Showing papers on "Homolysis published in 2010"
TL;DR: In this paper, the effect of CN groups and solvent on the enthalpies of homolytic and heterolytic N-H bond cleavage was studied using DFT/UB3LYP/6-31++G∗∗ method.
Abstract: In this article, we have studied p-phenylenediamine (PPD) and tetracyano-p-phenylenediamine (TCPPD) molecules in order to study the effect of CN groups and the solvent effect on the enthalpies of homolytic and heterolytic N–H bond cleavage. Geometries of the molecules and reaction enthalpies related to hydrogen atom transfer, single electron transfer–proton transfer (SET–PT) mechanism and sequential proton loss electron transfer (SPLET) mechanisms were studied using DFT/UB3LYP/6-31++G∗∗ method. Ab initio MP2/6-31++G∗∗ method was used as the reference for the geometry calculation of the two molecules in vacuum. Solvent contribution to the enthalpies was computed employing integral equation formalism IEF-PCM method. Obtained results show that solvent is able to cause significant change in the reaction enthalpies of the stepwise SET–PT and SPLET mechanisms of hydrogen splitting-off from NH2 group. This may result in the change in thermodynamically preferred mechanism. Solvents also attenuate the CN-substituent effect in the case of SET–PT and SPLET mechanisms.
257 citations
TL;DR: In this article, the brominated aromatic molecule 1,3,5-tris(4-bromophenyl)benzene has been studied by variable-temperature scanning tunneling microscopy (STM).
Abstract: Adsorption of the brominated aromatic molecule 1,3,5-tris(4-bromophenyl)benzene on different metallic substrates, namely Cu(111), Ag(111), and Ag(110), has been studied by variable-temperature scanning tunneling microscopy (STM). Depending on substrate temperature, material, and crystallographic orientation, a surface-catalyzed dehalogenation reaction is observed. Deposition onto the catalytically more active substrates Cu(111) and Ag(110) held at room temperature leads to cleavage of carbon−bromine bonds and subsequent formation of protopolymers, i.e., radical metal coordination complexes and networks. However, upon deposition on Ag(111) no such reaction has been observed. Instead, various self-assembled ordered structures emerged, all based on intact molecules. Also sublimation onto either substrate held at ∼80 K did not result in any dehalogenation, thereby exemplifying the necessity of thermal activation. The observed differences in catalytic activity are explained by a combination of electronic and g...
147 citations
TL;DR: Investigations detailed herein demonstrate that the formation of either alkoxy- or hydroxy-Ti(III) complexes considerably decreases the energy of activation for C-O bond homolysis, and two new synthetic applications of Nugent's reagent in organic chemistry are described.
Abstract: Investigations detailed herein, including density functional theory (DFT) calculations, demonstrate that the formation of either alkoxy− or hydroxy−Ti(III) complexes considerably decreases the energy of activation for C−O bond homolysis. As a consequence of this observation, we described two new synthetic applications of Nugent’s reagent in organic chemistry. The first of these applications is an one-step methodology for deoxygenation−reduction of alcohols, including benzylic and allylic alcohols and 1,2-dihydroxy compounds. Additionally, we have also proved that Ti(III) is capable of mediating carbonyl coupling−olefination. In this sense, and despite the fact that for over 35 years it has been widely accepted that either Ti(II) or Ti(0) was the active species in the reductive process of the McMurry reaction, the mechanistic evidence presented proves the involvement of Ti(III) pinacolates in the deoxygenation step of the herein described Nugent’s reagent-mediated McMurry olefination. This observation shed...
123 citations
TL;DR: The results have revealed a detailed experimental mechanistic picture of the oxidation reactions catalyzed by 1, based on direct characterization of the intermediates and products, and kinetic analysis of the individual reaction steps.
