Showing papers on "Homolysis published in 1996"
TL;DR: In this article, electron spin resonance (ESR) spectroscopy was used to investigate the formation of radicals in a hematin/tert-butyl hydroperoxide system.
120 citations
116 citations
TL;DR: In this article, the dissociation energy of the C−H bond in benzene was predicted at the BLYP/6-311G level of theory, and the results showed that the removal of hydrogens is accompanied by highly regular changes in the molecular geometries, namely a widening of the ipso bond angle by 6.0° and a concomitant shortening of adjacent C−C bonds by ca. 0.02 A.
Abstract: Electronic structure calculations carried out at the BLYP/6-311G** level of theory accurately predict the dissociation energy of the C−H bond in benzene. The analogous energies of the homolytic C−H bond cleavage in the other nine polycyclic aromatic hydrocarbons (PAHs) are found to be governed almost entirely by steric factors, the hydrogens from congested regions of the PAHs being removed preferentially. The removal of hydrogens is accompanied by highly regular changes in the molecular geometries, namely a widening of the ipso bond angle by ca. 6.0° and a concomitant shortening of the adjacent C−C bonds by ca. 0.02 A. These observations suggest an almost complete localization of the unpaired σ electrons on single carbon atoms and the separation of the local σ and π effects in the aryl radicals under study. This localization is confirmed by the computed charges and spin populations of atoms in the phenyl, 1-naphthalenyl, and 2-naphthalenyl radicals. In contrast with their UHF counterparts, the UBLYP elect...
95 citations
Book•
07 Nov 1996
TL;DR: In this article, the authors propose a method for determining activation energy of organic compounds by means of NMR and NMR-13 NMR spectroscopy of carbon-carbon bond formation.
Abstract: Preface. Preface to the First Edition. 1 Functional Groups and Chemical Bonding. Functional Groups. Orbitals. Bonding Schemes. Antibonding Orbitals. Resonance. Conjugated pi Systems. Aromaticity. Bibliography. Problems. 2 Oxidation States of Organic Compounds. Oxidation Levels. Oxidation States in Alkanes. Oxidation States in Alkenes. Oxidation States in Common Functional Groups. Oxidation Level Changes During Reactions. Bibliography. Problems. 3 Acidity and Basicity. Bronsted and Lewis Acids and Bases. Acid Strength. Acid-Base Equilibria. Amphoteric Compounds. Structural Effects on Acidity. Electronegativity. Inductive Effects. Resonance Effects. Bibliography. Problems. 4 Curved-Arrow Notation. Electron Movement. Heterolytic Bond Cleavages. Heterolytic Bond Formation. Homolytic Bond Making and Bond Breaking. Resonance Structures. Depiction of Mechanism. Bibliography. Problems. 5 Mechanisms of Organic Reactions Activation Energy. Activated Complex. Reaction Energetics. Structure of the Activated Complex. Hammond Postulate. Reaction Kinetics. Determining Activation Energies. Isotope Effects. Electronic Effects. Hammett Equation. Bibliography. Problems. 6 Stereochemical and Conformational Isomerism. Stereochemical Structures. Chirality. Configuration of Chiral Centers. Multiple Stereocenters. Optical Activity. Absolute Configuration. Physical Properties of Enantiomers. Resolution of Enantiomers. Stereoselective Reactions. Formation of Enantiomers. Formation of Diastereomers. Stereochemistry to Deduce Mechanism. Conformational Analysis. Conformational Energies. A Values. Strain in Ring Systems. Stereoelectronic Effects. Bibliography. Problems. 7 Functional Group Synthesis. Functional Group Manipulation. Carboxylic Acids. Esters. Amides. Acid Chlorides. Aldehydes. Ketones. Imines and Imine Derivatives. Alcohols. Amines. Alkenes. Alkanes. Bibliography. Problems. 