Evidence for HOOO radicals in the formation of alkyl hydrotrioxides (ROOOH) and hydrogen trioxide (HOOOH) in the ozonation of C-H bonds in hydrocarbons.

Low-temperature ozonation of cumene (1a) in acetone, methyl acetate, and tert-butyl methyl ether at -70 degrees C produced the corresponding hydrotrioxide, C(6)H(5)C(CH(3))(2)OOOH (2a), along with hydrogen trioxide, HOOOH. Ozonation of triphenylmethane (1b), however, produced only triphenylmethyl hydrotrioxide, (C(6)H(5))(3)COOOH (2b). These observations, together with the previously reported experimental evidence, seem to support the "radical" mechanism for the first step of the ozonation of the C-H bonds in hydrocarbons, i.e., the formation of the caged radical pair (R(**)OOOH), which allows both (a) collapse of the radical pair to ROOOH and (b) the abstraction of the hydrogen atom from alkyl radical R(*) by HOOO(*) to form HOOOH. The B3LYP/6-311++G(d,p) (ZPE) calculations revealed that HOOO radicals are considerably stabilized by forming intermolecularly hydrogen-bonded complexes with acetone (BE = 8.55 kcal/mol) and dimethyl ether (7.04 kcal/mol). This type of interaction appears to be crucial for the relatively fast reactions (and the formation of the polyoxides in relatively high yields) in these solvents, as compared to the ozonations run in nonbasic solvents. However, HOOO radicals appear to be not stable enough to abstract hydrogen atoms outside the solvent cage, as indicated by the absence of HOOOH among the products in the ozonolysis of triphenylmethane. The decomposition of alkyl hydrotrioxides 2a and 2b involves a homolytic cleavage of the RO-OOH bond with subsequent "in cage" reactions of the corresponding radicals, while the decomposition of HOOOH is most likely predominantly a "pericyclic" process involving one or more molecules of water acting as a bifunctional catalyst to produce water and singlet oxygen (Delta(1)O(2)).     

Mechanistical studies on the formation of isotopomers of hydrogen peroxide (HOOH), hydrotrioxy (HOOO), and dihydrogentrioxide (HOOOH) in electron-irradiated H(2)(18)O/O(2) ice mixtures

In order to investigate the chemical reactions inside water-oxygen ice mixtures in extreme environments, and to confirm the proposed reaction mechanisms in pure water ice, we conducted a detailed infrared spectroscopy and mass spectrometry study on the electron irradiation of H(2)(18)O/O(2) ice mixtures. The formation of molecular hydrogen, isotopically substituted oxygen molecules (18)O(18)O and (16)O(18)O, ozone ((16)O(16)O(16)O, (16)O(16)O(18)O, and (16)O(18)O(16)O), hydrogen peroxide (H(18)O(18)OH, H(16)O(16)OH and H(16)O(18)OH), hydrotrioxy (HOOO), and dihydrogentrioxide (HOOOH) were detected. Kinetic models and reaction mechanisms are proposed to form these molecules in water and oxygen-rich solar system ices.     

17O NMR spectroscopic characterization and the mechanism of formation of alkyl hydrotrioxides (ROOOH) and hydrogen trioxide (HOOOH) in the low-temperature ozonation of isopropyl alcohol and isopropyl methyl ether: water-assisted decomposition

