Captodative Substitution Research Articles

Designing an Efficient Singlet Fission Material with B-N Substitution in Pyrene: A Model Exact Study.

The electronic structure of boron (B)-nitrogen (N)-substituted pyrene molecules is the center of attraction in designing an efficient intermolecular singlet fission (x-SF) material. Thermodynamic energy criteria required for x-SF are obtained by captodative substitution with B and N in pristine pyrene to increase the lowest singlet-triplet energy gap. We computed low-lying excited states of BN-embedded pyrene molecules by exactly solving the Pariser-Parr-Pople (PPP) model Hamiltonian and compared these results with the TDDFT and EOM-CCSD values. Exact diagonalization of the PPP model Hamiltonian suggests that pristine pyrene, which is endothermic for x-SF, becomes isoergic with certain (BN)2 substitution. The low-lying excited state energies calculated using the model Hamiltonian match very well with experimental values over EOM-CCSD and TDDFT. Moreover, the low value of the spin-orbit coupling constant calculated for BN-substituted pyrene strengthens its applicability as an SF material.

Controlling Möbius-Type Helicity and the Excited-State Properties of Cumulenes with Carbenes.

Diradical character and excited-state aromaticity serve as guidelines to identify molecules that show nonlinear optical properties. Cumulenes are known to have small singlet-triplet gaps resulting in significant diradical character. Herein, we report a computational investigation on the electronic structure and excited-state properties of cumulenes of different lengths and with various terminal carbene groups. Intriguingly, cumulenes with an even number of cumulative double bonds, which barely have been studied experimentally, are predicted to be thermodynamically more stable than their odd counterparts. We propose that this is due to the stabilizing effect of electron delocalization in the helical Möbius-type frontier orbitals. Accordingly, we delineate how to control the energies of the excited states by the choice of carbene and length of the cumulene. We find that π-acceptor carbenes decrease the diradical character, whereas donors as well as captodative substitution or potentially a biscationic charge leads to an open-shell ground state. We also predict that bent allenes are better in stabilizing organic radicals than carbenes. Eventually, we identify suitable candidates for experimental endeavors toward new singlet fission molecules.

Captodative Substitution Enhances the Diradical Character of Compounds, Reduces Aromaticity, and Controls Single-Molecule Conductivity Patterns: A Valence Bond Study.

The present contribution uses a valence bond (VB) perspective to consider the captodative substitution strategy, a method to enhance the diradical character of (potentially aromatic) compounds. We confirm the qualitative reasoning that has generally been used to rationalize the diradical-character-enhancing effect of captodative substitution: this type of substitution scheme disproportionally stabilizes specific Dewar/diradical(oid) VB structures, thus increasing their weight in the full ground-state wave function. Furthermore, we assess the effect of captodative substitution on the aromaticity of the considered compound. We observe a clear trade-off between diradical character and aromaticity for our model systems: as one of these properties increases, the other decreases. This finding is especially significant within the field of single-molecule electronics because it enables unification of the previously observed inverse proportionality between the aromaticity of a compound and the magnitude of conductance through that molecule, with the observed proportionality between diradical character and the magnitude of conductance associated with a compound. To some extent, both properties, i.e., aromaticity and diradical character, appear to be the flip-sides of the same coin.

Designing Long-Range Charge Delocalization from First-Principles.

Efficient electronic communication over long distances is a desirable property of molecular wires. Charge delocalization in mixed-valence (MV) compounds where two redox centers are linked by a molecular bridge is a particularly well-controlled instance of such electronic communication, thus lending itself to comparisons between theory and experiment. We study how to achieve and control long-range charge delocalization in cationic organic MV systems by means of Kohn-Sham density functional theory (DFT) and show that a captodative substitution approach recently suggested for molecular conductance ( Stuyver et al. J. Phys. Chem. C 2018 , 122 , 3194 ) greatly enhances charge delocalization in p-phenylene-based wires. To ensure the adequacy of our DFT methods, we validate different protocols for organic MV systems of different lengths. The BLYP35 hybrid functional combined with a polarizable continuum model, established by Renz and Kaupp, is indeed capable of correctly describing experimentally observed length-dependent charge delocalization, in contrast to the long-range corrected functionals ω-B97X-D and ω-PBE. We also discuss the implications of these results for a first-principles description of the transition between coherent tunneling and incoherent hopping regimes in molecular conductance.