8 Carbon-Carbon Bond Formation between Carbon Nucleophiles and Carbon Electrophiles. Synthetic Strategy. Nucleophilic Carbon. Electrophilic Carbon. Reactivity Matching. Generation of Nucleophilic Carbon Reagents. Generation of Electrophilic Carbon Reagents. Matching Nucleophiles with Electrophiles. Enolates. Enolate Regioisomers. Diastereoselection in Aldol Reactions. Organometallic Compounds. Neutral Carbon Nucleophiles. C=C Formation. Cyclopropanation Reactions. Metal-Catalyzed Carbon-Carbon Bond Formation. Pd(0)-Catalyzed Carbon-Carbon Bond Formation. Heck Reaction. Suzuki Coupling. Stille Coupling. Olefin Metathesis. Bibliography. Problems. 9 Carbon-Carbon Bond Formation by Free-Radical Reactions. Free-Radical Reactions. Free-Radical Polymerization. Nonpolymerization Reactions. Free-Radical Initiation. Free-Radical Cyclization. Bibliography. Problems. 10 Planning Organic Syntheses. Retrosynthetic Analysis. Carbon Skeleton Synthesis. Umpolung Synthons. Acetylide Nucleophiles. Ring Construction. Robinson Annulation. Diels-Alder Reaction. HOMO-LUMO Interactions. Stereoelectronic Factors. 1,3-Dipolar Cycloadditions. Bibliography. Problems. 11 Structure Determination of Organic Compounds. Structure Determination. Chromatographic Purification. Instrumental Methods. Nuclear Magnetic Resonance. Chemical Shift. Spin-Spin Coupling. Descriptions of Spin Systems. Second-Order Splitting. Structure Identification by 1H NMR. Carbon-13 NMR. Infrared Spectroscopy. IR Stretching Frequencies. Use of IR Spectroscopy for Structure Determination. Mass Spectrometry. Fragmentation Processes. Bibliography. Problems. Solutions to Chapter Problems. Index.
90 citations
TL;DR: The homolytic bond dissociation energies (BDEs) of the O−H bonds in DMSO solution for (a) phenol and a number of its derivatives, (b) three oximes, (c) three alcohols, (d) three hydroxylamines, and two hydroxamic acids have been estimated by eq 1: BDEHA = 1.37pKHA + 23.1Eox(A-) + 73.3 kcal/mol as discussed by the authors.
Abstract: The homolytic bond dissociation energies (BDEs) of the O−H bonds in DMSO solution for (a) phenol and a number of its derivatives, (b) three oximes, (c) three alcohols, (d) three hydroxylamines, and (e) two hydroxamic acids have been estimated by eq 1: BDEHA = 1.37pKHA + 23.1Eox(A-) + 73.3 kcal/mol. For most of these hydroxylic acids, the BDEs of the O−H bonds estimated by eq 1 are within ±2 kcal/mol of the literature values in nonpolar solvents or in the gas phase. There is no reason to believe, therefore, that these BDEs are “seriously in error because of failure to correct for solvent effects” as has been claimed on the basis that BDEs in highly polar solvents estimated for the O−H bond in phenol by photoacoustic calorimetry must be so corrected.
84 citations
71 citations
TL;DR: Gas-phase BDE values of H-B(+) bonds were found to be 5-11 kcal/mol higher than the corresponding gas phase BDE(HB(+))(g) values, interpreted as being due to the greater solvation energy of the HB(+) cation than that of the B(+*) radical cation.
Abstract: The oxidation potentials of 19 nitrogen bases (abbreviated as B: six primary amines, five secondary amines, two tertiary amines, three anilines, pyridine, quinuclidine, and 1,4-diazabicyclo[2,2,2]octane), i.e., Eox(B) values in dimethyl sulfoxide (DMSO) and/or acetonitrile (AN), have been measured. Combination of these Eox(B) values with the acidity values of the corresponding acids (pKHB+) in DMSO and/or AN using the equation: BDEHB+ = 1.37pKHB+ + 23.1 Eox(B) + C (C equals 59.5 kcal/mol in AN and 73.3 kcal/mol in DMSO) gave estimates of solution phase homolytic bond dissociation energies of H−B+ bonds. Gas-phase BDE values of H−B+ bonds were estimated from updated proton affinities (PA) and adiabatic ionization potentials (aIP) using the equation, BDE(HB+)g = PA + aIP − 314 kcal/mol. The BDEHB+ values estimated in AN were found to be 5−11 kcal/mol higher than the corresponding gas phase BDE(HB+)g values. These bond-strengthening effects in solution are interpreted as being due to the greater solvation ...