Low-temperature ozonation of isopropyl alcohol (1a) and isopropyl methyl ether (1b) in [D6]acetone, methyl acetate, and tert-butyl methyl ether at -78 degrees C produced the corresponding hydrotrioxides, Me2C(OH)(OOOH) (2a) and Me2C(OMe)(OOOH) (2b), along with hydrogen trioxide (HOOOH). All the polyoxides investigated were characterized for the first time by 17O NMR spectroscopy of highly 17O-enriched species. The assignment was confirmed by GIAO/MP2/6-31++G* calculations of 17O NMR chemical shifts, which were in excellent agreement with the experimental values. Ab initio density functional (DFT) calculations at the B3LYP/ 6-31G*+ZPE level have clarified the transition structure (TS1, deltaE = 7.4 and 10.6 kcalmol(-1), relative to isolated reactants and the complex 1a-ozone, respectively) for the ozonation of 1a: this, together with the formation of HOOOH and some other products, indicates the involvement of radical intermediates (R*, *OOOH) in the reaction. The activation parameters for the decomposition of the hydrotrioxides 2a and 2b (Ea, = 23.5+/-1.5 kcalmol(-1), logA = 16+/-1.8) were typical for a homolytic process in which cleavage of the ROOOH molecule occurs to yield a radical pair [RO* *OOH] and represents the lowest available energy pathway. Significantly the lower activation parameters for the decomposition of HOOOH (Ea = 16.5+/-2.2 kcalmol(-1), logA = 9.5+/-2.0) relative to those expected for the homolytic scission of the HO-OOH bond [bond dissociation energy (BDE) = 29.8 kcalmol(-1), CCSD(T)/6-311++G**] are in accord with the proposal that water behaves as a bifunctional catalyst and therefore participates in a "polar" (non-radical) decomposition process of this polyoxide. A relatively large acceleration of the decomposition of the hydrotrioxide 2a in [D6]acetone, accompanied by a significant lowering of the activation energies, was observed in the presence of a large excess of water. Thus intramolecular 1,3-proton transfer probably also involves the participation of water and is similar to the mechanism proposed for the decomposition of HOOOH. This hypothesis was further substantiated by the B3LYP/6-31++ G*+ZPE calculations for the participation of water in the decomposition of CH3OOOH, which revealed two stationary points on the potential energy surface corresponding to a CH3OOOH-HOH complex and a six-membered cyclic transition state TS2. The energy barriers were comparable with those calculated for HOOOH, that is, deltaE = 15.0 and 21.5 kcalmol(-1) relative to isolated reactants and the CH3OOOH-HOH complex, respectively.     

Peculiar structure of the HOOO(-) anion

The HOOO(-) anion (1) can adopt a triplet state (T-1) or a singlet state (S-1), where the former is 9.8 kcal/mol (DeltaH(298) = 10.3 kcal/mol) more stable than the latter. S-1 possesses a strong O-OOH bond with some double bond character and a weakly covalent OO-OH bond (1.80 A) according to CCSD(T)/6-311++G(3df,3pd) calculations (the longest O-O bond ever found for a peroxide). In aqueous solution, S-1 adopts a geometry closely related to that of HOOOH (OO(O), 1.388 A; (O)OO(H), 1.509 A; tau(OOOH), 78.3 degrees ), justifying that S-1 is considered the anion of HOOOH. Dissociation into HO anion and O(2)((1)Delta(g)) requires 15.4 (DeltaH(298) = 14.3; DeltaG(298) = 8.9) kcal/mol. Structure T-1 corresponds to a van der Waals complex between HO anion and O(2)((3)Sigma(g)(-)) having a binding energy of 2.7 (DeltaH(298) = 2.1) kcal/mol. Modes of generating S-1 in aqueous solution are discussed, and it is shown that S-1 represents an important intermediate in ozonation reactions.     

15N CIDNP study of formation and decay of peroxynitric acid: evidence for formation of hydroxyl radicals

