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Showing papers on "Homolysis published in 2002"


Journal ArticleDOI
TL;DR: The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products.
Abstract: The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methods for water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide, photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradation by reaction with intermediates generated by photolytic or radiolytic means.

563 citations


Journal ArticleDOI
TL;DR: From the reactions of [Mn(TF(4)TMAP)](5+) with various oxidants in the pH range 3-11, the O-O bond cleavage of hydroperoxides was found to be sensitive to the hydroperoxide substituent and the pH of the reaction solution.
Abstract: The reaction of [Mn(TF(4)TMAP)](CF(3)SO(3))(5) (TF(4)TMAP=meso-tetrakis(2,3,5,6-tetrafluoro-N,N,N-trimethyl-4-aniliniumyl)porphinato dianion) with H(2)O(2) (2 equiv) at pH 10.5 and 0 degrees C yielded an oxomanganese(V) porphyrin complex 1 in aqueous solution, whereas an oxomanganese(IV) porphyrin complex 2 was generated in the reactions of tert-alkyl hydroperoxides such as tert-butyl hydroperoxide and 2-methyl-1-phenyl-2-propyl hydroperoxide. Complex 1 was capable of epoxidizing olefins and exchanging its oxygen with H(2) (18)O, whereas 2 did not epoxidize olefins. From the reactions of [Mn(TF(4)TMAP)](5+) with various oxidants in the pH range 3-11, the O-O bond cleavage of hydroperoxides was found to be sensitive to the hydroperoxide substituent and the pH of the reaction solution. Whereas the O-O bond of hydroperoxides containing an electron-donating tert-alkyl group is cleaved homolytically, an electron-withdrawing substituent such as an acyl group in m-chloroperoxybenzoic acid (m-CPBA) facilitates O-O bond heterolysis. The mechanism of the O-O bond cleavage of H(2)O(2) depends on the pH of the reaction solution: O-O bond homolysis prevails at low pH and O-O bond heterolysis becomes a predominant pathway at high pH. The effect of pH on (18)O incorporation from H(2) (18)O into oxygenated products was examined over a wide pH range, by carrying out the epoxidation of carbamazepine (CBZ) with [Mn(TF(4)TMAP)](5+) and KHSO(5) in buffered H(2) (18)O solutions. A high proportion of (18)O was incorporated into the CBZ-10,11-oxide product at all pH values but this proportion was not affected significantly by the pH of the reaction solution.

140 citations


Journal ArticleDOI
TL;DR: These measurements and the determinations of various electrochemical potentials were used to determine 11 of 12 possible homolytic and heterolytic solution Co-H bond dissociation free energies of [H(2)Co(dppe)(2)](+) and its monohydride derivatives.
Abstract: A detailed structural and thermodynamic study of a series of cobalt-hydride complexes is reported. This includes structural studies of [H2Co(dppe)2]+, HCo(dppe)2, [HCo(dppe)2(CH3CN)]+, and [Co(dppe)2(CH3CN)]2+, where dppe = bis(diphenylphosphino)ethane. Equilibrium measurements are reported for one hydride- and two proton-transfer reactions. These measurements and the determinations of various electrochemical potentials were used to determine 11 of 12 possible homolytic and heterolytic solution Co−H bond dissociation free energies of [H2Co(dppe)2]+ and its monohydride derivatives. These values provide a useful framework for understanding observed and potential reactions of these complexes. These reactions include the disproportionation of [HCo(dppe)2]+ to form [Co(dppe)2]+ and [H2Co(dppe)2]+, the reaction of [Co(dppe)2]+ with H2, the protonation and deprotonation reactions of the various hydride species, and the relative ability of the hydride complexes to act as hydride donors.

136 citations


Journal ArticleDOI
TL;DR: In this paper, the authors employed density functional theory to investigate the reactivity of Fe(TPA) complexes, which are known to catalyze stereospecific hydrocarbon oxidation when H2O2 is used as oxidant.
Abstract: Density functional theory using the B3LYP hybrid functional has been employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H2O2 is used as oxidant. The reaction pathway leading to O−O bond heterolysis in the active catalytic species FeIII(TPA)−OOH has been explored, and it is shown that a high-valent iron-oxo intermediate is formed, where an FeV oxidation state is attained, in agreement with previous suggestions based on experiments. In contrast to the analogous intermediate [(Por·)FeIVO]+1 in P450, the TPA ligand is not oxidized, and the electrons are extracted almost exclusively from the mononuclear iron center. The corresponding homolytic O−O bond cleavage, yielding the two oxidants FeIVO and the OH· radical, has also been considered, and it is shown that this pathway is inaccessible in the hydrocarbon oxidation reaction with Fe(TPA) and hydrogen peroxide. Investigations have also been perfor...

