Homolytic Dissociation Research Articles

Hydride, Hydrogen, Proton, and Electron Affinities of Imines and Their Reaction Intermediates in Acetonitrile and Construction of Thermodynamic Characteristic Graphs (TCGs) of Imines as a “Molecule ID Card”

A series of 61 imines with various typical structures were synthesized, and the thermodynamic affinities (defined as enthalpy changes or redox potentials in this work) of the imines to abstract hydride anions, hydrogen atoms, and electrons, the thermodynamic affinities of the radical anions of the imines to abstract hydrogen atoms and protons, and the thermodynamic affinities of the hydrogen adducts of the imines to abstract electrons in acetonitrile were determined by using titration calorimetry and electrochemical methods. The pure heterolytic and homolytic dissociation energies of the C=N pi-bond in the imines were estimated. The polarity of the C=N double bond in the imines was examined using a linear free-energy relationship. The idea of a thermodynamic characteristic graph (TCG) of imines as an efficient "Molecule ID Card" was introduced. The TCG can be used to quantitatively diagnose and predict the characteristic chemical properties of imines and their various reaction intermediates as well as the reduction mechanism of the imines. The information disclosed in this work could not only supply a gap of thermodynamics for the chemistry of imines but also strongly promote the fast development of the applications of imines.

THE INFLUENCES OF OXIDATION AND CATIONIZATION ON THE N-GLYCOSIDIC BOND STABILITY OF 8-OXO-2′-DEOXYADENOSINE — A THEORETICAL STUDY

This work is an attempt to evaluate theoretically the influences of oxidation and cationization on the N-glycosidic bond stability and the proton and sodium affinities on 8-oxo-2′-deoxyadenosine (8-oxodA) by using the density functional theory (DFT) B3LYP with basis set 6-31++G(d,p). This work shows that the cation attachment to 8-oxodA may modify the equilibrium geometry and bond dissociation. In all modified forms, the length of the N9–C1′ bond in which there is no intramolecular interaction, i.e. O8–H(Na)⋯O4′, increases relative to the neutral system 8-oxodA but that of others decreases. The analysis for the proton and sodium affinity energies indicates that the N1 and N3 atoms are the favorable sites for proton while the N1 atom is the favorable site for Na+. From the dissociation energies of the N-glycosidic bond, it has been found that the homolytic dissociation becomes more difficult upon introducing positive charge in the base ring. In contrast, these systems favor the heterolytic dissociation significantly. The influence is most prominent with the monocation obtained by O8 cationization.

Homolytic versus Heterolytic Dissociation of Alkalimetal Halides: The Effect of Microsolvation

Herein we report density functional calculations of homolytic and heterolytic dissociation energies of the diatomic alkalimetal halides MX (M=Li, Na, K, Rb, and Cs and X=F, Cl, Br, I, and At) and their corresponding microsolvated structures MX(H(2)O)(n) (n=1 to 4). Our results show that the homolytic dissociation energy of the MX(H(2)O)(n) species increases with the number of water molecules involved in the microsolvated salts. On the other hand, the heterolytic dissociation energy follows exactly the opposite trend. As a result, while for the isolated diatomic alkalimetal halides, homolytic dissociation is always favored over heterolytic dissociation, the latter is preferred for CsF and CsCl already for n=2, and for n=4 it is the preferential mode of dissociation for more than half of the species studied.

Rearrangement and thermal decomposition of nitromethane confined inside an armchair (5, 5) single-walled carbon nanotube

In this work, the thermal decomposition and rearrangement of nitromethane (NMT) confined inside an armchair (5, 5) single-walled carbon nanotubes [CNT(5.5)] were investigated by using the ONIOM calculations in order to understand the confinement effect of CNT on the initial reactions of nitro-energetic compounds. We found that the activation energy barriers for the NMT–MNT (methyl nitrite) rearrangement, H-migration rearrangement and C–N homolytic dissociation were significantly decreased by the confinement effect of CNT(5, 5), suggesting that the CNT(5, 5) can promote the rearrangement and thermal decomposition reactions of the NMT. In both the isolated and confined cases, the C–N homolytic cleavage was kinetically favorable in view of the relatively low activation barrier. However, the energy barrier of the NMT–MNT rearrangement was lower than that of the H-migration rearrangement reaction due to their confinement in CNT(5, 5). Furthermore, the relative energies of the intermediates generated from the rearrangement and thermal decomposition were also modified by the confinement of CNT(5, 5).

