Showing papers on "Homolysis published in 2005"

Polar, Steric, and Stabilization Effects in Alkoxyamines C−ON Bond Homolysis: A Multiparameter Analysis

Abstract: We present measurements of the rate constants (kd) of the C−ON bond cleavage in new alkoxyamine models containing the N-(2-methyl-2-propyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl (SG1) moiety. The homolysis rate constants of 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO)- and SG1-based alkoxyamines are analyzed in terms of polar inductive/field (σU), steric (υ), and radical stabilization (σRS) contributions of the leaving alkyl radicals, using a multiparameter equation, i.e., log(kd/kd,0) = ρUσU + δυ + ρRSσRS. The rate constants increase with increasing electron withdrawing, steric, and stabilization demands of the leaving alkyl radicals. Good correlations are found for TEMPO (log(kd/kd,0) = 13.6σU + 6.6υ + 13.9σRS) and SG1 (log(kd/kd,0) = 19.5σU + 7.0υ + 15.3σRS) derivatives, highlighting the polar sensitivity of the leaving alkyl radical to the nitroxyl moiety. Such correlations should facilitate the design of new alkoxyamines as initiators/regulators and help to improve the tuning of NMP exper...

Theoretical studies on the vibrational spectra, thermodynamic properties, detonation properties, and pyrolysis mechanisms for polynitroadamantanes.

Abstract: To look for high energy density materials (HEDM), the relationships between the structures and the performances of polynitroadamantanes (PNAs) were studied. The assigned infrared spectra of PNAs obtained at the density functional theory (DFT) B3LYP/6-31G* level were used to compute the thermodynamic properties on the basis of the principle of statistical thermodynamics. The thermodynamic properties are linearly related with the number of nitro groups as well as with the temperatures. Detonation properties of PNAs were evaluated by using the Kamlet-Jacobs equation based on the calculated densities and heats of formation for titled compounds, and it is found that only when the number of nitro groups of PNA is equal to or more than eight can it be possible for PNAs to be used as HEDMs. The relative stabilities of PNAs were studied by the pyrolysis mechanism using the UHF-PM3 method. The homolysis of the C-NO 2 bond is predicted to be the initial step of thermal decomposition. The activation energies (E a ) for the homolysis decrease with the number of nitro groups being increased on the whole. The stability order of dinitroadamantane isomers derived from the interactions among nitro groups is consistent with what is determined by E a . The relations between the E a 's and the electronic structure parameters were discussed. In combination with the stability, PNA (1,2,3,4,5,6,7,8,9,10-) is recommended as the target of HEDM with insensitivity.

Estimation of the bond dissociation energies from the kinetic characteristics of liquid-phase radical reactions

Abstract: Three methods used in the kinetics of liquid-phase radical reactions for estimating the dissociation energies of individual bonds in polyatomic molecules are described. The first approach is based on the study of the equilibrium in radical abstraction reactions involving stable radicals and measurements of the equilibrium constants. The second method is based on the study of the kinetics of homolytic decomposition of molecules. Measuring the activation energy of these reactions makes it possible to estimate the dissociation energies of the weakest bonds, e.g., the O–O bonds in various peroxides. The essence of the third approach developed in the framework of the model of intersecting parabolas is calculations of the bond dissociation energy from the activation energy of a radical reaction involving the molecules under consideration. This method allowed the dissociation energies of the C–H, N–H, O–H and S–H bonds in a large number of organic compounds to be estimated. The scope and the specific features of application of each method are discussed and the bond dissociation energies determined by these methods are given.

Preparation of iron amido complexes via putative Fe(IV) imido intermediates.

