Hydrogen transfer between sulfuric acid and hydroxyl radical in the gas phase: competition among hydrogen atom transfer, proton-coupled electron-transfer, and double proton transfer

In an attempt to assess the potential role of the hydroxyl radical in the atmospheric degradation of sulfuric acid, the hydrogen transfer between H2SO4 and HO* in the gas phase has been investigated by means of DFT and quantum-mechanical electronic-structure calculations, as well as classical transition state theory computations. The first step of the H2SO4 + HO* reaction is the barrierless formation of a prereactive hydrogen-bonded complex (Cr1) lying 8.1 kcal mol(-1) below the sum of the (298 K) enthalpies of the reactants. After forming Cr1, a single hydrogen transfer from H2SO4 to HO* and a degenerate double hydrogen-exchange between H2SO4 and HO* may occur. The single hydrogen transfer, yielding HSO4* and H2O, can take place through three different transition structures, the two lowest energy ones (TS1 and TS2) corresponding to a proton-coupled electron-transfer mechanism, whereas the higher energy one (TS3) is associated with a hydrogen atom transfer mechanism. The double hydrogen-exchange, affording products identical to reactants, takes place through a transition structure (TS4) involving a double proton-transfer mechanism and is predicted to be the dominant pathway. A rate constant of 1.50 x 10(-14) cm(3) molecule(-1) s(-1) at 298 K is obtained for the overall reaction H2SO4 + HO*. The single hydrogen transfer through TS1, TS2, and TS3 contributes to the overall rate constant at 298 K with a 43.4%. It is concluded that the single hydrogen transfer from H2SO4 to HO* yielding HSO4* and H2O might well be a significant sink for gaseous sulfuric acid in the atmosphere.     

Reactions of large water cluster anions with hydrogen chloride: formation of atomic hydrogen and phase separation in the gas phase

The reactions of water cluster anions (H2O)n-, n = 30-70, with hydrogen chloride have been studied by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The first HCl taken up by the clusters is presumably ionically dissolved. The solvated electron recombines with the proton, which is thereby reduced to atomic hydrogen and evaporates from the cluster. This process is accompanied by blackbody radiation and collision induced loss of water molecules. Subsequent collisions lead to uptake of HCl and loss of H2O, yielding mixed clusters Cl-(HCl)m(H2O)n until they are saturated with HCl. Those saturated clusters lose H2O and HCl in a characteristic sequence. The final stage of the reaction, involving clusters with m = 0-4 and n = 0-6, is studied in detail with density functional theory calculations. The Cl-(HCl)4(H2O)6 cluster represents an example for supramolecular self-organization in the gas phase: it consists of a tetrahedral Cl-(HCl)4, connected on one side of the tetrahedron to a compact water hexamer.     

Energetics and dynamics of electron transfer and proton transfer in dissociation of metal(III)(salen)-peptide complexes in the gas phase

Time- and collision energy-resolved surface-induced dissociation (SID) of ternary complexes of Co(III)(salen)+, Fe(III)(salen)+, and Mn(III)(salen)+ with several angiotensin peptide analogues was studied using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved fragmentation efficiency curves (TFECs) were modeled using an RRKM-based approach developed in our laboratory. The approach utilizes a very flexible analytical expression for the internal energy deposition function that is capable of reproducing both single-collision and multiple-collision activation in the gas phase and excitation by collisions with a surface. The energetics and dynamics of competing dissociation pathways obtained from the modeling provides important insight on the competition between proton transfer, electron transfer, loss of neutral peptide ligand, and other processes that determine gas-phase fragmentation of these model systems. Similar fragmentation behavior was obtained for various Co(III)(salen)-peptide systems of different angiotensin analogues. In contrast, dissociation pathways and relative stabilities of the complexes changed dramatically when cobalt was replaced with trivalent iron or manganese. We demonstrate that the electron-transfer efficiency is correlated with redox properties of the metal(III)(salen) complexes (Co > Fe > Mn), while differences in the types of fragments formed from the complexes reflect differences in the modes of binding between the metal-salen complex and the peptide ligand. RRKM modeling of time- and collision-energy-resolved SID data suggests that the competition between proton transfer and electron transfer during dissociation of Co(III)(salen)-peptide complexes is mainly determined by differences in entropy effects while the energetics of these two pathways are very similar.     

