Dynamics on the HOCO potential energy surface studied by dissociative photodetachment of HOCO- and DOCO-

An experimental study of the dissociative photodetachment (DPD) dynamics of HOCO(-) and DOCO(-) at a photon energy of 3.21 eV has been carried out to probe the potential energy surface of the HOCO free radical and the dynamics of the OH+CO-->H+CO(2) reaction. These photoelectron-photofragment coincidence experiments allow the identification of photodetachment processes leading to the production of stable HOCO free radicals and both the H+CO(2) and OH+CO dissociation channels on the neutral surface. Isotopic substitution by deuterium in the parent ion is observed to reduce the product branching ratio for the D+CO(2) channel, consistent with tunneling playing a role in this dissociation pathway. Other isotope effects on the detailed partitioning of kinetic energy between photoelectrons and photofragments are also discussed. The results are compared to recent theoretical predictions of this DPD process, and evidence for the involvement of vibrationally excited HOCO(-) anions is discussed.     

Assisted intramolecular proton transfer in (uracil)2Ca2+ complexes

The structure and relative stability of the complexes between uracil dimers and Ca²⁺, as well as the proton transfer (PT) processes within these dimers, have been investigated by the density functional theory methods. Although in uracil dimers PT occurs as an almost synchronous double PT processes that connect the diketo dimer with a keto-enol dimer, the process within the most stable (uracil)₂Ca²⁺ complexes is much more complicated, and the product of the reaction looks like the result of an intramolecular PT from one of the NH groups of one monomer to one of the carbonyl groups of the same monomer. An analysis of the force profile along the reaction coordinate shows that the intimate mechanism implies three elementary steps, two intermolecular PTs, and an in-plane displacement of one monomer with respect to the other. The result of this so-called assisted intramolecular proton transfer is the formation of a dimer in which only one monomer is a keto-enol derivative, the other monomer being apparently unchanged, although it suffers significant structural rearrangements along the reaction coordinate. Quite importantly, this dimer is significantly stabilized upon Ca²⁺ association; therefore, while the most stable uracil dimers correspond systematically to associations involving only the diketo forms, in (uracil)₂Ca²⁺ complexes the most stable structures correspond to those in which one of the monomers is a keto-enol uracil isomer.     

Measurements of the kinetics of the OH‐initiated oxidation of isoprene: Radical propagation in the OH + isoprene + O2 + NO reaction system

The mechanism of the OH‐initiated oxidation of isoprene in the presence of NO and O₂ has been investigated using a discharge‐flow system at 298 K and 2 torr total pressure. OH radical concentration profiles were measured using laser‐induced fluorescence as a function of reaction time. The rate constant for the reaction of OH + isoprene was measured to be (1.10 ± 0.05) × 10⁻¹⁰ cm³ mol⁻¹ s⁻¹. In the presence of NO and O₂, regeneration of OH radicals by the reaction of isoprene‐based peroxy radicals with NO was measured and compared to simulations of the kinetics of this system. The results of these experiments are consistent with an overall rate constant of 9 × 10⁻¹² cm³ mol⁻¹ mol⁻¹ (with an uncertainty factor of 2) for the reaction of isoprene‐based hydroxyalkyl peroxy radicals with NO. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 637–643, 1999     

Energetic Ordering of Hydrogen Bond Strengths in Methanol-Water Clusters: Insights via Molecular Tailoring Approach

In this work, we examine the strength of various types of individual hydrogen bond (HB) in mixed methanol-water M_nW_m, (n+m=2 to 7) clusters, with an aim to understand the relative order of their strength, using our recently proposed molecular tailoring-based approach (MTA). Among all the types of HB, it is observed that the O_M−H…O_W HBs are the strongest (6.9 to 12.4 kcal mol⁻¹). The next ones are O_M−H…O_M HBs (6.5 to 11.6 kcal mol⁻¹). The O_W−H…O_W (0.2 to 10.9 kcal mol⁻¹) and O_W−H…O_M HBs (0.3 to 10.3 kcal mol⁻¹) are the weakest ones. This energetic ordering of HBs is seen to be different from the respective HB energies in the dimer i. e., O_M−H…O_M (5.0 to 6.0 kcal mol⁻¹)>O_W−H…O_M (1.5 to 6.0 kcal mol⁻¹)>O_M−H…O_W (3.8 to 5.6 kcal mol⁻¹)>O_W−H…O_W (1.2 to 5.0 kcal mol⁻¹). The plausible reason for the difference in the HB energy ordering may be attributed to the increase or decrease in HB strengths due to the formation of cooperative or anti-cooperative HB networks. For instance, the cooperativity contribution towards the different types of HB follows: O_M−H…O_W (2.4 to 8.6 kcal mol⁻¹)>O_M−H…O_M (1.3 to 6.3 kcal mol⁻¹)>O_W−H…O_W (−1.0 to 6.5 kcal mol⁻¹)>O_W−H…O_M (−1.2 to 5.3 kcal mol⁻¹). This ordering of cooperativity contribution is similar to the HB energy ordering obtained by the MTA-based method. It is emphasized here that, the interplay between the cooperative and anti-cooperative contributions are indispensable for the correct energetic ordering of these HBs.     