Captodative Substitution: A Strategy for Enhancing the Conductivity of Molecular Electronic Devices

We explore a new strategy to tune the conductivity of molecular electronic devices: captodative substitution. We demonstrate that a careful design of such substitution schemes on a benzene parental structure can enhance the conductivity by almost an order of magnitude under small bias. Once this new strategy has been established, we apply it to molecular wires and demonstrate that it enables the unprecedented anti-Ohmic design of wires whose conductivity increases with the length. Overall, the captodative substitution approach provides a very promising pathway toward full chemical control of the conductivity of molecules which opens up the possibility to design molecular switches with an improved on/off ratio among others.

Captodative substitution induced acceleration effect towards 4π electrocyclic ring-opening of substituted cyclobutenes

Significance of quantum chemical calculations as a theoretical tool to identify the optimum substitution pattern for a rapid 4π electrocyclic ring-opening of substituted cyclobutenes is presented.

ChemInform Abstract: The Captodative Effect

Publisher Summary The concept of captodative substitution implies the simultaneous action of a captor (acceptor) and a donor substituent on a molecule. The chapter analyzes whether the claim of the proponents of the captodative effect has found experimental or theoretical support. The postulate of a synergetic action of a captor and a donor substituent at a radical centre and its chemical consequences has been discussed. Captodative substitution is that chemical consequences should be connected with this substitution pattern. For synthetic purposes, a captodative effect of a few kcal mol- might be helpful for the selection of a lower energy pathway in cases where a radical can react by several reaction paths, discriminated by activation barriers of closely similar energy. Synthetic applications have shown that captodative-substituted radicals generally behave like other stabilized, short-lived radicals. Left alone these radicals usually give dimers in high yield, as, for instance, allylic or benzylic radicals also do. As captodative substitution provides stability to a radical centre it is no surprise that captodative olefins are goodpartners in (2 + 2)-cycloadditions.

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An expedient synthesis and the reactivity of donor substituted cyanoketenes 7 (captodative substitution) are described. 3-Cyano β-lactams are formed from 7 and imines.

8161 POSTER “Sports were my whole life – I had a perfect body before getting cancer”: young athletes rediscover aspects of former body identity during exercise – qualitative findings

An expedient synthesis and the reactivity of donor substituted cyanoketenes 7 (captodative substitution) are described. 3-Cyano β-lactams are formed from 7 and imines.

How to Promote Sluggish Electrocyclization of 1,3,5-Hexatrienes by Captodative Substitution

Hexatriene electrocyclization, if not disfavored by its harsh reaction conditions, can be highly useful for the synthesis of complex organic molecules. Herein we developed a two-layer ONIOM method which could predict the activation free energy of hexatriene electrocyclization with an accuracy of about 1.0 kcal/mol. Using this carefully benchmarked method, we calculated the activation free energies for a variety of substituted hexatrienes. It was found that extraordinarily rapid electrocyclization could occur for certain patterns of captodative substituted hexatrienes, including 2-acceptor-3-donor hexatrienes, 2-acceptor-5-donor hexatrienes, and 3-acceptor-5-donor hexatrienes. The activation free energies for these systems could be up to 10 kcal/mol lower than that of the unsubstituted hexatriene, and therefore, their electrocyclization could proceed smoothly even at room temperature. The mechanism for the captodative effect on hexatriene electrocyclization could be understood by calculating the affinity between the donor and acceptor group in the reactant state and transition state of the reaction. If the affinity was stronger in the transition state, captodative substitution would produce an extra acceleration effect. It was shown that our theoretical results were in excellent agreement with the experimental data from the recent synthetic studies of hexatriene electrocyclizations. Thus, the theoretical tools developed in the present study could be used to predict not only how to accelerate the hexatriene electrocyclization via substituent manipulation but also under what conditions each particular electrocyclization could be accomplished in the real experiment.