65 citations
TL;DR: In this article, potential energy curves have been calculated for a number of excited states of Mn2(CO)10, along the Mn−Mn bond dissociation coordinate and along Mn−COax and Mn− COeq coordinates, in order to understand why irradiation into the σ → σ* band does not only lead to Mn−mn bond breaking but also to Mn −CO dissociation.
Abstract: Potential energy curves (PECs) have been calculated for a number of excited states of Mn2(CO)10, along the Mn−Mn bond dissociation coordinate and along Mn−COax and Mn−COeq coordinates, in order to understand why irradiation into the σ → σ* band does not only lead to Mn−Mn bond breaking but also to Mn−CO dissociation. Mn−Mn bond homolysis can straightforwardly occur along the dissociative σ → σ* 3B2 PEC. The σ → σ* excited state is not itself Mn−CO dissociative. CO dissociation occurs since PECs that correspond at equilibrium geometry to dπ* → σ* 1,3E1 excited states (nearly degenerate with the σ → σ* excited state) are Mn−COax dissociative (both 1E1 and 3E1, 1,3E in C4v) or Mn−COeq dissociative (only just, and only the b3A‘ component (in Cs) of 3E1). The Mn−CO dissociative character has been traced to the precipitous lowering of the initially high-lying Mn−CO σ-antibonding (3d(eg)-like) orbitals upon Mn−CO bond lengthening, making them considerably lower than σ* in Mn2(CO)9. Excitations to these orbitals ...
60 citations
TL;DR: In this article, it was shown that the reaction proceeds via a σ(Re-R)π* excited state that is rapidly populated by a nonradiative transition from the optically excited MLCT state.
Abstract: MLCT excitation of the complexes [Re(R)(CO)3(α-diimine)] (R = Me, Et, benzyl (Bz); α-diimine = iPr-PyCa, R′-DAB) results in the homolysis of the Re-R bond leading to the formation of radicals R. and [Re(CO)3(α-diimine)]. as primary photoproducts. The quantum yield of this photoprocess is dependent on the alkyl group used. For R = Me, the quantum yield is low (10−2) and depends on the temperature and excitation wave-length, whereas for R = Et and Bz the quantum yield is near unity and independent of T and λexc. The reaction is shown to proceed via a σ(Re-R)π* excited state that is rapidly (< 20 ps) populated by a nonradiative transition from the optically excited MLCT state. Time-resolved IR and UV/Vis absorption spectra studied in the ns-μs and ps-μs time domains, respectively, show that the σπ* excited state is rather long-lived (τ ≈ 250 ns) in noncoordinating solvents; the dissociation of the Re-R bond from this state is strongly accelerated by polar or coordinating solvents (τ< 20 ps). The σπ* excited state is spectroscopically characterized by a (presumably σπ* → MLCT) transition at approximately 500 nm and by CO stretching frequencies closely resembling their ground-state values. The relative energies of the MLCT and reactive σπ* states, controlled by the nature of the alkyl lig-and, determine the photoreactivity of the complexes.
59 citations
TL;DR: In this paper, the one-electron reduction potential in water of 28 alkyl peroxyl radicals and 10 aromatic peroxideyl radicals has been estimated from previously published kinetic data on electron-transfer reactions.