Homolytic Reactive Mass Spectrometry of Fullerenes: Interaction of C60 and C70 with Organo- and Organoelement Mercurials in the Electron Impact Ion Source of a Mass Spectrometer; EPR, CIDEP, and MS Studies of Several Analogous Reactions of C60 Performed in Solution

Interaction of C(60) with organo- and organoelement mercurials (CF(3)HgBr, PhHgBr, p-CH(3)C(6)H(4)HgBr, p-CH(3)OC(6)H(4)HgCl, CF(3)HgPh, Ph(2)Hg, (o-carborane-9-yl)(2)Hg, (m-carborane-9-yl)(2)Hg, (p-carborane-9-yl)(2)Hg, and (m-carborane-9-yl)HgCl) in the ionization chamber (IC) of the electron impact (EI) ion source of a mass spectrometer at 250-300 degrees C results in the transfer of the corresponding organic or organoelement radicals from the mercurials to the fullerene. Some of the processes are accompanied by hydrogen addition. C(70) reacts with Ph(2)Hg and (o-carborane-9-yl)(2)Hg at 300 degrees C in a similar fashion. A homolytic reaction path is considered for the reactions. It suggests both the thermal and EI initiated homolytic dissociation of the mercurials to the intermediate organic or organoelement radicals followed by their interaction with the fullerenes at the metallic walls of the IC. When EI is involved, the dissociation is supposed to occur via superexcited states (the excited states with the electronic excitation energies higher than the first ionization potentials) of the mercury reagents, with possible contribution of the process proceeding via their molecular ions. In line with the results obtained in the IC, C(60) reacts with Ph(2)Hg and (o-carborane-9-yl)(2)Hg under UV-irradiation in benzene and toluene solutions to furnish phenyl and carboranyl derivatives of the fullerene, respectively, some also containing the acquired hydrogen atoms. EPR monitoring of the processes has shown the formation of phenylfullerenyl and o-carborane-9-yl-fullerenyl radicals. g-Factors and hyperfine coupling (hfc) constants with (10)B, (11)B, and (13)C nuclei of both the latter and m-carborane-9-yl-fullerenyl radical formed in the reaction of C(60) with (m-carborane-9-yl)(2)Hg have been determined by the special EPR studies. The unusually great chemically induced dynamic electron polarization (CIDEP) of the latter radical where even the (13)C satellite lines are polarized has been observed and is discussed in terms of both radical-triplet-pair and radical-pair mechanisms. The similar CIDEP effect is also intrinsic to the o-carborane-9-yl-fullerenyl radical obtained under the same conditions. The analogous transfer of the carboranyl radicals from (o-carborane-9-yl)(2)Hg to C(60) occurs when their mixture is boiled in (t)BuPh for 10-15 h.

Modelling the structure of the water molecule under both local environmental constraints and explosive conditions

A model proposed by Gabbay and March allowed the geometry of the water molecule to be studied as the OH bond length was compressed from its equilibrium value; the corresponding H–O–H bond angle opened up from 104° and under extreme compression a limiting angle of 180° was predicted. Local environmental changes were later found to increase the bond angle of 104° by some degrees. Such structural changes are compared here and contrasted with molecular dynamics results of Schwegler et al. (Schwegler, Galli and Gygi, Phys. Rev. Lett. 84, 2429 (2000)) on water under pressure, and with subsequent simulations by Wu et al. (Wu, Fried, Yang, Goldman and Bastea, Nat. Chem. 1, 57 (2009)) involving the high-explosive pentaerythritol tetranitrate. Extreme conditions of both pressure and temperature lead to a homolytic dissociation reaction involving free atoms and radicals: H2O ⇌ H + OH, in marked contrast to ambient conditions that favour the heterolytic self-ionisation. The X-ray induced homolytic dissociation of water and formation of an O2 + H2 crystalline alloy at high pressure as observed by Mao et al. (Mao, Mago, Meng, Eng, Hu, Chow, Cai, Shu and Hemley, Science 314, 636 (2006)) are also noted.