Synthesis, stabilities, and redox behavior of Di(1-azulenyl)(6-azulenyl)methylium hexafluorophosphates. Generation of a donor-acceptor-substituted neutral radical by azulenes.

Several di(1-azulenyl)(6-azulenyl)methanes and 1,3-bis[(1-azulenyl)(6-azulenyl)methyl]azulenes were prepared by the condensation reaction of azulenes with diethyl 6-formylazulene-1,3-dicarboxylate under acidic conditions. The products were converted into di(1-azulenyl)(6-azulenyl)methylium hexafluorophosphates and azulene-1,3-diylbis[(1-azulenyl)(6-azulenyl)methylium] bis(hexafluorophosphate)s via hydride abstraction reaction with DDQ following the exchange of counterions. These mono- and dications exhibited high stability with large pK(R)(+) values (5.6-10.1), despite the captodative substitution of azulenes. The electrochemical reduction of the monocations upon cyclic voltammetry (CV) exhibited a reversible two-step, one-electron reduction wave with a small difference between the first reduction potential (E(1)(red)) and the second one (E(2)(red)), which exhibited the generation of highly amphoteric neutral radicals in solution. The electrochemical reduction of dications showed voltammograms, which were characterized by subsequent two single-electron waves and a two-electron transfer upon CV attributable to the formation of a radical cation, a diradical (or twitter ionic structure), and a dianionic species, respectively. Formation of a persistent neutral radical from a monocation was revealed by ESR and UV-vis spectroscopies and theoretical calculations. The ESR spectra of the neutral radical gave two hyperfine coupling constants: a(H) = 0.083 (6H) and 0.166 mT (9H) (g = 2.0024), indicating that an unpaired electron delocalizes over all three of the azulene rings. The stable monoanion, which shows the localization of the charge on the 6-azulenyl substituent, was also successfully generated from the di(1-azulenyl)(6-azulenyl)methane derivative.

Radical Polymerization of Methyl Methacrylate by Captodative Substituted Morpholino Succinonitrile

Radical polymerization of methyl methacrylate (MMA) by captodatively substituted morpholino succinonitrile was studied kinetically at 90 and 130°C by comparison with that of styrene. The polymerizations of MMA and styrene at 90°C proceeded at moderate rates to give polymers which show unimodal size exclusion chromatography (SEC) curves. Both polymerizations at 130°C were significantly fast to give polymers with bimodal SEC curve. A higher molecular weight fraction of the obtained polymer may mostly formed by a spontaneous thermal polymerization in styrene polymerization, but in MMA polymerization produced mainly by dissociation of the domant species. The rate constant of the activation of captodative-capped poly(MMA) domant was smaller than that reported for nitroxide-mediated styrene polymerization.

Controlled Radical Polymerization of Captodative Substituted Methyl α-Methoxyacrylate with Captodative Substituted Morpholino Succinonitrile

Radical polymerization of captodative (cd) substituted methyl α-methoxyacrylate (MMOA) with cd substituted morpholino succinonitrile is studied kinetically at 90, 110, and 130°C. The polymerization of MMOA proceeds at moderate rate to give a cd-capped dormant polymer in contrast to the polymerization with conventional azo initiator and the rate constant of the activation of the dormant is estimated to be intermediate between that of the nitroxide-mediated styrene polymerization and cd substituted succinonitrile initiated methyl methacrylate polymerization. On the basis of these results, the relationship between cd substitution effect and the stability of the dormants is discussed.

