Intramolecular, intermolecular, and heterogeneous nonadiabatic dissociative electron transfer to peresters

The electron transfer to peresters was studied by electrochemical means in N,N-dimethylformamide. The reduction was carried out by three independent methods: (i) heterogeneously, by using glassy carbon electrodes, (ii) homogeneously, by using electrogenerated radical anions as the donors, and (iii) intramolecularly, by using purposely synthesized donor-spacer-acceptor (D-Sp-A) systems. Convolution analysis of the heterogeneous data led to results in excellent agreement with the dissociative electron transfer theory. The homogeneous redox catalysis data also confirmed the reduction mechanism. The cyclic voltammetries of the D-Sp-A molecules could be simulated, leading to determination of the corresponding intramolecular dissociative rate constants. Analysis of the results showed that, regardless of the way by which the acceptor is reduced, the investigated dissociative electron transfers are strongly nonadiabatic and, particularly, that the experimental rates are several orders of magnitude smaller than the adiabatic limit. A possible mechanism responsible for the observed behavior is discussed.     

Sticky dissociative electron transfer to polychloroacetamides. In-cage ion-dipole interaction control through the dipole moment and intramolecular hydrogen bond

The reductive cleavage of chloro- and polychloroacetamides in N,N-dimethylformamide gives new insights into the nature of the in-cage ion radical cluster formed upon dissociative electron transfer. Within the family of compounds investigated, the electrochemical reduction leads to the successive expulsion of chloride ions. At each stage the electron transfer is concerted with the breaking of the C-Cl bond and acts as the rate-determining step. The reduction further leads to the formation of the corresponding carbanion with the injection of a second electron, which is in turn protonated by a weak acid added to the solution. From the joint use of cyclic voltammetric data, the sticky dissociative electron-transfer model and quantum ab initio calculations, the interaction energies within the cluster fragments (*R, Cl-) resulting from the first electron transfer to the parent RCl molecule are obtained. It is shown that the stability of these adducts, which should be viewed as an essentially electrostatic radical-ion pair, is mainly controlled by the intensity of the dipole moment of the remaining radical part and may eventually be strengthened by the formation of an intramolecular hydrogen bond, as is the case with 2-chloroacetamide.     

Anomalous distance dependence of electron transfer across peptide bridges

The first investigation on the distance dependence of a dissociative electron transfer process across peptide bridges is reported. This study was carried out by using a series of donor-peptide-acceptor systems in which the donor is a phthalimido moiety, the peptide bridges are provided by alpha-aminoisobutyric acid (Aib) homooligomers, and the acceptor is a peroxide functional group. The intramolecular electron transfer from the electrogenerated phthalimido radical anion to the peroxide was studied in comparison with the thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular rate constants were determined in N,N-dimethylformamide by taking into account the corresponding intermolecular values. The experimental results point to an unusual non-exponential dependence of the intramolecular electron transfer rate on the number of bridge units. The same trend could be verified also by taking into account the actual donor-acceptor edge-to-edge distance. The peculiar distance dependence that was observed for the intramolecular electron transfer rate is attributed to the mediating effect of the intramolecular C=O...H-N hydrogen bonds.     

Evidence for concerted pathways in ion-pairing coupled electron transfers

Ion-pairing with electro-inactive metal ions may change drastically the thermodynamic and kinetic reactivity of electron transfer in chemical and biochemical processes. Besides the classical stepwise pathways (electron-transfer first, followed by ion-pairing or vice versa), ion-pairing may also occur concertedly with electron transfer. The latter pathway avoids high-energy intermediates but a key issue is that of the kinetic price to pay to benefit from this thermodynamic advantage. A model is proposed leading to activation/driving force relationships characterizing such concerted associative electron transfers for intermolecular and intramolecular homogeneous reactions and for electrochemical reactions. Contrary to previous assertions, the driving force of the reaction (defined as the opposite of the reaction standard free energy), as well as the intrinsic barrier, does not depend on the concentration of the ion-pairing agent, which simply plays the role of one of the reactants. Besides solvent and intramolecular reorganization, the energy of the bond being formed is the main component of the intrinsic barrier. Application of these considerations to reactions reported in recent literature illustrates how concerted ion-pairing electron-transfer reactions can be diagnosed and how competition between stepwise and concerted pathways can be analyzed. It provided the first experimental evidence of the viability of concerted ion-pairing electron-transfer reactions.     