Abstract: The isolation and characterization of monomeric Fe(III) amido complexes with hybrid ureate/amidate ligands is described. An aryl azide serves as the source of the amido ligand in preparing the complexes from trigonal monopyramidal Fe(II) precursors. Aryl azides more commonly react with transition metal complexes by a two-electron oxidation process to yield imido complexes, suggesting that the Fe(III) amido complexes may be formed from high valent species by hydrogen atom abstraction from an external species. The mechanistic basis for formation of the amido complexes is investigated using substrates that readily donate hydrogen atoms. Results from these experiments suggest that the Fe(III) amido complexes are generated from Fe(IV) imido intermediates that can facilitate homolytic X−H bond cleavage. The Fe(III) amido complexes are high spin (S = 5/2) with a strong absorbance band at λmax ≈ 600 nm and extinction coefficients between 2000 and 3000 M-1 cm-1. These complexes are hygroscopic, reacting with 1 equ...

Synthesis of heteroarenes using cascade radical cyclisation via iminyl radicals.

Abstract: Cascade radical cyclisation involving homolytic aromatic substitution has been used to synthesise new tetracycles. Treatment of vinyl iodide radical precursors with Me3Sn˙ radicals (from hexamethylditin) yielded intermediate vinyl radicals which undergo 5-exo cyclisation onto suitably placed nitrile groups to yield intermediate iminyl radicals. The iminyl radicals undergo aromatic homolytic substitution via 6-endo cyclisation (or 5-exo cyclisation followed by neophyl rearrangement) with loss of hydrogen (H˙) in a H-abstraction step. We propose that this abstraction was facilitated by tert-butoxyl (t-BuO˙) radicals from di-tert-butyl peroxide or methyl radicals, generated from breakdown of trimethylstannyl radicals (Me3Sn˙). The biologically active alkaloids mappicine and luotonin A were synthesised using the new methodology. A novel radical conversion of nitriles to primary amides is proposed.

Mechanisms of the aerobic oxidation catalyzed by N-hydroxyderivatives. Enthalpic, polar and solvent effects. Molecole-induced homolysis and synthetic involvements

Abstract: Abstract Aminoxyl (R 2 N O ), amidoxyl (RCO N(O ) R) and imidoxyl ((RCO) 2 N O ) radicals play a key role in the aerobic oxidation catalyzed by N -hydroxyderivatives. The rationalization of the mechanisms of a variety of oxidations is based on thermochemical, kinetic and spectroscopic investigations and on solvent effects and it has suggested new selective synthetic developments. In collaboration with CIBA Speciality Chemicals a new catalytic system resulting from the combination of a persistent macrocyclic aminoxyl radical and the couple Mn(NO 3 ) 2 /Co(NO 3 ) 2 has been developed; it is particularly effective for the aerobic oxidation of alcohols to aldehydes and ketones under mild conditions (air at room temperature and atmospheric pressure); above all it presents the great advantage of an easy recovery and recycling providing the possibility of practical applications. The kinetic investigation of the substituent effect in the oxidation of benzyl alcohols has allowed identifying the rate-determining step of the oxidation. The amidoxyl radicals, generated “in situ” from N -hydroxyamide, revealed particularly effective catalysts for the aerobic peroxidation of polyunsaturated fatty acids and esters, which is involved in the origin of several important pathologies, such as tumor initiation and atherosclerosis. The kinetic investigation has contributed to explain the mechanism of the oxidation and to develop the most effective methodology for the synthesis of hydroperoxides. The importance of enthalpic, polar, captodative, solvent effects and “molecule-induced homolysis” has been emphasized in the oxidation, halogenation and acetoxylation of a variety of classes of organic compounds (hydrocarbons, alcohols, aldehydes, amines, amides, silanes) by O 2 and N -hydroxyimide catalysis. The high selectivity often observed and the very mild experimental conditions, based on the mechanistic interpretation, provide industrial potentiality to the catalytic processes.

Time-resolved measurements of the photolysis and recombination of adenosylcobalamin bound to glutamate mutase.