Abstract: The one-electron reduction potential in water of 28 alkyl peroxyl radicals and 10 aromatic peroxyl radicals have been estimated from previously published kinetic data on electron-transfer reactions The kinetic data were calibrated against thermochemical data on peroxyl radicals From the estimated one-electron reduction potentials and acidity data on some hydroperoxides, the pKa values of the corresponding hydroperoxides were estimated The homolytic O−H bond dissociation energies of the hydroperoxides were calculated from the estimated one-electron reduction potentials and pKa values The results have been summarized in linear free energy relationships based partly on an operational substituent constant scale derived in this work In addition the ionization potentials of 16 peroxide anions, the absolute hardness and electronegativity of the corresponding peroxyl radicals and the O−H bond dissociation energies of the corresponding hydroperoxides have been calculated by AM1 semiempirical quantum chemical
59 citations
TL;DR: In this article, the UHF-SCF-AM1 method has been employed to study the pyrolysis mechanism and impact sensitivity of nitro derivatives of benzene.
Abstract: The UHF-SCF-AM1 method has been employed to study the pyrolysis mechanism and impact sensitivity of nitro derivatives of benzene. Potential energy curves, transition states and activation energies of seven pyrolysis initiation reactions (homolysis of CNO2 bond into radicals) have been obtained here first. The molecular geometries of reactants, transition states and products of the seven reactions were fully optimized. It is found that there is a good linear relationship between the activation energies and bond orders of the weakest bond CNO2 in the molecule of each reactant. The linear correlation coefficient is 0.996. This result gives “the principle of the smallest bond order” powerful support and shows that the activation energy can also be used as a dynamic criterion of impact sensitivity.
TL;DR: The photochemistry of N-hydroxypyridine-2(1H)-thione (N-HPT) has been investigated in aqueous and organic solvents using laser flash photolysis (λexc = 308 or 355 nm) as discussed by the authors.
Abstract: The photochemistry of N-hydroxypyridine-2(1H)-thione (N-HPT) has been investigated in aqueous and organic solvents using laser flash photolysis (λexc = 308 or 355 nm). Independent of the environment, UV excitation of N-HPT causes homolytic N−O bond cleavage, which leads to formation of the 2-pyridylthiyl (PyS•) and hydroxyl (•OH) radicals. In aqueous media, this process occurs efficiently from both the anionic and neutral forms (ΦN-O ≈ 0.20−0.30). In addition to N−O bond scission, N-HPT undergoes other primary photoprocesses which are pH-dependent. At pH = 7, photoionization (Φe− = 0.09 (λexc = 308 nm) and 0.05 (λexc = 355 nm)) of the anionic form generates the hydrated electron as well as the semioxidized radical of N-HPT. Fast rearrangement of the latter species produces the N-oxy-2-pyridylthiyl radical. At pH = 2, where the uncharged structure predominates, formation of an excited triplet state (ET ≥ 59.5 kcal mol-1) is observed (ΦT ≥ 0.05 using λexc = 355 nm) but photoionization does not take place. T...
TL;DR: In this article, the spectroscopic properties and photochemistry of the complexes Mn(R)(CO)3(R′-DAB) (R=Me, Bz; R′=iPr, pTol) are reported.
TL;DR: In this paper, it was shown that homolytic cleavage reactions, involving C-H cleavage as an initial rate-determining step, operate during the charge-remote fragmentation observed for high-energy collision-induced dissociation of [M+Li]- ions of n-butyl palmitate and correspond to a major fragmentation route.
Abstract: The high-energy collision-induced dissociation (CID) of [M+Li]+ ions of n-butyl ester derivatives of palmitic acid and oleic acid as well as 9,9-2H2-palmitic acid and 11,11-2H2-oleic acid has been studied in order to obtain information on the charge-remote fragmentation mechanism of saturated and mono-unsaturated fatty acid ions containing a stable charge centre. The results obtained in the present study indicate that homolytic cleavage reactions, involving C—H cleavage as an initial rate-determining step, operate during the charge-remote fragmentation observed for high-energy CID of [M+Li]+ ions of n-butyl palmitate and correspond to a major fragmentation route. With respect to the charge-remote fragmentation of n-butyl oleate, our 2H-labelling results point to the same mechanism, involving an initial C—H cleavage at allylic positions, for the formations of ions corresponding to a formal homo-allylic cleavage, and are also consistent with a direct allylic C—C cleavage for the formation of ions due to a formal allylic C—C cleavage. These results, however, do not exclude the possibility of other minor homolytic fragmentation pathways for the formation of ions involving formal allylic and homo-allylic cleavages.
