ground state. However, with 3,3‘,4,4‘-benzophenonetetracarboxylic dianhydride as the triplet sensitizer, the triplet ion pair is generated, but can undergo BCET to 3Q efficiently, which then relaxes to Q. To our knowledge, this is the first example of an efficient electron-transfer mechanism to photoisomerize NB to Q without the intermediacy of 3NB. Consequently, in the valence isomerization of NB to Q, it is possible to directly access the NB-Q triplet surface by controlling the energy of the triplet ion pair photogenerated, and in effect, the quantum yield can be modulated.
By use of Fourier transform near-infrared (NIR) absorption spectroscopy and the aid of a kinetic model, we have investigated the conversion of quadricyclane to norbornadiene catalyzed by anhydrous CuSO4 and SnCl2 in chloroform. The reaction mixture is not agitated so as to avoid the effect of sample heterogeneity. The NIR absorption spectra are acquired, at a position 2 mm above the catalyst surface, at 30-s intervals during 4 h. The concentrations of quadricyclane and norbornadiene are determined with the analysis of partial least squares. The isomerization of quadricyclane, as numerically solved from the model, is expected to describe its behavior more accurately in the catalytic system than that obtained previously. In addition to the isomerization rate, the kinetic model takes into account the contribution of diffusion. The diffusion coefficients of quadricyclane can be determined to be 3.8 × 10-5 cm2 s-1 in chloroform and 1.14 × 10-5 and 2.85 × 10-6 cm2 s-1 inside the CuSO4 and SnCl2 stacks, respectively. Diffusion is slowed inside the solid stacks, and thus the molecular mechanism cannot be suitable for this system. Given the diffusion coefficients, the pseudo-first-order depletion rate constants are evaluated to be (3.7 ± 0.1) × 10-3 and (3.8 ± 0.1) × 10-3 s-1 for CuSO4 and SnCl2, respectively. The corresponding second-order rate constants are determined to be (1.3 ± 0.2) × 10-5 and (2.0 ± 0.1) × 10-6 s-1 A-1 by considering the density and the size of the catalyst particles; A denotes the total catalyst surface area per unit effective volume of solvent. The rate constant with the CuSO4 catalyst is consistent with others obtained in a continuously stirred mixture. In the surface-mediated reaction, the catalytic isomerization is subject to one-site coordination (1:1 complex) between the reactant and the catalyst. Nevertheless, a two-site coordinated reaction cannot be excluded unless the interstitial size dependence of the depletion rate is known.
The quantum mechanical effective Hamiltonian of crystal field (EHCF) methodology (previously developed for describing electronic structure of transition metal complexes) is combined with the Gillespie−Kepert version of molecular mechanics (MM) in order to describe multiple potential energy surfaces (PES) of the Werner-type complexes corresponding to different spin states of the latter. The procedure thus obtained is a special version of the hybrid quantum mechanics/molecular mechanics approach. The MM part is responsible for representing the whole molecule, including ligand atoms and metal ion coordination sphere, but leaving aside the effects of the d shell. The quantum mechanics part (EHCF) is restricted to the metal ion d shell. The method reproduces with considerable accuracy geometry and spin states of a wide range of Fe(II) and Co(II) complexes of various total spin and coordination polyhedra and containing both monodentate and polydentate ligands with aliphatic and aromatic nitrogen donor atoms. In this setting, a single MM parameters set is shown to be sufficient for dealing with all spin states and coordination numbers of the complexes.
The norbornadiene triplet is directly observed and its energy measured for the first time by time-resolved photoacoustic calorimetry. PAC affords a transient subsequent to the sensitizer triplet, with a lifetime of 6.20 +/- 0.03 ns and an energy of 61.37 +/- 0.25 kcal/mol relative to ND, which we have ascribed to the norbornadiene tripler. The lifetime of the triplet, which is anomalously short, is determined by the rate at which the species undergoes rearrangement along the path to quadricyclane. Calculations for the norbornene triplet indicate that its short lifetime is consistent with T-1 --> S-0 intersystem crossing.
The electronic spectrum of the nonplanar, indirectly conjugated molecule norbornadiene has been studied theoretically, using multiconfigurational second-order perturbation theory (CASPT2) with extended ANO-type basis sets, and experimentally, using optical absorption and polarization-selected two-photon resonant-enhanced multiphoton ionization (REMPI) spectroscopies. The calculations comprise five valence excited states and the 3s, 3p, and 3d members of the Rydberg series converging on the first two ionizations. The two lowest triplet states have also been studied. The experiments enabled definitive assignments to be made for four Rydberg transitions and three valence transitions. The computed excitation energies were found to be within 0.2 eV of the experimental energies for correctly correlated transitions; the computed oscillator strengths were in good relative agreement with their experimental values. Comparisons are made between this and other theoretical calculations and also between the electronic spectra of norbornadiene, cis-butadiene, and cyclopentadiene.