Abstract: Femtosecond to nanosecond transient absorption spectroscopy is used to investigate the photolysis of 5'-deoxyadenosylcobalamin (coenzyme B12, AdoCbl) bound to glutamate mutase. The photochemistry of AdoCbl is found to be inherently dependent upon the environment of the cofactor. Excitation of AdoCbl bound to glutamate mutase results in formation of a metal-to-ligand charge transfer intermediate state which decays to form cob(II)alamin with a time constant of 105 ps. This observation is in contrast to earlier measurements in water where the photohomolysis proceeds through an intermediate state in which the axial dimethylbenzimidazole ligand appears to have dissociated, and measurements in ethylene glycol where prompt bond homolysis is observed (Yoder, L. M.; Cole, A. G.; Walker, L. A., II; Sension, R. J. J. Phys. Chem. B 2001, 105, 12180-12188). The quantum yield for formation of stable radical pairs in the enzyme is found to be phi = 0.05 +/- 0.03, and the resulting intrinsic rate constants for geminate recombination and "cage escape" are 1.0 +/- 0.1 and 0.05 +/- 0.03 ns(-1), respectively. The rate constant for geminate recombination is 30% less than that observed for AdoCbl in water or ethylene glycol. This reduction is insufficient to account for the 10(12)-fold increase in the homolysis rate observed when substrate is bound to the protein. Finally, the protein provides a cage to prevent diffusive loss of the adenosyl radical; however, the ultimate yield for long-lived radicals is determined by the evolution from a singlet to a triplet radical pair as proposed for AdoCbl in ethylene glycol.

Steric and polar effects of the cyclic nitroxyl fragment on the C-ON bond homolysis rate constant

Abstract: Alkoxyamines and persistent nitroxyl radicals are important regulators of nitroxide mediated radical polymerization (NMP). Because polymerization times decrease with increasing rate constant of the homolysis of the C−ON bond between the polymer chain and the nitroxyl moiety, the factors influencing the cleavage rate constant are of considerable interest. Here, we present the measurements of the rate constants (kd) of the C−ON bond cleavage in new cyclic alkoxyamine models. The homolysis rate constants of 9 new alkoxyamines and 33 others given by the literature are analyzed with regards to the contributions of the polar inductive/field (σI) effect, the steric (Es) effect and the intramolecular hydrogen bonding (IHB) effect of the nitroxyl moieties, using the multiparameter equation established by Marque, i.e., log(kd/kd,0) = −3.07σI − 0.88Es − 5.88. Cyclic steric constants r(Ri) for seven- and eight-membered rings are developed. Analysis of the results provides new insight on the importance of the conforma...

Homolytic Cleavage of the O-glycoside Bond in Carbohydrates: A Steady-state Radiolysis Study

Abstract: Carbohydrates/Radiolysis/O-glycoside bond/Cleavage. The formation of products resulting from the O-glycoside bond cleavage following radiolysis of aqueous solutions of methyl- α -D-glucopyranoside (I), 3-O-methyl- α -D-glucopyranose (II), maltose, lactose, gentiobiose and cellobiose were studied. Radiation-induced destruction yields were also determined for dextran, laminarin and trimethylcelulose upon irradiation of their aqueous solutions. Oxygen, quinones and compounds capable of forming quinoid structures were found to inhibit radiation-induced homolytic destruction processes taking place in glycosides, di- and polysaccharides. The data obtained in this study enabled the authors to demonstrate an important role played by the fragmentation reaction of C-2 radicals which were generated from the starting substances in the formation of final radiolysis products.

Tin‐Free Radical Alkoxyamine Addition and Isomerization Reactions by Using the Persistent Radical Effect: Variation of the Alkoxyamine Structure

Abstract: Various C-centered radicals can efficiently be generated through thermal C-O-bond homolysis of alkoxyamines. This method is used to perform environmentally benign radical cyclization and intermolecular addition reactions. These alkoxyamine isomerizations and intermolecular carboaminoxylations are mediated by the persistent radical effect (PRE). In the paper, the effect of the variation of the alkoxyamine structure--in particular steric effects in the nitroxide moiety--on the outcome of the PRE mediated radical reactions will be discussed. Fourteen different nitroxides were used in the studies. It will be shown that reaction times can be shortened about 100 times upon careful tuning of the alkoxyamine structure. Activation energies for the C-O-bond homolysis of the various alkoxyamines are provided. The kinetic data are used to explain the reaction outcome of the PRE-mediated processes.