Intra- and intermolecular reaction selectivities of γ-substituted adamantanylidenes

A study of adamantanylidenes having a γ-substituent (R) was undertaken to gauge how inductive and steric effects of remotely positioned functional groups influence intra- and intermolecular product selectivity. 3H-Diazirines were thermolyzed or photolyzed to generate the corresponding carbenes. On rapid heating, the resulting carbenes isomerized to 2,4-didehydroadamantanes by intramolecular 1,3-CH insertions. When R was an electron donor (R(D)) mostly asymmetric 1-substituted derivatives were produced but when it was an electron acceptor (R(A)) the symmetric 7-substituted ones were formed. When solutions were exposed to UV-A light, intermolecular adducts from the carbenes and solvent predominated with lesser amounts of intramolecular product being formed. Valence isomerization of 3H-diazirines also afforded diazo compounds. In methanol, protonation of diazo compounds to give the corresponding 2-adamantyl cations exceeds their coupling. This diversion was controlled with fumaronitrile by trapping the diazo compounds. The adducts possessed mostly anti configurations with R = R(D) and syn arrangements with R = R(A). The connection between as- and anti-product formation and that of s- and syn-products was deemed to be the consequence of a rapid equilibrium between two distinct carbene conformations. This was qualified and quantified using ab initio calculations and NBO analyses.     

Solvent dependence of the charge-transfer properties of a quaterthiophene–anthraquinone dyad

An electron donor–acceptor dyad (quaterthiophene–anthraquinone) mediates ultrafast intramolecular photoinduced charge separation and consequent charge recombination when in polar or moderately polar solvents. Alternatively, non-polar media completely impedes the initial photoinduced electron transfer by causing enough destabilization of the charge-transfer state and shifting its energy above the energy of the lowest locally excited singlet state. Furthermore, femtosecond transient-absorption spectroscopy reveals that for the solvents mediating the initial photoinduced electron-transfer process, the charge recombination rates were slower than the rates of charge separation. This behavior of donor–acceptor systems is essential for solar-energy-conversion applications. For the donor–acceptor dyad described in this study, the electron-transfer driving force and reorganization energy place the charge-recombination processes in the Marcus inverted region.     

Quantum Dot‐Catalyzed Photoreductive Removal of Sulfonyl‐Based Protecting Groups

This Communication describes the use of CuInS₂/ZnS quantum dots (QDs) as photocatalysts for the reductive deprotection of aryl sulfonyl‐protected phenols. For a series of aryl sulfonates with electron‐withdrawing substituents, the rate of deprotection for the corresponding phenyl aryl sulfonates increases with decreasing electrochemical potential for the two electron transfers within the catalytic cycle. The rate of deprotection for a substrate that contains a carboxylic acid, a known QD‐binding group, is accelerated by more than a factor of ten from that expected from the electrochemical potential for the transformation, a result that suggests that formation of metastable electron donor–acceptor complexes provides a significant kinetic advantage. This deprotection method does not perturb the common NHBoc or toluenesulfonyl protecting groups and, as demonstrated with an estrone substrate, does not perturb proximate ketones, which are generally vulnerable to many chemical reduction methods used for this class of reactions.     