Abstract: Mechanism of substrate oxidations with hydrogen peroxide in the presence of a highly reactive, biomimetic, iron aminopyridine complex, (Fe II -(bpmen)(CH 3 CN) 2 ][ClO 4 ] 2 (1; bpmen= N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine), is elucidated. Complex 1 has been shown to be an excellent catalyst for epoxidation and functional-group-directed aromatic hydroxylation using H 2 O 2 , although its mechanism of action remains largely unknown. [1,2] Efficient intermolecular hydroxylation of unfunctionalized benzene and substituted benzenes with H 2 0 2 in the presence of 1 is found in the present work. Detailed mechanistic studies of the formation of iron(III)-phenolate products are reported. We have identified, generated in high yield, and experimentally characterized the key Fe III (OOH) intermediate (λ max = 560 nm, rhombic EPR signal with g=2.21, 2.14, 1.96) formed by 1 and H 2 O 2 . Stopped-flow kinetic studies showed that Fe III (OOH) does not directly hydroxylate the aromatic rings, but undergoes rate-limiting self-decomposition producing transient reactive oxidant. The formation of the reactive species is facilitated by acid-assisted cleavage of the O-O bond in the iron-hydroperoxide intermediate. Acid-assisted benzene hydroxylation with 1 and a mechanistic probe, 2-Methyl-1-phenyl-2-propyl hydroperoxide (MPPH), correlates with O-O bond heterolysis. Independently generated Fe IV =O species, which may originate from O-O bond homolysis in Fe III -(OOH), proved to be inactive toward aromatic substrates. The reactive oxidant derived from 1 exchanges its oxygen atom with water and electrophilically attacks the aromatic ring (giving rise to an inverse H/D kinetic isotope effect of 0.8). These results have revealed a detailed experimental mechanistic picture of the oxidation reactions catalyzed by 1, based on direct characterization of the intermediates and products, and kinetic analysis of the individual reaction steps. Our detailed understanding of the mechanism of this reaction revealed both similarities and differences between synthetic and enzymatic aromatic hydroxylation reactions.
116 citations
TL;DR: The homolysis of peracetic acid as a relevant source of free radicals (e.g., *OH) was studied in detail and an artificial neural network (ANN) was designed to associate the removal efficiency of phenol with relevant process parameters such as concentrations of both the catalyst and PAA and the reaction time.
Abstract: The homolysis of peracetic acid (PAA) as a relevant source of free radicals (e.g., •OH) was studied in detail. Radicals formed as a result of chain radical reactions were detected with electron spin resonance and nuclear magnetic resonance spin trapping techniques and subsequently identified by means of the simulation-based fitting approach. The reaction mechanism, where a hydroxyl radical was a primary product of O−O bond rupture of PAA, was established with a complete assessment of relevant reaction thermochemistry. Total energy analysis of the reaction pathway was performed by electronic structure calculations (ab initio and semiempirical methods) at different levels and basis sets [e.g., HF/6-311G(d), B3LYP/6-31G(d)]. Furthermore, the heterogeneous MnO2/PAA system was tested for the elimination of a model aromatic compound, phenol from aqueous solution. An artificial neural network (ANN) was designed to associate the removal efficiency of phenol with relevant process parameters such as concentrations ...
96 citations
TL;DR: Redox-active ligand allows high-valent rhenium to utilize a mechanism for O(2) activation that is atypical of oxorhenium(V) but more typical for oxygenase enzymes and models based on 3d transition metal ions.
Abstract: Five-coordinate oxorhenium(V) anions with redox-active catecholate and amidophenolate ligands are shown to effect clean bimetallic cleavage of O2 to give dioxorhenium(VII) products. A structural homologue with redox-inert oxalate ligands does not react with O2. Redox-active ligands lower the kinetic barrier to bimetallic O2 homolysis at five-coordinate oxorhenium(V) by facilitating formation and stabilization of intermediate O2 adducts. O2 activation occurs by two sequential Re−O bond forming reactions, which generate mononuclear η1-superoxo species, and then binuclear trans-μ-1,2-peroxo-bridged complexes. Formation of both Re−O bonds requires trapping of a triplet radical dioxygen species by a cis-[ReV(O)(cat)2]− anion. In each reaction the dioxygen fragment is reduced by 1e−, so generation of each new Re−O bond requires that an oxometal fragment is oxidized by 1e−. Complexes containing a redox-active ligand access a lower energy reaction pathway for the 1e− Re−O bond forming reaction because the metal f...
88 citations
TL;DR: Cleavage of a C-P bond with the weak Cl(-) nucleophile to release the reactive NHC moiety is a definite experimental indication of the dative nature of the N(2)C-P Bond of amidiniophosphines, which are better described as NHC→phosphenium adducts.