Hydride affinities of borane derivatives: novel approach in determining the origin of lewis acidity based on triadic formula.

Abstract: The problem of intrinsic Lewis acidities of simple boron compounds (BH3-mXm, m = 0−3, X = F, Cl, Br, CH3, and OH) is assessed by their gas-phase hydride affinities (HAs). A simple and intuitively appealing picture of the interaction process including detachment of an electron from the hydride ion H-, capture of the pruned electron to the investigated Lewis acid (LA), and subsequent formation of the homolytic chemical bond between two newly created radicals is proposed. It enables transparent and straightforward dissection of the initial and final state effects, which taken together with the intermediate relaxation stabilization determine the trend of changes in the hydride affinities. The former effect is reflected in the electron affinities of the neutral Lewis acids given within Koopmans' approximation, while the final state effect involves properties of the formed Lewis acid−base adducts mirrored in the bond dissociation energy of the formed [LA−H]- chemical bond. It is demonstrated that unexpectedly l...

Computational study on the difference between the Co–C bond dissociation energy in methylcobalamin and adenosylcobalamin

Abstract: The bond dissociation energies of the Co–C bonds in the cobalamin cofactors methylcobalamin and adenosylcobalamin were calculated using the hybrid quantum mechanics/molecular mechanics method IMOMM (integrated molecular orbital and molecular mechanics). Calculations were performed on models of differing complexities as well as on the full systems. We investigated the origin of the different experimental values for the Co–C bond dissociation energies in methylcobalamin and adenosylcobalamin, and have provided an explanation for the difficulties encountered when we attempt to reproduce this difference in quantum chemistry. Additional calculations have been performed using the Miertus–Scrocco–Tomasi method in order to estimate the influence of solvent effects on the homolytic Co–C bond cleavage. Introduction of these solvation effects is shown to be necessary for the correct reproduction of experimental trends in bond dissociation energies in solution, which consequently have no direct correlation with dissociation processes in the enzyme.

Homolysis of weak Ti-O bonds: experimental and theoretical studies of titanium oxygen bonds derived from stable nitroxyl radicals.

Abstract: Titanium−oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be homolyzed at 60 °C. The strength of these bonds depends sensitively on the ancillary ligation at titanium. Direct measurements of the rate of Ti−O bond homolysis in Ti−TEMPO complexes Cp2TiCl(TEMPO) (3) and Cp2TiCl(4-MeO-TEMPO) (4) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl, 4-MeO-TEMPO = 2,2,6,6-tetramethyl-4-methoxypiperidine-N-oxyl) were conducted by nitroxyl radical exchange experiments. Eyring plots gave the activation parameters, ΔH⧧ = 27(±1) kcal/mol, ΔS⧧ = 6.9(±2.3) eu for 3 and ΔH⧧ = 28(±1) kcal/mol, ΔS⧧ = 9.0(±3.0) eu for 4, consistent with a process involving the homolysis of a weak Ti−O bond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPO complexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxide followed by trapping by the nitroxyl radical. The X-ray crystal structure of the Ti−TEMPO derivative, Cp2TiCl(4-M...

The C-H and α(C-X) Bond Dissociation Enthalpies of Toluene, C6H5-CH2X (X = F, Cl), and Their Substituted Derivatives: A DFT Study

Abstract: The homolytic C-H bond dissociation enthalpies (BDEs) of toluene and its para- and meta-substituted derivatives have been estimated by using the (RO)B3LYP/6-311++G(2df,2p)//(U)B3LYP/6-311G(d,p) procedure. The performance of two other hybrid functionals of DFT, namely, B3PWP91 and O3LYP, has also been evaluated using the same basis sets and molecules. Our computed results are compared with the available experimental values and are found to be in good agreement. The (RO)B3LYP and (RO)O3LYP procedures are found to produce reliable BDEs for the C-H bonds in toluene and the C-X (X = F, Cl) bond in alpha-substituted toluene (C6H5-CH2X) and their substituted derivatives. The substituent effect on the BDE values has been analyzed in terms of the ground-state effect and the radical effect. The effect of polarization of the C-H bond on the substituent effect is also analyzed. The BDE(C-H) and BDE(C-X) values for alpha-substituted (X = F and Cl) toluenes with a set of para substituents are presented for the first time.