Electronic coupling matrix elements from charge constrained density functional theory calculations using a plane wave basis set

We present a plane wave basis set implementation for the calculation of electronic coupling matrix elements of electron transfer reactions within the framework of constrained density functional theory (CDFT). Following the work of Wu and Van Voorhis [J. Chem. Phys. 125, 164105 (2006)], the diabatic wavefunctions are approximated by the Kohn-Sham determinants obtained from CDFT calculations, and the coupling matrix element calculated by an efficient integration scheme. Our results for intermolecular electron transfer in small systems agree very well with high-level ab initio calculations based on generalized Mulliken-Hush theory, and with previous local basis set CDFT calculations. The effect of thermal fluctuations on the coupling matrix element is demonstrated for intramolecular electron transfer in the tetrathiafulvalene-diquinone (Q-TTF-Q(-)) anion. Sampling the electronic coupling along density functional based molecular dynamics trajectories, we find that thermal fluctuations, in particular the slow bending motion of the molecule, can lead to changes in the instantaneous electron transfer rate by more than an order of magnitude. The thermal average, (<|H(ab)|(2)>)(1/2)=6.7 mH, is significantly higher than the value obtained for the minimum energy structure, |H(ab)|=3.8 mH. While CDFT in combination with generalized gradient approximation (GGA) functionals describes the intermolecular electron transfer in the studied systems well, exact exchange is required for Q-TTF-Q(-) in order to obtain coupling matrix elements in agreement with experiment (3.9 mH). The implementation presented opens up the possibility to compute electronic coupling matrix elements for extended systems where donor, acceptor, and the environment are treated at the quantum mechanical (QM) level.     

Quantum Dot-Catalyzed Photoreductive Removal of Sulfonyl-Based Protecting Groups

This Communication describes the use of CuInS₂/ZnS quantum dots (QDs) as photocatalysts for the reductive deprotection of aryl sulfonyl-protected phenols. For a series of aryl sulfonates with electron-withdrawing substituents, the rate of deprotection for the corresponding phenyl aryl sulfonates increases with decreasing electrochemical potential for the two electron transfers within the catalytic cycle. The rate of deprotection for a substrate that contains a carboxylic acid, a known QD-binding group, is accelerated by more than a factor of ten from that expected from the electrochemical potential for the transformation, a result that suggests that formation of metastable electron donor–acceptor complexes provides a significant kinetic advantage. This deprotection method does not perturb the common NHBoc or toluenesulfonyl protecting groups and, as demonstrated with an estrone substrate, does not perturb proximate ketones, which are generally vulnerable to many chemical reduction methods used for this class of reactions.     

Electrochemically induced electron transfer through molecular bridges

In this Opinion, we address some of the most important results obtained electrochemically in the area of intramolecular electron transfer (ET). The focus is on freely diffusing molecular systems in which a donor D and an acceptor A are separated by a well-defined bridge B (D-B-A systems). B can be a saturated spacer, a delocalized bridge, or the more complex peptide backbones. As to the acceptors, the selected examples encompass species that can be charged reversibly but a special emphasis is on ETs associated with the concerted cleavage of a sigma bond (dissociative ETs). Our goal is to showcase the essential background, the most appropriate electrochemical tools and methodologies, and a series of selected examples where molecular electrochemistry has provided invaluable information on the mechanisms of intramolecular ET and electronic communication through bridges.     

Observation of the Low‐Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces

Using the helium nanodroplet isolation setup at the ultrabright free‐electron laser source FELIX in Nijmegen (BoHeNDI@FELIX), the intermolecular modes of water dimer in the frequency region from 70 to 550 cm^?1 were recorded. Observed bands were assigned to donor torsion, acceptor wag, acceptor twist, intermolecular stretch, donor torsion overtone, and in‐plane and out‐of‐plane librational modes. This experimental data set provides a sensitive test for state‐of‐the‐art water potentials and dipole moment surfaces. Theoretical calculations of the IR spectrum are presented using high‐level quantum and approximate quasiclassical molecular dynamics approaches. These calculations use the full‐dimensional ab initio WHHB potential and dipole moment surfaces. Based on the experimental data, a considerable increase of the acceptor switch and a bifurcation tunneling splitting in the librational mode is deduced, which is a consequence of the effective decrease in the tunneling barrier.     

Tetrathiafulvalene derivatives as NLO-phores: synthesis, electrochemistry, Raman spectroscopy, theoretical calculations, and NLO properties of novel TTF-derived donor-pi-acceptor dyads.