TL;DR: In this paper, the effect of transition-metal-substituted heteropolyanions (HPAs) in the presence of isobutyraldehyde (IBA) and tetrabutylammonium salts of PW11MOn−39 (PW11M; M = CoII, MnII, CuII, PdII, TiIV, RuIV, and VV), was studied in acetonitrile using trans-stilbene as a model substrate.
Abstract: Alkene epoxidation by dioxygen in the presence of isobutyraldehyde (IBA) and tetrabutylammonium salts of transition-metal-substituted heteropolyanions (HPAs), PW11MOn−39 (PW11M; M = CoII, MnII, CuII, PdII, TiIV, RuIV, and VV), has been studied in acetonitrile using trans-stilbene as a model substrate. The selectivity of epoxidation attains 95% at complete alkene conversion. The epoxide formation shows an induction period and is inhibited by 2,6-di-tert-butyl-4-methylphenol, indicating chain radical mechanism of the reaction. The formation of perisobutyric acid (PIBAc) was detected during oxidation process after the completion of the induction period. Decomposition of PIBAc as well as its interaction with trans-stilbene in the presence of PW11M has been stuided. PW11M with M = CoII, PdII, and RuIV greatly enhance PIBAc homolytic decomposition and thus increase the rate of the degenerate branching in the chain radical process of alkene-aldehyde co-oxidation. HPAs with the other metals are much less active in PIBAc decomposition and most likely catalyze the co-oxidation process by generating chain-initiating acyl radicals from IBA followed by the addition of O2 with the formation of acylperoxy radicals responsible for epoxidation. The influence of PW11M on the different elementary steps of the chain radical process depends on the M nature and HPA concentration. In most cases HPAs behave as catalysts at their low concentrations and as inhibitors at high concentrations.
TL;DR: In this article, homolytic M−H bond dissociation enthalpies (BDEs) of mononuclear cationic metal hydride complexes HMLn+, where MLn = Cr(CO)2(dppm)2, Mo(CpRu(PMe3)2I, CpRu[L−L]H, CmRu[PPh3]2H], Cp*2O(PR3), CpO[O] 2
Abstract: Homolytic M−H bond dissociation enthalpies (BDEs) of the mononuclear cationic metal hydride complexes HMLn+, where MLn = Cr(CO)2(dppm)2, Mo(CO)2(L−L)2, W(CO)3(PR3)3, W(CO)2(dppm)2, W(CO)3(tripod), W(CO)3(triphos), Cp*Re(CO)2(PR3), Fe(CO)3(PR3)2, Fe(CO)3(L−L), Cp*2Ru, CpRu(PMe3)2I, CpRu(L−L)H, CpRu(PPh3)2H, Cp*2Os, CpOs(PR3)2Br, CpOs(PPh3)2Cl, CpOs(PPh3)2H, CpIr(CO)(PR3), CpIr(CS)(PPh3), (C5MenH5-n)Ir(COD), Cp*Ir(CO)(PR3), and Cp*Ir(CO)2, have been estimated by use of a thermochemical cycle that requires a knowledge of the heats of protonation (ΔHHM) and redox potentials (E1/2) for the oxidation of the neutral metal complexes (MLn). Excellent correlations were found between −ΔHHM and E1/2 within related series of complexes. The BDE values obtained by this method fall in the range 56−75 kcal/mol. For related complexes of a given metal, the energy required for homolytic M−H bond cleavage (BDE) increases linearly as −ΔHHM for heterolytic M-H bond cleavage increases. For analogous complexes with different meta...
TL;DR: In this article, the equilibrium acidities (pKHA values) in DMSO of the following hydroxylamines have been measured: (1) Nphenylhydroxylamine and its p-bromo and p-cyano derivatives, (2) N-benzyl-N-phenyl hydroxylpiperidine.