Kinetics of the gas-phase reactions of NO3 radicals and O3 with 3-methylfuran and the OH radical yield from the O3 reaction

Using relative rate methods, rate constants have been measured for the gas-phase reactions of 3-methylfuran with NO₃ radicals and O₃ at 296 ± 2 K and atmospheric pressure of air. The rate constants determined were (1.31 ± 0.461) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ for the NO₃ radical reaction and (2.05 ± 0.52) × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ for the O₃ reaction, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference reactions. Based on the cyclohexanone plus cyclohexanol yield in the presence of sufficient cyclohexane to scavenge > 95% of OH radicals formed, it is estimated that the O₃ reaction leads to the formation of OH radicals with a yield of 0.59, uncertain to a factor of ca. 1.5. In the troposphere, 3-methylfuran will react dominantly with the OH radical during daylight hours, and with the NO₃ radical during nighttime hours for nighttime NO₃ radical concentrations > 10⁷ molecule cm ⁻³. © 1996 John Wiley & Sons, Inc.     

Pulse radiolysis investigation of <Superscript><Emphasis Type="Bold">·</Emphasis></Superscript>OH and <Superscript><Emphasis Type="Bold">·</Emphasis></Superscript>H radicals initiated degradation reaction of sulfonated aromatics as model compounds for proton exchange membrane

In order to predict ^· OH and ^· H radicals initiated degradation of proton exchange membrane (PEM) based on polymers obtained by radiation-induced grafting of styrene or α-methylstyrene on different polyolefin, isopropyl benzene sulfonic acid (IPBS) and tert-butyl benzene sulfonic acid (TBBS) were taken as model compounds for PEM with and without α-H, respectively. Results indicated that ^· OH radicals played a vital role in PEM degradation. Eighty-nine percent of ^· OH radicals reacted with IPBS to form benzylic radicals by electron transfer and subsequent deprotonation reactions. Benzylic radicals can react with O₂ to form peroxyl radicals, which leads to PEM degradation. TBBS, without α-H, reacted with ^· OH or ^· H radicals, benzylic radicals would not be generated, and only adducts were formed. Adducts have less influence on the stability of TBBS than benzylic radicals. In this work, we elucidated that PEM, without α-H, with similar structures to TBBS, had a stronger resistance to oxidative degradation than PEM with α-H, with similar structures to IPBS.     

Living Carbocationic Polymerization. LIX. The Synthesis of Novel Asymmetric Telechelic Polyisobutylenes

Abstract

The synthesis of novel asymmetric telechelic polyisobutylenes (PIB) carrying a CH₃OCO— headgroup and a —CH₂C(CH₃)₂C1 tailgroup by the use of novel initiators mediating the living carbocationic polymerization (LC⁺Pzn) of isobutylene (IB) is described. Subsequently, the parent headgroup has been quantitatively converted into a HOCO— group, and the parent tailgroup into a —pC₆H₄OH group. Scheme 1 summarizes the synthesis routes to the initiators, as well as the polymerizations and functionalizations leading to various asymmetric telechelic PIBs. The CH₃OCO— headgroup of the initiator most likely functions as an internal electron donor during the LC⁺Pzn of IB.     

The kinetic isotope effect for carbon and oxygen in the reaction CO + OH

The kinetic isotope effect (KIE) for carbon and oxygen in the reaction CO + OH has been measured over a range of pressures of air and at 0.2 and 1.0 atm of oxygen, argon, and helium. The reaction was carried out with 21–86% conversion under static conditions, utilizing the photolysis of H₂O₂ as a source of OH radicals. The value of the KIE for carbon varies with pressure and the kind of ambient gas; for air the ratio of the reaction rates ¹²k/¹³k has the value 1.007 at 1.00 atm and decreases to 0.997 at 0.2 atm; for oxygen and argon over the same pressure range the values are 1.002–0.994 and 1.000–0.991, respectively. The value of the KIE for the CO oxygen atom is ¹⁶k/¹⁸k = 0.990 over the pressure range 0.2–1.0 atm and is independent of the kind of ambient gas. No exchange of the oxygen atoms in the activated complex, followed by decomposition to the starting molecules, was observed. From the mechanistic standpoint the normal KIE observed for carbon at the high pressure is attributed to the initial formation of the activated HOCO radical, whereas the inverse KIE observed at low pressures is a result of the KIE for the reverse reaction HOCO? → CO + OH being greater than that for the forward reaction HOCO^? → CO₂ + H. The derived isotopic equilibrium constant for HOCO ?CO favors the enrichment of ¹³C in the more strongly bound HOCO.     