135 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present a review of O-Radical reactions and their application in organic synthesis, focusing on 5-exo-triggered addition reactions and homolysis of a β-C-C bond.
Abstract: In recent years, powerful new methods for the generation of alkoxyl radicals under mild and neutral conditions have been developed. This progress has led to a thorough investigation of most O-radical elementary reactions.Today, sufficiently reliable thermodynamic and kinetic data are available either from experimental or from theoretical studies in order to predict alkoxyl radical reactivities and selectivities in synthesis. For instance, alkoxyl radicals readily add to carbon-carbon and carbon-nitrogen double bonds. Due to generally low activation barriers and strongly negative reaction enthalpies, inter- and intramolecular addition reactions proceed under kinetic control and are associated with high rate constants. Nevertheless, intramolecular 5-exo-trig additions, i.e. cyclizations, proceed with an astonishing degree of diastereoselectivity and often provide complementary selectivities if compared to commonly used methods such as the bromine cyclization of alkenols. Therefore, several useful applications of O-radical cyclizations in the synthesis of functionalized tetrahydrofurans have been discovered in the last few years. A second major reaction channel of alkoxyl radicals is associated with the homolysis of a β-C-C bond. This fragmentation proceeds under thermodynamic control and affords a carbonyl compound besides an alkyl radical from the starting alkoxyl radical. Regioselectivities for C-C bond homolysis may be predicted by considering strain release (cyclic carbon framework) and the stability of the newly formed carbon radical (cyclic and open chain carbon skeletons). The third major group of alkoxyl radical-based transformations are connected with homolytic substitutions such as intramolecular 1,5-hydrogen shifts which have been applied with considerable success to remote functionalization reactions. In view of the diversity of alkoxyl radical reactions it is the aim of this review to organize basic principles of this type of chemistry and to present its latest useful application in organic synthesis.

132 citations


Journal ArticleDOI
TL;DR: In this article, supported metallophthalocyanine catalysts were studied in the oxidation of 2-methylnaphthalene (2MN) to 2-1,4-naphthoquinone (Vitamin K3, VK3), 2,3,6-trimethylphenol (TMP) to trimethyl-1-4-benzoquinone and in the epoxidation of olefins.
Abstract: This article summarizes our research in catalytic oxidation on the design and study of supported metallophthalocyanine catalysts. The catalytic properties of these materials were studied in the oxidation of 2-methylnaphthalene (2MN) to 2-methyl-1,4-naphthoquinone (Vitamin K3, VK3), 2,3,6-trimethylphenol (TMP) to trimethyl-1,4-benzoquinone (precursor of Vitamin E) and in the epoxidation of olefins. Iron tetrasulfophthalocyanine (FePcS) covalently grafted in the dimeric form yielded catalyst more active and selective that those containing monomeric species but suffered from a lack of stability transforming into less selective monomer complexes during catalysis. The stabilization of supported dimer form by covalent link of two adjacent phthalocyanine molecule through appropriate diamine spacer provided more selective and stable catalysts. Trimethyl-1,4-benzoquinone was obtained with 87% yield at 97% conversion of TMP. More demanding oxidation of 2MN afforded 45% yield of VK3. Particular emphasis is placed on the mechanistic aspects of these oxidations using two mechanistic probes, 2-methyl-1-phenylpropan-2-yl hydroperoxide (MPPH) to distinguish between homolytic versus heterolytic cleavage of O–O bond during the formation of active species and thianthrene 5-oxide (SSO) to evaluate nucleophilic versus electrophilic character of formed active species. To illustrate a versatility of the phthalocyanine-based supported catalysts we prepared a novel phthalocyanine complex with eight triethoxysylil substituents which can be directly anchored to the silica without any modification of the silica support. This new catalyst shows good catalytic activity in epoxidation of olefins by dioxygen in the presence of isobutyraldehyde. The same catalytic system was also active in the oxidation of phenols to biphenols with 86% yields. This catalytic system is complementary to previous one that selectively oxidizes phenols to quinones. An appropriate choice of the reaction conditions allows selective oxidation either to quinones or to biaryl compounds.

