Showing papers on "Homolysis published in 1992"
TL;DR: In theory, activation could involve (1) heterolysis to OH- and NO2+ (delta rxn Gzero' = 13 kcal/mol at pH 7) or (2) homolysis
Abstract: Peroxynitrite [oxoperoxonitrate(1-), ONOO-] may be formed in vivo from superoxide and nitric oxide. The anion is stable, but the acid (pKa = 6.8) decays to nitrate with a rate of 1.3 s-1 at 25 degrees C. The experimental activation parameters of this process are delta H++ = +18 +/- 1 kcal/mol, delta S++ = +3 +/- 2 cal/(mol.K), and delta G++ = +17 +/- 1 kcal/mol. Peroxynitrite (or its protonated form) oxidizes some compounds such as thiols and thioethers in a biomolecular reaction. The reactions with glutathione and cysteine have activation enthalpies of 10.9 and 9.7 kcal/mol, respectively, which are lower than that of the isomerization reaction. Peroxynitrite reacts with other compounds such as dimethyl sulfoxide and deoxyribose in a unimolecular reaction for which the activation of peroxynitrite is rate-limiting. In theory, activation could involve (1) heterolysis to OH- and NO2+ (delta rxn Gzero' = 13 kcal/mol at pH 7) or (2) homolysis to .OH and .NO2 (delta rxn Gzero = 21 kcal/mol), and these processes also could be involved in the isomerization to nitrate. However, thermodynamic and kinetic considerations indicate that neither process is feasible, although binding to metal ions may reduce the large activation energy associated with heterolysis. An intermediate closely related to the transition state for isomerization of ONOOH to HONO2 may be the strongly oxidizing intermediate responsible for hydroxyl radical-like oxidations mediated by ONOOH. Thus, peroxynitrite reacts with different compounds by at least two distinct mechanisms, and the hydroxyl radical is not involved in either.
1,280 citations
TL;DR: In this paper, a qualitative theoretical treatment of the singlet and triplet potential energy curves that arise when a single bond formed by an electron pair is cleaved either homolytically or H2-2H dissociation is presented.
Abstract: : When considering the fragmentation of a single bond, the attractive singlet and repulsive triplet potential energy curves of the prototype H2 - 2 H dissociation often come to mind. For species in which homolytic bond cleavage is energetically favored, such comparisons are reasonable. For other species where heterolytic cleavage gives lower-energy products, the H2 analogy is inappropriate. This paper offers a qualitative theoretical treatment of the singlet and triplet potential energy curves that arise when a single bond formed by an electron pair is cleaved either homolytically or heterolytically. This analysis is shown to provide insight into several problems involving transition metal systems: transition metal carbonyls, metal ion-ligand complexes, and transition metal dimers.
196 citations
TL;DR: In this paper, the free energies of solvation of the noble gases in water are linearly related to the atomic radii, and the standard potential for reduction of the proton in water (reaction i) was determined using the correlation line.
Abstract: Free energies of solvation [ΔG solv (gas) aq 's] of the noble gases in water are linearly related to the atomic radii. These data allow ΔG solv (H . ) aq to be calculated from the correlation line. The standard potential [E o (H + /H . aq ] for reduction of the proton in water (reaction i) was then determined using eq ii
162 citations
TL;DR: These rate enhancements are greater than the analogous enzyme-induced Co-C cleavage rate enhancements in adenosylcobalamin (Coenzyme B12, AdoCbl)-dependent enzymes, however, electron transfer is not predicted for the mechanism of any adeno-benzimidazole-coordinated base-on form and the two distinct yet similar non- coordinated forms: the base-off and the so-called "tuck-in" forms.