Rate constants for the reactions of Cl atoms with HCOOH and with HOCO radicals

Rate constants have been measured at room temperature for the reactions of Cl atoms with formic acid and with the HOCO radical: Cl + HCOOH → HCl + HOCO (R1) Cl + HOCO → HCl + CO₂ (R2) Cl atoms were generated by flash photolysis of Cl₂ and the progress of reaction was followed by time‐resolved infrared absorption measurements using tunable diode lasers on the CO₂ that was formed either in the pair of reactions ( R1 ) plus ( R2 ), or in reaction ( R1 ) followed by O₂ + HOCO → HO₂ + CO₂ (R3) In a separate series of experiments, conditions were chosen so that the kinetics of CO₂ formation were dominated either by the rate of reaction ( R1 ) or by that of reactions ( R1 ) and ( R2 ) combined. The results of our analysis of these experiments yielded: k₁ = (1.8₃ ± 0.12) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ k₂ = (4.8 ± 1.0) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 85–91, 2000     

On-the-Fly Ring-Polymer Molecular Dynamics Calculations of the Dissociative Photodetachment Process of the Oxalate Anion

The dissociative photodetachment dynamics of the oxalate anion, C₂O₄H⁻ + hν → CO₂ + HOCO + e⁻, were theoretically studied using the on-the-fly path-integral and ring-polymer molecular dynamics methods, which can account for nuclear quantum effects at the density-functional theory level in order to compare with the recent experimental study using photoelectron–photofragment coincidence spectroscopy. To reduce computational time, the force acting on each bead of ring-polymer was approximately calculated from the first and second derivatives of the potential energy at the centroid position of the nuclei beads. We find that the calculated photoelectron spectrum qualitatively reproduces the experimental spectrum and that nuclear quantum effects are playing a role in determining spectral widths. The calculated coincidence spectrum is found to reasonably reproduce the experimental spectrum, indicating that a relatively large energy is partitioned into the relative kinetic energy between the CO₂ and HOCO fragments. This is because photodetachment of the parent anion leads to Franck–Condon transition to the repulsive region of the neutral potential energy surface. We also find that the dissociation dynamics are slightly different between the two isomers of the C₂O₄H⁻ anion with closed- and open-form structures.     

Infrared Multiple Photon Dissociation Spectroscopy of Hydrated Cobalt Anions Doped with Carbon Dioxide CoCO2(H2O)n−, n=1–10, in the C−O Stretch Region

We investigate anionic [Co,CO₂,nH₂O]⁻ clusters as model systems for the electrochemical activation of CO₂ by infrared multiple photon dissociation (IRMPD) spectroscopy in the range of 1250–2234 cm⁻¹ using an FT-ICR mass spectrometer. We show that both CO₂ and H₂O are activated in a significant fraction of the [Co,CO₂,H₂O]⁻ clusters since it dissociates by CO loss, and the IR spectrum exhibits the characteristic C−O stretching frequency. About 25 % of the ion population can be dissociated by pumping the C−O stretching mode. With the help of quantum chemical calculations, we assign the structure of this ion as Co(CO)(OH)₂⁻. However, calculations find Co(HCOO)(OH)⁻ as the global minimum, which is stable against IRMPD under the conditions of our experiment. Weak features around 1590–1730 cm⁻¹ are most likely due to higher lying isomers of the composition Co(HOCO)(OH)⁻. Upon additional hydration, all species [Co,CO₂,nH₂O]⁻, n≥2, undergo IRMPD through loss of H₂O molecules as a relatively weakly bound messenger. The main spectral features are the C−O stretching mode of the CO ligand around 1900 cm⁻¹, the water bending mode mixed with the antisymmetric C−O stretching mode of the HCOO⁻ ligand around 1580–1730 cm⁻¹, and the symmetric C−O stretching mode of the HCOO⁻ ligand around 1300 cm⁻¹. A weak feature above 2000 cm⁻¹ is assigned to water combination bands. The spectral assignment clearly indicates the presence of at least two distinct isomers for n ≥2.     