Formation and reactivity of metal carbonyl anions in the gas phase

The reactions of metal carbonyl anions (M(CO)_n^?; M = Cr, Mn and Fe; n = 1–3) with n-heptane, water and methanol were studied with use of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an external ion source. The M(CO)_n^? ions were formed in the FT-ICR cell by collision-induced dissociation of the most abundant primary ion generated by electron impact of the appropriate metal carbonyl compound present in the external ion source. The M(CO)_n^? ions were allowed subsequently to undergo non-reactive collisions with argon in order to remove possible excess internal/translational energy prior to the ion/molecule reaction. Only the Cr(CO)₃^?, Mn(CO)₃^? and Fe(CO)₂^? ions react with n-heptane. This reaction proceeds by loss of H₂ from the collision complex and the Cr(CO)₃^? and Fe(CO)₂^? ions react about three times more efficiently than the Mn(CO)₃^? ion. With water, Mn(CO)^? and Fe(CO)₃^? are unreactive, whereas the other ions react by loss of one or two CO molecules from the collision complex. The rate of the reaction with water decreases in the order Cr(CO)₃^?, Fe(CO)₂^?, Cr(CO)₂^?, Fe(CO)^?, Mn(CO)₃^? and Mn(CO)₂^?. With methanol, the Cr(CO)₂^? ion reacts by loss of two CO molecules from the collision complex, whereas loss of one CO molecule and elimination of CO + H₂ occur in the reaction with Cr(CO)₃^?. Competing loss of CO and one or two H₂ molecules occurs in the reactions of Mn(CO)₃^? and Fe(CO)₂^? with methanol. The rate of the reaction with methanol decreases in the order Cr(CO)₃^?, Fe(CO)₂^?, Cr(CO)₂^? and Mn(CO)₃^?.     

Thermochemical aspects of proton transfer in the gas phase

The beginning of the twentieth century saw the development of new theories of acidity and basicity, which are currently well accepted. The thermochemistry of proton transfer in the absence of solvent attracted much interest during this period, because of the fundamental importance of the process. Nevertheless, before the 1950s, few data were available, either from lattice energy evaluations or from calculations using the emerging molecular orbital theory. Advances in mass spectrometry during the last 40 years allowed studies of numerous systems with better accuracy. Thousands of accurate gas-phase acidities or basicities are now available, for simple atomic and molecular systems and for large biomolecules. The intrinsic effect of structure on the Br?nsted basic or acidic properties of molecules and the influence of solvents have been unravelled. In this tutorial, the basics of the thermodynamic principles involved are given, and the mass spectrometric techniques are briefly reviewed. Advances in the design and measurements of gas-phase superacids and superbases are described. Recent studies concerning biomolecules are also evoked.     

Complex mechanism of the gas phase reaction between formic acid and hydroxyl radical. Proton coupled electron transfer versus radical hydrogen abstraction mechanisms

The gas phase reaction between formic acid and hydroxyl radical has been investigated with high level quantum mechanical calculations using DFT-B3LYP, MP2, CASSCF, QCISD, and CCSD(T) theoretical approaches in connection with the 6-311+G(2df,2p) and aug-cc-pVTZ basis sets. The reaction has a very complex mechanism involving several elementary processes, which begin with the formation of a reactant complex before the hydrogen abstraction by hydroxyl radical. The results obtained in this investigation explain the unexpected experimental fact that hydroxyl radical extracts predominantly the acidic hydrogen of formic acid. This is due to a mechanism involving a proton coupled electron-transfer process. The calculations show also that the abstraction of formyl hydrogen has an increased contribution at higher temperatures, which is due to a conventional hydrogen abstraction radical type mechanism. The overall rate constant computed at 298 K is 6.24 x 10(-13) cm3 molecules(-1) s(-1), and compares quite well with the range from 3.2 +/- 1 to 4.9 +/- 1.2 x 10(-13) cm3 molecules(-1) s(-1), reported experimentally.     

