Homolytic Bond Dissociation Research Articles

Distinguishing homolytic vs heterolytic bond dissociation of phenylsulfonium cations with localized active space methods

Modeling chemical reactions with quantum chemical methods is challenging when the electronic structure varies significantly throughout the reaction and when electronic excited states are involved. Multireference methods, such as complete active space self-consistent field (CASSCF), can handle these multiconfigurational situations. However, even if the size of the needed active space is affordable, in many cases, the active space does not change consistently from reactant to product, causing discontinuities in the potential energy surface. The localized active space SCF (LASSCF) is a cheaper alternative to CASSCF for strongly correlated systems with weakly correlated fragments. The method is used for the first time to study a chemical reaction, namely the bond dissociation of a mono-, di-, and triphenylsulfonium cation. LASSCF calculations generate smooth potential energy scans more easily than the corresponding, more computationally expensive CASSCF calculations while predicting similar bond dissociation energies. Our calculations suggest a homolytic bond cleavage for di- and triphenylsulfonium and a heterolytic pathway for monophenylsulfonium.

A physical organic strategy to predict and interpret stabilities of chemical bonds in energetic compounds for the discovery of thermal-resistant properties.

The in-depth understanding about the stability of chemical bonds in energetic compounds plays a central role for molecular design and safety-related evaluations. Most energetic compounds contain nitro as explosophores, and nitro cleavage is fundamental for thermal and mechanical stability. However, the quantum chemistry approach to accurately predict energy and temperature properties related to bond stability is challenging, due to the tradeoff between computational costs and deviations. Herein, the bond orders are proposed as accurate and computational-cost efficient descriptors for predicting the chemical bond stability and thermal-resistant properties. The intrinsic bond strength index (IBSI) demonstrates the best prediction for experimental homolytic bond dissociation energies (R2 > 0.996), which is on par with the results from high-precision quantum chemistry methods. The effects from bond connectivity and steric hindrance hierarchy were analyzed to reveal underlying mechanisms. Additionally, the IBSI descriptors are successfully applied to predict the thermal decomposition temperatures of 24 heat-resistant energetic compounds (R2 = 0.995), thus validating the effectiveness for the prediction and interpretation of chemical bond stability in energetic compounds via a physical organic approach. All DFT calculations were performed with Gaussian 09 software. To investigate the dependence of the method on functionals and basis sets, 9 DFT methods were considered (B3LYP/6-31G(d,p), B3LYP/6-311G(d,p), B3LYP/def2-TZVP, M062X/6-31G(d,p), M062X/6-311G(d,p), M062X/def2-TZVP, ωB97XD/6-31G(d,p), ωB97XD/6-311G(d,p), and ωB97XD/def2-TZVP). The bond order descriptors LBO and IBSI are obtained through the bond order analysis module in the Multiwfn software.

Predicting bond dissociation energies of cyclic hypervalent halogen reagents using DFT calculations and graph attention network model.

Although hypervalent iodine(III) reagents have become staples in organic chemistry, the exploration of their isoelectronic counterparts, namely hypervalent bromine(III) and chlorine(III) reagents, has been relatively limited, partly due to challenges in synthesizing and stabilizing these compounds. In this study, we conduct a thorough examination of both homolytic and heterolytic bond dissociation energies (BDEs) critical for assessing the chemical stability and functional group transfer capability of cyclic hypervalent halogen compounds using density functional theory (DFT) analysis. A moderate linear correlation was observed between the homolytic BDEs across different halogen centers, while a strong linear correlation was noted among the heterolytic BDEs across these centers. Furthermore, we developed a predictive model for both homolytic and heterolytic BDEs of cyclic hypervalent halogen compounds using machine learning algorithms. The results of this study could aid in estimating the chemical stability and functional group transfer capabilities of hypervalent bromine(III) and chlorine(III) reagents, thereby facilitating their development.

Unveiling distinct bonding patterns in noble gas hydrides via interference energy analysis.

Despite their apparent simplicity, the helium hydride ion (HeH+) and its analogues with heavier noble gas (Ng) atoms present intriguing challenges due to their unusual electronic structures and distinct ground-state heterolytic bond dissociation profiles. In this work, we employ modern valence bond calculations and the interference energy analysis to investigate the nature of the chemical bond in NgH+ (Ng = He, Ne, Ar). Our findings reveal that the energy well formation in their ground-state potential energy curves is driven by a reduction in kinetic energy caused by quantum interference, identical to cases of homolytic bond dissociation. However, clear differences in bonding situation emerge: in HeH+ and ArH+, electron charge transfer leads to Ng+-H covalent bonds, while in NeH+, a preferred Ne + H+ valence bond structure suggests the formation of a dative bond. This study highlights the distinct bonding mechanisms within the NgH+ series, showcasing the interplay between quantum interference and quasi-classical effects in molecules featuring noble gases.