121 citations


Journal ArticleDOI
TL;DR: Kinetic analysis has shown that the reaction consists of two distinct steps, where the first step involves a binding of DBP to the trinuclear complex to give a certain intermediate that further reacts with the second molecule ofDBP to give another intermediate, from which the final products are released.
Abstract: Copper(I)-dioxygen reactivity has been examined using a series of 2-(2-pyridyl)ethylamine bidentate ligands (R1)Py1(R2,R3). The bidentate ligand with the methyl substituent on the pyridine nucleus (Me)Py1(Et,Bz) (N-benzyl-N-ethyl-2-(6-methylpyridin-2-yl)ethylamine) predominantly provided a (mu-eta(2):eta(2)-peroxo)dicopper(II) complex, while the bidentate ligand without the 6-methyl group (H)Py1(Et,Bz) (N-benzyl-N-ethyl-2-(2-pyridyl)ethylamine) afforded a bis(mu-oxo)dicopper(III) complex under the same experimental conditions. Both Cu(2)O(2) complexes gradually decompose, leading to oxidative N-dealkylation reaction of the benzyl group. Detailed kinetic analysis has revealed that the bis(mu-oxo)dicopper(III) complex is the common reactive intermediate in both cases and that O[bond]O bond homolysis of the peroxo complex is the rate-determining step in the former case with (Me)Py1(Et,Bz). On the other hand, the copper(I) complex supported by the bidentate ligand with the smallest N-alkyl group ((H)Py1(Me,Me), N,N-dimethyl-2-(2-pyridyl)ethylamine) reacts with molecular oxygen in a 3:1 ratio in acetone at a low temperature to give a mixed-valence trinuclear copper(II, II, III) complex with two mu(3)-oxo bridges, the UV-vis spectrum of which is very close to that of an active oxygen intermediate of lacase. Detailed spectroscopic analysis on the oxygenation reaction at different concentrations has indicated that a bis(mu-oxo)dicopper(III) complex is the precursor for the formation of trinuclear copper complex. In the reaction with 2,4-di-tert-butylphenol (DBP), the trinuclear copper(II, II, III) complex acts as a two-electron oxidant to produce an equimolar amount of the C[bond]C coupling dimer of DBP (3,5,3',5'-tetra-tert-butyl-biphenyl-2,2'-diol) and a bis(mu-hydroxo)dicopper(II) complex. Kinetic analysis has shown that the reaction consists of two distinct steps, where the first step involves a binding of DBP to the trinuclear complex to give a certain intermediate that further reacts with the second molecule of DBP to give another intermediate, from which the final products are released. Steric and/or electronic effects of the 6-methyl group and the N-alkyl substituents of the bidentate ligands on the copper(I)-dioxygen reactivity have been discussed.

104 citations


Journal ArticleDOI
TL;DR: The extent of conversion to NO3– and that of homolysis is significantly less than that reported in the literature, according to analyses of peroxynitrite, carbon dioxide and nitrogen monoxide.
Abstract: CO2 catalyses the isomerization of the biological toxin ONOO– to NO3– via an intermediate, presumably ONOOCO2–, which has an absorption maximum near 650 nm. The reflection spectrum of solid NMe4+ONOO– exposed to CO2 shows a similar band near 650 nm; this absorption decays over minutes. Stopped-flow experiments in which CO2 solutions were mixed with alkaline ONOO– solutions indicate the formation of at least one intermediate. The initial absorption at 302 nm is less than that of ONOO–, which indicates that reactions take place within the mixing time, and this absorption is dependent (but not linearly) on the ONOO– and CO2 concentrations. We found that reaction of peroxynitrite with carbon dioxide forms some trioxocarbonate(•1–) (CO3•–) and nitrogen dioxide (NO2•) radicals via homolysis of the O-O bond in ONOOCO2–. We determined the extent of radical formation by mixing peroxynitrite, carbon dioxide and nitrogen monoxide. The later reacts with CO3•– and NO2• radicals to form, effectively, three NO2– per homolysis; ONOOCO2– that does not undergo homolysis yields NO3– and CO2. Based on the NO3– and NO2– analyses, the extent of conversion to NO3– is 96±1% and that of homolysis is 3±1%, respectively, significantly less than that reported in the literature.

92 citations


Journal ArticleDOI
TL;DR: An ultrafast transient absorption study of the primary photolysis of ethyl- and n-propylcobalamin in water is presented, demonstrating the formation of one major photoproduct on a picosecond time scale.
Abstract: An ultrafast transient absorption study of the primary photolysis of ethyl- and n-propylcobalamin in water is presented. Data have been obtained for two distinct excitation wavelengths, 400 nm at the edge of the UV γ-band absorption, and 520 nm in the strong visible αβ-band absorption. These data are compared with results reported earlier for the B12 coenzymes, methyl- and adenosylcobalamin. The data obtained for ethylcobalamin and n-propylcobalamin following excitation at 400 nm demonstrate the formation of one major photoproduct on a picosecond time scale. This photoproduct is spectroscopically identifiable as a cob(II)alamin species. Excitation of methyl-, ethyl-, and n-propylcobalamin at 520 nm in the low-lying αβ absorption band results in bond homolysis proceeding via a bound cob(III)alamin MLCT state. For all of the cobalamins studied here competition between geminate recombination of caged radical pairs and cage escape occurs on a time scale of 500 to 700 ps. The rate constants for geminate recomb...