Co-C bond activation in methylmalonyl-CoA mutase by stabilization of the post-homolysis product Co2+ cobalamin.

Abstract: Despite decades of research, the mechanism by which coenzyme B 12 (adenosylcobalamin, AdoCbl)-dependent enzymes promote homolytic cleavage of the cofactor's Co-C bond to initiate catalysis has continued to elude researchers. In this work, we utilized magnetic circular dichroism spectroscopy to explore how the electronic structure of the reduced B 12 cofactor (i.e., the post-homolysis product Co 2+ Cbl) is modulated by the enzyme methylmalonyl-CoA mutase. Our data reveal a fairly uniform stabilization of the Co 3d orbitals relative to the corrin π/π*-based molecular orbitals when Co 2+ Cbl is bound to the enzyme active site, particularly in the presence of substrate. Contrastingly, our previous studies (Brooks, A. J.; Vlasie, M.; Banerjee, R.; Brunold, T. C. J. Am. Chem. Soc. 2004, 126, 8167-8180.) showed that when AdoCbl is bound to the MMCM active site, no enzymatic perturbation of the Co 3+ Cbl electronic structure occurs, even in the presence of substrate (analogues). Collectively, these observations provide direct evidence that enzymatic Co-C bond activation involves stabilization of the post-homolysis product, Co 2+ -Cbl, rather than destabilization of the Co 3+ Cbl ground state.

Determination of N-NO bond dissociation energies of N-methyl-N-nitrosobenzenesulfonamides in acetonitrile and application in the mechanism analyses on NO transfer

Abstract: The heterolytic and homolytic N-NO bond dissociation energies of seven substituted N-methyl-N-nitrosobenzenesulfonamides (abbreviated as G-MNBS, G = p-OCH(3), p-CH(3), p-H, p-Cl, p-Br, 2,5-2Cl, m-NO(2)) in acetonitrile solution were evaluated for the first time by using titration calorimetry and relative thermodynamic cycles according to Hess' law The results show that the energetic scales of the heterolytic and homolytic N-NO bond dissociation energies of G-MNBS in acetonitrile solution cover the ranges from 443 to 495 and from 330 to 349 kcal/mol for the neutral G-MNBS, respectively, which indicates that N-methyl-N-nitrosobenzenesulfonamides are much easier to release a NO radical (NO(*)) than to release a NO cation (NO(+)) The estimation of the heterolytic and homolytic (N-NO)(-)(*) bond dissociation energies of the seven G-MNBS radical anions in acetonitrile solution gives the energetic ranges of -158 to -129 and -31 to 18 kcal/mol for the (N-NO)(-)(*) bond homolysis and heterolysis, respectively, which means that G-MNBS radical anions are very unstable at room temperature and able to spontaneously or easily release a NO radical or NO anion (NO(-)), but releasing a NO radical is easier than releasing NO anion These determined N-NO bond dissociation energies of G-MNBS and their radical anions have been successfully used in the mechanism analyses of NO transfer from G-MNBS to 3,6-dibromocarbazole and the reactions of NO with the substituted N-methyl-benzenesulfonamide nitranions (G-MBSN(-)) in acetonitrile solution

Rearrangements and stereomutations of metallacycles derived from allenes and imidozirconium complexes.

Abstract: The mechanisms of the rearrangements and stereoinversion of azametallacyclobutenes generated via [2+2] cycloaddition of allenes and imidozirconium complexes have been studied. Metallacycles derived from allenes bearing β-hydrogen atoms racemize at room temperature by reversible β-hydride elimination, a process which is also responsible for their eventual conversion to monoazadiene complexes. Metallacycles derived from diarylallenes racemize by reversible thermal bond homolysis at 95 °C; racemization of these metallacycles is also catalyzed by mild oxidants.