Contact-electro-catalysis for Direct Synthesis of H2O2 under Ambient Conditions

Hydrogen peroxide (H₂O₂) is an indispensable basic reagent in various industries, such as textile bleach, chemical synthesis, and environmental protection. However, it is challenging to prepare H₂O₂ in a green, safe, simple and efficient way under ambient conditions. Here, we found that H₂O₂ could be synthesized using a catalytic pathway only by contact charging a two-phase interface at room temperature and normal pressure. Particularly, under the action of mechanical force, electron transfer occurs during physical contact between polytetrafluoroethylene particles and deionized water/O₂ interfaces, inducing the generation of reactive free radicals (⋅OH and ⋅O₂⁻ ), and the free radicals could react to form H₂O₂, yielding as high as 313 μmol L⁻¹ h⁻¹. In addition, the new reaction device could show long-term stable H₂O₂ production. This work provides a novel method for the efficient preparation of H₂O₂, which may also stimulate further explorations on contact-electrification-induced chemistry process.     

Atmospheric Chemistry of Camphor

Rate constants have been measured at 296 ± 2 K for the gas‐phase reactions of camphor with OH radicals, NO₃ radicals, and O₃. Using relative rate methods, the rate constants for the OH radical and NO₃ radical reactions were (4.6 ± 1.2) × 10⁻¹² cm³ molecule⁻¹ s⁻¹ and <3 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹, respectively, where the indicated error in the OH radical reaction rate constant includes the estimated overall uncertainty in the rate constant for the reference compound. An upper limit to the rate constant for the O₃ reaction of <7 × 10⁻²⁰ cm³ molecule⁻¹ s⁻¹ was also determined. The dominant tropospheric loss process for camphor is calculated to be by reaction with the OH radical. Acetone was identified and quantified as a product of the OH radical reaction by gas chromatography, with a formation yield of 0.29 ± 0.04. In situ atmospheric pressure ionization tandem mass spectrometry (API‐MS) analyses indicated the formation of additional products of molecular weight 166 (dicarbonyl), 182 (hydroxydicarbonyl), 186, 187, 213 (carbonyl‐nitrate), 229 (hydroxycarbonyl‐nitrate), and 243. A reaction mechanism leading to the formation of acetone is presented, as are pathways for the formation of several of the additional products observed by API‐MS. © 2000 John Wiley and Sons, Inc. Int J Chem Kinet 33: 56–63, 2001     

Crystal structure and Hirshfeld surface analysis of a salt of antineoplastic kinase inhibitor vandetanib

A salt of vandetanib, namely, 4-({4-[(4-bromo-2-fluorophenyl)amino]-6-methoxyquinazolin-7-yl}methoxy)-1-methylpiperazin-1-ium 2-(butylamino)-4-phenoxy-6-sulfamoylbenzoate acetonitrile monosolvate, C₂₂H₂₅BrFN₄O₂⁺·C₁₇H₁₉N₂O₅S⁻·C₂H₃N, composed of kinase inhibitor vandetanib and sulfamyl diuretic bumetanide in a 1:1 molar ratio, is reported. There is proton transfer between the piperidine ring of vandetanib and the carboxyl group of bumetanide to form the salt. In the vandetanib cation, the arene and pyrimidine rings are not coplanar, their planes subtending a dihedral angle of 60.47 (14)°. The roles of the intermolecular interactions in the crystal packing were clarified using Hirshfeld surface analysis, and two-dimensional fingerprint plots indicate that the most important contributions to the crystal packing are from H…H (40.5%), O…H/H…C (20.7%), C…H/ H…C (18.8%) and N…C/C…N (9.0%) contacts.     

Direct ESR detection and spin trapping of radicals generated by reaction of oxygen radicals with sulfonated poly(ether ether ketone) (SPEEK) membranes

The stability of membranes under the strong oxidizing conditions in fuel cells is one of the major challenges in the development of fuel cells based on proton exchange membranes (PEMs). This study is centered on the determination of the susceptibility to degradation of SPEEK membranes exposed to OH radicals, using both direct ESR and spin trapping with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). In order to achieve a complete picture on SPEEK degradation, two types of experiments were performed: 1. UV irradiation at 77 K of SPEEK membranes swollen by aqueous solutions of H₂O₂; 2. UV irradiation of SPEEK membranes swollen by aqueous solutions of H₂O₂ in the presence of DMPO as a spin trap. UV irradiation without oxygen of SPEEK at 77 K in acid or basic form in the presence of H₂O₂/H₂O produced phenoxyl radicals as the predominant radicals detected by direct ESR or spin trapping methods. At pH 4, the oxygen radicals produced phenyl radicals as the predominant species detected by spin trapping methods. The hydroperoxyl radical, as DMPO/OOH adduct, was detected only when the DMPO/OH adduct was absent. The appearance of phenyl and phenoxyl radicals provides the evidence that OH radicals react with the aromatic ring of SPEEK or leading to the scission of its ether bridge.     