The reaction of nitrous acid with hydrogen peroxide leads to nitric acid as the only stable product. In the course of this reaction, peroxynitrous acid (ONOOH) and, in the presence of CO(2), a peroxynitrite-CO(2) adduct (ONOOCO(2)(-)) are intermediately formed. Both intermediates decompose to yield highly oxidizing radicals, which subsequently react with excess hydrogen peroxide to yield peroxynitric acid (O(2)NOOH) as a further intermediate. During these reactions, (15)N chemically induced dynamic nuclear polarization (CIDNP) effects are observed, the analysis of the pH dependency of which allows the elucidation of mechanistic details. The formation and decay of peroxynitric acid via free radicals NO(2)(*) and HOO(*) is demonstrated by the appearance of (15)N CIDNP leading to emission (E) in the (15)N NMR signal of O(2)NOOH during its formation and to enhanced absorption (A) during its decay reaction. Additionally, the (15)N NMR signal of the nitrate ion (NO(3)(-)) appears in emission at pH approximately 4.5. These observations are explained by proposing the intermediate formation of short-lived radical anions O(2)NOOH(*)(-) probably generated by electron transfer between peroxynitric acid and peroxynitrate anion, followed by decomposition of O(2)NOOH(*)(-) into NO(3)(-) and HO(*) and NO(2)(-) and HOO(*) radicals, respectively. The feasibility of such reactions is supported by quantum-chemical calculations at the CBS-Q level of theory including PCM solvation model corrections for aqueous solution. The release of free HO(*) radicals during decomposition of O(2)NOOH is supported by (13)C and (1)H NMR product studies of the reaction of preformed peroxynitric acid with [(13)C(2)]DMSO (to yield the typical "HO(*) products" methanesulfonic acid, methanol, and nitromethane) and by ESR spectroscopic detection of the HO(*) and CH(3)(*) radical adducts to the spin trap compound POBN in the absence and presence of isotopically labeled DMSO, respectively.     

Dihydrogen trioxide clusters, (HOOOH)n (n = 2-4), and the hydrogen-bonded complexes of HOOOH with acetone and dimethyl ether: implications for the decomposition of HOOOH

Hydrogen-bonded gas-phase molecular clusters of dihydrogen trioxide (HOOOH) have been investigated using DFT (B3LYP/6-311++G(3df,3pd)) and MP2/6-311++G(3df,3pd) methods. The binding energies, vibrational frequencies, and dipole moments for the various dimer, trimer, and tetramer structures, in which HOOOH acts as a proton donor as well as an acceptor, are reported. The stronger binding interaction in the HOOOH dimer, as compared to that in the analogous cyclic structure of the HOOH dimer, indicates that dihydrogen trioxide is a stronger acid than hydrogen peroxide. A new decomposition pathway for HOOOH was explored. Decomposition occurs via an eight-membered ring transition state for the intermolecular (slightly asynchronous) transfer of two protons between the HOOOH molecules, which form a cyclic dimer, to produce water and singlet oxygen (Delta (1)O 2). This autocatalytic decomposition appears to explain a relatively fast decomposition (Delta H a(298K) = 19.9 kcal/mol, B3LYP/6-311+G(d,p)) of HOOOH in nonpolar (inert) solvents, which might even compete with the water-assisted decomposition of this simplest of polyoxides (Delta H a(298K) = 18.8 kcal/mol for (H 2O) 2-assisted decomposition) in more polar solvents. The formation of relatively strongly hydrogen-bonded complexes between HOOOH and organic oxygen bases, HOOOH-B (B = acetone and dimethyl ether), strongly retards the decomposition in these bases as solvents, most likely by preventing such a proton transfer.     

INDO study of 1,2-fluorine atom migration in 1,2-difluoroethyl and 1,1,2-trifluoroethyl radicals

INDO molecular orbital calculations have been carried out to estimate the barrier heights to the 1,2-migration of fluorine and hydrogen atoms in 1,2-difluoroethyl and 1,1,2-trifluoroethyl radicals. The calculated results suggest that (1) the 1,2-fluorine atom migration through a fluorine atom bridging intermediate will occur more readily than the 1,2-hydrogen atom migration through a hydrogen atom bridging intermediate in both radicals, (2) a fluorine atom will undergo 1,2-migration in 1,1,2-trifluoroethyl radical more readily than in 1,2-difluoroethyl radical. The enthalpy change accompanied by the 1,2-fluorine atom migration in 1,1,2-trifluoroethyl radical was estimated to be 1.7 kcal/mol, which was in good agreement with the value(1.6 kcal/mol) obtained experimentally.     