Generation of O− Radical Anions on MgO Surface: Long-Distance Charge Separation or Homolytic Dissociation of Chemisorbed Water?

O− radical anions were observed over a partially hydroxylated MgO surface after illumination by monochromatic light with λ = 280 and 303 nm. As all corner oxygen atoms are covered with adsorbed hydroxyl groups after activation at 450 °C, this process is initiated by selective excitation of 3-coordinated complexes [Mg2+−O2−]3C containing chemisorbed water. A new mechanism for generation of the O− radical anions is suggested. It is based on homolytic dissociation of chemisorbed water followed by migration of a hydrogen atom, probably, to a different nanoparticle rather than on long-distance separation of charges. The surface structure remaining after its detachment consists of an •OH radical stabilized near the corner oxygen atom. The imminent charge transfer generates a hole at the corner oxygen atom stabilized by a hydroxyl group [OH−···Mg2+−O−]3C. DFT simulation of this structure showed that it reproduces the main characteristics of the radical anions O−3C. The overall structure is electrically neutral. The charge of the hole is compensated by the nearby hydroxyl group. So, no long-distance charge separation is required.

Establishment of the C−NO Bond Dissociation Energy Scale in Solution and Its Application in Analyzing the Trend of NO Transfer from C-Nitroso Compound to Thiols

The first set of experimentally determined C-NO bond homolytic and heterolytic dissociation enthalpies in solution is derived by using direct titration calorimetry combined with appropriate electrode potentials through thermodynamic cycles. The homolytic bond dissociation energy scale (BDEs) of the corresponding C-NO bonds in the gas phase was also calculated at the MP2/6-311+G**//B3LYP/6-31G* level and BP86/6-31G*//B3LYP/6-31G* level of theory for the purpose of comparison. The C-NO and S-NO bond thermodynamic parameters were used to predict the trend of NO transfer from C-nitroso substrates to thiols in acetonitrile solution.

Formation and Interconversion of Organo-Cobalt Complexes in Reactions of Cobalt(II) Porphyrins with Cyanoalkyl Radicals and Vinyl Olefins

Observation of the formation and interconversion of organo-cobalt complexes ((TMP)Co-R) is used to reveal mechanistic features in the living radical polymerization (LRP) of methyl acrylate (MA) mediated by cobalt porphyrins. Both dissociative and associative exchange of radicals in solution with organo-cobalt complexes contribute to controlling the radical polymerization. The sequence of organo-cobalt species formed during the induction period for the (TMP)Co-R mediated LRP of MA indicates that homolytic dissociation is a prominent pathway for the interconversion of organo-cobalt complexes which contrasts with the corresponding vinyl acetate (VAc) system where associative radical exchange totally dominates these processes. The dissociation equilibrium constant (K(d(333 K))) for organo-cobalt complexes formed in methyl acrylate polymerization ((TMP)Co-CH(CO(2)CH(3))CH(2)P) was estimated as 1.15 x 10(-10) from analysis of the polymerization kinetics and (1)H NMR. The ratio of the rate constants (333 K) for the cyanoisopropyl radical (*C(CH(3))(2)CN) adding with monomer (k(1)) to the process of transferring a hydrogen atom to (TMP)Co(II)* (k(2)) was evaluated for the methyl acrylate system as 2 x 10(-3) which is larger than that for vinyl acetate LRP (9 x 10(-5)). Kinetic analysis places the rate constant for associative radical interchange (333 K) at approximately 7 x 10(5) M(-1) s(-1). The larger radical stabilization energy and lower energy of the singly occupied molecular orbital (SOMO) for methyl acrylate based radicals (*CH(CO(2)CH(3))CH(2)P) compared to vinyl acetate contribute to the observed prominence of organo-cobalt homolytic dissociation and much smaller chain transfer which result in substantially better control for living radical polymerization of methyl acrylate than that observed for vinyl acetate.