82 citations


Journal ArticleDOI
TL;DR: The precise structure of the dimer was determined for the first time by X-ray crystallography, and its homolytic dissociation as well as spectroscopic and electrochemical properties were clarified.

77 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used high level ab initio molecular orbital theory including extrapolation to the complete basis set limit to calculate the heats of formation of reactants and products for the homolytic bond fission pathways for decomposition of HOONO and the molecular pathway that yields HNO and 1O2 as well as the transition state for the latter process.
Abstract: It was recently suggested that HOONO, which forms after protonation of the ONOO- anion under biological conditions, decomposes into HNO and (1Δg)O2. Subsequent workers argued that the mechanism for HOONO decomposition proceeds via homolytic bond fission, producing the radical pair OH and NO2, and another recent study argued that a cyclic form of the peroxynitrous acid results in the products H+ + O2(1Δg) + NO-. Calculations on the reaction pathway for the process showed that it required a high activation energy, and is thus implausible. High level ab initio molecular orbital theory including extrapolation to the complete basis set limit has been used to calculate the heats of formation of reactants and products for the homolytic bond fission pathways for decomposition of HOONO and the molecular pathway that yields HNO and 1O2 as well as the transition state for the latter process. These data are used to evaluate the probability of whether the decomposition of peroxynitrous acid can produce HNO and 1O2. Is...

Journal ArticleDOI
TL;DR: In this paper, four density-functional methods (B3LYP, B3PW91, MPW1PW 91, and B 3P86) are employed to compute the C−C homolytic bond dissociation enthalpies (BDE) of a set of aromatic hydrocarbons related to coal structures with aliphatic linkages.
Abstract: Four density-functional methods (B3LYP, B3PW91, MPW1PW91, and B3P86) are employed to compute the C−C homolytic bond dissociation enthalpies (BDE) of a set of aromatic hydrocarbons related to coal structures with aliphatic linkages. In comparison with the available experimental data, the B3P86 method can provide reasonably reliable BDE values for these model compounds. The BDE values for large aromatic hydrocarbon systems of interest are computed, and the substituent effects are discussed.


Journal ArticleDOI
TL;DR: Structural and spectroscopic studies indicate the presence of two hydrogen-bonding interactions involving the oxygen atom of the zinc-bound anion in each complex, which indicates that spontaneous zinc alkoxide formation from a hydroxide precursor occurs in this system at low temperature.
Abstract: The synthesis and properties of mononuclear zinc methoxide ([(ebnpa)Zn-OCH3]ClO4) (1) and hydroxide ([(ebnpa)Zn-OH]ClO4) (2) complexes of a new mixed nitrogen/sulfur ligand (ebnpa = N-2-(ethylthio)ethyl-N,N-bis(6-neopentylamino-2-pyridylmethyl)amine) are reported. The structures of 1 and 2 were determined by X-ray diffraction. Each possesses a single zinc-coordinated anion (methoxide or hydroxide) and exhibits an overall trigonal bipyramidal geometry. Structural and spectroscopic studies indicate the presence of two hydrogen-bonding interactions involving the oxygen atom of the zinc-bound anion in each complex. Treatment of [(ebnpa)Zn-OH]ClO4 with CH3OH results in the formation of an equilibrium mixture of 1 and 2. 1H NMR spectroscopic methods were used to examine the equilibrium as a function of temperature, yielding KMe (304 K) = 0.30(8), DeltaHMe = -0.9(1) kcal/mol, and DeltaSMe = -5(1) eu. The negative enthalpy indicates that spontaneous zinc alkoxide formation from a hydroxide precursor occurs in this system at low temperature. Using the experimentally determined DeltaHMe value, we found the homolytic Zn-O bond dissociation energy (BDE) in the Zn-OCH3 unit to be approximately -14 kcal/mol relative to the Zn-O BDE in the Zn-OH unit.

Journal ArticleDOI
TL;DR: The origin of the above remarkably different substituent effects on the Si-X BDEs was found to be associated with the ability of the substituents to stabilize or destabilize the starting material as well as the product of the homolysis.
Abstract: UB3LYP/6-31G(d) and ROMP2/6-311++G(d,2p) methods were used to calculate the Si-X bond dissociation energies (BDEs) of a number of para-substituted aromatic silanes (4-Y-C(6)H(4)-SiH(2)X, where X = H, F, Cl, or Li). It was found that the substituent effect on the Si-H BDE of 4-Y-C(6)H(4)-SiH(3) was small, as the slope (rho(+)()) of the BDE- regression was only 0.09 kJ/mol. In comparison, the substituent effect on the Si-F BDE of 4-Y-C(6)H(4)-SiH(2)F was much stronger, whose rho(+ )()value was -2.34 kJ/mol. The substituent effect on the Si-Cl BDE of 4-Y-C(6)H(4)-SiH(2)Cl was also found to be strong with a rho(+)() value of -1.70 kJ/mol. However, the substituent effect on the Si-Li BDE of 4-Y-C(6)H(4)-SiH(2)Li was found to have a large and positive slope (+9.12 kJ/mol) against. The origin of the above remarkably different substituent effects on the Si-X BDEs was found to be associated with the ability of the substituent to stabilize or destabilize the starting material (4-Y-C(6)H(4)-SiH(2)X) as well as the product (4-Y-C(6)H(4)-SiH(2)* radical) of the homolysis. Therefore, the direction and magnitude of the effects of Y-substituents on the Z-X BDEs in compounds such as 4-YC(6)H(4)Z-X should have some important dependence on the polarity of the Z-X bond undergoing homolysis. This conclusion was in agreement with that from earlier studies (for example, J. Am. Chem. Soc. 1991, 113, 9363). However, it indicated that the proposal from a recent work (J. Am. Chem. Soc. 2001, 123, 5518) was unfortunately not justified.