Mathematical simulation as a method to study the influence of oxygen, hydrogen peroxide, and phosphate and carbonate ions on the kinetics of ozone decomposition in aqueous solution

The influence of oxygen, hydrogen peroxide, and phosphate and carbonate ions on the kinetics of ozone decomposition in an aqueous solutions was studied by using mathematical simulation. The apparent rate constants of ozone decomposition and the hydroxyl radical concentration in the presence of compounds accelerating and inhibiting the ozone decomposition process were calculated. The calculated data were compared with the experimental results taken from literature sources.     

The role of the HOOO(-) anion in the ozonation of alcohols: large differences in the gas-phase and in the solution-phase mechanism

The mechanism of the ozonation of isopropyl alcohol was investigated for the gas and the solution phase using second-order many body perturbation theory and density functional theory (DFT) with the hybrid functional B3LYP and a 6-311++G(3df,3pd) basis set. A careful analysis of calculated energies (considering thermochemical corrections, solvation energies, BSSE corrections, the self-interaction error of DFT, etc.) reveals that the gas-phase mechanism of the reaction is dominated by radical or biradical intermediates while the solution-phase mechanism is characterized by hydride transfer and the formation of an intermediate ion pair that includes the HOOO(-) anion. The product distribution observed for the ozonation in acetone solution can be explained on the basis of the properties of the HOOO(-) anion. General conclusions for the ozonation of alcohols and the toxicity of ozone (inhaled or administered into the blood) can be drawn.     

A theoretical characterization of reactions of HOOO radical with guanine: formation of 8-oxoguanine

Hydrogen trioxide (HOOO) radical and other polyoxides of general formula, RO_nR (where R stands for hydrogen, other atoms or groups and n?≥?3), are believed to be key intermediates in atmospheric chemistry and biological oxidation reactions. In this contribution, DFT calculations using M06-2X density functional and the 6-31G(d,p) and 6-311+G(d,p) basis sets have been carried out to study different reactions of HOOO radical with guanine such as addition of HOOO radical at the C2, C4, C5, and C8 sites of guanine, abstraction of hydrogen atoms (H1, H2a, and H8) of guanine, and the mechanisms of oxidation of guanine with HOOO radical yielding 8-oxoguanine(a highly mutagenic derivative of guanine) and its radical in gas phase and aqueous media. The polarizable continuum model (PCM) has been used for solvation calculations in aqueous media. Our calculations reveal that the C8 site of guanine is the most reactive site for addition of HOOO radical, and adduct formed at this site would be appreciably stable. The rate constant (\( =\frac{K_bT}{h}{e}^{-\frac{\Delta {E}^b}{RT}} \)) at the C8 site is found to be 6.07?×?10⁷ (2.89?×?10⁷) s^?1 at the M06-2X/6-311+G(d,p) level of theory in gas phase (aqueous media). The calculated barrier energy and heat of formation of hydrogen abstraction reactions show that HOOO radical would not abstract hydrogen atoms of guanine. Oxidation of guanine with HOOO radical can occur following two schemes (Scheme 1 and Scheme 2). It is found that formation of 8-oxoguanine radical via Scheme 1 would predominate over formation of 8-oxoguanine via Scheme 2, in a reaction of HOOO radical and guanine. Thus, HOOO radical can be treated as a member of reactive oxygen species (ROS) which play key roles in biological oxidation reactions, in agreement with previous literature reports.     

Comparative studies of reactions of commercial polymers with molecular oxygen,singlet oxygen,atomic oxygen and ozone—II. Reactions with 1,2-polybutadiene

The oxidations of 1,2-polybutadiene by molecular oxygen, singlet oxygen, atomic oxygen and ozone have been studied using u.v. and i.r. spectroscopic methods. Some possible implications of the results of oxidation in the presence of singlet oxygen (parallel free radical oxidation) and atomic oxygen (formation of NO₂ and its reaction with polymer) are discussed. Crosslinking was observed during all types of oxidation. A new mechanism involving formation of free radicals has been considered in detail. During ozonization of 1,2-polybutadiene, formation of formaldehyde and formic acid were detected. An ozonization mechanism has been proposed.     