Theoretical investigations into the intermediacy of chlorinated vinylcobalamins in the reductive dehalogenation of chlorinated ethylenes

Abstract: The reductive dehalogenation of perchloroethylene and trichloroethylene by vitamin B12 produces ∼95% (Z)-dichloroethylene (DCE) and small amounts of (E)-DCE and 1,1-DCE, which are further reduced to ethylene and ethane. Chloroacetylene and acetylene have been detected as intermediates, but not dichloroacetylene. Organocobalamins (RCbls) have been proposed to be intermediates in this process. Density functional theory based approaches were employed to investigate the properties of chlorinated vinylcobalamins and chlorinated vinyl radicals. They reveal that all vinyl radicals studied have reduction potentials more positive (E° ≥ −0.49) than that of the CoII/CoI couple of B12 (E° = −0.61 V), indicating that any (chlorinated) vinyl radicals formed in the reductive dehalogenation process should be reduced to the corresponding anions by cob(I)alamin in competition with their combination with Co(II) to yield the corresponding vinylcobalamins. The computed Co−C homolytic bond dissociation enthalpies (BDEs) of the...

Carbon-to-Metal Hydrogen Atom Transfer: Direct Observation Using Time-Resolved Infrared Spectroscopy

Abstract: We report the direct spectroscopic observation of hydrogen atom transfer reactions from carbon to metals, in which homolytic cleavage of a C−H bond is accomplished at a single metal center Laser flash photolysis (355 nm) of a solution of [Cp(CO)2Os]2 leads to homolysis of the Os−Os bond and formation of the osmium-centered radical, Cp(CO)2Os•, as observed by time-resolved infrared (TRIR) spectroscopy DFT computations on Cp(CO)2Os• support this assignment Continuous photolysis (λ > 300 nm) of [Cp(CO)2Os]2 in the presence of excess 1,4-cyclohexadiene produces the osmium hydride Cp(CO)2OsH The kinetics of this carbon-to-metal hydrogen atom transfer were examined by TRIR spectroscopy The second-order rate constant for hydrogen atom transfer from 1,4-cyclohexadiene to Cp(CO)2Os• in hexane at 23 °C is kH = (21 ± 02) × 106 M-1 s-1 The pKa of Cp(CO)2OsH was determined as 327 in CH3CN, and use of a thermochemical cycle provided an estimated lower limit of 82 kcal/mol for the Os−H bond dissociation energy,

Isotope effects for deuterium transfer between substrate and coenzyme in adenosylcobalamin-dependent glutamate mutase.

Abstract: A key step in the mechanism of all adenosylcobalamin-dependent enzymes is the abstraction of a hydrogen atom from the substrate by a 5‘-deoxyadenosyl radical generated by homolytic fission of the coenzyme cobalt−carbon bond We have investigated the isotope effects associated with this process for glutamate mutase reacting with deuterated glutamate The kinetics of deuterium incorporation into 5‘-deoxyadenosine (5‘-dA) during the reaction were followed by rapid chemical quench, using HPLC and electrospray mass spectrometry to analyze the 5‘-dA formed The kinetics of 5‘-dA formation are biphasic, comprising a rapid phase kapp = 37 ± 3 s-1 and a slower phase kapp = 09 ± 04 s-1 The mass spectral data clearly show that the faster phase is associated with the formation of monodeuterated 5‘-dA whereas the slower phase is associated with the incorporation of a second and then a third deuterium into 5‘-dA This observation implies that a large inverse equilibrium secondary isotope effect is associated with th