Journal ArticleDOI
TL;DR: In this article, the origin of the intrinsic basicity of neutral nitrogen bases, as reflected in their gas phase proton affinities, is addressed and a simple solution is found, rooted in an intuitively appealing picture involving ionization of the base in question by pruning an electron, subsequent creation of the hydrogen atom with the incoming proton, and the formation of the homolytic chemical bond between a radical cation and the hydrogen.
Abstract: The problem of the origin of the intrinsic basicity of neutral nitrogen bases, as reflected in their gas phase proton affinities, is addressed and a simple solution is found. It is rooted in an intuitively appealing picture involving ionization of the base in question by pruning an electron, subsequent creation of the hydrogen atom with the incoming proton, and the formation of the homolytic chemical bond between a radical cation and the hydrogen. The role of the initial state (base) is mirrored by the ionization potential of the pruned electron given by Koopmans' approximation, whereas the contribution of the final state (conjugate acid) encompasses the electron affinity of the proton, the relaxation energy of the produced radical cation, and finally the homolytic bond association energy of the newly formed N−H bond. This dissection of the protonation process into three sequential steps has a high cognitive value, enabling classification of bases into three categories at the same time. The first is given...

Journal ArticleDOI
TL;DR: Results have an important bearing on the generalization of the Baldwin-Beckwith rules, which classified the 5-endo-trig radical cyclization as a "disfavored" process.
Abstract: Relative kinetic data were determined for the 5-endo-trig cyclization of radical 12 compared to hydrogen abstraction from (TMS)(3)SiH in the temperature range of 344-430 K, which allows for the estimation of a rate constant of 2 x 10(4) s(-)(1) at 298 K with an activation energy of ca. 9 kcal/mol for the cyclization process. The 5-endo-trig cyclization of a variety of radicals that afford five-membered nitrogen-containing heterocycles was addressed computationally at the UB3LYP/6-31G level. The 5-endo vs 4-exo mode of cyclication and the effect of delocalization of the unpaired electron in the transition state were investigated. Because the ring formed during cyclization contains five sp(2) centers, electrocyclization via a pentadienyl-like resonance form was also considered. For comparison, similar calculations were performed for 4-penten-1-yl and related radicals. The factors that affect the activation energies of homolytic 5-endo-trig cyclization were determined. In the absence of steric or conformational effects, the endo cyclization to form the five-membered ring was strongly favored over exo cyclization to form the four-membered ring not only on thermodynamic grounds but also kinetically. When a substituent on the double bond was able to delocalize the unpaired electron in the transition state of the 4-exo path, the two modes of cyclization became kinetically comparable. These results have an important bearing on the generalization of the Baldwin-Beckwith rules, which classified the 5-endo-trig radical cyclization as a "disfavored" process.

Journal ArticleDOI
TL;DR: In this paper, the authors describe the synthesis of β-phosphorus nitroxides bearing a β-hydrogen, that present very interesting properties for the control of the radical polymerization of styrenes, acrylates and other monomers.
Abstract: Persistent nitroxides and their corresponding alkoxyamines are important regulators of living radical polymerization. Here we describe the synthesis of β-phosphorus nitroxides bearing a β-hydrogen, that present very interesting properties for the control of the radical polymerization of styrenes, acrylates and other monomers. A large series of alkoxyamines derived from these nitroxides was prepared, and Electron Spin Resonance (ESR) was used to determine both the temperature (Tc) and the rate constant (kd) for their homolysis. For the whole series of alkoxyamines (27 compounds), a very good linear correlation was found between Tc and logkd. Satisfactory linear correlations were found between Tc and calculated (PM3 method) Bond Dissociation Energy (BDE) of the NO-C bond, for series of alkoxyamines with the same type of leaving radical. The characteristics of free radical polymerization of styrene carried out in the presence of these new nitroxides and alkoxyamines will be discussed.

