Reactivity Of Different Sites Research Articles

Mechanistic Studies of Regiocontrolled Bisaddition of Fullerenes Driven by Oriented External Electric Fields.

The challenge of achieving regioselective multifunctionalization on highly symmetric C60 and C70 fullerenes persists as a significant hurdle. In this study, we present a novel approach involving the participation of an oriented external electric field (OEEF) to facilitate the regioselective formation of bisadducts in C60/C70 fullerenes. These products are obtained through consecutive Diels-Alder cycloaddition reactions. We constructed the field strength-barrier relationship and elucidated the OEEF-driven modulation mechanisms quantitatively. Leveraging the interplay between molecular dipoles and electric fields, the diverse reactions at distinct sites exhibit varying degrees of sensitivity to the applied electric fields, thereby leading to a pronounced regioselectivity in the bisaddition process. Our proposition suggests that the angle formed between the bonding direction (referred to as the reaction axis) and the external field can conveniently function as a predictive descriptor for the reactivity of different sites on the fullerene surface when subjected to electric fields.

Atmospheric oxidation of furanones by •OH and •Cl radicals: In situ FTIR rate coefficient determinations, SAR and theoretical studies

Gas-phase kinetics of the overall reactions of •OH and •Cl radicals with dihydrofuran-3(2H)-one (oxolan-3-one) and dihydro-2-methyl-3(2H)-furanone (2MTHF-3-one) were studied at 298 K and atmospheric pressure. The rate coefficients were determined using the relative method in a 480 L multipass glass reactor coupled to an FT-IR detection system. The rate coefficients found for oxolan 3-one and 2MTHF-one with •OH radicals (k1 and k2) and with •Cl atoms (k3 and k4) at 298 K and atmospheric pressure (in cm3 molecule−1 s−1) were: k1 = (1.86 ± 0.29) × 10−11, k2 = (2.64 ± 0.47) × 10−11, k3= (1.15 ± 0.28) × 10 −10, and k4 = (1.33 ± 0.32) × 10−10, respectively. Reactivity trends were developed by comparison with other similar structures and Fukui indices employed to determine the reactivity of different sites on the ring. The singularity of the reaction with •OH was assessed by computational studies which showed the formation of several stable hydrogen bonded complexes, explaining the difference with the reaction with the •Cl atom. SAR estimations of the rate coefficients were calculated and compared to the experimental values.

Chemodivergent assembly of ortho-functionalized phenols with tunable selectivity via rhodium(III)-catalyzed and solvent-controlled C-H activation

Ortho-functionalized phenols and their derivatives represent prominent structural motifs and building blocks in medicinal and synthetic chemistry. While numerous synthetic approaches exist, the development of atom-/step-economic and practical methods for the chemodivergent assembly of diverse ortho-functionalized phenols based on fixed catalyst/substrates remains challenging. Here, by selectively controlling the reactivities of different sites in methylenecyclopropane core, Rh(III)-catalyzed redox-neutral and tunable C-H functionalizations of N-phenoxyacetamides are realized, providing access to both ortho-functionalized phenols bearing linear dienyl, cyclopropyl or allyl ether groups, and cyclic 3-ethylidene 2,3-dihydrobenzofuran frameworks under mild cross-coupling conditions. These divergent transformations feature broad substrate compatibility, synthetic applications and excellent site-/regio-/chemoselectivity. Experimental and computational mechanistic studies reveal that distinct catalytic modes involving selective β-C/β-H elimination, π-allylation, inter-/intramolecular nucleophilic substitution cascade and β-H’ elimination processes enabled by different solvent-mediated and coupling partner-controlled reaction conditions are crucial for achieving chemodivergence, among which a structurally distinct Rh(V) species derived from a five-membered rhodacycle is proposed as the corresponding active intermediates.

Photodegradation of C-PCPDTBT and Si-PCPDTBT: influence of the bridging atom on the stability of a low-band-gap polymer for solar cell application.