Abstract: Whereas the external nucleophilic reactivity of α-amidiniophosphines has been previously illustrated by their complexation to transition-metal centers, their internal electrophilic reactivity is herein investigated by using BIMIONAP (BIMIONAP=N-methylated BIMINAP cation, BIMINAP=formal contraction of the acronyms BIMIP=2,2'-bis(diphenylphosphino)-1,1'-bibenzimidazole and BINAP=2,2'-bis(diphenylphosphino)-1,1'-binaphthyl). Reaction of tetraethylammonium chloride with free BIMIONAP is found to induce heterolytic cleavage of the N(2)C-P bond to give chlorodiphenylphosphine and a transient phosphine-N-heterocyclic carbene (NHC) species that is trapped in situ by protonation to the corresponding phosphine-benzimidazolium cation. When the chloride anion reacts with the cationic [Pd(η(2)-BIMIONAP)Cl(2)] complex, the same cleavage occurs and the phosphine-NHC moiety is trapped in the corresponding [PdCl(2)(η(2)-phosphine-NHC)] complex. When the chloride anion reacts with the dicationic [Pd(π-allyl)(η(2)-BIMIONAP)](+) complex, allyldiphenylphosphine is produced, and the [PdCl(η(2)-phosphine-NHC)(PPh(2)CH(2)CH=CH(2))](+) complex is obtained. Reaction of free BIMIONAP with the harder n-butyllithium nucleophile also induces heterolytic cleavage of the N(2)C-P bond, from which the phosphine-NHC moiety is trapped by hydrolysis of the benzimidazole ring or by P,C-sulfurization. Cleavage of a C-P bond with the weak Cl(-) nucleophile to release the reactive NHC moiety (according to the unusual scheme C-P+Cl(-)→C:+Cl-P) is a definite experimental indication of the dative nature of the N(2)C-P bond of amidiniophosphines, which are, therefore, better described as NHC→phosphenium adducts. This interpretation is supported by the calculation, at the DFT level, of a heterolytic dissociation mode of the N(2)C-P bond lower in energy than the homolytic one. A mesomeric description of the NHC→phosphenium entity is also proposed on the basis of electron localization function (ELF) and atoms in molecules (AIM) analyses. Finally ELF and AIM-based Fukui indices, molecular orbitals, and MESP analyses show that the initial attack of Cl(-) takes place at the carbenic atom of BIMIONAP.
87 citations
TL;DR: Arylboronic acids are shown to be valuable precursors for aryl radicals upon treatment with manganese triacetate and subsequent homolytic aromatic substitution provide tetrahydronaphthaline derivatives B in moderate to good yields.
77 citations
TL;DR: The analysis of charge- and spin-density distributions shows that spin delocalization in the boryl radical complexes constitutes one of the mechanisms of radical stabilization.
Abstract: Radical stabilization energies (RSE)s have been calculated for a variety of boryl radicals complexed to Lewis bases at the G3(MP2)-RAD level of theory. These are referenced to the B-H bond dissociation energy (BDE) in BH(3) determined at W4.3 level. High RSE values (and thus low BDE(B-H) values) have been found for borane complexes of a variety of five- and six-membered ring heterocycles. Variations of RSE values have been correlated with the strength of Lewis acid-Lewis base complex formation at the boryl radical stage. The analysis of charge- and spin-density distributions shows that spin delocalization in the boryl radical complexes constitutes one of the mechanisms of radical stabilization.
73 citations
TL;DR: The scope of the N- OC homolysis side reaction in nitroxide-mediated polymerization was determined and it was found that the free energies of NO-C and N-OC homolyses are not correlated, with no-C homolytic activity being more dependent upon the properties of the alkyl fragment andN-OC Homolysis being moredependent upon the structure of the aminyl fragment.
Abstract: Free energies for the homolysis of the NO−C and N−OC bonds were compared for a large number of alkoxyamines at 298 and 393 K, both in the gas phase and in toluene solution. On this basis, the scope...
67 citations
TL;DR: Ultrafast photochemical triggers hold the promise of providing information on the dynamics of peptide and protein folding by providing a means to explore the free energy landscape of folding and misfolding, but there remain significant challenges.