First Principles Investigation of H Addition and Abstraction Reactions on Doped Aluminum Clusters

We have investigated axial interactions of H(2) with Al(12)X (X = Mg, Al, and Si) clusters and found that homolytic dissociation leading to Al(12)XH and H atom proceeds without a barrier but is an extremely endothermic process. The calculated difference in energy of the addition and abstraction reactions indicates that any Al(12)X-based hydrogen storage technology that involves predissociation of H(2) will be limited by the competing processes. We have also discovered that while there is a modest barrier for dissociation of H(2) on a single Al(12)Mg cluster to give the dihydride, the process occurs spontaneously between two closely spaced Al(12)Mg clusters, resulting in the formation of two Al(12)MgH species. Doping of the cluster with an electropositive atom (Mg) enables the transfer of electron density to the Al cage, which enhances H(2) dissociation. The information gained can contribute to the design of novel solid-state materials made of doped Al clusters, which may ultimately be suitable for catalytic processes.

Trifluoromethylation of C60 and C70 with Mercury(II) Trifluoroacetate

Fullerenes C60 and C70 are shown to react with Hg(CF3COO)2 in the ionization chamber of the electron impact source of a mass spectrometer at 300 and 310°C, yielding trifluoromethylated derivatives of the fullerenes with up to six and two CF3 addends, respectively. For some of the products the reactions are accompanied by hydrogen addition. Based on previous mechanistic studies of the analogous reactions of C60 with the Scherer radical (C9F19•), a mixture of perfluoronones (C9F18), and acetone, a plausible scheme is proposed for the processes. It involves formation of the intermediate CF3 radicals owing to electron impact or thermally initiated homolytic dissociation of the mercury(II) salt, and their further reaction with the fullerenes at the surface of the ionization chamber. Heating mixtures of the reagents under argon at 300–310°C also yields trifluoromethylated fullerenes, in agreement with the results obtained with the ionization chamber. *Deceased on 1 February 2006

Concept of acid–base catalysis by metal sulfides

The structure of the active component of the sulfide hydrodesulfurization (HDS) catalysts is considered. The active component consists of a single slab of MoS 2 (WS 2) with Ni or Co atoms being located in its edge plane. The essential element of the active component is hydrogen occluded into the MoS 2 matrix, which localizes in the center of “empty” trigonal prism of a MoS 2 matrix made from six sulfur atoms under Ni (Co) atom at the distance of 1.5 Å. Occluded hydrogen appeared in the active component during the catalyst sulfidation as a result of its oxidative addition to the Ni (Co) atoms. This occluded hydrogen creates an electronic d 6 configuration of Ni(IV) or Co(III) atoms, which are the Lewis acid sites with two vacant d z2 and d xy orbitals. Therefore, these active centers can adsorb donor molecules of thiophene. The activation of hydrogen occurs on the couplet sulfur atoms at the edge planes of a single MoS 2 (WS 2) slab. The driving force of this process is the presence of the S S chemical bond at the terminal sulfur atoms that restrict an electrically neutral macromolecule of the active component and act as the Lewis basic sites. An excess of electron density is transferred from these atoms to a hydrogen molecule, which results in its homolytic dissociation. The high rate of catalytic transformation is ensured by the fast transfer of electrons and protons along the system of chemical bonds within an electrically neutral macromolecule of the active component.

Two alternative mechanisms of aromatization of light hydrocarbons on Ga/ZSM-5 and Zn/ZSM-5 zeolites

By means of IR diffuse reflectance spectroscopy, it was shown that aromatization of light paraffins on the GaZSM-5 and ZnZSM-5 catalysts occurs through different routes. The first step of transformation on GaZSM-5 is the oxidative addition of paraffin molecules to Ga+ ions resulting in homolytic dissociation and the appearance of particles [H-Ga-R]+, whereas dehydrogenation on zinc-containing high-silica zeolites results from heterolytic dissociative adsorption on acid-base pairs yielding Bronsted acid centers and alkyl groups bonded to zinc. The subsequent degradation of the alkyl fragments leads to the formation of zinc hydrides or gallium dihydrides and olefins. The oligimerization of olefins in the case of ZnZSM-5 can occur on both acidic hydroxyl groups produced via paraffin dissociation and on the Zn2+ cations per se already at ambient temperature. In the case of GaZSM-5, the appearance of Bronsted sites in the catalyst is achieved via incomplete replacement of OH groups with gallium cations.