The kinetics of photodegradation and the reactivity of different sites of the low-band-gap polymers poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and poly[2,6-(4,4-bis-(2-ethylhexyl)dithieno[3,2-b:2',3'-d]silole)-alt-4,7-(2,1,3-benzothiadiazole)] (Si-PCPDTBT) are investigated as thin films and are compared to those of poly(3-hexylthiophene) (P3HT). The decay kinetics are monitored with UV/Vis spectroscopy and the reactivity and product evolution are investigated with X-ray photoelectron spectroscopy (XPS). Both polymers exhibit higher stability than P3HT. The bridging atom in the cyclopentadithiophene (CPDT) subunit has a significant influence on the stability. Varying oxidation rates for the different elements were observed. In the case of Si-PCPDTBT, the silicon atom is oxidized primarily, whereas the photooxidation rates of the other elements are reduced relative to C-PCPDTBT. Additionally, XPS experiments with varying excitation energies reveal a significant reaction gradient within a few nanometers of the surface of degraded thin films of C-PCPDTBT.

Why bistetracenes are much less reactive than pentacenes in Diels-Alder reactions with fullerenes.

The Diels-Alder (DA) reactions of pentacene (PT), 6,13-bis(2-trimethylsilylethynyl)pentacene (TMS-PT), bistetracene (BT), and 8,17-bis(2-trimethylsilylethynyl)bistetracene (TMS-BT) with the [6,6] double bond of [60]fullerene have been investigated by density functional theory calculations. Reaction barriers and free energies have been obtained to assess the effects of frameworks and substituent groups on the DA reactivity and product stability. Calculations indicate that TMS-BT is about 5 orders of magnitude less reactive than TMS-PT in the reactions with [60]fullerene. This accounts for the observed much higher stability of TIPS-BT than TIPS-PT when mixed with PCBM. Surprisingly, calculations predict that the bulky silylethynyl substituents of TMS-PT and TMS-BT have only a small influence on reaction barriers. However, the silylethynyl substituents significantly destabilize the corresponding products due to steric repulsions in the adducts. This is confirmed by experimental results here. Architectures of the polycyclic aromatic hydrocarbons (PAHs) play a crucial role in determining both the DA barrier and the adduct stability. The reactivities of different sites in various PAHs are related to the loss of aromaticity, which can be predicted using the simple Hückel molecular orbital localization energy calculations.

A theoretical insight into surface reactivity of nitrogen-doped BC3 nanotubes

Chemical doping with foreign atoms offers a practical path to modify the properties of host materials. Among them, nitrogen-doping plays an important role in regulating the electronic and mechanical properties of carbon-based materials. In the present study, density functional theory calculations are performed to predict the relative reactivities of different sites on the external surface of pristine and nitrogen-doped BC3 nanotubes (BC3NTs). The properties determined include the electrostatic potential VS(r) and average local ionization energy ĪS(r) on the surfaces of the investigated tubes. It is revealed that doping BC3NTs with N impurity can significantly improve the tube׳s surface reactivity and it allows their surface properties to be controlled. There is an obvious increase in the magnitude of VS(r) minima for all the BC3 doped with N atom, compared to the pristine one. There is a good correlation between chemisorption energies and average local ionization energies, indicating that ĪS(r) provides an effective means for rapidly predicting the relative reactivities of finite-sized BC3NTs. These results could be useful for designing and developing metal-free catalyst based on N-doped BC3NTs.

Reactivity of the interior surface of (5,5) single-walled carbon nanotubes with and without a Stone–Wales defect