Abstract: The equilibrium acidities (pKHA values) in DMSO of the following hydroxylamines have been measured: (1) N-phenylhydroxylamine and its p-bromo and p-cyano derivatives, (2) N-benzyl-N-phenylhydroxylamine and its p-bromo and p-cyano derivatives, (3) O-benzyl-N-phenylhydroxylamine, (4) N-benzoylphenylhydroxylamine and its p-bromo and p-cyano derivatives, and (5) N-hydroxylpiperidine. The BDEs of the O−H and/or N−H bonds in these 11 weak acids have been estimated by combining their pKHA values with the oxidation potentials of their conjugate bases according to the following equation: BDEHA = 1.37pKHA + 23.1Eox(A-) + 73.3 kcal/mol.
TL;DR: In this article, the photoinduced homolysis and heterolysis of (4-methylphenyl)phenylmethyl chloride and bis(4-methylthenyl)methyl chloride in acetonitrile were examined.
Abstract: The picosecond dynamics for the photoinduced homolysis and heterolysis of (4-methylphenyl)phenylmethyl chloride and bis(4-methylphenyl)methyl chloride in acetonitrile are examined. In less than 20 ...
TL;DR: In this paper, the thermolysis of the Co−C bond in adocobinamide (AdoCbi+BF4-) in anaerobic ethylene glycol has been studied as a function of a series of para-substituted pyridine axial bases using the TEMPO radical-trapping method.
Abstract: The thermolysis of the Co−C bond in adocobinamide (AdoCbi+BF4-) in anaerobic ethylene glycol has been studied as a function of a series of para-substituted pyridine axial bases using the TEMPO radical-trapping method. In contrast to the slower rates of Co−C cleavage previously found for benzylcobinamide, neopentylcobinamide, and the (α-phenylethyl)cobaloxime coenzyme B12 models, for AdoCbi+ the rate of total Co−C cleavage becomes faster as the para-substituted pyridines become more electron-donating. Specifically, the 110 °C kobsd for AdoCbi+BF4- total Co−C cleavage increased 23-fold on going from 1 M pyridine (py) to 1 M p-(dimethylamino)pyridine (Me2N-py). However, HPLC product studies reveal that the percentage of abiological Co−C heterolysis increases (to a limiting value); that is, Co−C heterolysis is a major reason for the observed rate increase seen for Me2N-py. Deconvolution of the kobsd rate constant into its heterolysis and homolysis components yields values of the 110 °C kheterolysis and khomol...
TL;DR: The photophysics and photochemistry of the metal-metal bonded complexes LnM′Re(CO)3(α-diimine) have been studied in this article, where the lowest excited state has a 3 σ(M′ - Re )π ∗ character.
TL;DR: In this article, Hartree-Fock (HF) or relativistic analogue compact effective potential (CEP) was used to replace the core electrons for the ab initio and hybrid density functional theory (DFT) levels and methods have been tested and compared with each other and with experiment, where available.
TL;DR: In this article, two methods are described and illustrated for the measurement of organo−cobalt bond homolysis energies through reactions of tetra(p-anisyl)porphyrinato cobalt(II), (TAP)CoII•, with organic radicals of the form •C(CH3)(R)CN in the presence of olefins.
Abstract: Two methods are described and illustrated for the measurement of organo−cobalt bond homolysis energies through reactions of tetra(p-anisyl)porphyrinato cobalt(II), (TAP)CoII•, with organic radicals of the form •C(CH3)(R)CN in the presence of olefins. Thermodynamic values for bond homolysis have been determined directly for (TAP)Co-C(CH3)2CN (ΔH° = 17.8±0.5 kcal mol-1, ΔS° = 23.1 ± 1.0 cal K-1 mol-1) and (TAP)Co-CH(CH3)C6H5 (ΔH° = 19.5 ± 0.6 kcal mol-1, ΔS° = 24.5 ± 1.1 cal K-1 mol-1) from evaluation of the equilibrium constants for the dissociation process (Co−R ⇌ CoII• + R•) in chloroform. The bond homolysis enthalpy for (TAP)Co-C5H9 (ΔH° = 30.9 kcal mol-1) was determined indirectly by measuring the thermodynamic values for the competition reaction (TAP)Co-C(CH3)2CN + C5H8 ⇌ (TAP)Co-C5H9 + CH2C(CH3)CN (ΔH° = 0.9 ± 0.3 kcal mol-1) in conjunction with a thermochemical cycle. This indirect approach was also used to evaluate (TAP)Co-CH(CH3)C6H5 BDE (20.5 kcal mol-1) which agrees favorably with the value dete...