Abstract: Methylcobalamin (MeCbl, MeB12) thermolyzed in ethylene glycol from 120 to 141 degrees C with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) as a Me(.) trap gives the homolysis products Co(II)B12 and TEMPO-Me quantitatively. The 5,6-dimethylbenzimidazole axial-base-off-base-on equilibrium in ethylene glycol has an enthalpy change of -5.1 (+/-2) kcal mol(-1) and an entropy change of -10.5 (+/-4) cal mol(-1) K(-1), equilibrating between the 5,6-dimethylbenzimidazole-coordinated base-on form and the two distinct yet similar non-coordinated forms: the base-off and the so-called "tuck-in" forms. The MeB12 Co-CH3 homolysis rates indicate an activation enthalpy of 41 +/- 3 kcal mol(-1), an activation entropy of 24 +/- 6 cal mol(-1) K(-1), and an estimated methylcob(III)alamin Co-CH3 bond dissociation energy of 37 +/- 3 kcal mol(-1). This is the strongest Co-C cobamide bond measured. Comparison of the MeCbl homolysis rate constant (10(-19 +/- 4) s(-1)) extrapolated to -30 degrees C with the known reduced-methylcobamide-radical-anion values indicates rate enhancements of 10(22 +/- 4) (at -30 degrees C) following electrochemical reduction, or still over 10(15) at 25 degrees C. Such reduction provides an antibonding electron which weakens the Co-C bond from 37 kcal mol(-1) down to ca. 12 kcal mol(-1). These rate enhancements are greater than the analogous enzyme-induced Co-C cleavage rate enhancements in adenosylcobalamin (Coenzyme B12, AdoCbl)-dependent enzymes. However, electron transfer is not predicted for the mechanism of any adenosylcobalamin-dependent or methylcobalamin-dependent enzymes.
98 citations
TL;DR: In this paper, a free-radical mechanish involving H atom abstraction from the ketones is proposed to convert the resulting radicals to the observed esters by reaction with acyloxy radicals derived from homolysis of the dioxiranes.
Abstract: Thermolysis of dioxiranes in solutions of their parent ketones or in mixtures of the parent ketone and a foreign ketone leads to the formation of esters. The results are explained by postulating a free-radical mechanish involving H atom abstraction from the ketones. The resulting radicals are converted to the observed esters by reaction with acyloxy radicals derived from homolysis of the dioxiranes. Autodecomposition of dimethyldioxirane in acetone solution at room temperature gives methyl acetate at a very slow rate
74 citations
TL;DR: In this article, the reaction of p-toluenesulphonyl cyanide or methanesulfonyl cyanides with carbon radicals, generated from the corresponding O-acyl-N-hydroxy-2-thiopyridone derivatives by visible light photolysis gives nitriles in good yield.
67 citations
TL;DR: In this article, the homolytic substitutions of pyrrole, indole, and some pyrroles derivatives have been carried out using ambiphilic and electrophilic carbon centered radicals, generated in DMSO by Fe 2+ /H 2 O 2 and α-cyano-, α-carbonyl-, and α,α'-dicarbonylalkyl iodides.
Abstract: Efficient and selective homolytic substitutions (yield between 55 and 90%) of pyrrole, indole, and some pyrrole derivatives have been carried out using ambiphilic and electrophilic carbon centered radicals, generated in DMSO by Fe 2+ /H 2 O 2 and α-cyano-, α-carbonyl-, and α,α'-dicarbonylalkyl iodides. The reaction is highly successfull also with pyrroles substituted by electron-withdrawing groups, which has allowed an efficient synthesis of Tolmetin
67 citations
66 citations
TL;DR: This process is examined with the help of 4-hydroperoxy-4-methyl-2-6-di-tert-butyl-cyclohexa-2,5-dien-1-one (BHTOOH), a compound that is converted heterolytically to an alcohol and homolytic to several rearranged products.