Synthesis of Closed‐Heterohelicenes Interconvertible between Their Monomeric and Dimeric Forms

AbstractOxidative fusion reaction of cyclic heteroaromatic pentads consisting of pyrrole and thiophene gave closed‐heterohelicene monomers and dimers depending on the oxidation conditions. Specifically, oxidation with [bis(trifluoroacetoxy)iodo]benzene (PIFA) gave closed‐[7]helicene dimers connected at the β‐position of one of the pyrrole units with remarkably elongated C−C bonds of about 1.60 Å. Although this bond was intact against thermal and physical activations, homolytic bond dissociation took place in DMSO upon irradiation with UV light to give the corresponding monomers. Thus, interconversion between the closed‐helicene monomer and dimer was achieved. The optically pure dimer was photo‐dissociated into the monomers associated with a turn‐on of circularly polarized luminescence (CPL).

Synthesis of Closed-Heterohelicenes Interconvertible between Their Monomeric and Dimeric Forms.

Oxidative fusion reaction of cyclic heteroaromatic pentads consisting of pyrrole and thiophene gave closed-heterohelicene monomers and dimers depending on the oxidation conditions. Specifically, oxidation with [bis(trifluoroacetoxy)iodo]benzene (PIFA) gave closed-[7]helicene dimers connected at the β-position of one of the pyrrole units with remarkably elongated C-C bonds of about 1.60 Å. Although this bond was intact against thermal and physical activations, homolytic bond dissociation took place in DMSO upon irradiation with UV light to give the corresponding monomers. Thus, interconversion between the closed-helicene monomer and dimer was achieved. The optically pure dimer was photo-dissociated into the monomers associated with a turn-on of circularly polarized luminescence (CPL).

Experimental and Computational Determination of a M-Cl Homolytic Bond Dissociation Free Energy: Mn(III)Cl-Mediated C-H Cleavage and Chlorination.

This study confirms the hypothesis that [MnCl3(OPPh3)2] (1) and acetonitrile-solvated MnCl3 (i.e., [MnCl3(MeCN)x]) can be used as synthons to prepare Mn(III) chloride complexes with facially coordinating ligands. This was achieved through the preparation and characterization of six new {MnIIICl} complexes using anionic ligands TpH (tris(pyrazolyl)borate) and TpMe (tris(3,5-dimethylpyrazolyl)borate). The MnIII-chloride dissociation and association equilibria (Keq) and MnIII/II reduction potentials were quantified in DCM. These two thermochemical parameters (Keq and E1/2), in addition to the known Cl-atom reduction potential in DCM, enabled the quantification of the Mn-Cl bond dissociation (homolysis) free energy of 21 and 23 ± 7 kcal/mol at room temperature for R = H and Me, respectively. These are in reasonable agreement with the bond dissociation free energy (BDFEM-Cl) of 34 ± 6 kcal/mol calculated using density functional theory. The BDFEM-Cl of 1 was also calculated (25 ± 6 kcal/mol). These energies were used in predictive C-H bond reactivity.

Corresponding Active Orbital Spaces along Chemical Reaction Paths.

The accuracy of reaction energy profiles calculated with multiconfigurational electronic structure methods and corrected by multireference perturbation theory depends crucially on consistent active orbital spaces selected along the reaction path. However, it has been challenging to choose molecular orbitals that can be considered corresponding in different molecular structures. Here, we demonstrate how active orbital spaces can be selected consistently along reaction coordinates in a fully automatized way. The approach requires no structure interpolation between reactants and products. Instead, it emerges from a synergy of the Direct Orbital Selection orbital mapping ansatz combined with our fully automated active space selection algorithm autoCAS. We demonstrate our algorithm for the potential energy profile of the homolytic carbon-carbon bond dissociation and rotation around the double bond of 1-pentene in the electronic ground state. However, our algorithm also applies to electronically excited Born-Oppenheimer surfaces.