Novel pi-conjugated donor-acceptor chromophores, based on the strong electron-donating tetrathiafulvalene moiety and different electron-withdrawing acceptors, exhibit large second-order optical nonlinearities. The effect of increasing the length of the polyenic spacer and the influence of the nature of the acceptor moiety on the NLO properties have been studied by using the electric field-induced second-harmonic generation (EFISH) technique as well as by semiempirical and ab initio theoretical calculations. A charge-transfer band has been observed in the absorption spectra of these D-pi-A compounds that undergoes an hypsochromic shift when increasing the number of vinylenic spacer units connecting both donor and acceptor moieties. The degree of the intramolecular charge transfer from the donor to the acceptor has also been analyzed by means of Raman spectroscopy.     

Mechanism of the hydrogen transfer from the OH group to oxygen-centered radicals: proton-coupled electron-transfer versus radical hydrogen abstraction

High-level ab initio electronic structure calculations have been carried out with respect to the intermolecular hydrogen-transfer reaction HCOOH+.OH-->HCOO.+H(2)O and the intramolecular hydrogen-transfer reaction .OOCH2OH-->HOOCH(2)O.. In both cases we found that the hydrogen atom transfer can take place via two different transition structures. The lowest energy transition structure involves a proton transfer coupled to an electron transfer from the ROH species to the radical, whereas the higher energy transition structure corresponds to the conventional radical hydrogen atom abstraction. An analysis of the atomic spin population, computed within the framework of the topological theory of atoms in molecules, suggests that the triplet repulsion between the unpaired electrons located on the oxygen atoms that undergo hydrogen exchange must be much higher in the transition structure for the radical hydrogen abstraction than that for the proton-coupled electron-transfer mechanism. It is suggested that, in the gas phase, hydrogen atom transfer from the OH group to oxygen-centered radicals occurs by the proton-coupled electron-transfer mechanism when this pathway is accessible.     

Studying and switching electron transfer: from the ensemble to the single molecule

We report here on the systematic investigation of photoinduced intramolecular electron transfer (IET) in a series of donor-bridge-acceptor molecules as a means of understanding electron transport through the bridge. Perylenebisimide chromophores connected by various oligophenylene bridges are studied because their electron-transfer behavior can readily be monitored by following changes in the fluorescence intensity. We find dramatic switching of the IET behavior as the solvent polarity (dielectric constant) is increased. By combining steady-state and time-resolved fluorescence spectroscopy in a variety of solvents at multiple temperatures with standard theories of electron transfer, we determine parameters governing the IET behavior of these dimers, such as the electronic coupling through the bridges. We also deploy available ab initio quantum chemical methods to calculate the through-space component of the electronic coupling matrix element. Single-molecule investigations of the electron-transfer behavior also show that IET can be switched reversibly by a similar mechanism in an isolated individual molecule.     

Intramolecular dissociative electron transfer

Dissociative electron transfers (ET) are reactions in which the ET is associated with the cleavage of a sigma bond. Although a rather satisfactory amount of information is currently available on the intermolecular and heterogeneous dissociative ET reactions, less is known for the corresponding intramolecular processes, despite the relevance of these reactions in both chemistry and biochemistry. This tutorial review focuses on the most recent developments in this area, with particular emphasis on the reactions occurring in well-defined Donor-Spacer-Acceptor molecular systems. The goal is to provide the reader with the essential background to understand and possibly predict the feasibility and rates of these reactions, as well as to stimulate the application of the intramolecular dissociative ET concepts and related issues to still unexplored molecular systems.     