TL;DR: In this paper, the alkyl group bearing polymer (PEG) was prepared by way of sulfide 8 and thiol 11 in high yields, and the degradation of the chain was either by homolysis under radical conditions or by desulfurization using Raney nickel to yield the new carbon-hydrogen bond product.
TL;DR: In this paper, the primary and subsequent photochemistry of N-hydroxy-2(1H)-pyridone (N-HP) has been investigated in aqueous and nonaqueous media by laser flash photolysis (λexc = 308 nm).
Abstract: The primary and subsequent photochemistry of N-hydroxy-2(1H)-pyridone (N-HP) has been investigated in aqueous and nonaqueous media by laser flash photolysis (λexc = 308 nm). In organic solvents, as well as in buffers at pH ≤ 7, the initial photochemistry of N-HP consists of homolytic N−O bond cleavage leading to the formation of the 2-pyridyloxyl (PyO•) and hydroxyl (•OH) radicals, the quantum yield (ΦN-O = Φ•OH) varying from 0.25 to 0.6, depending on the solvent. Quenching experiments have demonstrated that PyO• is relatively unreactive and is removed mainly via a bimolecular radical reaction. In highly basic aqueous media, N-HP exists in the anionic form and is much less photolabile. At pH = 10, in addition to a low yield of N−O bond cleavage (ΦN-O = 0.037), N-HP undergoes photoionization, but solvated electron production was found to be very inefficient (Φe- = 0.003). Thus, under biologically relevant conditions, N-HP has a much simpler photochemical behavior than that of the closely related N-hydroxyp...
TL;DR: The effect of homogeneous catalysis, Fe and Ru, on the conversion of guaiacol in tetralin to catechol and phenol has been investigated as a model for the behavior of the aryl-oxy linkage that is found in wood, peat and younger coals as discussed by the authors.
TL;DR: In this paper, a broad spectrum of organic compounds including decomposition of explosives have been measured with 1 μs duration and peak temperatures from 900 to 1400 K and pressures from 6 to 16 GPa.
Abstract: Shock waves of 1 μs duration and peak temperatures from 900 to 1400 K and pressures from 6 to 16 GPa have been applied to measurement of the rates of a broad spectrum of organic compounds including decomposition of explosives. All reaction rates are consistent with known activation parameters and reaction conditions, and there is no indication of unique chemical processes connected with the subnanosecond rise time of the shock front. Reactions having both positive and negative activation volumes have been studied. Extreme pressure favors ionic reaction mechanisms, whereas extreme temperature favors homolytic mechanisms. Either influence may dominate; thus, cyclohexyl nitrate in benzene gives cyclohexylbenzene by a reaction of Friedel−Crafts type, whereas neopentyl nitrate undergoes homolysis, β-scission, and recombination to give 2-nitro-2-methylpropane. When methanol is used as a solvent, it ionizes extensively and promotes a variety of reactions which ordinarily require acid or base catalysis such as es...
TL;DR: In this article, a Fourier transform EPR (FT-EPR) was used to monitor the formation and decay of alkyl radicals generated by metal-R bound homolysis.