Solvent-free oxidation of toluene to benzaldehyde using electron-rich Au clusters confined in Silicalite-1

The selective oxidation of the primary CH bonds in hydrocarbons to corresponding high-valued oxygenates is highly desired but challenging to realize. The difficulty arises from the high homolytic bond dissociation enthalpy of the CH bond, the relatively high reactivity of oxygenated intermediates, and the diversity of resultant products. Herein, we reported the in situ confinement of Au clusters in Silicalite-1 for the selective solvent-free oxidation of toluene to benzaldehyde. The doping of an alkali metal (K or Na) resulted in the formation of electron-rich Au species, which readily facilitate the activation of oxygen, In turn, toluene is activated by the produced reactive oxygen species in a single step, as confirmed through density functional theory (DFT) calculations. At optimal alkali metal doping, the selectivity to benzaldehyde could reach > 90 %, with the toluene conversion being 19.8 %. Remarkably, the total turnover number (TTN) of Au@Silicalite-1–0.2 K was found to be 29,658, which is one or two orders of magnitudes higher than the previously reported values. These findings are expected to inspire the design of highly efficient catalysts for the activation of fundamental feedstock with CH bonds.

Machine Learning to Predict Homolytic Dissociation Energies of C-H Bonds: Calibration of DFT-based Models with Experimental Data.

Random Forest (RF) QSPR models were developed with a data set of homolytic bond dissociation energies (BDE) previously calculated by B3LYP/6-311++G(d,p)//DFTB for 2263 sp3C-H covalent bonds. The best set of attributes consisted in 114 descriptors of the carbon atom (counts of atom types in 5 spheres around the kernel atom and ring descriptors). The optimized model predicted the DFT-calculated BDE of an independent test set of 224 bonds with MAE=2.86 kcal/mol. A new data set of 409 bonds from the iBonD database (http://ibond.nankai.edu.cn) was predicted by the RF with a modest MAE (5.36 kcal/mol) but a relatively high R2 (0.75) against experimental energies. A prediction scheme was explored that corrects the RF prediction with the average deviation observed for the k nearest neighbours (KNN) in an additional memory of experimental data. The corrected predictions achieved MAE=2.22 kcal/mol for an independent test set of 145 bonds and the corresponding experimental bond energies.

A practical fluorosulfonylating platform via photocatalytic imidazolium-based SO2F radical reagent

Sulfonyl fluorides are key components in the fields of chemical biology, materials science and drug discovery. In this line, the highly active SO2F radical has been employed for the construction of sulfonyl fluorides, but the utilization of gaseous ClSO2F as radical precursor is limited due to the tedious and hazardous preparation. Meanwhile, the synthesis of sulfonyl fluorides from inert SO2F2 gas through a fluorosulfonyl radical (·SO2F) process has met with inevitable difficulties due to the high homolytic bond dissociation energy of the S(VI)-F bond. Here we report a radical fluorosulfonylation strategy for the stereoselective synthesis of alkenyl sulfonyl fluorides and functional alkyl sulfonyl fluorides with an air-stable crystalline benzimidazolium fluorosulfonate cationic salt reagent. This bench-stable redox-active reagent offers a useful and operational protocol for the radical fluorosulfonylation of unsaturated hydrocarbons with good yield and high stereoselectivity, which can be further transformed into valuable functional SO2F moieties.

A quantum chemistry study on C–H homolytic bond dissociation enthalpies of five-membered and six-membered heterocyclic compounds

The C–H homolytic bond dissociation enthalpies (BDEs) of 27 heterocyclic compounds of small systems (less than 8 non-hydrogen atoms) were evaluated by the composite ab initio methods of G4 and CBS-Q. In addition, the C–H BDEs of an extended database including 60 heterocyclic compounds were assessed by 16 DFT functionals. The correlation between the theoretical and experimental values reveals that the BMK functional provided the lowest root mean square error (RMSE) of 10.2 kJ/mol, and the correlation coefficient (R2) was 0.955. The mean deviation (MD), mean absolute deviation (MAD) of BMK are 0.1 kJ/mol and 7.9 kJ/mol separately. Therefore, we utilized BMK to research C–H BDEs together with the substituent effects of five-membered and six-membered heterocyclic compounds including large systems. The nature of C–H BDE change pattern was analyzed by the natural bond orbital (NBO).

Thermally Driven Diselenide Metathesis: Polarization Process vs Radical Process.

Diselenide, as a dynamic covalent bond, has been widely applied in functional materials due to its response to light, heat, sonication, pH, and other stimuli. Herein, a polarization-induced metathesis mechanism for diselenides under heating conditions in the dark is proposed. First, a radical trap experiment is used to prove that the exchange reaction of diselenides in the dark does not involve any radicals. Second, the dynamic exchange reaction of diselenides is found to be affected not only by the polarity of the solvent but also by the introduction of polar groups into the molecular skeleton. Furthermore, DFT calculations also support the notion that polarity has a large effect on the heterolytic rather than homolytic bond dissociation energies. The experimental results for allyl selenide small molecules, polymers, and polymer materials catalyzed by diselenide all support the polarization-induced metathesis mechanism. In short, we successfully enhanced the understanding of the mechanism for diselenide metathesis.