Carbon–hydrogen bond dissociation enthalpies in ethers: a theoretical study

Abstract: Density functional theory calculations based on different representations of the exchange-correlation functional (BLYP, B3LYP, B3PW91, mPW1PW91, B1LYP, BHandHLYP, BHandH, and B3P86) were carried out to predict C–H bond dissociation enthalpies in selected ethers (dimethyl ether, tetrahydrofuran (thf), and 1,4-dioxane), and some related molecules. Comparison with available experimental information shows that bond dissociation enthalpies are accurately predicted by several functionals when isodesmic and isogyric reactions are considered. However, for bond homolysis reactions only the hybrid functional B3P86 has an acceptable performance, slightly underestimating bond dissociation enthalpies. DFT calculations were further compared with the multilevel CBS-Q method. The analysis of structural properties of the radical species suggests that hyperconjugation and ring strain are the main features to consider for understanding their thermodynamic stabilization. These effects are also useful to explain some trends of bond dissociation enthalpies that are observed for the compounds involved in the present study. The C–H bond dissociation enthalpies of thf (leading to β-furanyl) and 1,4-dioxane, for which no experimental information is available, are 410 and 381 kJ mol −1 , respectively, at the CBS-Q level.

Interconversions of aryl radicals by 1,4-shifts of hydrogen atoms. A synthesis of benzo[a]corannulene.

Abstract: Aryl radicals generated ortho to aryl substituents by flash vacuum pyrolysis (FVP) of the corresponding aryl chlorides are shown to be capable of transferring hydrogen atoms between the ortho and ortho‘ positions (1,4-shifts of hydrogen atoms). In the examples described here, the rearranged aryl radicals are trapped by subsequent radical cyclization reactions. For example, FVP of 2-(o-chlorophenyl)benzo[c]phenanthrene gives 1-phenylbenzo[ghi]fluoranthene as the major product by homolysis of the C−Cl bond, 1,4-shift of a hydrogen atom out of the sterically congested cove region to the radical center, cyclization of the rearranged radical, and rearomatization of the molecule by loss of the other cove region hydrogen. In a control experiment run under the same conditions, FVP of 2-phenylbenzo[c]phenanthrene, which lacks a radical precursor, gave primarily recovered starting material. When the FVP was repeated using 2-(2,6-dichlorophenyl)benzo[c]phenanthrene as the starting material, benzo[a]corannulene was o...

Electronic structure studies of the adenosylcobalamin cofactor in glutamate mutase.

Abstract: Glutamate mutase (GM) is a cobalamin-dependent enzyme that catalyzes the reversible interconversion of L-glutamate and L-threo-3-methylaspartate via a radical-based mechanism. To initiate catalysis, the 5'-deoxyadenosylcobalamin (AdoCbl) cofactor's Co-C bond is cleaved homolytically to generate an adenosyl radical and Co 2 + Cbl. In this work, we employed a combination of spectroscopic and computational tools to evaluate possible mechanisms by which the Co-C bond is activated for homolysis. Minimal perturbations to the electronic absorption (Abs), circular dichroism (CD), and magnetic CD (MCD) spectra of AdoCbl are observed upon formation of holoenzyme, even in the presence of substrate (or a substrate analogue), indicating that destabilization of the Co 3 + Cbl "ground state" is an unlikely mechanism for Co-C bond activation. In contrast, striking alterations are observed in the spectroscopic data of the post-homolysis product Co 2 + Cbl when bound to glutamate mutase in the presence of substrate (or a substrate analogue) as compared to unbound Co 2 + Cbl. These enzymatic perturbations appear to most strongly affect the metal-to-ligand charge-transfer transitions of Co 2 + Cbl, suggesting that the cofactor/active-site interactions give rise to a fairly uniform stabilization of the Co 3d orbitals. Remarkable similarities between the results obtained in this study and those reported previously for the related Cbl-dependent isomerase methylmalonyl-CoA mutase indicate that a common mechanism by which the cofactor's Co-C bond is activated for homolytic cleavage may be operative for all base-off/His-on Cbl-dependent isomerases.