Abstract: Despite extensive work on homolytic versus heterolytic peroxide bond cleavage by iron porphyrins, the nature of the corresponding reactions catalyzed by myoglobin (Mb) and hemoglobin (Hb) remains unclear. These hemoproteins react with peroxides to give a ferryl (Fe IV =O) complex and a protein radical, but the coupling of dioxygen bond cleavage to protein radical formation is obscure. This process is examined here with the help of 4-hydroperoxy-4-methyl-2,6-di-tert-butyl-cyclohexa-2,5-dien-1-one (BHTOOH), a compound that is converted heterolytically to an alcohol and homolytically to several rearranged products
62 citations
TL;DR: In this article, the authors used thermodynamic data extracted from electrochemical studies for estimations of M-H bond dissociation energies (BDEs) in solution, and the relative yields of homolytic and heterolytic cleavage of m-H resulting from 1e{sup {minus}} oxidation processes were considered.
Abstract: The authors used thermodynamic data extracted from electrochemical studies for estimations of M-H bond dissociation energies (BDEs) in solution. Here, estimates of M-H BDEs in metal hydride cation radicals are made and the relative yields of homolytic and heterolytic cleavage of M-H resulting from 1e{sup {minus}} oxidation processes are considered. 18 refs., 2 figs., 2 tabs.
57 citations
TL;DR: In this article, a homolytic mechanism involving oxidation of α-benzylmzalonate (1) to the corresponding malonyl radical, its addition to a triple bond, and intramolecular homolyty aromatic substitution of the vinyl radical adducts, is suggested.
Abstract: Oxidation of diethy α-benzylmzalonate (1) by Mn(III) acetate in acetic acid at 70 o C in the presence of alkynes 2a-j leads to dihydronaphalene derivatives 3a-j in moderate to good yields. A homolytic mechanism, involving oxidation of 1 to the corresponding malonyl radical, its addition to a triple bond, and intramolecular homolytic aromatic substitution of the vinyl radical adducts, is suggest. Propargylic hydrogen abstraction and dimerization at the malonic position for less reactive alkynes are the main side reactions observed
TL;DR: The equilibrium acidities of 14 aldoximes, 4 amidoximes and 19 ketoximes have been measured in DMSO solution as discussed by the authors, and the acidity of the corresponding radical cations (pK HA+ I) were estimated for 32 of these oximes.
Abstract: The equilibrium acidities of 14 aldoximes, 4 amidoximes, and 19 ketoximes have been measured in DMSO solution. Homolytic bond dissociation energies (BDEs) for the O-H bonds in these oximes have been estimated by combining the pK HA values thus obtained with the oxidation potentials of their conjugate bdases. The acidities of the corresponding radical cations (pK HA+ I.) were estimated for 32 of these oximes in DMSO as for phenols. The pK HA values for syn-benzaldoxime and acetaldoxime were found to be 20.2 abd 28.5, respectively
TL;DR: In this article, anhydrous single crystals of thymine have been X-irradiated at 10 K and studied using K-band EPR, ENDOR, and FSE spectroscopy.
Abstract: Anhydrous single crystals of thymine have been X-irradiated at 10 K and studied using K-band EPR, ENDOR, and FSE spectroscopy. In addition to several radical pairs, five monoradicals were formed and stabilized at this temperature. It is demonstrated that the dominating radical pair species analyzed previously consists of two radicals, each formed by net hydrogen abstraction from the methyl group. The five monoradicals were identified to be the O4-protonated thymine anion, the thymine-7-yl radical formed by net hydrogen addition to C6, and the 1-yl radical formed by net hydrogen abstraction from N1. Upon thermal annealing, the protonated anion decays before 60 K apparently with no successor, and the 6-yl radical. Possible mechanisms for the formation of these radicals are discussed. These involve processes initiated by primary ionization followed by charge neutralization by protonation/deprotonation or by geminate recombination leading to the formation of hydrogen atoms by homolytic bond scission from molecules in (super) excited states. 23 refs., 5 figs., 3 tabs.
TL;DR: A series of 4-substituted cubyl radicals was generated by bromine atom abstraction from 1-bromo-4-substantuted cubanes as discussed by the authors.