Abstract: Ultrafast photochemical triggers hold the promise of providing information on the dynamics of peptide and protein folding.[1] Prior to photolysis, bonding to the trigger constrains the peptide to have a narrow structure distribution. Photochemical triggering releases the constraints, permitting the molecule to evolve to a different equilibrium distribution. The structure evolution, even when ultrafast, can be followed by infrared probe or two-dimensional infrared spectroscopy. Fast phototriggering can thus reveal early kinetic events in protein dynamics by providing a means to explore the free energy landscape of folding and misfolding. Several phototriggers have been developed for this purpose,[1,2] but there remain significant challenges. For example, disulfide bonds in peptides can be used as a phototriggers. Deep UV light severs the disulfide bond and releases the structural constraints; such experiments have been carried out in short helical peptides,[1a,b] cyclic peptides,[1d] and β hairpins.[1c] Although disulfide photolysis offers the capability of initiating ultrafast structure equilibration, limitations preclude broad generality of this technique. Homolytic S–S bond scission reveals two reactive radicals that can undergo geminate recombination, as well as reactions with protein sidechains. Moreover, the UV excitation required to dissociate the disulfide bond also excites the peptide backbone. Another example is azobenzene which undergoes fast, reversible photoisomerization. When designed into a peptide a light pulse can be used to cause the system to reversibly shift between significantly different equilibrium configurations.[3]
TL;DR: In this article, the gas-phase fragmentation reactions of the group 11 organometallates [CH3MR] were studied by density functional theory (DFT) calculations and, for those accessible in the gas phase, via collision-induced dissociation (CID) and selected deuterium labeling experiments.
TL;DR: In this article, the effects of ancillary ligands on the metal-alkyl bond dissociation energy in OMRP reagents are investigated and the involvement of radical intermediates in oxidative addition of secondary alkyls for catalytic cross-coupling reactions catalyzed by first row transition metals is examined for relevant design concepts.
Abstract: Organometallic-mediated radical polymerization (OMRP) has emerged as a powerful new class of living controlled radical polymerization. In order to fulfill its potential in the polymerization of vinyl acetate (VOAc) and other challenging monomers, the effects of ancillary ligands on the metal-alkyl bond dissociation energy in OMRP reagents must be thoroughly explored. Recent results investigating structure-activity relationships in well-defined cobalt, iron and chromium complexes will be discussed. The involvement of radical intermediates in oxidative addition of secondary alkyls for catalytic cross-coupling reactions catalyzed by first row transition metals will also be examined for relevant design concepts.
TL;DR: Comparison of the fragmentation reactions of regiospecifically generated radicals suggests that hydrogen atom transfer (HAT) between the α C-H of adjacent residues and the cysteinyl radical can occur and is shown to give rise to the protonated, truncated N-formyl peptide.
TL;DR: Whereas [P4O10](·+) reacts by homolytic C-H bond cleavage and electron transfer, the isostructural [V4O 10](·-) shows only oxygen-atom transfer (see picture).
Abstract: Why so different? The comparison of the reaction of "bare" [P4O10](·+) and [V4O10](·+) with ethene by mass-spectrometric and computational studies permits insight into mechanistic aspects of the competition between C-H bond activation and oxygen-atom and electron transfers. Whereas [P4O10](·+) reacts by homolytic C-H bond cleavage and electron transfer, the isostructural [V4O10](·+) shows only oxygen-atom transfer (see picture).
TL;DR: The density functional theory (DFT method), B3P86/6-311++G(2df,2p)//B3LYP/ 6-31+G(d), is found to be reliable to predict the C-COOH BDE of various structurally unrelated carboxylic acids.
Abstract: The knowledge of C−COOH homolytic bond dissociation enthalpies (BDEs) is of great importance in understanding various chemical and biochemical processes involving the decarboxylation reaction. In t...
TL;DR: The idea of a thermodynamic characteristic graph (TCG) of imines as an efficient "Molecule ID Card" can be used to quantitatively diagnose and predict the characteristic chemical properties of imine and their various reaction intermediates as well as the reduction mechanism of the imines.
