Traditional and Ion-Pair Halogen-Bonded Complexes Between Chlorine and Bromine Derivatives and a Nitrogen-Heterocyclic Carbene

A theoretical study of the halogen-bonded complexes (A–X···C) formed between halogenated derivatives (A–X; A = F, Cl, Br, CN, CCH, CF₃, CH₃, H; and X = Cl, Br) and a nitrogen heterocyclic carbene, 1,3-dimethylimidazole-2-ylidene (MeIC) has been performed using MP2/aug′-cc-pVDZ level of theory. Two types of A–X:MeIC complexes, called here type-I and -II, were found and characterized. The first group is described by long C–X distances and small binding energies (8–54 kJ·mol^–1). In general, these complexes show the traditional behavior of systems containing halogen-bonding interactions. The second type is characterized by short C–X distances and large binding energies (148–200 kJ·mol^–1), and on the basis of the topological analysis of the electron density, they correspond to ion-pair halogen-bonded complexes. These complexes can be seen as the interaction between two charged fragments: A^– and ⁺[X–CIMe] with a high electrostatic contribution in the binding energy. The charge transfer between lone pair A­(LP) to the σ* orbital of C–X bond is also identified as a significant stabilizing interaction in type-II complexes

Photoemission Spectra and Density Functional Theory Calculations of 3d Transition Metal–Aqua Complexes (Ti–Cu) in Aqueous Solution

Photoelectron spectroscopy measurements and density functional calculations are combined to determine the lowest electron binding energies of first-row transition-metal aqua ions, titanium through copper, with 3d¹ through 3d⁹ electronic configurations, in their most common oxidation states. Vertical ionization energies are found to oscillate considerably between 6.76 and 9.65 eV for the dications and between 7.05 and 10.28 eV for the respective trivalent cations. The metal cations are modeled as [M­(H₂O)_<i>n</i>]^<i>q</i>+ clusters (<i>q</i> = 2, 3, and 4; <i>n</i> = 6 and 18) surrounded by continuum solvent. The performance of 10 exchange–correlation functionals, two GGAs, three MGGAs, two HGGAs and three HMGGAs, combined with the MDF10­(ECP)/6-31+G­(d,p) basis set is assessed for 11 M–O bond distances, 10 vertical ionization energies, 6 adiabatic ionization energies, and the associated reorganization free energies. We find that for divalent cations the HGGA and HMGGA functionals in combination with the 18 water model show the best agreement with experimental vertical ionization energies and geometries; for trivalent ions, the MGGA functionals perform best. The corresponding reorganization free energies (λ_o) of the oxidized ions are significantly underestimated with all DFT functionals and cluster models. This indicates that the structural reorganization of the solvation shell upon ionization is not adequately accounted for by the simple solvation models used, emphasizing the importance of extended sampling of thermally accessible solvation structures for an accurate computation of this quantity. The photoelectron spectroscopy measurements reported herein provide a comprehensive set of transition-metal redox energetic quantities for future electronic structure benchmarks

DFT Study on the Relative Stabilities of Substituted Ruthenacyclobutane Intermediates Involved in Olefin Cross-Metathesis Reactions and Their Interconversion Pathways

DFT (M06-L) calculations have been used to determine the relative stabilities of the metallacyclobutane intermediates arising from the cross-metathesis reactions of terminal olefins as well as to get insights into the origin of the nondetection of the α,β-substituted species. For that, we discuss the structures, NMR signatures, stabilities with respect to separated reactants, and experimentally proposed interconversion pathways of all potential metallacyclobutane intermediates arising from propene and styrene homocoupling. For the case of propene, the unsubstituted and mono- and disubstituted metallacycles are lower in Gibbs energy than the separated reactants under the NMR experimental conditions. Moreover, for the same number of substituents, regardless of their nature, the metallacycles presenting substituents at the C_α carbons are always lower in energy than those presenting substituents at C_β, the energy difference being between 1.7 and 8.8 kcal mol^–1. The computed energy barriers associated with the olefin and carbene rotation processes, two of the experimentally proposed pathways for the metallacycle interconversion, are low and are in excellent agreement with the values previously determined through NMR studies. Cycloaddition and cycloreversion energy barriers are also low, and in fact, there is not a significant difference between the barrier heights of the processes leading to observed or nonobserved intermediates. Therefore, the nondetection of metallacyclobutane intermediates with substituents in C_β seems to arise from their lower stability in comparison with the isomers with substituents in C_α, which makes their detection not feasible under thermodynamic equilibrium conditions. That is, for cross-metathesis processes involving small terminal alkenes and activated carbenes, the nature of the observed metallacycles is based on thermodynamic control. The preference of having the substituents in C_α is attributed to the formation of stronger M–C and C–C bonds during the cycloaddition when the substituents are in an α position due to higher charge transfer from the original alkene fragment to the metal carbene

DFT Study on the Relative Stabilities of Substituted Ruthenacyclobutane Intermediates Involved in Olefin Cross-Metathesis Reactions and Their Interconversion Pathways