Behavior of two different constituents of natural organic matter in the removal of sodium dodecylbenzenesulfonate by O3 and O3-based advanced oxidation processes

The objective of this study was to analyze the role played by two components of natural organic matter (NOM), gallic acid (GAL) and humic acid (HUM), in the removal of the surfactant sodium dodecylbenzenesulfonate (SDBS) from waters by O(3)-based oxidation processes, i.e., O(3)/H(2)O(2), O(3)/granular activated carbon (GAC), and O(3)/powdered activated carbon (PAC). It was found that the presence of low concentrations of these compounds (1 mg/L) during SDBS ozonation increases both the ozone decomposition rate and the rate of SDBS removal from the medium. Because of the low reactivity of SDBS with ozone, these effects are mainly due to an increase in the transformation rate of ozone into HO(*) radicals. Results obtained demonstrate that the presence of GAL and HUM during SDBS ozonation increases the concentration of O(2)(-*) radicals in the medium, confirming that GAL and HUM act as initiating agents of ozone transformation into HO(*). It was also found that this effect was smaller with a larger molecular size of the acid. Presence of GAL and HUM during SDBS removal by O(3)/H(2)O(2), O(3)/GAC, and O(3)/PAC systems also increases the SDBS degradation rate, confirming the role of these compounds as initiators of ozone transformation into HO(*) radicals.     

Hemiortho esters and hydrotrioxides as the primary products in the low-temperature ozonation of cyclic acetals: an experimental and theoretical investigation

Low-temperature ozonation (-78 degrees C) of 1,3-dioxolanes 1a-1f and 1,3-dioxanes 1g and h in acetone-d6, methyl acetate, and tert-butyl methyl ether produced both the corresponding hemiortho esters (2a-h, ROH) and acetal hydrotrioxides (3a-h, ROOOH) in molar ratios ROH/ROOOH ranging from 0.5 to 23. Both types of intermediates were fully characterized by 1H, 13C, and 17O NMR spectroscopy. DFT calculations suggest that ozone abstracts a hydride ion from 1 to form an ion pair, R+ -OOOH, which subsequently collapses to either the corresponding hemiortho ester (ROH) or the acetal hydrotrioxide (ROOOH). Hemiortho esters decomposed quantitatively into the corresponding hydroxy esters. Experimentally obtained activation parameters for the decomposition of 2a (E(a) = 13.5 +/- 1.0 kcal/mol, log A = 8.3 +/- 1.0) are in accord with a highly oriented transition state involving, according to B3LYP calculations (deltaH(a)(298) = 13.2 kcal/mol), two molecules of water as a bifunctional catalyst. This mechanism is also supported by the magnitude of the solvent isotope effect for the decomposition of 2e, i.e., k(H2O)/k(D2O) = 4.6 +/- 1.2. Besides the hydroxy esters and oxygen (3O2/1O2), dihydrogen trioxide (HOOOH) was formed in the decomposition of most of the acetal hydrotrioxides (ROOOH) investigated. The activation parameters for the decomposition of the hydrotrioxides 3a-e in various solvents were E(a) = 20 +/- 2 kcal/mol, log A = 13.5 +/- 1.5. Several mechanistic possibilities for the decomposition of ROOOH were tested by experiment and theory. The formation of the hydroxy esters and oxygen could be explained by the intramolecular transfer of the proton to form the hydroxy ester. The assistance of water in the decomposition of ROOOH to form the hydroxy esters, either directly or via hemiortho esters, was also investigated. According to DFT calculations, the formation of a hydroxy ester via hemiortho ester is energetically more favorable (deltaH(a)(298) = 14.5 kcal/mol), again due to the catalytic effect of two water molecules. HOOOH generation requires the involvement of water in the decomposition of ROOOH where the direct formation out of ROOOH is energetically preferred. The energy for a reaction between two molecules of water and singlet oxygen (delta1O2) is too high to occur in solution.     