Journal ArticleDOI
TL;DR: It was strongly suggested that the Co-C bond becomes largely activated (labilized) when the coenzyme binds to the apoenzyme even in the absence of substrate and undergoes homolysis through the substrate-induced conformational changes of the enzyme.
Abstract: Substrate binding triggers catalytic radical formation through the cobalt-carbon bond homolysis in coenzyme B12-dependent enzymes. We have determined the crystal structure of the substrate-free form of Klebsiella oxytoca diol dehydratase*cyanocobalamin complex at 1.85 A resolution. The structure contains two units of the heterotrimer consisting of alpha, beta, and gamma subunits. As compared with the structure of its substrate-bound form, the beta subunits are tilted by approximately 3 degrees and cobalamin is also tilted so that pyrrole rings A and D are significantly lifted up toward the substrate-binding site, whereas pyrrole rings B and C are only slightly lifted up. The structure revealed that the potassium ion in the substrate-binding site of the substrate-free enzyme is also heptacoordinated; that is, two oxygen atoms of two water molecules coordinate to it instead of the substrate hydroxyls. A modeling study in which the structures of both the cobalamin moiety and the adenine ring of the coenzyme were superimposed onto those of the enzyme-bound cyanocobalamin and the adenine ring-binding pocket, respectively, demonstrated that the distortions of the Co-C bond in the substrate-free form are already marked but slightly smaller than those in the substrate-bound form. It was thus strongly suggested that the Co-C bond becomes largely activated (labilized) when the coenzyme binds to the apoenzyme even in the absence of substrate and undergoes homolysis through the substrate-induced conformational changes of the enzyme. Kinetic coupling of Co-C bond homolysis with hydrogen abstraction from the substrate shifts the equilibrium to dissociation.

Journal ArticleDOI
TL;DR: In this article, the photochemical Pt-C bond homolysis reactions of metal carbon-bonded platinum compounds are considered. And three types of complexes are considered: [Pt(R) 2 (COD), [Ptp(R), 2 (α-diimine)] and [pt(Me) 4 (α-, diimine)].

Journal ArticleDOI
Yao Fu1, Yi Mou1, Lin1, Lei Liu1, Qing-Xiang Guo1 
TL;DR: In this paper, high-level theoretical methods (UB3LYP/6-311++g(2df,p), RMP2/6 -311++G(d,p, CBS-4M), CBS-Q, and G3) were used to study the structures and bond dissociation energies (BDE) of the X-Y-NO molecules.
Abstract: High-level theoretical methods (UB3LYP/6-311++g(2df,p), RMP2/6-311++g(d,p), CBS-4M, CBS-Q, and G3) were used to study the structures and bond dissociation energies (BDE) of the X-Y-NO molecules. The data were used to evaluate the previous experimental and theoretical results. It was found that the syn conformation is favored by CH 3 -Y-NO, C 2 H 5 -Y-NO, and CH 3 O-Y-NO (Y = C, N, O, S), whereas the anti conformation is favored by CH 3 CO-Y-NO and Ph-Y-NO (Y = C, N, O). For Ph-S-NO, the syn conformation is preferred because of the long S-N bond. When X is an alkyl substituent, the Y-NO BDEs increase in the order X-S-NO (∼30 kcal/mol) < X-CH 2 -NO (∼40 kcal/mol) < X-O-NO (∼43 kcal/ mol) < X-NH-NO (48 kcal/mol). When X is an aromatic substituent, the Y-NO BDEs increase in the order X-O-NO (∼21 kcal/mol) < X-S-NO (26 kcal/mol) < X-CH 2 -NO (∼30 kcal/mol) < X-NH-NO (∼35 kcal/mol). The solvent effects of acetonitrile on the free energy change of C-NO and N-NO homolysis are significant, which are about 3-5 kcal/mol. The solvent effects of acetonitrile on the free energy change of O-NO and S-NO homolysis are relatively small, which are about 1-2 kcal/mol. Finally, we found that the remote substituent effects on C-NO, N-NO, O-NO, and S-NO BDEs have ρ + values of -0.4∼-0.9, 1.7-1.8, 3.2-3.9, and 1.2-1.7 kcal/mol. These values are significantly different from those on the C-H (0.4-0.6 kcal/mol), N-H (3.4-4.6 kcal/mol), O-H (4.1-5.7 kcal/mol), and S-H (2.0-3.8 kcal/ mol) BDEs. Therefore, the ground effects are important for the net substituent effects on BDEs.

Journal ArticleDOI
TL;DR: Alyl- and alkyl-derived azidoacyl radicals, generated from thiolesters by intramolecular homolytic substitution at the sulfur, can undergo five- and six-membered cyclization onto the azido moiety to give cyclized lactams.