Theoretical analysis of the energy barriers for the rotational isomerization of the allyl and the 1-cyano-, 1-hydroxy- and 1-cyano-1-hydroxyallyl radicals

An ab initio computational study was carried out on the ground- and transition-state structures for the rotational isomerization of the allyl and the 1-cyano, 1-hydroxy- and 1-cyano-1-hydroxyallyl radical systems in an attempt to gain an understanding of the factors affecting the relative energy barriers for rotational isomerization. The results of ESR rate measurements have indicated that there exists the presence of an ‘extra’ lowering of the energy barrier for the rotational isomerization of the 1-cyano-1-hydroxylallyl radical relative to the sum of the lowerings of the energy barriers for the rotational isomerization of the 1-cyano- and 1-methoxylallyl radical compared with the energy barrier for the rotational isomerization of the allyl radical. The ‘extra’ lowering of the energy barrier for the rotational isomerization of the 1-cyano-1-hydroxyallyl radical has been attributed to captodative stabilization of the transition structure for rotational isomerization. Prior calculational studies in the author's and other laboratories have resulted in the suggestion that captodative substitution does not always lead to ‘extra’ stabilization to a radical center. Recent calculations in the author's laboratories and the results reported in the present paper strongly suggest that ground-state effects can be dominant in determining the rate of a radical-forming reaction. The present paper describes the results of theoretical calculations on the allyl and substituted allyl radical systems that indicate that ground-state effects appear to dominate the relative rates of the rotational isomerization of the substituted allyl radicals. © 1997 John Wiley & Sons, Ltd.

Conversion-Dependent Molecular Weight of the Polymer in Free Radical Polymerization of Captodative Substituted (Acyloxy)acrylates

The radical homopolymerization of captodatively substituted methyl α-acetoxyacrylate (MAA) is studied by comparing it with that of the α-trifluoroacetoxy, the α-butyryloxy, and the ethyl ester analogs at 40 °C. The yield of the polymer is much influenced by the acyloxy substituents, and the polymer molecular weight increases with conversion in bulk MAA and ethyl ester polymerizations in contrast to other polymerizations. Dilution of the MAA polymerization system, however, results in no increase in the polymer molecular weight during the polymerization. Some polymer−polymer interactions between captive and dative moieties in MAA polymer are discussed on the basis of the measurement of the viscosity of the polymerization system and the polymer solution.

Kinetic Study of Radical Homopolymerization of Captodative Substituted Methyl .alpha.-(Acyloxy)acrylates

Radical homopolymerizations of some captodative substituted methyl α-(acyloxy)acrylates are studied kinetically by using azo initiators at 30-60 °C. Polymerization rate and the properties of the polymer obtained are much influenced by the acyloxy substituents. Methyl α-acetoxyacrylate (MAA), which was already found to give a polymer with excellent optical properties, shows the highest reactivity and gives a high molecular weight polymer with very poor solubility. Kinetic study shows that the propagation rate constant (k p ) of the polymerization of MAA is larger than that of methyl α-(butyryloxy)-acrylate (MBA) at every temperature, k p = 255 and 114 L/mol.s at 50 °C for the polymerization of MAA and MBA, respectively. The activation energy of the overall polymerization of MAA is estimated to be E a = 16.0 kcal/mol, smaller than that of MBA (E a = 17.4 kcal/mol), although the energies of propagation and termination of MAA are larger than those of the bulky-substituted MBA. The mechanism of the unique polymerization of MAA is discussed based on the kinetic study, ESR spectroscopy, and the physical property measurements of the polymer obtained.

Theoretical Investigation of Some Evidences of the Captodative Effect

AbstractIn this paper, the stability and reactivity of some gem‐disubstituted closed‐shell and open‐shell systems are investigated using the unifying concept of thermodynamic stabilization energy. We deduce this quantity either from available experimental data or from enthalpies of formation obtained at various theoretical levels. The energies of open‐shell species, medium‐size closed‐shell systems and large diamagnetic compounds are respectively calculated at the CISD/6‐31 G(d,p), MP2 = FULL/6‐31 G(d,p) and HF/6‐31 G(d,p) levels.It is found that captodative (cd) substitution significantly stabilizes carbon‐centred radicals and ethylenic compounds but generally destabilizes methane derivatives. Didative (dd) substitution most often stabilizes both the methyl radicals and their parent molecules. Finally most cc‐substituted species are destabilized. On the whole, the cdCH2 compounds do actually have the smallest C‐H bond strengths due to the joint action of the captodative and stereoelectronic effects. For this reason, they deserve to be named proradical. Consequently hydrogen transfer reactions are strongly favoured by captodative substitution which explains the selectivity induced by this substitution in dehydrodimerization reactions. Radical additions to alkenes are facilitated by dicaptor and captodative substitutions while donor groups have a relatively small influence on the thermochemistry of these reactions. In the absence of steric effects, didative and captodative olefines could probably give rise to radical polymerization.