Electronic Database on Experimental Bond Dissociation Energies of Organic Compounds

The presented web database on experimental homolytic bond dissociation energies in organic compounds is intended for use by a wide range of theoreticians and practitioners in free access. The paper provides a brief overview of the sources of the dissociation energies of bonds of organic molecules, which are calculated theoretically, measured experimentally and estimated from kinetic and thermochemical experimental data, their presentation in the Internet database. A web database on homolytic bond dissociation energies of organic compounds is presented. The reported bond dissociation energies are calculated from experimental kinetic and thermochemical data. Descriptions of experimental data sources, classes of organic compounds and calculation methods are given. The logical structure of the database and the description of the main fields of its tables are given. The main search form of the database interface is presented and an example of a search result for a specific organic compound is given. Bond dissociation energies are calculated at a temperature of 298.15 K, which is usually absent in most sources. The analogs of the present base are inferior to the latter in taking into account temperature correlations. Currently, work is underway to analyze and analyze the published data taking into account the entropy effects.

Stimuli-Responsive Covalent Adaptable Hydrogels Based on Homolytic Bond Dissociation and Chain Transfer Reactions

Covalent adaptable hydrogels containing dynamic covalent bonds are gaining significant interest based on their ability of stimuli-controlled reversible bond dissociation, structural reorganization, color change, and self-healing through covalent bond exchange or dissociation. Potential applications of such hydrogels have been explored in coatings, sealants, tissue adhesives, soft robotics, tissue engineering, and stimuli-responsive lithography. Stimuli-induced homolytic bond cleavage leads to the formation of radicals with the ability of recombination or transfer to induce bond exchange and color variation. The incorporation of such stimuli-responsive homolytically cleavable bonds in hydrogels can lead to stimuli-controlled plasticity, stress relaxation, self-healing, structural reorganization, mechanochromism, and mechanoluminescence. The resultant smart materials are interesting for different applications, ranging from patterning and shape-shifting, cell encapsulation and culturing, and protein binding to strain sensing and damage reporting. The recent progress in the preparation of covalently adaptable hydrogels based on stimuli-responsive homolytical bond dissociation and recombination or chain transfer will be discussed in this review. More specifically, the different types of chemistry that can be used for development of covalent adaptable hydrogels based on light-induced, temperature-induced, and mechanically induced homolytic bond dissociation will be discussed. Moreover, the applications of the resultant covalent adaptable hydrogels will be highlighted, focusing on stress sensing and damage reporting, tissue engineering, and self-healable hydrogels, as well as stimuli-controlled patterning and shape-shifting.

On the energetics of radical adduct formation of OH• with phenol analogs and aniline

Abstract Theoretical studies on aniline, phenol, benzenethiol, benzeneselenol, and their corresponding adducts with hydroxyl radical in possible positions on a hydrocarbon ring are presented. Bond dissociation enthalpies, related to radical scavenging of primary antioxidants, were calculated using the M06–2X/6–311+G** method. Calculated data were compared with available experimental data. Preferable homolytic bond dissociation of the presented molecules with OH• through functional groups X—OH followed by the m–OH ones has been confirmed. The highest antioxidant activity among the investigated positions is predicted for benzeneselenol. Also, the formation of non-covalent van der Waals structures has been shown as important in radical scavenging.

Computational Study on Homolytic Bond Energies of the Ag-X (X = C, O, and H) Complexes and Hammett-Type Analysis of Reactivity.

Thirty-seven calculation methods were benchmarked against the available experimental bond lengths and energies data regarding the Ag–X bonds. The theoretical protocol PBE0/VDZ//ωB97x-D/mVTZ was found to be capable of accurately predicting the homolytic bond dissociation energies (BDEs) of Ag–X complexes with a precision of 1.9 kcal/mol. With the available method in hand, a wide range of different Ag–X BDEs were estimated. BDE(Ag–CH2X), BDE(Ag–PhX), BDE(Ag–OPhX), and BDE(Ag–OCOPhX) (X = NH2, OMe, Me, H, Cl, and NO2) were found to be in the ranges of 27–47, 51–54, 19–39, and 64–70 kcal/mol, respectively. Subsequently, Hammett-type analysis was carried out with reactivity parameters. Good positive linear relationships were found for BDE of Ag−O bands and decarboxylation barriers of Ag–OCOPhX with the Hammett constant σ. It suggested that electron-donating substituents could promote either the homolytic cleavage of the Ag–OPhX bond to undergo a radical process or Ag–OCOPhX decarboxylation. Moreover, ligand effects on Ag–H bonds were investigated using BDE(Ag–H) and related NPA charges on Ag. In the case of P-ligands, carbene ligands, and other small molecule ligands (i.e., CO, CO2, and H2O), a good negative linear relationship was found. In contrast, N-ligands could have a reverse effect. Understanding the intrinsic relationships of BDE(Ag–X) with related reactivity parameters might help gain insights into the structure–reactivity relationships in Ag–X-assisted C–H activation/decarboxylation.