Abstract: A series of 4-substituted cubyl radicals was generated by bromine atom abstraction from 1-bromo-4-substituted cubanes. EPR observations showed that the cubyl radicals abstracted secondary hydrogen atoms from the ethyl groups of triethylsilane and decayed mainly by second-order combination reactions. tert-Butoxyl radicals abstracted hydrogen atoms from cubane at least 26 times more rapidly than from cyclopropane at -90 o C. Electron-withdrawing substituents on cubane greatly reduced this rate. tert-Butoxyl radicals selectively abstracted the cage hydrogens rather than primary methyl hydrogens from methylcubane
TL;DR: A method for estimating the homolytic bond dissociation energies of the benzyclic C-H bonds in radical anions, BDE HA -, has been devised in this paper.
Abstract: A method for estimating the homolytic bond dissociation energies of the benzyclic C-H bonds in radical anions, BDE HA. -, has been devised. The method, which is potentially applicable to all weak acids, has been applied to the radical anions derived from 19 fluorenes, 15 triphenylmethanes, p-nitrotoluene, diphenylmethane, bis(4-nitrophenyl)methane, and three xanthenes. (In this abstract and in the text, the C-H bonds referred to are all of the benzyclic type)
TL;DR: The homolytic bond dissociation energies of the acid C-H bonds in radical cations of the type HA + have been estimated by using the equation BDE HA+. =BDE HA +23.1[E ox (A. )-E ox(HA)].
Abstract: The homolytic bond dissociation energies (BDE HA+. ) of the acidic C-H bonds in radical cations of the type HA +. have been estimated by using the equation BDE HA+. =BDE HA +23.1[E ox (A . )-E ox (HA)]. The DBE HA values of the acidic H-A bonds in neutral molecules were estimated from pK HA and E ox (A - ) values in DMSO
TL;DR: Tris(trimethylsilyl)silane (2) is an efficient hydrosilylating agent of 1,6-dienes operating via a free-radical chain mechanism.
Abstract: Tris(trimethylsilyl)silane (2) is an efficient hydrosilylating agent of 1,6-dienes 1 operating via a free-radical chain mechanism. The intermediate alkyl radical 3 attacks the second olefinic bond of the diene 1 and forms cyclopentylalkyl radical 4. Using a low concentration of the hydrogen donor 2, a radical substitution reaction at the Si-center of cis-4 occurs and yields the bicyclic silane 6. The rate of this homolytic substitution at the Si-atom is 2.4 · 105 s-1 (80°).
TL;DR: X-ray edge and extended x-ray absorption fine structure (EXAFS) techniques provide powerful tools for analysis of local molecular structure of complexes in solution and this paper presents the first direct evidence that formation of the Co(I) B12 intermediate must involve breaking of theCo-N (DMB) bond.
TL;DR: In this article, GVB-LSDC was used for the calculation of homolytic dissociation energies, with an average error of 4 kcal/mol. But the average error was not as high as that of GVB.
TL;DR: In this paper, the energy barrier for the attack on methaneselenol by the alkyl radicals was shown to be at 96-105 kJ mol −1.
TL;DR: In this article, 2,2-Dimethylaziridines 1 carrying a COR group on nitrogen are opened by PhS⊖ with a regioselectivity RS = abnormal:normal ⩾ 20 in MeOH and 5-16 in THF.
TL;DR: In this paper, the phenyl groups in triphenylmethane, 9-phenyl-9,10-dihydroanthracene, and 9phenylranthene are shown to be constrained in their ability to delocalize either the negative charges in the anions formed by loss of a proton or the odd electrons released by a hydrogen atom.
Abstract: Equilibrium acidities in DMSO and BDEs for the benzylic C-H bonds are reported for 19 triphenylmethanes, three 9,10-dihydroanthracenes, and nine xanthenes. The phenyl groups in triphenylmethane, 9-phenyl-9,10-dihydroanthracene, and 9-phenylranthene are shown to be constrained in their ability to delocalize either the negative charges in the anions formed by loss of a proton or the odd electrons released by loss of hydrogen atom
TL;DR: In this paper, the photodissociation of HMn(CO) 5 has been studied through contracted configuration interaction calculations of the potential energy surfaces for the metal-hydrogen bond homolysis and the dissociation of the carbonyl ligand.