Kinetics of Coloration in Photochromic Organoammonium Polyoxomolybdates

The excellent photochromic properties of (H(2)DABCO)(2)(HDMA)(0.5)Na(0.75)(H(3)O)(0.75)[Mo(8)O(27)] x 3 H(2)O (4), a new member of the (H(2)DABCO)(2)(A)(x)[Mo(8)O(27)] x n H(2)O series, are compared with those of (H(2)DABCO)(2)(NH(4))(2)[Mo(8)O(27)] x 4 H(2)O (1), (H(2)DABCO)(2)(H(2)pipz)[Mo(8)O(27)] (2), and (H(2)pipz)(3)[Mo(8)O(27)] (3). All these powdered materials turn from white to purple under illumination at 365 nm, which is associated with photoreduction of Mo(6+) cations into Mo(5+) cations. We show that the rates of coloration, which increase in the order 1 < 3, 2 < 4, are related to the decrease in the concentration of reducible Mo(6+) centers with irradiation time and follow a second-order reaction law because the event of light absorption at a reducible Mo(6+) site does not necessarily coincide with that of the N(+)-H bond breaking in the N(+)-H...O hydrogen bond associated with the Mo(6+) site. First-principles density functional electronic structure calculations were carried out to find that this trend correlates with the homolytic dissociation energies of the N(+)-H bonds in the organic cations HDMA(+), H(2)pipz(2+), H(2)DABCO(2+), and NH(4)(+). This observation is consistent with a photochromic mechanism based on the homolytic cleavage of N(+)-H bonds rather than on the heterolytic cleavage of N(+)-H bonds.

Organic Molecular Probes in Heterogeneous Catalysis. Hydrogenation of Norbornadiene on Gold

A few important examples are described to show that well-chosen molecular probes in organic reactions on heterogeneous catalysts can reveal much about the nature of the active sites and reaction mechanisms. This approach using norbornadiene in hydrogenation casts much light on the reasons why highly dispersed gold is an active selective catalyst. In the new polar mechanism, H2 undergoes heterolytic fission in the very act of addition to adsorbed alkene, in contrast to the conventional Horiuti-Polanyi mechanism based on prior homolytic dissociation of H2 on the metal surface.

A Brief Computational Study of Decamethyldizincocene Formation via Diethylzinc and Decamethylzincocene

We have used density functional theory and ab initio methods to study different mechanistic possibilities for the formation of decamethyldizincocene from the reaction between decamethylzincocene and diethylzinc. Our results suggest that decamethyldizincocene could form from the combination of two pentamethylcyclopentadienylzinc radicals. More importantly, our data show that homolytic dissociation of decamethylzincocene into pentamethylcyclopentadienylzinc and pentamethylcyclopentadienyl radicals is 6.7 kcal/mol less costly than the analogous dissociation of zincocene. If such an energy difference is coupled with the fact that the activation barrier to form the half-sandwich product pentamethylcyclopentadienylethylzinc is 11.8 kcal/mol more costly than to form cyclopentadienylethylzinc, we can rationalize why dizincocene does not form experimentally.

Photochemistry of Protonated Nitrosamine: Chemical Inertia of NH2NOH+ Versus Reactivity of NH3NO+