The atmospheric chemistry of hydrogen peroxide: a review

This paper reviews the atmospheric chemistry of hydrogen peroxide, taking into account the formation processes of both gas-phase and aqueous H2O2, and the reactions involving hydrogen peroxide in the gas phase and in atmospheric hydrometeors. Gas-phase hydrogen peroxide mainly forms upon dismutation of the hydroperoxyl radical, a product of the reactions between atmospheric hydrocarbons, hydroxyl radicals, nitric oxide, and oxygen. Aqueous hydrogen peroxide originates from the dissolution of the gaseous one, the reduction of molecular oxygen, a series of reactions involving dissolved ozone, and the irradiation of anthraquinones, aromatic carbonyls, and semiconductor oxides. The reactions involving aqueous H2O2 are very important in the context of the chemistry of the atmosphere. They include oxidation of S(IV) to S(VI), photolysis, the Fenton reaction in the presence of Fe(II), and possibly the formation of peroxynitrous acid. Within this framework, the correlation of hydrogen peroxide with other atmospheric components and the time trends of hydrogen peroxide in the atmosphere are easily accounted for.     

1，2-环己二羧酸及芳香含氮配体的双核镉、锰配合物的合成和晶体结构

用水热法合成了两种新的配合物[Cd2(e,e-trans-chdc)2(bipy)2(H2O)2].H2O(1)和[Mn2(e,a-cis-chdc)2(phen)2(H2O)2].2H2O(2)(chdc=1,2-环己二羧酸,bipy=2,2′-联吡啶和phen=1,10-邻菲咯啉),用X-射线单晶衍射分析确定了配合物的晶体结构。配合物1和2均为双核分子。配合物1中,2个镉髤离子由2个1,2-环己二羧酸根以e,e-trans配位方式桥联,每个镉髤离子与1个2,2′-联吡啶的2个氮原子、2个1,2-环己二羧酸根的4个氧原子及1个水分子中的氧原子配位,形成了单帽变形三棱柱构型。配合物2中,2个锰髤离子由2个1,2-环己二羧酸根以e,a-cis配位方式桥联,每个锰髤离子与1个1,10-邻菲咯啉的2个氮原子、2个1,2-环己二羧酸根的3个氧原子及1个水分子中的氧原子配位,形成了畸变的八面体构型。配合物1和2分子之间都存在π-π堆积和O-H…O、C-H…O弱作用,进而将双核分子连接成三维超分子网络结构。配合物的荧光均来自于配体的荧光。     

The ozonation of silanes and germanes: an experimental and theoretical investigation

Ozonation of various silanes and germanes produced the corresponding hydrotrioxides, R3SiOOOH and R3GeOOOH, which were characterized by 1H, 13C, 17O, and 29Si NMR, and by infrared spectroscopy in a two-pronged approach based on measured and calculated data. Ozone reacts with the E-H (E = Si, Ge) bond via a concerted 1,3-dipolar insertion mechanism, where, depending on the substituents and the environment (e.g., acetone-d6 solution), the H atom transfer precedes more and more E-O bond formation. The hydrotrioxides decompose in various solvents into the corresponding silanols/germanols, disiloxanes/digermoxanes, singlet oxygen (O2(1delta(g))), and dihydrogen trioxide (HOOOH), where catalytic amounts of water play an important role as is indicated by quantum chemical calculations. The formation of HOOOH as a decomposition product of organometallic hydrotrioxides in acetone-d6 represents a new and convenient method for the preparation of this simple, biochemically important polyoxide. By solvent variation, singlet oxygen (O2(1delta(g))) can be generated in high yield.     