Radiolytic degradation of 4-nitrophenol in aqueous solutions: Pulse and steady state radiolysis study

The radiation induced degradation of 4-nitrophenol (4-NP) has been studied by gamma irradiation, while the reactivity and spectral features of the short lived transients formed by reaction with primary transient radicals at different pHs has been investigated by pulse radiolysis technique. In steady state radiolysis a dose of 4.4 k Gy is able to degrade 98% of 1×10⁻⁴ mol dm⁻³ 4-NP. 4-NP has pK_a at 7.1, above which it is present in the anionic form. At pH 5.2, ^•OH and N₃^• radicals were found to react with 4-NP with rate constants of 4.1×10⁹ dm³ mol⁻¹ s⁻¹ and 2.8×10⁸ dm³ mol⁻¹ s⁻¹, respectively. Differences in the absorption spectra of species formed in the reactions of 4-NP with ^•OH and N₃^• radicals suggested that ^•OH radicals add to the aromatic ring of 4-NP along with electron transfer reaction, whereas N₃^• radicals undergo only electron transfer reaction. At pH 9.2, rate constants for the reaction of ^•OH radicals with 4-NP was found to be higher by a factor of 2 compared to that at pH 5.2. This has been assigned to the deprotonation of 4-NP at pH 9.2.     

Screening of intramolecular hydrogen bonds in monosalts of diols and IR continuum

Solutions of salts of the diols o-xylidenedicarbinol, o-xylidenetetraethyldicarbinol and o-xylidenetetrahexyldicarbinol were studied by IR and NMR spectroscopy. Within this series the IR continuum caused by intramolecular OH…O^? ? ^?O…HO bonds vanishes, a band at 1950 cm^?1 being observed. Thus occurence of the continuum requires interaction of the hydrogen bonds with great proton polarizability with their solvent environments.     

Characterization of [M–H] cations,radicals and anions of glycine in the gas phase: a combined experimental and ab initio study

The gas phase structures of the [M–H] cations and anions of glycine have been studied by using a combination of ab initio calculations (at the MP2(FC)/6–31+G1 level of theory) and tandem mass spectrometry (MS/MS). It was found that the ab initio stability order for the anions is [H₂NCH₂CO₂]⁻ > [H₂NCHCO₂H]⁻ > [HNCH₂CO₂H]⁻. In contrast, the cations exhibit different behaviour, whereas [H₂NCHCO₂H]⁺ is predicted to be a stable structure, [H₂NCH₂CO₂]⁺ spontaneously fragments to the ion–molecule complex [H₂NCH₂⁺ ⋯ (OCO)] and the singlet [HNCH₂CO₂H]⁺ isomer is predicted to undergo a skeletal rearrangement to form [CH₂NHCO₂H]⁺. MS/MS spectra of [M–H]⁺ cations of various glycine isotopomers were obtained via: (i) collisional activation of electron impact generated cations and (ii) charge reversal of anions formed via HO⁻ negative ion chemical ionization. The resulting spectra were significantly different, suggesting different structures were involved. Neutralization–reionization experiments were performed on [M–H]⁻ anions in order to gain insights into the structures of the intermediate radicals.     

The mechanism of the OH + CO reaction and the stability of the HOCO radical

The mechanism of the reaction between OH radicals and CO is discussed in relation to recent experiments which indicate that the rate constant, k = ?(dln[OH]/dt)/[CO], depends on total pressure. It is shown that this observation is quite consistent with the known spectroscopic and thermodynamic properties of the HOCO radical, as long as the dissociation of HOCO to H + CO₂ is no faster than that to OH + CO.     

Quantum dynamics study of the dissociative photodetachment of HOCO-

Six-dimensional wave packet calculations are carried out to study the behavior of HOCO subsequent to the photodetachment of an electron from the negative anion, HOCO-. It is possible to form stable and/or long-lived HOCO complexes, as well as the dissociative products OH+CO and H+CO2. A variety of observables are determined: the electron kinetic energy (eKE) distributions associated with the OH+CO and H+CO2 channels, the correlated eKE and product translational energy distribution for the OH+CO channel, and product branching ratios. Most of our results are in good accord with the experimental results of Clements, Continetti, and Francisco [J. Chem. Phys. 117, 6478 (2002)], except that the calculated eKE distribution for the H+CO2 channel is noticeably colder than experiment. Reasons for this discrepancy are suggested.