Abstract: A Fourier transform EPR (FT-EPR) study was made of the photochemistry of Re(R)(CO){sub 3}({alpha}-diimine) and Ru(I)(R)(CO){sub 2}({alpha}-diimine) complexes, where R = benzyl, 2-propyl, or ethyl and {alpha}-diimine = 4,4`-dimethyl-2, 2`-bipyridine or N,N`-diisopropyl-1,4-diazabutadiene. The FT-EPR technique was used to monitor the formation and decay of alkyl radicals generated by metal-R bound homolysis. The spectra showed pronounced chemically induced dynamic electron polarization (CIDEP) effects with polarization patterns that depend strongly on metal ion, alkyl substituent, and solvent. To varying degrees, all spectra shows a contribution form ST{sub 0} radical pair mechanism (RPM) CIDEP. From the low-field-emission/high-field-adsorption polarization pattern given by this mechanism, it can be deduced that the free radicals are formed via a triplet-state precursor, confirming conclusions reached in earlier spectroscopic studies. In addition to the RPM contribution, some spectra also show a net-absorptive CIDEP contribution while others show a net-emissive contribution. These net spin polarization contributions are attributed tentatively to triplet mechanism and spin-orbit coupling CIDEP. From the fact that the alkyl radical spectra exhibit strong spin polarization, it is concluded that metal ion atomic orbital contributions to the electronic states of precursor triplet and radical pair must be small. 17 refs., 4 figs.
TL;DR: In this article, the effect of the apparent population of these additional conformations at elevated temperatures on the activation parameters for thermal carbon-cobalt bond homolysis is discussed in the light of molecular-mechanics calculations on the influence of Co C bond geometry and acetamide side-chain rotational motions on the energetivs of this reaction.
TL;DR: In this article, the (porphyrinato)iron(III)-catalyzed oxidation of styrene and α-methylstyrene by O2 in benzene-ethanol containing NaBH4 was investigated.
Abstract: Mechanisms have been proposed for the (porphyrinato)iron(III)-catalyzed oxidation of styrene and α-methylstyrene by O2 in benzene-ethanol containing NaBH4. The product analysis and the deuterium incorporation using NaBD4 suggest that the (σ-alkyl)FeIIIPor complex, [C6H5CH(CH3)]FeIIIPor, is formed as an intermediate in the reaction of styrene. Insertion of O2 to the (σ-alkyl)FeIIIPor complex having a radical character yields a (peroxy)iron(III) complex, [C6H5CH(CH3)OO]FeIIIPor. The homolytic fission of the OO bond followed by the hydrogen abstraction within the radical pair affords acetophenone and (HO)FeIIIPor. Acetophenone is readily reduced with NaBH4 to give l-phenylethanol. Meanwhile, the reaction of α-methylstyrene with BH−4 in the presence of PorFeIIICl may also yield the (σ-alkyl)FeIIIPor complex, which takes up O2 to form a (peroxy)iron(III) complex, (C6H5C(CH3)2OO)FeIIIPor. The (peroxy)iron(III) complex is directly reduced by BH−4 to give 2-phenyl-2-propanol and (HO)FeIIIPor. In the reaction of styrene, such direct reduction of the (peroxy)iron(III) complex as a minor pathway competes with the homolytic fission of its OO bond.
TL;DR: In this paper, the homolytic Se−H bond energy of benzeneselenol, a very efficient radical trap in solution and in the gas phase, was determined experimentally by using two independent approaches in a Fourier transform ion cyclotron resonance mass spectrometer.
Abstract: The homolytic Se−H bond energy of benzeneselenol, a very efficient radical trap in solution and in the gas phase, was determined experimentally by using two independent approaches in a Fourier-transform ion cyclotron resonance mass spectrometer. The hydrogen−selenium bond strength was concluded to be 76−80 kcal mol-1 based on the measurement of the efficiency of hydrogen atom abstraction from benzeneselenol by several radical cations. The Se−H bond energy was also determined indirectly through the use of a thermochemical cycle. This approach required the measurement of the adiabatic ionization energy of benzeneselenol and the proton affinity of C6H5Se•. An adiabatic ionization energy of 8.3 ± 0.1 eV was obtained by measurement of the efficiencies of various electron transfer reactions in the forward and reverse directions. The proton affinity of C6H5Se• (acidity of the benzeneselenol radical cation) was found to be 200 ± 3 kcal mol-1 based on the measured efficiencies of several proton transfer reactions....