The adsorption of hydrogen and fluorine atoms on the interior surface of defect-free and Stone–Wales defect armchair (5,5) single-walled carbon nanotubes were investigated using density functional theory. The reaction energy values for the hydrogenation and fluorination at all the possible unique sites were obtained at the UB3LYP/6-31G* level. The calculated reaction energy values were used to corroborate the reactivities of different sites on the interior surface of SWCNTs. The results indicate that for defect-free nanotube, endohedral adsorption is rather unfavorable both for hydrogen and fluorine. The computed exothermicities range from 4.8 to 13.4kcal/mol for hydrogenation and from 16.6 to 23.2kcal/mol for fluorination. However, the introduction of a SW defect on the (5,5) SWCNT improves the chemical reactivity of the interior surface of the defected tube. The computed exothermicities for the endohedral hydrogen and fluorine atoms are within the ranges of 7.0–33.4 and 18.5–41.8kcal/mol, respectively. The most exothermic and reactive site on the interior surface of SW defective (5,5) SWCNT is the C2 site shared by two seven-membered rings and one five-membered ring. This is different from the results reported on the reactivity of the exterior surface of SW defective nanotubes. The electronic and vibrational properties of the (5,5) SWCNTs with adsorption of hydrogen and fluorine atoms on the interior surface were also explored.

Diels–Alder Reactions of Graphene: Computational Predictions of Products and Sites of Reaction

The cycloaddition reactions and noncovalent π interactions of 2,3-dimethoxybutadiene (DMBD), 9-methylanthracene (MeA), tetracyanoethylene (TCNE), and maleic anhydride (MA) with graphene models have been investigated using density functional theory (DFT) calculations. Reaction enthalpies have been obtained to assess the reactivity and selectivity of covalent and noncovalent functionalization. Results indicate that graphene edges may be functionalized by the four reagents through cycloaddition reactions, while the interior regions cannot react. Noncovalent complexation is much more favorable than cycloaddition reactions on interior bonds of graphene. The relative reactivities of different sites in graphene are related to loss of aromaticity and can be predicted using Hückel molecular orbital (HMO) localization energy calculations.

Nitrogen-doping improves surface reactivity of carbon nanocone

A density functional theory study was carried out to predict the relative reactivities of different sites on the external surface of pristine and nitrogen-doped carbon nanocones (CNCs). The properties determined include the electrostatic potential VS(r) and average local ionization energy I¯S(r) on the surfaces of the investigated CNCs. Our results reveal that the surface potentials of the N-doped CNCs are considerably affected by the presence of the dopant atom. More especially, the carbon atoms located in the cone apex show different reactivity pattern than those at the edge regions of the CNC. There is an obvious increase in the magnitude of VS(r) minima (VS,min) for the all CNCs doped with N atom, compared to the pristine one. This significant increase in the value of VS,min parameter stands for the activation of CNC surface toward electrophilic attack. There is a good correlation between chemisorption energies and average local ionization energies, indicating that I¯S(r) provides an effective means for rapidly predicting the relative reactivities of finite-sized CNCs.

Surface Reactivity for Chlorination on Chlorinated (5,5) Armchair SWCNT: A Computational Approach

In the present work, we first attached one chlorine radical and then examined the site selectivity for the addition of a second chlorine radical on the external surface of (5,5) armchair single-walled carbon nanotubes (SWCNTs) of 9 and 15 carbon layers. Optimized geometries and energetics were determined using density functional theory (DFT), at the M06-2X/6-31G(d,p) level. Computed reaction energy values for second chlorination have been used to corroborate the prediction of the reactivities of different sites based on the electron density distribution in the singly occupied molecular orbital (SOMO), total spin density, the DFT based local reactivity descriptors (viz., Fukui function, local softness), and the average local ionization energy. Natural bond orbital (NBO) analysis was performed to confirm the covalent bonding between the chlorine and carbon of the SWCNT. The present study reveals that the first chlorine attached to the SWCNT behaves like an ortho- and para-director. The addition of a second chlorine on an already chlorinated SWCNT exhibits positional preference.

Hydrogenation and Fluorination of Graphene Models: Analysis via the Average Local Ionization Energy

We have investigated the use of the average local ionization energy, I[combining overline](S)(r), as a means for rapidly predicting the relative reactivities of different sites on two model graphene surfaces toward the successive addition of one, two, and three hydrogen or fluorine atoms. The I[combining overline](S)(r) results were compared with directly computed interaction energies, at the B3LYP/6-311G(d,p) level. I[combining overline](S)(r) correctly predicts that the edges of graphene sheets are more reactive than the interior portions. It shows that added hydrogens activate the adjoining (ortho) sites and deactivate those that are separated by one site (meta). Overall, I[combining overline](S)(r) is effective for rapidly (single calculations) estimating the relative site reactivities of these large systems, although it reflects only the system prior to an interaction and cannot take into account postinteraction factors, e.g., structural distortion.