Chemistry of Organic Nitrates: Thermal Chemistry of Linear and Branched Organic Nitrates

Abstract: The objective of our research was to measure accurate rate constants for the thermal, unimolecular decomposition of organic nitrates. Our research confirms that the rate-determining step is homolytic cleavage of the weak O−N bond to form alkoxy radical and NO2, but the rate constants reported in the past are incorrect. The alkoxy radical and NO2 engage in secondary reactions that ultimately generate stable products such as carbonyl and nitro compounds. Infrared spectroscopy (IR) and gas chromatography/mass spectrometry (GC/MS) were used to monitor the time dependent loss of organic nitrate and to characterize the products of the thermal reaction. Past research indicates that oxygen slows the rate of homolytic O−N bond cleavage to form radicals. Our research shows that the rates are the same in nitrogen as they are in air. The reaction in air produces α−β unsaturated ketones/aldehydes, which are not generated in a nitrogen atmosphere. The unsaturated ketone/aldehydes complicate the IR analysis, giving the ...

Mechanisms of Thermal Decompositions of Polysulfones: A DFT and CBS-QB3 Study

Abstract: The thermal stabilities of polysulfones with different lengths of the carbon-linker between sulfone groups have been explored. B3LYP and CBS-QB3 calculations on possible thermal decomposition mechanisms of model compounds show that even carbon-linked polysulfonescopolymers of alkenes and SO2decompose by a radical chain mechanism involving facile depolymerization after initial homolysis. Sulfinate impurities [−S(O)−O−CH2−], formed as a minor component in polysulfone synthesis, can serve as good initiators for the radical mechanism. Polysulfones with odd-carbon linkers do not readily depolymerize in this fashion owing to high barriers of depropagation; these polymers undergo thermal degradation by the slower cyclic β-elimination processes.

Density functional theory study on the thermodynamic properties of aminophenols

Abstract: Density Functional Theory was used to investigate several gas-phase thermodynamic parameters of the o-, m-, and p-aminophenol isomers. Within the DFT approach, the B3LYP method and the 6-31G(d) and 6-311+G(2d,2p) basis sets were used to compute standard enthalpies of formation. Calculated data are in excellent agreement with the experimental work of Nunez et al. [J Chem Thermodyn 1996, 18, 575–579] but differs significantly from the values of Sabbah et al. [Can J Chem 1996, 74, 500–507]. In this work, other properties such as homolytic O-H and N-H bond dissociation energies, gas-phase acidities, and proton or electron affinities were also obtained and confirm the few experimental results available for these properties in these kind of compounds, except the O-H homolytic dissociation energy of 2-aminophenol. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Gas phase chemistry in gallium nitride CVD: Theoretical determination of the Arrhenius parameters for the first Ga-C bond homolysis of trimethylgallium.

Abstract: Experimental evidence suggests that the energy of activation for the first homolytic Ga−C bond fission of GaMe3 of Ea = 249 kJ/mol, measured by Jacko and Price in a hot-wall tube reactor, is affected by surface catalytic effects. In this contribution, the rate constant for this crucial step in the gas-phase pyrolysis of GaMe3 has been calculated by variational transition state theory. By a basis set extrapolation on the MP2/cc-pVXZ level and a correlation correction from CCSD(T)/cc-pVDZ level, a theoretical “best estimate” for the bond energy of ΔH289K = 327.2 kJ/mol was derived. For the VTST calculation on the B3LYP/cc-pVDZ level, the energies were corrected to reproduce this bond energy. Partition functions of the transitional modes were approximated by a hindered rotor approximation to be valid along the whole reaction coordinate defined by the Ga−C bond length. On the basis of the canonical transition state theory, reaction rates were determined using the maxima of the free energy ΔG⧧. An Arrhenius-ty...

Is there a homolytic substitution chemistry (SH2) of sulfones

Abstract: [reaction: see text] A series of 2-alkylsulfonyl-2'-biphenyl radicals, in which the alkyl group is primary, secondary, or tertiary, were generated and the products of their reactions investigated. Dibenzothiophene S,S-dioxide was not identified among the products, which arose mainly from intramolecular hydrogen abstraction from the alkyl group or addition to the solvent, benzene. On this basis, it is concluded that homolytic substitution at sulfonyl sulfur, if possible at all, is too slow to take precedence over a number of competing decomposition pathways. Previous literature results suggesting the possibility of intramolecular homolytic substitution at sulfonyl sulfur may be explained by alternative processes.