Homolytic decomposition of hydrotrioxides

The rate constants and fraction of radical decomposition of the hydrotrioxides (CH₃)₂C(OH)OOOH, (CH₃)₂CHOC(CH₃)₂OOOH, and C₆H₅C(O)OOOH were determined by the free radical acceptor method. It was found that the dependence of the rate constant of radical decomposition on the nature of the solvent obeys the Koppel-Palm equation. The fraction of radical decomposition is a function of the structure of the hydrotrioxide and the temperature.Translated from Ivzestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2208–2211, October, 1989.We would like to thank N. M. Korotaev for his assistance in conducting the experiments.     

Lasagna-type arrays with halide-nitromethane cluster filling. The first recognition of the Hal(-)···HCH2NO2 (Hal = Cl, Br, I) hydrogen bonding

The previously predicted ability of the methyl group of nitromethane to form hydrogen bonding with halides is now confirmed experimentally based on X-ray data of novel nitromethane solvates followed by theoretical ab initio calculations at the MP2 level of theory. The cationic (1,3,5-triazapentadiene)Pt(II) complexes [Pt{HN=C(NC(5)H(10))N(Ph)C(NH(2))=NPh}(2)](Cl)(2), [1](Hal)(2) (Hal = Cl, Br, I), and [Pt{HN=C(NC(4)H(8)O)N(Ph)C(NH(2))=NPh}(2)](Cl)(2), [2](Cl)(2), were crystallized from MeNO(2)-containing systems providing nitromethane solvates studied by X-ray diffraction. In the crystal structure of [1][(Hal)(2)(MeNO(2))(2)] (Hal = Cl, Br, I) and [2][(Cl)(2)(MeNO(2))(2)], the solvated MeNO(2) molecules occupy vacant spaces between lasagna-type layers and connect to the Hal(-) ion through a weak hydrogen bridge via the H atom of the methyl thus forming, by means of the Hal(-)···HCH(2)NO(2) contact, the halide-nitromethane cluster "filling". The quantum-chemical calculations demonstrated that the short distance between the Hal(-) anion and the hydrogen atom of nitromethane in clusters [1][(Hal)(2)(MeNO(2))(2)] and [2][(Cl)(2)(MeNO(2))(2)] is not just a consequence of the packing effect but a result of the moderately strong hydrogen bonding.     

Intermolecular hydrogen transfer between guest species in small and large cages of methane + propane mixed gas hydrates

To investigate the molecular interaction between guest species inside of the small and large cages of methane + propane mixed gas hydrates, thermal stabilities of the methyl radical (possibly induced in small cages) and the normal propyl and isopropyl radicals (induced in large cages) were investigated by means of electron spin resonance measurements. The increase of the total amount of the normal propyl and isopropyl radicals reveals that the methyl radical in the small cage withdraws one hydrogen atom from the propane molecule enclathrated in the adjacent large cage of the structure-II hydrate. A guest species in a hydrate cage has the ability to interact closely with the other one in the adjacent cages. The clathrate hydrate may be utilized as a possible nanoscale reaction field.     

Photoinduced water splitting with oxotitanium porphyrin: a computational study

The photochemistry of the hydrogen-bonded oxotitanium porphyrin-water complex (TiOP-H(2)O) has been explored with electronic-structure calculations. It is shown that intramolecular charge-transfer processes, which are initiated by the excitation of the Soret band of TiOP, accumulate electronic charge on the oxygen atom of TiOP, which in turn abstracts a hydrogen atom from water by an exoenergetic and essentially barrierless hydrogen-transfer reaction, resulting in the TiPOH˙-OH˙ biradical. About 75% of the absorbed photon energy is thus stored as chemical energy in two ground-state radicals. Absorption of a second photon by TiPOH˙ can result in the detachment of the H˙ radical and recovery of the photocatalyzer TiOP. Again, about 75% of the photon energy is stored in the dissociation energy of TiPOH˙. Overall, a water molecule is decomposed into H˙ and OH˙ radicals by the absorption of two visible photons. Exoenergetic radical recombination reactions can yield molecular hydrogen, molecular oxygen or hydrogen peroxide as closed-shell products.