ChemInform Abstract: Electronic Structure Engineering in Heterogeneous Catalysis: Identifying Novel Alloy Catalysts Based on Rapid Screening for Materials with Desired Electronic Properties

AbstractReview: 56 refs.

Electronic Structure Engineering in Heterogeneous Catalysis: Identifying Novel Alloy Catalysts Based on Rapid Screening for Materials with Desired Electronic Properties

The immense phase space of multimetallic materials spanned by structural and compositional degrees of freedom precludes thorough screening for efficient alloy catalysts, even with combinatorial high-throughput experiments or quantum-chemical calculations. Based on X-ray absorption spectroscopy measurements and density functional theory calculations, we have identified critical electronic structure descriptors that govern local chemical reactivity of different sites in metal alloys. These descriptors were used to develop a model that allows us to predict variations in the adsorption energy of various adsorbates on alloy surfaces based on easily accessible physical characteristics of the constituent elements in alloys, mainly their electronegativity, atomic radius, and the spatial extent of valence orbitals. We show that this model, which is grounded on validated theories of chemisorption on metal surfaces, can be used to rapidly screen through a large phase space of alloy catalysts and identify optimal alloys for targeted catalytic transformations. We underline the potential of the electronic structure engineering, relating alloy geometry to its catalytic performance using simple electronic structure descriptors, in catalysis.

Structural and electronic properties of Ce nO 2n ( n = 1–5) nanoparticles: A computational study

Understanding the electronic structures and properties of different size CeO 2 nanoparticles is very important for further application in the field of catalysis used in several promising materials. In this study, we have elucidated the electronic analyses of (CeO 2) n ( n = 1–5) nanoparticles through first-principle density-functional theory approach. All structures with the global minimal energies are obtained by fire algorithm combining simulated annealing method and then are further re-optimized by DMol 3 program with double numerical atomic basis sets. Two useful analyzed methods (Fukui function and electronic density of state) are calculated to explain the chemical reactivity of different sites for (CeO 2) n ( n = 1–5) nanoparticles.

Analysis of protein adduction kinetics by quantitative mass spectrometry: Competing adduction reactions of glutathione- S-transferase P1-1 with electrophiles

Defining the mechanisms and consequences of protein adduction is crucial to understanding the toxicity of reactive electrophiles. Application of tandem mass spectrometry and data analysis algorithms enables detection and mapping of chemical adducts at the level of amino acid sequence. Nevertheless, detection of adducts does not indicate relative reactivity of different sites. Here, we describe a method to measure the kinetics of competing adduction reactions at different sites on the same protein. Adducts are formed by electrophiles at Cys14 and Cys47 on the metabolic enzyme glutathione- S-transferase P1-1 and modification is accompanied by a loss of enzymatic activity. Relative quantitation of protein adducts was done by tagging N-termini of peptide digests with isotopically labeled phenyl isocyanate and tracking the ratio of light-tagged peptide adducts to heavy-tagged reference samples in liquid chromatography–tandem mass spectrometry analyses using a multiple reaction monitoring method. This approach was used to measure rate constants for adduction at both positions with two different model electrophiles, N-iodoacetyl- N-biotinylhexylenediamine and 1-biotinamido-4-(4′-[maleimidoethyl-cyclohexane]-carboxamido)butane. The results indicate that Cys47 was approximately two- to three-fold more reactive toward both electrophiles than was Cys14. This result was consistent with the relative reactivity of these electrophiles in a complex proteome system and with previously reported trends in reactivity of these sites. Kinetic analyses of protein modification reactions provide a means of evaluating the selectivity of reactive mediators of chemical toxicity.