Abstract: A series of 61 imines with various typical structures were synthesized, and the thermodynamic affinities (defined as enthalpy changes or redox potentials in this work) of the imines to abstract hydride anions, hydrogen atoms, and electrons, the thermodynamic affinities of the radical anions of the imines to abstract hydrogen atoms and protons, and the thermodynamic affinities of the hydrogen adducts of the imines to abstract electrons in acetonitrile were determined by using titration calorimetry and electrochemical methods. The pure heterolytic and homolytic dissociation energies of the C═N π-bond in the imines were estimated. The polarity of the C═N double bond in the imines was examined using a linear free-energy relationship. The idea of a thermodynamic characteristic graph (TCG) of imines as an efficient “Molecule ID Card” was introduced. The TCG can be used to quantitatively diagnose and predict the characteristic chemical properties of imines and their various reaction intermediates as well as the ...
TL;DR: In this paper, the Nitro derivatives of phenols are optimized to obtain their molecular geometries and electronic structures at the DFT-B3LYP/6-31G* level.
Abstract: The nitro derivatives of phenols are optimized to obtain their molecular geometries and electronic structures at the DFT-B3LYP/6-31G* level. Detonation properties are evaluated using the modified Kamlet–Jacobs equations based on the calculated densities and heats of formation. It is found that there are good linear relationships between density, detonation velocity, detonation pressure, and the number of nitro and hydroxy groups. Thermal stability and pyrolysis mechanism of the title compounds are investigated by calculating the bond dissociation energies (BDEs) at the unrestricted B3LYP/6-31G* level. The activation energies of H-transfer reaction is smaller than the BDEs of all bonds and this illustrates that the pyrolysis of the title compounds may be started from breaking OH bond followed by the isomerization reaction of H transfer. Moreover, the CNO2 bond with the smaller bond overlap population and the smaller BDE will also overlap may be before homolysis. According to the quantitative standard of energetics and stability as a high-energy density compound, pentanitrophenol essentially satisfies this requirement. In addition, we have discussed the effect of the nitro and hydroxy groups on the static electronic structural parameters and the kinetic parameter. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010
TL;DR: Comparison of the gasphase decomposition reactions of [CH 3CuCH3]- and [CH3AgCH 3]- reveals that [ CH3CuCH 3] - undergoes a competition between a dyotropic rearrangement and bond homolysis, whilst [CH2AgCH3] - only undergoes bondhomolysis.
Abstract: All change! Comparison of the gasphase decomposition reactions of [CH CuCH ] and [CH AgCH ] reveals that [CH CuCH ] undergoes a competition between a dyotropic rearrangement and bond homolysis, whilst [CH AgCH ] only undergoes bond homolysis. Ab initio calculations reveal that the different behavior in [CH AgCH ] stems from both the lowering of the homolytic bond dissociation energy and an increase in dyotropic rearrangement activation energy. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA. 3 3 3 3 3 3 3 3 3 3 - - - - -
TL;DR: A range of paramagnetic Cr(III) monohydrocarbyl complexes CpCr[(ArNCMe)2CH](R) were synthesized to investigate how varying the steric and electronic properties of the R group affected their propensity for Cr-R bond homolysis.
Abstract: A range of paramagnetic Cr(III) monohydrocarbyl complexes CpCr[(ArNCMe)2CH](R) (Ar = ortho-disubstituted aryl; R = primary alkyl, trimethylsilylmethyl, benzyl, phenyl, alkenyl, or alkynyl) were synthesized to investigate how varying the steric and electronic properties of the R group affected their propensity for Cr-R bond homolysis. Most complexes were prepared by salt metathesis of known CpCr[(ArNCMe)2CH](Cl) compounds in Et2O with commercial RMgCl solutions, although more sterically demanding combinations of Ar and R groups necessitated the use of halide-free MgR2 reagents and the Cr(III) tosylate or triflate derivatives. Alternative synthetic routes to Cr(III)-R species using the previously reported Cr(II) compounds CpCr[(ArNCMe)2CH] and sources of R· radicals (e.g., BEt3 and air) were also explored. The UV-vis spectra of the CpCr[(ArNCMe)2CH](R) complexes possessed two strong bands with maximum absorbances in the ranges 395-436 nm and 535-582 nm, with the band in the latter range being particularly characteristic of the Cr(III)-R compounds. The Cr-CH2R bond lengths as determined by single-crystal X-ray diffraction were longer than those in the corresponding Cr-CH3 complexes, typically falling in the range 2.10 to 2.13 A. The Cr(III) benzyl compounds displayed longer Cr-CH2Ph distances, while the bond lengths for the alkenyl and alkynyl species were substantially shorter. The rate of Cr-R bond homolysis at room temperature was determined by monitoring the reaction of Cr(III) neopentyl, benzyl, and isobutyl complexes with excess PhSSPh using UV-vis spectroscopy. Although the other primary alkyl, phenyl, and alkenyl compounds did not undergo appreciable homolysis under these conditions, they were cleanly converted to CpCr[(ArNCMe)2CH](SPh) by photolysis.