Quantification of intramolecular nonbonding interactions in organochalcogens

Intramolecular nonbonding interactions between chalcogen atoms in a series of ortho substituted arylselenides (S/O...Se-Y, with Y = -Me, -CN, -Cl, and -F) are quantified using the coupled cluster CCSD(T)/cc-pVDZ level of theory. A homodesmic reaction method as well as an ortho-para approach are employed in evaluating the strength of intramolecular interactions. Comparison of the results obtained using the ab initio MP2 method and pure and hybrid density functional theories are performed with that of the coupled cluster values to assess the quality of different density functionals in evaluating the strength of nonbonding interactions. The interaction energies are found to be higher when the thioformyl group acts as the donor and the Se-F bond acts as the acceptor. In a given series with the same donor atom, the strength of the interaction follows the order Me < CN < Cl < F, exhibiting fairly high sensitivity to the group attached to selenium (Se-Y). Analysis of electron density at the S/O...Se bond critical point within the Atoms in Molecule framework shows a very good correlation with the computed intramolecular interaction energies.     

Evidences of strong C-H...O bond in an o-carboranyl beta-lactoside in solution

In this communication, the presence of a persistent intramolecular C-H....O bond in solution, has been evidenced by NMR spectroscopy and theoretical calculations. The molecules under study were o-carboranyl beta-glycoside derivatives, in which, the donor group was the activated CH of the carboranyl residue and the acceptor atom was the glycosidic oxygen. The strength of the interaction and the geometry of the atoms involved were determined by ab initio calculations.     

Ab initio based calculations of electron-transfer rates in metalloproteins

A long-standing challenge in electron-transfer theory is to compute accurate rates of long-distance reactions in proteins. We describe an ab initio Hartree-Fock approach to compute electronic-coupling interactions and electron-transfer rates in proteins that allows the favorable comparison with experiment. The method includes the following key features; each is essential for reliable rate computations: (1) summing contributions over multiple tunneling pathways, (2) averaging couplings over thermally accessible protein conformations, (3) describing donor and acceptor electronic structure explicitly, including solvation effects, and averaging coupling over multiple energy-level crossings of the nearly degenerate donor-acceptor ligand-field states, and (4) eliminating basis set artifacts associated with diffuse basis functions. The strong dependence of coupling on donor-acceptor distance and on pathway interferences causes large variations of the computed electron-coupling values with protein geometry, and the strongest coupled conformers dominate the electron-transfer rate. As such, averaging over thermally accessible conformers of the protein and of the redox cofactors is essential. This approach was tested on six ruthenium-modified azurin derivatives using the high temperature nonadiabatic rate expression and compared with simpler pathways, average barrier, and semiempirical INDO models. Results of ab initio Hartree-Fock calculations with a split-valence basis set are in good agreement with the experimental rates. Predicted rates in the longer-distance derivatives are underestimated by 3-8-fold. This analysis indicates that the key ingredients needed for quantitatively reliable protein electron-transfer rate calculations are accessible.     

Molecular Self‐Recognition: Rotational Spectra of the Dimeric 2‐Fluoroethanol Conformers

Fluoroalcohols show competitive formation of intra‐ and intermolecular hydrogen bonds, a property that may be crucial for the protein‐altering process in a fluoroalcohol/water solution. In this study, we examine the intra‐ and intermolecular interactions of 2‐fluoroethanol (FE) in its dimeric conformers by using rotational spectroscopy and ab initio calculations. Three pairs of homo‐ and heterochiral dimeric FE conformers are predicted to be local minima at the MP2/6‐311++G(d,p) level of theory. They are solely made of the slightly distorted most stable G+g?/G?g+ FE monomer units. Jet‐cooled rotational spectra of four out of the six predicted dimeric conformers were observed and unambiguously assigned for the first time. All four observed dimeric conformers have compact geometries in which the fluoromethyl group of the acceptor tilts towards the donor and ensures a large contact area. Experimentally, the insertion of the O? H group of one FE subunit into the intramolecular O? H???F bond of the other was found to lead to a higher stabilisation than the pure association through an intermolecular O? H???O? H link. The hetero‐ and homochiral combinations were observed to be preferred in the inserted and the associated dimeric conformers, respectively. The experimental rotational constants and the stability ordering are compared with the ab initio calculations at the MP2 level with the 6‐311++G(d,p) and aug‐cc‐pVTZ basis sets. The effects of fluorination and the competing inter‐ and intramolecular hydrogen bonds on the stability of the dimeric FE conformers are discussed.