A general procedure for analyzing correlated wave functions is explicited and applied to the double bridge of diborane. The procedure consists of four steps. First, a complete-active-space multiconfigurational SCF (CASSCF) calculation provides optimized valence molecular orbitals and treats the electronic correlation internal to the space associated with the given chemical group. Then, these orbitals are localized into a minimal set of molecularly adapted atomic orbitals called nearly atomic molecular orbitals (NAMOs). Next, these orbitals are used to build a basis set of orthogonal valence-bond (OVB) determinants on which the CASSCF wave function is reexpressed, weighting the various distributions of the active electrons in these local cells. The information is lastly reduced by further processing the OVB expansion to figure out the electron populations and their fluctuations within one NAMO or a subset of NAMOs. To measure the extent of interaction between given orbitals, an index is proposed that is related to the covalent organization residual two-electron probability (CORP). The whole method is proved to avoid the shortcomings of basis set dependence. For B2H6, the NAMOs are hydrogen-centered orbitals and B-H directed boron hybrids. The OVB expansion exhibits a clear hierarchy of situations, reflecting a compromise between the separation of the two bridges (91% probability for up/down 2e/2e partitioning) and the tendency to maintain the neutrality of the atoms. All indexes indicate that the B-H interactions definitely prevail over the B-B ones, although the latter are not negligible. This is further confirmed by a direct estimate of charge-transfer interactions in the OVB-CI matrix. In a way, each three-center two-electron bridge in diborane is roughly halfway between an allyl-like cation and a cyclopropenyl-like cation.
The results of studies on intramolecular interconversions in systems of norbornadiene quadricyclane and their derivatives are classified and discussed. The mechanisms of the forward photoreaction and reverse thermal process in relation to the nature of the substituents and carbocycles, type of sensitiser and catalyst, and properties of the medium are discussed in detail and classified. An analysis is made of the influence of these factors on the spectroscopic, kinetic, and thermodynamic properties of this system as an accumulator and converter of light energy. Methods of improving its characteristics — light absorption, the quantum yield of the photoreaction, and stability of the photoproduct — have been established. The bibliography includes 188 references.
Data on the valence isomerisation of norbornadiene and its derivatives into the corresponding quadricyclanes published between 1990 and 2001 are considered and described systematically. The applicability of this reaction for the storage of solar energy is discussed. The bibliography includes 112 references.
Momentum Distributions (MDs), obtained using high-resolution electron momentum spectroscopy (HREMS), are reported for norbornadiene's 18 valence orbitals. Corresponding theoretical results, using generalized gradient approximation density functional theory (DFT) together with TZVP, DZVP, and DZVP2 basis functions and a plane wave impulse approximation to describe the ionization process, are also detailed. This work represents the first comprehensive HREMS/DFT investigation into the complete valence electronic structure of norbornadiene (NBD), with significant results being obtained. In particular, an exacting comparison between our experimental and theoretical MDs enables us to define the "optimum" basis for NBD, from those we studied. This "optimum" basis is then used to extract a wide range of NBD's important molecular property information, which are subsequently compared with the results of independent measurements and calculations. Agreement between our results and those from independent measurements was generally very good, highlighting the utility of HREMS in a priori basis set evaluation.
Electron transfer catalysis is an efficient method for the catalysis of symmetry-forbidden or slow pericyclic reactions. Accurate quantum mechanical calculations are an important tool for gaining insights into the mechanistic details of these fast reactions involving radical cations. The current “state of the art” of computational studies of pericyclic reactions of radical cations is reviewed. In particular, four parent reaction types are discussed: (i) the ring-opening of the cyclobutane radical cation; (ii) the [2+2] cycloreversion of the cyclobutane radical cation; (iii) the radical cation Diels−Alder reaction of 1,3-butadiene and ethylene; and (iv) the [1,3] methylene shift in the vinylcyclopropane radical cation. The transfer of these findings to chemically more relevant substituted systems is also briefly discussed. The potential energy hypersurfaces obtained are very flat and have activation barriers that are significantly lower than the ones for the corresponding neutral reactions, which is in agreement with the large rate acceleration observed experimentally. Many of the located radical cation structures closely resemble their biradical counterparts in the neutral, stepwise pathways. The reactions generally follow a lower symmetry pathway, due to Jahn−Teller distortions induced by the unpaired electron. Finally, the results from computationally efficient B3LYP/6-31G* calculations are found to be in good agreement with those from highly correlated MO calculations.