The photochemical behavior of the protonated simplest nitrosamine [NH2NO-H](+) has been addressed by means of the CASPT2//CASSCF methodology in conjunction with the ANO-L basis sets. The relative stability of the different tautomers, namely, (1) NH2NOH(+), (2) NH3NO(+), and (3) NH2NHO(+), has been considered, and the corresponding tautomerization transition states have been characterized. With respect to the most chemically relevant species, it has been found that NH2NOH(+) corresponds to a bound structure, while NH3NO(+) corresponds to an adduct between NH3 and NO(+) at both CASSCF and CASPT2 levels of theory. Vertical transition calculations and linear interpolations on the homolytic dissociation of NH3NO(+) in combination with previous results on neutral nitrosamine [J. Chem. Phys. 2006, 125, 164311] and neutral N,N-dimethylnitrosamine [J. Org. Chem. 2007, 72, 4741] indicate that, in acidic diluted solutions, the protonation of nitrosamine takes place on the excited surface. The N-N dissociation channels have been studied both in ground and first excited singlet state. An S1/S0 conical intersection is found to be responsible for the photostability of NH2NOH(+). On the contrary, NH3NO(+) is photochemically unstable as its first excited state is purely dissociative. The latter species is characterized by a twofold reactivity: the formation of nitrosyl cation (NO(+)) in the ground state and the photorelease of physiologically relevant nitric oxide radical (NO) in its first excited state.

The Mechanism of Propane Oxidation over Iron Antimony Oxide

Density functional theory has been used to investigate propane oxidation over iron antimony oxide, an important process in the petrochemical industry. Detailed microscopic mechanisms have been revealed for the following three key reactions: (i) the initial hydrogen abstraction from propane, the rate-determining step in propane oxidation to acrolein (CH2═CH−CHO), (ii) the formation of propene, a possible intermediate in the process, and (iii) the production of acrolein via the economically valuable one-stage mechanism. We have found that in the initial C−H splitting, the most feasible cleavage takes place via a homolytic dissociation, involving two oxygen atoms along the [010] direction of the surface and occurring in the methylene group. The heterolytic dissociation paths are generally less favorable than the homolytic processes; and in the former, there is almost no preference in abstracting H from methylene or methyl group. Subsequent reactions, after the first H abstraction via homolytic splitting, can...

Kinetics of Dissociation of Molecular Oxygen from a Superoxorhodium(III) Complex and Reactivity of a Macrocyclic Rhodium(II) Ion

The kinetics of disappearance of the superoxorhodium complex L2(H2O)RhOO2+ (L2 = meso-hexamethylcyclam) were determined in the presence of several oxidants (H2O2, (NH3)5CoBr2+, and IrCl62-) in both air-free and air-saturated aqueous solutions. Under air-free conditions, the reaction obeyed first-order kinetics. After the correction for the appropriate stoichiometric factors, the value of the rate constant kh was the same irrespective of the oxidant, kh = 2.18 (+/-0.37) x 10(-4) s(-1) at 25.0 degrees C in acidic solutions. The disappearance of L2(H2O)RhOO2+ was slower in the presence of O2. All the data suggest a sequence of reactions beginning with homolytic dissociation of O2 from L2(H2O)RhOO2+, followed by capture of the newly generated L2(H2O)Rh2+ by added oxidants in competition with O2. The equilibrium constant for O2 binding by L2(H2O)Rh2+ is 109-fold greater than that for the cobalt analogue. This difference is attributed to the lower reduction potential of the rhodium complex.

Failures and limitations of quantum chemistry for two key problems in the atmospheric chemistry of peroxy radicals

Quantum chemical calculations have obtained significant and useful information about chemical reactions relevant to atmospheric chemistry. This article reviews two problems for which it would seem that such calculations could readily provide a great deal of helpful information, but for which that potential has not been fully realized. The first is the formation of alkylnitrates (RONO 2) from the reaction of peroxy radicals (ROO ) with NO. Several studies of this process have been carried out, but most clearly yielded unphysical results due to methodological pitfalls. The one study which appears valid [Zhao, Y., Houk, K.N., Olson, L.P., 2004. Mechanisms of peroxynitrous acid and methyl peroxynitrite, ROONO (R=H, Me), rearrangements: a conformation-dependent homolytic dissociation. Journal of Physical Chemistry A 108, 5864–5871] suggests an indirect route between ROONO and RONO 2, and implies that statistical rate theory will be insufficient to understand the yields of RONO 2. The second problem discussed here is the competition between radical production and disproportionation in the self-reaction of organic peroxy radicals, for which only one quantum chemical study has found a plausible mechanism connecting reactants with products.