Journal ArticleDOI
TL;DR: The O-O bond homolysis of cis,cis-ONOonO is particularly interesting because it has a very low barrier and arises from the most stable ONOONO conformer, and also due to obvious similarities to the well-known [3,3]-sigmatropic shift of 1,5-hexadiene.
Abstract: ONOONO has been proposed as an intermediate in the oxidation of nitric oxide by dioxygen to yield nitrogen dioxide. The O−O bond breaking reactions of this unusual peroxide, and subsequent rearrangements, were evaluated using CBS-QB3 and B3LYP/6-311G* hybrid density functional theory. The three stable conformers (cis,cis-, cis,trans-, and trans,trans-ONOONO, based on the O−N−O−O dihedral angles of either ∼0° or ∼180°) are predicted to have very different O−O cleavage barriers: 2.4, 13.0, and 29.8 kcal/mol, respectively. These large differences arise because bond breaking leads to correlation of the nascent NO2 fragments with either the ground 2A1 state or the excited 2B2 state of NO2, depending on the starting ONOONO conformation. A cis-oriented NO2 fragment correlates with the 2A1 state, whereas a trans-oriented NO2 fragment correlates with the 2B2 state. Each NO2 fragment that correlates with 2A1 lowers the O−O homolysis energy by ∼15 kcal/mol, similar to the ∼17−25 kcal/mol 2A1 → 2B2 energy difference...


Journal ArticleDOI
TL;DR: In the proposed mechanism, the initially produced peroxynitrito complex, Cr(aq)OONO(2+), undergoes O-O bond homolysis followed by some known and some novel chemistry of Cr(q)O( 2+) and NO(2).
Abstract: The kinetics of the rapid reaction between CraqOO2+ and NO were determined by laser flash photolysis of CraqNO2+ in O2-saturated acidic aqueous solutions, k = 7 × 108 M-1 s-1 at 25° C. The reaction produces an intermediate, believed to be NO2, which was scavenged with ([14]aneN4)Ni2+. With limiting NO, the CraqOO2+/NO reaction has a 1:1 stoichiometry and produces both free NO3- and a chromium nitrato complex, CraqONO22+. In the presence of excess NO, the stoichiometry changes to [NO]/[CraqOO2+] = 3:1, and the reaction produces close to 3 mol of nitrite/mol of CraqOO2+. An intermediate, identified as a nitritochromium(III) ion, CraqONO2+, is a precursor to a portion of free NO2-. In the proposed mechanism, the initially produced peroxynitrito complex, CraqOONO2+, undergoes O−O bond homolysis followed by some known and some novel chemistry of CraqO2+ and NO2. The reaction between CraqO2+ and NO generates CraqONO2+, k > 104 M-1 s-1. CraqOO2+ reacts with NO2 with k = 2.3 × 108 M-1 s-1.

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TL;DR: Calculations show that the formation of CuI-RSNO intermediates results in weakening of the S-N bond and strengthening of the N-O bond, which would promote S-n bond breaking and NO release from S-nitrosothiols.
Abstract: The degradation of S-nitrosothiols (RSNOs) to release NO is believed to be catalyzed by CuI ions, but the mechanism remains unclear. Kinetic experiments have shown that decomposition rates vary significantly with the chemical nature of the RSNO considered. On the basis of first-principles calculations, the catalytic role of CuI ion is investigated for the decomposition of S-nitrosocysteine and its N-acetylated and ethyl ester derivatives, and for S-nitrosohomocysteine. This preliminary study focuses on the CuI−RSNO intermediates involved in the decomposition pathway. The model chemistry has been validated by comparing calculated CuI-ligand binding energies and S−N bond homolysis energies with available experimental data. Calculations show that the formation of CuI−RSNO intermediates results in weakening of the S−N bond and strengthening of the N−O bond, which would promote S−N bond breaking and NO release from S-nitrosothiols.