TL;DR: Ion-molecule reactions of [CH(3)AuCH( 3)](-) with methyl iodide did not yield any products even at relatively high concentrations of the neutral substrate and longer reaction times, indicating a reaction efficiency of less than 1 in 20 000 collisions.
Abstract: A combination of multistage mass spectrometry experiments and DFT calculations were used to examine the synthesis and reactivity of dimethylaurate. Collision induced dissociation (CID) of [(CH3CO2)4Au]− proceeded via reductive elimination of acetylperoxide to yield the diacetate [CH3CO2AuO2CCH3]−, which in turn underwent sequential CID decarboxylation reactions to yield the organoaurates [CH3CO2AuCH3]− and [CH3AuCH3]−. The unimolecular chemistry of the dimethylaurate proceeds via a combination of bond homolysis to yield the methyl aurate radical anion [CH3Au]˙− as well as formation of the gold dihydride [HAuH]−. DFT calculations reveal that the latter anion is formed via a 1,2-dyotropic rearrangement to yield the isomer [CH3CH2AuH]−, followed by a β-hydride elimination reaction. Ion-molecule reactions of [CH3AuCH3]− with methyl iodide did not yield any products even at relatively high concentrations of the neutral substrate and longer reaction times, indicating a reaction efficiency of less than 1 in 20 000 collisions. DFT calculations were carried out on two different potential energy surfaces (PES) for the reaction of [CH3AuCH3]− with CH3I: (i) an SN2 mechanism proceeding via a side-on transition state; and (ii) a stepwise mechanism proceeding via oxidative addition followed by reductive elimination. Both pathways have significant endothermic barriers, consistent with the lack of C–C bond coupling products being formed in the experiments. Finally, the reactivity of [CH3AuCH3]− is compared to the previously studied [CH3AgCH3]− and [CH3CuCH3]−, as well as condensed phase studies on dimethylaurate salts.
TL;DR: In this paper, the initiation of explosive decomposition in two energetic crystals, diamino−dinitroethylene (DADNE, C2H4N4O4) and triamino−trinitrobenzene (TATB, C6H6N6O6), was investigated using density functional theory.
Abstract: The initiation of explosive decomposition in two energetic crystals, diamino−dinitroethylene (DADNE, C2H4N4O4) and triamino−trinitrobenzene (TATB, C6H6N6O6), was investigated using density functional theory. The initial chemical reactions in ideal TATB crystals were found to be determined by three main decomposition mechanisms that are almost unaffected by a shear-strain-induced deformation, C−NO2 homolysis, nitro−nitrite isomerization, and proton transfer. The two latter reactions are nearly isoenergetic and have lower activation energies than the first reaction. At the same time, decomposition of DADNE is found to depend strongly on the molecular environment; molecules in ideal DADNE crystals favor nitro−nitrite isomerization, while molecules located at shear planes decompose via the C−NO2 homolysis pathway. We also established that the shear-strain accumulated in the DADNE dashboard-shaped molecular layers triggers an exothermic regime at fairly early stages of decomposition. In contrast, the structure...
TL;DR: It is found that three of the most often encountered radicals located on the enzyme protein chain (tyrosyl, cysteinyl and glycyl radicals) are of similar stability, despite the fact that O–H, S–H and C–H bonds have intrinsically very different homolytic bond dissociation energies.
Abstract: The thermodynamic stability of radicals involved in enzymatic catalysis has been quantified using a series of theoretical methods. It is found that three of the most often encountered radicals located on the enzyme protein chain (tyrosyl, cysteinyl and glycyl radicals) are of similar stability. This is despite the fact that O–H, S–H and C–H bonds have intrinsically very different homolytic bond dissociation energies. The cofactor-derived 5′-adenosyl radical, in contrast, is significantly less stable than these protein-bound radicals.