The Chemical Bond: When Atom Size Instead of Electronegativity Difference Determines Trend in Bond Strength.

We have quantum chemically analyzed element−element bonds of archetypal HnX−YHn molecules (X, Y=C, N, O, F, Si, P, S, Cl, Br, I), using density functional theory. One purpose is to obtain a set of consistent homolytic bond dissociation energies (BDE) for establishing accurate trends across the periodic table. The main objective is to elucidate the underlying physical factors behind these chemical bonding trends. On one hand, we confirm that, along a period (e. g., from C−C to C−F), bonds strengthen because the electronegativity difference across the bond increases. But, down a period, our findings constitute a paradigm shift. From C−F to C−I, for example, bonds do become weaker, however, not because of the decreasing electronegativity difference. Instead, we show that the effective atom size (via steric Pauli repulsion) is the causal factor behind bond weakening in this series, and behind the weakening in orbital interactions at the equilibrium distance. We discuss the actual bonding mechanism and the importance of analyzing this mechanism as a function of the bond distance.

Surprising Homolytic Gas Phase Co-C Bond Dissociation Energies of Organometallic Aryl-Cobinamides Reveal Notable Non-Bonded Intramolecular Interactions.

Aryl‐cobalamins are a new class of organometallic structural mimics of vitamin B12 designed as potential ‘antivitamins B12’. Here, the first cationic aryl‐cobinamides are described, which were synthesized using the newly developed diaryl‐iodonium method. The aryl‐cobinamides were obtained as pairs of organometallic coordination isomers, the stereo‐structure of which was unambiguously assigned based on homo‐ and heteronuclear NMR spectra. The availability of isomers with axial attachment of the aryl group, either at the ‘beta’ or at the ‘alpha’ face of the cobalt‐center allowed for an unprecedented comparison of the organometallic reactivity of such pairs. The homolytic gas‐phase bond dissociation energies (BDEs) of the coordination‐isomeric phenyl‐ and 4‐ethylphenyl‐cobinamides were determined by ESI‐MS threshold CID experiments, furnishing (Co−Csp2 )‐BDEs of 38.4 and 40.6 kcal mol−1, respectively, for the two β‐isomers, and the larger BDEs of 46.6 and 43.8 kcal mol−1 for the corresponding α‐isomers. Surprisingly, the observed (Co−Csp2 )‐BDEs of the Coβ‐aryl‐cobinamides were smaller than the (Co−Csp3 )‐BDE of Coβ‐methyl‐cobinamide. DFT studies and the magnitudes of the experimental (Co−Csp2 )‐BDEs revealed relevant contributions of non‐bonded interactions in aryl‐cobinamides, notably steric strain between the aryl and the cobalt‐corrin moieties and non‐bonded interactions with and among the peripheral sidechains.

Bismuth Amides Mediate Facile and Highly Selective Pn-Pn Radical-Coupling Reactions (Pn=N, P, As).

The controlled release of well‐defined radical species under mild conditions for subsequent use in selective reactions is an important and challenging task in synthetic chemistry. We show here that simple bismuth amide species [Bi(NAr2)3] readily release aminyl radicals [NAr2]. at ambient temperature in solution. These reactions yield the corresponding hydrazines, Ar2N−NAr2, as a result of highly selective N−N coupling. The exploitation of facile homolytic Bi−Pn bond cleavage for Pn−Pn bond formation was extended to higher homologues of the pnictogens (Pn=N–As): homoleptic bismuth amides mediate the highly selective dehydrocoupling of HPnR2 to give R2Pn−PnR2. Analyses by NMR and EPR spectroscopy, single‐crystal X‐ray diffraction, and DFT calculations reveal low Bi−N homolytic bond‐dissociation energies, suggest radical coupling in the coordination sphere of bismuth, and reveal electronic and steric parameters as effective tools to control these reactions.