Abstract: The photodissociation of HMn(CO) 5 has been studied through contracted configuration interaction calculations of the potential energy surfaces for the metal-hydrogen bond homolysis and the dissociation of the carbonyl ligand The corresponding potential energy curves connect the ground and excited states of the reactant to the ground and excited states of the primary products The calculations were carried out under C 4υ constraint with a basis set that is at least of double-ζ quality
TL;DR: In this article, the N-NO 2 bond was found to be a major point for homolysis in the solution thermolysis of dimethylnitramine, diisopropylnitramerine, and N-nitropiperidine.
Abstract: Kinetics and activation parameters of the solution thermolysis of nitramines dimethylnitramine, diisopropylnitramine, and N-nitropiperidine were determined over the temperature range 200-300 o C. Each formed the corresponding nitrosamine as the only, or major, condensed-phase product. For dimethylnitramine the products were completely characterized. The observations of a large positive activation volume in the thermolysis of dimethylnitramine and of the slowing of the decomposition reaction with increased solvent viscosity both suggest that the rate-determining step is homolysis, the N-NO 2 bond being the most obvious point for this
TL;DR: In the presence of chlorinated solvents, the platinum-containing product is [PtClMe3(bipy)] and the quantum yield in CH2Cl2 is 0.96 as mentioned in this paper.
TL;DR: In this paper, photolysis of the N-arylmethyl-3-amino-1,2,4-triazoles 2-5 in methanol or water-methanol mixture, induces homolytic and heterolytic cleavage of the arylmethyl-C-N bond giving rise to 3-AMINO-1.4.
TL;DR: In this paper, the homolytic photodissociation process of carbon-oxygen bonds in aromatic compounds of the common type R-OPh (R=carbon fragment) was investigated by using molecular orbital theory.
TL;DR: In this article, the properties of the benzylic C-H bonds of the conjugate bases of 23 indenes were measured in DMSO for three classes of indenes.
Abstract: Equilibrium acidities of the benzylic C-H bonds (pK HA ), oxidation potentials (E OX (HA)), and oxidation potentials of the conjugate bases (E ox (A - )) have been measured in DMSO for 23 indenes. Homolytic bond dissociation energies of the benzylic C-H bonds in these indenes (BDE HA ) and acidities (pK HA.+ ) of the corresponding radical cations were estimated by a suitable combination of these data
TL;DR: In this article, a novel mechanism for generating triboelectricity is proposed, where the charge carrer is an electron and the electron acceptor is the mechanoradical B·, which is produced by homolytic carbon-carbon bond scission of the main chain of polymer B. The mechanism deduced from the mechanism is PMMA-PP-PE-PVDF-PTFE.
Abstract: Mechanical fracture of solid polymer under vacuum at 77K can, in principle, produce both mechanoradicals and mechanoanions which are formed by homolytic and heterolytic scission of carbon-carbon bonds in the polymer main chain. The production of mechanoanions was claimed by a detection of tetracyanoethylene (TCNE) anion radical (TCNE−), which was observed by electron spin resonance (ESR) spectroscopy using the electron trapping method with TCNE. A novel mechanism for generating triboelectricity is proposed. The charge carrer is an electron. The electron donor is the mechanoanion A−, which is produced by heterolytic carbon-carbon bond scission of the main chain of polymer A. The electron acceptor is the mechanoradical B·, which is produced by homolytic carbon-carbon bond scission of the main chain of polymer B. Charge separation is due to an electron transfer from the mechanoanion to the mechanoradical when in contact. It is possible to speculate the sign of the charge induced by friction from the electron release potential of A−, Pr(A−), and the molecular electron affinity of B·, Ea(B·). The triboelectric series deduced from our mechanism is PMMA-PP-PE-PVDF-PTFE, in which polymers having a relatively low Pr(A−) to high Pr(A−) or polymers having a relatively low Ea(B·) to high Ea(B·) are arranged. This order is identical with well-known triboelectric series.