TL;DR: In this paper, Cohen et al. showed that 3,5-dinitro-2(or 4)-methyl phenoxy is more favor to decompose into CO and 3, 5-diminro-methyl-cyclopentadienyl than to loss NO following nitro-nitrite rearrangement.
Abstract: Simple C–NO2 homolysis, 4,6-dinitroanthranil (DNAt) production by dehydration, and the nitro-nitrite rearrangement–homolysis for gas-phase TNT decomposition were recently studied by Cohen et al. (J Phys Chem A 111:11074, 2007), based on DFT calculations. Apart from those three pathways, other possible initiation processes were suggested in this study, i.e., CH3 removal, O elimination, H escape, OH removal, HONO elimination, and nitro oxidizing adjacent backbone carbon atom. The intermediate, 3,5-dinitro-2(or 4)-methyl phenoxy, is more favor to decompose into CO and 3,5-dinitro-2(or 4)-methyl-cyclopentadienyl than to loss NO following nitro-nitrite rearrangement. Below ~1,335 K, TNT condensing to DNAt by dehydration is kinetically the most favor process, and the formations of substituted phenoxy and following cyclopentadienyl include minor contribution. Above ~1,335 K, simple C–NO2 homolysis kinetically dominates TNT decomposition; while the secondary process changes from DNAt production to CH3 removal above ~2,112 K; DNAt condensed from TNT by dehydration yields to that by sequential losses of OH and H above ~1,481 K and to nitro-nitrite rearrangement–fragmentation above ~1,778 K; O elimination replaces DNAt production above ~2,491 K, playing the third role in TNT decomposition; H escaping directly from TNT thrives in higher temperature (above ~2,812 K), as the fourth largest process. The kinetic analysis indicates that CH3 removal, O elimination, and H escape paths are accessible at the suggested TNT detonation time (~100–200 fs), besides C–NO2 homolysis. HONO elimination and nitro oxidizing adjacent backbone carbon atom paths are negligible at all temperatures. The calculations also demonstrated that some important species observed by Rogers and Dacons et al. are thermodynamically the most favor products at all temperatures, possibly stemmed from the intermediates including 4,6-dinitro-2-nitroso-benzyl alcohol, 3,5-dinitroanline, 2,6-dinitroso-4-nitro-phenylaldehyde, 3,5-dinitro-1-nitrosobenzene, 3,5-dinitroso-1-nitrobenzene, and nitrobenzene. All transition states, intermediates, and products have been indentified, the structures, vibrational frequencies, and energies of them were verified at the uB3LYP/6-311++G(d,p) level. Our calculated energies have mean unsigned errors in barrier heights of 3.4–4.2 kcal/mol (Lynch and Truhlar in J Phys Chem A 105:2936, 2001), and frequencies have the recommended scaling factors for the B3-LYP/6-311+G(d,p) method (Andersson and Uvdal in J Phys Chem A 109:2937, 2005; Merrick et al. in J Phys Chem A 111:11683, 2007). All calculations corroborate highly with the previous experimental and theoretical results, clarifying some pertinent questions.
TL;DR: In this paper, the authors used the CBS-QB3 method to provide the theoretical benchmark values for estimating the energetic C-N bond dissociation energies and showed that the three hybrid meta GGA functionals, BB1K, MPWB1K and M06-2X, can reproduce the experimental values well.
Abstract: The assessment of the C–N bond dissociation energies is performed by using the various density functionals at 6-31+g(d,p) level. CBS-QB3 method was used to provide the theoretical benchmark values. The present results show that the three hybrid meta GGA functionals, BB1K, MPWB1K and M06 reproduce the experimental values well. M06-2X could normally overestimate the homolytic C–N bond dissociation energies. For the hybrid functionals, B3P86 and PBE1PBE can also behave almost as well as the above meta GGA functionals. Thus, they should be recommended as the most reliable method to estimate the energetic C–N bond dissociation energies.
TL;DR: In this paper, the chemical reactivity of representative hydrocarbon organic light-emitting diode (OLED) materials was studied and cyclization was identified in photoexcited films of representative anthracene derivative and detected in electrically degraded OLEDs utilizing this material in the emissive layer.