Synthesis and reactivity of captodative cyanoketenes

An expedient synthesis and the reactivity of donor substituted cyanoketenes 7 (captodative substitution) are described. 3-Cyano β-lactams are formed from 7 and imines.

The influence of gew-substitution on the stability of open-shell and closed-shell systems. The captodative and anomeric effects re-examined

In this paper we examine the influence of gem-substitution on the stability of carbon-centred radicals and their parent molecules, using the unifying concept of thermodynamic stabilization energy. We deduce this quantity either from available experimental data or from theoretical enthalpies of formation calculated at various theoretical levels. It is found that captodative substitution significantly stabilizes carbon-icentred radicals and generally destabilizes methane. Moreover, most captodative (cd) radicals studied in this work possess a sizable extra stabilization which demonstrates a synergetic effect of the captor and donor substituents at the radical centre. Didative (dd)-substituted methyl radicals and parent molecules are most often stabilized by a non-negligible anomeric effect. Finally most cc-substituted species are destabilized. On the whole, cdCH 2 compounds have systematically lower C-H bond strengths than ccCH 2 and ddCH 2 species due to the joint action of the captodative and the generalized anomeric effects. We have also shown that enthalpies of reactions can not be used to analyse the influence of substituents on the stability of chemical species. More particularly, the energy changes of the processes R . + CH 4 → RH + ·H 3 and those of group separation reactions are far from being equal to radical and anomeric stabilization energies, respectively.

Diels‐Alder Reactions of Sulfur‐Substituted Allenecarboxylates An FMO Approach by the PM3‐Method. The effect of captodative (c,d) substitution of some olefins and allenes has been investigated by MNDO‐PM3 calculations. These results are compared with the Diels‐Alder reactions of α‐sulfur‐substituted allenecarboxylates. α‐Sulfonyl allenecarboxylates 6 are a new class of stable gem‐diacceptor‐substituted allenes.

Chemische BerichteVolume 124, Issue 4 p. 867-873 Article Diels-Alder Reactions of Sulfur-Substituted Allenecarboxylates An FMO Approach by the PM3-Method. The effect of captodative (c,d) substitution of some olefins and allenes has been investigated by MNDO-PM3 calculations. These results are compared with the Diels-Alder reactions of α-sulfur-substituted allenecarboxylates. α-Sulfonyl allenecarboxylates 6 are a new class of stable gem-diacceptor-substituted allenes. Martin Conrads, Martin Conrads Organisch-Chemisches Institut der Universität Münster, Orléansring 23, D-4400 MünsterSearch for more papers by this authorJochen Mattay, Corresponding Author Jochen Mattay Organisch-Chemisches Institut der Universität Münster, Orléansring 23, D-4400 MünsterOrganisch-Chemisches Institut der Universität Münster, Orléansring 23, D-4400 MünsterSearch for more papers by this author Martin Conrads, Martin Conrads Organisch-Chemisches Institut der Universität Münster, Orléansring 23, D-4400 MünsterSearch for more papers by this authorJochen Mattay, Corresponding Author Jochen Mattay Organisch-Chemisches Institut der Universität Münster, Orléansring 23, D-4400 MünsterOrganisch-Chemisches Institut der Universität Münster, Orléansring 23, D-4400 MünsterSearch for more papers by this author First published: April 1991 https://doi.org/10.1002/cber.19911240429Citations: 10AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume124, Issue4April 1991Pages 867-873 RelatedInformation