Application of density functional theory concepts to the study of the chemical reactivity of isomeric thiadiazolines

Several useful concepts derived from density functional theory have been applied to the study of the chemical reactivity of the more stable forms of the isomeric thiadiazolines. Dipole moments, total hardness of the molecules in terms of the calculated ionization potentials and electron affinities as a measure of aromaticity and the condensed Fukui functions related to the variation of the net charges of the atoms resulting from a Mulliken population analysis were calculated in order to determine the reactivity of different sites within the molecules studied. The net charges have been obtained from calculations made in the context of the Hartree–Fock and of local (SVWN) and nonlocal (BLYP, B3LYP) density functional approximations. The results were compared with the existing experimental evidence on thiadiazolines and related compounds. Electronic properties calculated using the hybrid B3LYP proved to be the most accurate. It is concluded that the 1,2,3-thiadiazoline molecule would be the best starting monomer of the four isomers to be considered for a polymerization reaction.

Application of density functional theory concepts to the study of the chemical reactivity of thiadiazoles

Several useful concepts derived from Density Functional Theory have been applied to study the chemical reactivity of 1,2,5- and 1,3,4-thiadiazoles. Total hardness of the molecules in terms of the HOMO-LUMO gap as a measure of aromaticity and the condensed Fukui functions related to the variations of the net charges of the atoms resulting from a Mulliken population analysis were calculated in order to determine the reactivity of different sites within the molecules studied. The net charges have been obtained from calculations made in the context of the Hartree-Fock-LCAO approximation and the results compared with the existing experimental evidence on thiadiazoles and related compounds.

Probing the reactivity of different sites within a molecule or solid by direct computation of molecular sensitivities via an extension of the electronegativity equalization method

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTProbing the reactivity of different sites within a molecule or solid by direct computation of molecular sensitivities via an extension of the electronegativity equalization methodBart G. Baekelandt, Wilfried J. Mortier, Jan L. Lievens, and Robert A. SchoonheydtCite this: J. Am. Chem. Soc. 1991, 113, 18, 6730–6734Publication Date (Print):August 1, 1991Publication History Published online1 May 2002Published inissue 1 August 1991https://doi.org/10.1021/ja00018a003RIGHTS & PERMISSIONSArticle Views199Altmetric-Citations83LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (516 KB) Get e-Alerts Get e-Alerts

Electron spin resonance studies. Part 69. Oxidation of some aliphatic carboxylic acids, carboxylate anions, and related compounds by the sulphate radical anion (SO4–˙)

The results of e.s.r experiments involving in situ photochemical generation of the sulphate radical anion in the presence of some aliphatic carboxylic acids and their anions are interpreted in terms of attack at the carboxy group (to give alkyl radicals via decarboxylation) and hydrogen-atom abstraction from carbon atoms in the chain. The general order of reactivity of different sites is CO2– > β-H, γ-H, etc. > CO2H > α-H. For example, reaction with EtCO2H gives ˙CH2CH2CO2H (k 2 × 105 dm3 mol–1 s–1) and Et˙(k 7 × 104 dm3 mol–1 s–1). Radicals formed by abstraction of α-H (e.g.˙CH2CO2H from ethanoic acid) are shown to result usually from further reactions of first-formed alkyl radicals. The reactions of SO4–˙ with some amides, nitriles, and esters have also been investigated.

Free radical substitution in aliphatic compounds. Part 33. Halogen atom abstraction from alkyl halides by trimethylgermanium radicals in the gas phase

Mixtures of trimethylgermanium hydride and halogenoalkanes have been irradiated by a medium pressure mercury lamp. A rapid reaction has been observed in which a halogen atom on the original halogenoalkane is displaced by a hydrogen atom and trimethylgermanium halide is formed. The reaction is exactly similar to that previously reported for trimethyltin hydride, and is interpreted as a radical chain reaction. The reactivity of different sites in monosubstituted alkanes is in the order tertiary > secondary > primary but trimethylgermanium radicals are less selective than trimethyltin radicals. However, in chlorine abstraction from a range of compounds with the general formula RCCl3, trimethylgermanium radicals are considerably more selective. These apparently conflicting results are interpreted in terms of polar forces in the transition state.