Abstract: We report the study of the chemical reactivity of representative hydrocarbon organic light-emitting diode (OLED) materials—fully aromatic derivatives of anthracene and tetracene in the OLED environment. In addition to the participation in free-radical chemistry initiated by homolytic bond dissociation reactions of arylamines, the hydrocarbons appear to initiate and undergo dehydrogenation reactions following the electronic excitation caused by the recombination of charge carriers or by the absorption of a photon. A chemical product of the intramolecular dehydrogenation reaction, cyclization, was identified in photoexcited films of representative anthracene derivative and detected in electrically degraded OLEDs utilizing this material in the emissive layer. Other analogous intra- and intermolecular dehydrogenation reactions initiated by the excited states of hydrocarbons are also expected to occur in operating OLEDs. The stepwise transfers of hydrogen atoms or ions to neighboring molecules are likely to yi...
TL;DR: In this article, a xanthate-based, solvent free, homolytic substitution on selected substituted pyrrole systems is described, followed by a double nucleophilic alkylation sequence.
TL;DR: The calculated free energy profiles for both metal thiolate complexes can reasonably account for the observed reversible H(2) activation by two metal thiolaate complexes under mild conditions.
Abstract: The molecular mechanism of H2 activation by two transition metal thiolate complexes [Cp*M(PMe3)(SDmp)](BArF4) (M = Ir, Rh) (Ohki, Y; Sakamoto, M; Tatsumi, K. J. Am. Chem. Soc., 2008, 130, 11610–11611) has been investigated using density functional theory calculations. According to our calculations, the reaction of the iridium thiolate complex with H2 is likely to proceed through the following steps: (1) the oxidative addition of H2 to the iridium center to generate a dihydride intermediate; (2) the reductive elimination of one Ir-bound hydrogen to produce the hydride thiol product. For the rhodium thiolate complex, its reaction with H2 is to form the dihydrogen intermediate first, and then the H–H bond is heterolytically cleaved at the Rh–S bond via a four-center transition state to yield the hydride thiol product. The rate-determining step is the oxidative addition step (with a barrier of 18.0 kcal/mol in the solvent) for the iridium complex, and the formation of the dihydrogen complex (with a barrier of 13.9 kcal/mol in the solvent) for the rhodium complex. The calculated free energy profiles for both metal thiolate complexes can reasonably account for the observed reversible H2 activation by two metal thiolate complexes under mild conditions.
TL;DR: In this article, the thermolysis of allyl acetate was investigated, using a stirred-flow type (“tank-flow”-) reactor operating at atmospheric pressure in the presence of nitrogen as the carrier gas.
Abstract: Within the scope of a general study on the gas phase chemistry of esters, the thermolysis of allyl acetate was investigated, using a stirred-flow type (“tank-flow”-) reactor operating at atmospheric pressure in the presence of nitrogen as the carrier gas.
Rate and product studies, including the use of toluene or cyclohexene as scavengers and the application of selectively deuterated compounds, demonstrated that under our conditions (400-500°C) the reaction mainly proceeds via. free radicals, allyl-oxygen homolysis probably being the intial step. The main decomposition products formed are carbon dioxide, 1-butene, carbon monoxide and methane; formation of these and other (minor) products may be accounted for in terms of an induced decomposition mainly effected by methyl radicals.
TL;DR: In this paper, the thermal and photochemical O−O bond dissociation mechanisms in the aromatic oxygen carrier photosensitizer anthracene-9,10-endoperoxide have been elucidated using high-level multiconfigurational ab initio calculations (CASSCF and MS-CASPT2).
Abstract: The thermal and photochemical O−O bond dissociation mechanisms in the aromatic oxygen carrier photosensitizer anthracene-9,10-endoperoxide have been elucidated using high-level multiconfigurational ab initio calculations (CASSCF and MS-CASPT2). Our results show that in both the thermal and the photochemical pathways, the system proceeds through a degeneracy of four singlet plus four triplet states. This high-order degeneracy provides an efficient funnel for radiationless deactivation from the lowest excited state, indicating that the system does not dissociate in the excited-state manifold but that the products are rather formed in the electronic ground state. Accordingly, the homolysis of the peroxide group leads to four plus four ground-state biradicals, which are the precursors of the experimentally observed rearrangement products.