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TL;DR: In this article, a simple reaction model is constructed, based on the assumption that the RSNO(n−1)− ligands are mostly responsible for the [Fe(CN)5N(O)(SR)](n+2)− reactivity and their electronic properties are discussed within the DFT framework.
Abstract: Reactions of the [Fe(CN)5NO]2− complex with biologically relevant thiols (HnRS = cysteine, N-acetylcysteine, ethyl cysteinate and glutathione) are initiated by the nucleophilic attack of a thiolate (RSn−) on the N atom of the NO+ ligand in the complex to form [Fe(CN)5N(O)SR](n+2)−. The N–S bond in the latter complex is, however, weak and can undergo both heterolytic and homolytic splitting. The former process makes the synthesis reaction reversible, whereas the latter is responsible for the spontaneous redox decomposition: [Fe(CN)5N(O)SR](n+2)− → [FeI(CN)5NO]3− + RS˙(n−1)−. The rate of the monomolecular reaction is controlled by an inductive effect in the thiol with an additional stabilisation coming from formation of a six-membered ring in the case of the N-acylated compounds. In the presence of thiolate excess, the RS˙(n−1)− radicals are transformed into the more stable RSSR˙(2n−1)− radicals, which are scavenged by both [Fe(CN)5N(O)SR](n+2)− and [Fe(CN)5NO]2−. The former reaction initiates, whereas the latter terminates, chain reactions of the catalysed redox decomposition. The catalytic decomposition (in the thiol excess) is much faster than the spontaneous decay (in the nitroprusside excess) but leads to the same final products. The Fe(I) reduction product is identified by UV/Vis, IR, electrochemical and EPR methods. The effect of molecular oxygen is investigated and explained. The mechanism is interpreted in terms of intermediate [Fe(CN)5N(O)(SR)2](2n+2)− complex formation via nucleophilic attack and its decay mainly via homolytic splitting of the N–S bond. To verify the mechanism, a simple reaction model is constructed, based on the assumption that the RSNO(n−1)− ligands are mostly responsible for the [Fe(CN)5N(O)(SR)](n+2)− reactivity and their electronic properties are discussed within the DFT framework.

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TL;DR: In this article, the authors used the density function UB3LYP/6-311++g(d,p) and perturbation theory ROMP2/6 -311+g (d, p) on 4-substituted thiophenols and their corresponding radicals.
Abstract: Density function UB3LYP/6-311++g(d,p) and perturbation theory ROMP2/6-311++g(d,p) calculations were performed on 4-substituted thiophenols and their corresponding radicals. It was found that although UB3LYP and ROMP2 methods underestimated the absolute S–H bond dissociation energies, they could predict almost as good relative S–H bond dissociation energies as a method of a considerably higher level, UCCSD(T)/6-311++g(d,p). From the calculation results it was determined that the S–H bond dissociation energies of thiophenols should have a positive correlation with the substituent σp+ constants whose slope was ca. 2.5 kcal mol−1. Such a slope indicated that the experimental S–H bond dissociation energies obtained from a previous solution phase measurement were reasonably accurate for para H, CH3, OCH3, Cl, and NO2 substituted thiophenols. However, the solution phase bond dissociation energy for 4-aminothiophenol was too low, which was found by the calculation in this study to be caused by the hydrogen bonding between the amino group and the solvent molecules. Finally, through the studies on the isodesmic reactions it was found that the substituent effects on the stability of neutral thiophenols had a fair and positive correlation with the substituent σp+ constants; the slope was 0.5 kcal mol−1. On the other hand, the substituent effects on the stability of thiophenol radicals had an excellent and negative correlation with the substituent σp+ constants and gave a slope of −1.8 kcal mol−1. Therefore, the major source of the substituent effects on S–H bond dissociation energies of thiophenols was the stability of the homolysis products, namely, thiophenol radicals.

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TL;DR: The reaction of BiCl(3) with the lithium salt of o-di-tert-butylphenol under nitrogen forms organic oxidation products rather than the expected Bi(OAr)(3) complex, and bismuth disproportionation products.
Abstract: The reaction of BiCl3 with the lithium salt of o-di-tert-butylphenol under nitrogen forms organic oxidation products rather than the expected Bi(OAr)3 complex, and bismuth disproportionation products. Likewise, the decomposition of Bi(III) aryloxides Bi(O-2,6-iPr2C6H3)3 and ClBi(O-2,4,6-tBu3C6H2)3 leads to corresponding organic oxidation products. These reactions can be explained by Bi−O bond homolysis to form unstable Bi(II) radicals, analogous to a fundamental step suggested to intervene in the SOHIO process.

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TL;DR: The conformational and spectroscopic properties of the tyrosyl radical dipeptide analogue (T(R)DA) are investigated both in gas phase and in aqueous solution by means of density functional calculations, and the calculated hyperfine coupling constants are in good agreement with the available experimental results.
Abstract: The conformational and spectroscopic properties of the tyrosyl radical dipeptide analogue (T(R)DA) are investigated both in gas phase and in aqueous solution by means of density functional calculations. Electronic interactions between backbone and side chain, determining the relative stability of the different energy minimums, depend on the electronic state of the phenoxy substituent. As a consequence, (i) the conformational behavior of T(R)DA is quite different from that of the tyrosine dipeptide analogue, and (ii) the energy required for the homolytic breaking of the OH bond depends on the adopted conformation. The calculated hyperfine coupling constants are in good agreement with the available experimental results. Side-chain-backbone interactions cause an asymmetrization of the magnetic properties of the phenoxy ring and deviations from McConnell relationship. Solvent effects, taken into account by means of a combined discrete/continuum model, significantly affect both the conformational and the magnetic behavior of T(R)DA.