DFT (M06-L) calculations have been used to determine the relative stabilities of the metallacyclobutane intermediates arising from the cross-metathesis reactions of terminal olefins as well as to get insights into the origin of the nondetection of the α,β-substituted species. For that, we discuss the structures, NMR signatures, stabilities with respect to separated reactants, and experimentally proposed interconversion pathways of all potential metallacyclobutane intermediates arising from propene and styrene homocoupling. For the case of propene, the unsubstituted and mono- and disubstituted metallacycles are lower in Gibbs energy than the separated reactants under the NMR experimental conditions. Moreover, for the same number of substituents, regardless of their nature, the metallacycles presenting substituents at the C_α carbons are always lower in energy than those presenting substituents at C_β, the energy difference being between 1.7 and 8.8 kcal mol^–1. The computed energy barriers associated with the olefin and carbene rotation processes, two of the experimentally proposed pathways for the metallacycle interconversion, are low and are in excellent agreement with the values previously determined through NMR studies. Cycloaddition and cycloreversion energy barriers are also low, and in fact, there is not a significant difference between the barrier heights of the processes leading to observed or nonobserved intermediates. Therefore, the nondetection of metallacyclobutane intermediates with substituents in C_β seems to arise from their lower stability in comparison with the isomers with substituents in C_α, which makes their detection not feasible under thermodynamic equilibrium conditions. That is, for cross-metathesis processes involving small terminal alkenes and activated carbenes, the nature of the observed metallacycles is based on thermodynamic control. The preference of having the substituents in C_α is attributed to the formation of stronger M–C and C–C bonds during the cycloaddition when the substituents are in an α position due to higher charge transfer from the original alkene fragment to the metal carbene

A Systematic DFT Study of Two Elementary Steps Involving Hydrogen Peroxide and the Hydroxyl Radical in the Fenton Reaction

The Fenton reaction plays a central role in many chemical and biological processes and has various applications as e.g. water remediation. The reaction consists of the iron-catalyzed homolytic cleavage of the oxygen-oxygen bond in the hydrogen peroxide molecule and the reduction of the hydroxyl radical. Here, we study these two elementary steps with high-level ab-initio calculations at the complete basis set limit and address the performance of different DFT methods following a specific classification based on the Jacob´s ladder in combination with various Pople's basis sets. Ab-initio calculations at the complete basis set limit are in agreement to experimental reference data and identified a significant contribution of the electron correlation energy to the bond dissociation energy (BDE) of the oxygen-oxygen bond in hydrogen peroxide and the electron affinity (EA) of the hydroxyl radical. The studied DFT methods were able to reproduce the ab-initio reference values, although no functional was particularly better for both reactions. The inclusion of HF exchange in the DFT functionals lead in most cases to larger deviations, which might be related to the poor description of the two reactions by the HF method. Considering the computational cost, DFT methods provide better BDE and EA values than HF and post--HF methods with an almost MP2 or CCSD level of accuracy. However, no systematic general prediction of the error based on the employed functional could be established and no systematic improvement with increasing the size in the Pople's basis set was found, although for BDE values certain systematic basis set dependence was observed. Moreover, the quality of the hydrogen peroxide, hydroxyl radical and hydroxyl anion structures obtained from these functionals was compared to experimental reference data. In general, bond lengths were well reproduced and the error in the angles were between one and two degrees with some systematic trend with the basis sets. From our results we conclude that DFT methods present a computationally less expensive alternative to describe the two elementary steps of the Fenton reaction. However, choice of approximated functionals and basis sets must be carefully done and the provided benchmark allows a systematic validation of the electronic structure method to be employe

Clean Singlet Oxygen Production by a Re^I Complex Embedded in a Flexible Self-Standing Polymeric Silsesquioxane Film

Rhenium complexes are versatile molecular building blocks whose tunable photophysical properties are useful in diverse opto-related applications. Herein we report the synthesis and characterization of a novel Re^I tricarbonyldiimine complex, [(<i>phen</i>)­Re­(CO)₃Br] (<i>phen</i>: 1,10-phenanthroline), which was found to be an efficient singlet oxygen [O₂(¹Δ_g)] photosensitizer in homogeneous solution [Φ_O₂(¹Δg) = 0.55 (dichloromethane) and 0.16 (dimethylformamide)]. The photophysical properties of [(<i>phen</i>)­Re­(CO)₃Br] were thoroughly characterized in solution and modeled by means of density functional theory (DFT) and time-dependent (TD)-DFT quantum mechanical calculations. The Re complex was incorporated into a flexible polymeric silsesquioxane (SSO) film, which has excellent dopant compatibility, chemical resistance, and mechanical properties. When [(<i>phen</i>)­Re­(CO)₃Br] is embedded in the SSO film, it is found to retain most of the photophysical characteristics observed for the complex in solution. In particular, the [(<i>phen</i>)­Re­(CO)₃Br]-doped SSO films were able to photosensitize O₂(¹Δ_g) when illuminated with blue light (∼405 nm). The O₂(¹Δ_g) sensitization by films in acetonitrile was followed by the photooxidation of the well-known O₂(¹Δ_g) chemical trap 9,10-dimethylanthracene (DMA) and confirmed by the direct observation of the O₂(¹Δ_g) luminescence spectrum (centered at 1270 nm) and the measurement of its kinetic profile. These results highlight the potential application of this type of polymeric material in the production of biological- or microbial-photoinactivating flexible surfaces or in the implementation of interfacial solid/liquid strategies for the photoinduced oxidation of organic compounds in solution