Reactions of simple and peptidic alpha-carboxylate radical anions with dioxygen in the gas phase

α-Carboxylate radical anions are potential reactive intermediates in the free radical oxidation of biological molecules (e.g., fatty acids, peptides and proteins). We have synthesised well-defined α-carboxylate radical anions in the gas phase by UV laser photolysis of halogenated precursors in an ion-trap mass spectrometer. Reactions of isolated acetate (˙CH(2)CO(2)(-)) and 1-carboxylatobutyl (CH(3)CH(2)CH(2)˙CHCO(2)(-)) radical anions with dioxygen yield carbonate (CO(3)˙(-)) radical anions and this chemistry is shown to be a hallmark of oxidation in simple and alkyl-substituted cross-conjugated species. Previous solution phase studies have shown that C(α)-radicals in peptides, formed from free radical damage, combine with dioxygen to form peroxyl radicals that subsequently decompose into imine and keto acid products. Here, we demonstrate that a novel alternative pathway exists for two α-carboxylate C(α)-radical anions: the acetylglycinate radical anion (CH(3)C(O)NH˙CHCO(2)(-)) and the model peptide radical anion, YGGFG˙(-). Reaction of these radical anions with dioxygen results in concerted loss of carbon dioxide and hydroxyl radical. The reaction of the acetylglycinate radical anion with dioxygen reveals a two-stage process involving a slow, followed by a fast kinetic regime. Computational modelling suggests the reversible formation of the C(α) peroxyl radical facilitates proton transfer from the amide to the carboxylate group, a process reminiscent of, but distinctive from, classical proton-transfer catalysis. Interestingly, inclusion of this isomerization step in the RRKM/ME modelling of a G3SX level potential energy surface enables recapitulation of the experimentally observed two-stage kinetics.     

Ultrafast electron dynamics at ice-metal interfaces: competition between heterogeneous electron transfer and solvation

Microscopic insight into heterogeneous electron transfer requires an understanding of the participating donor and acceptor states and of their respective interaction. In the regime of strong electronic coupling, two limits have been discussed where either the states overlap directly or the states are separated by a potential barrier. In both situations, the transfer probability is determined by the magnitude of the wave function overlap, whereby in the case of the potential barrier, its width and height are rate limiting. In our study, we observe a dynamical crossover between these two regimes by investigating the electron-transfer dynamics of localized, solvated electrons at ice-metal interfaces. Employing femtosecond time-resolved two-photon photoelectron spectroscopy, we analyze the population dynamics of excess electrons in the ice layer, which experience the competing processes of transfer to the metal electrode and energetic stabilization in the ice by molecular reorientation. Comparing the dynamics of D(2)O on Cu(111) and Ru(001), we observe an early regime at t < 300 fs, where the transfer time is determined by wave-function overlap with the metal and a second regime (t > 300 fs), where the transfer proceeds nearly independent of the substrate. The assignment of these two regimes to the established mechanisms of electron transfer is backed by an empirical model calculation that reproduces the experimental data in an excellent manner.     

Carbon-to-carbon identity proton transfer from allene, ketene, ketenimine, and thioketene to their respective conjugate anions in the gas phase. An ab initio study.

Gas-phase acidities of CH2=C=X (X = CH2, NH, O, and S) and barriers for the identity proton transfers (X=C=CH2 + HC triple bond C-X- right harpoon over left harpoon -X-C triple bond CH + CH2=C=X) as well as geometries and charge distributions of CH2=C=X, HC triple bond C-X- and the transition states of the proton transfer were determined by ab initio methods at the MP2/6-311+G(d,p)//MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The acidities were also calculated at the CCSD(T)/6-311+G(2df,p) level. A major objective of this study was to examine how the enhanced unsaturation of CH2=C=X compared to that of CH3CH=X may affect acidities, transition state imbalances, and intrinsic barriers of the identity proton transfer. The results show that the acidities are all higher while the barriers are lower than for the corresponding CH3CH=X series. The transition states are all imbalanced but less so than for the reactions of CH3CH=X.     

Reactions of first-row transition metal ions with propargyl alcohol in the gas phase

The gas-phase reactions with propargyl alcohol (PPA) of all the singly charged ions of the first-row transition metals, generated by laser ablation in an external ion source, were studied by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS.). The reactivities of the metal ions change irregularly across the periodic table, and the reactivity of each ion is a function of its electronic configuration and corresponding metal-oxygen (M-O) bond energies. The 10 metal ions were classified into three categories according to their reactivities: Sc(+), Ti(+) and V(+) are the most reactive ions which react with PPA to give many kinds of oxygen-rich products due to stronger M-O bonds; Fe(+), Co(+) and Ni(+) are less reactive; Cr(+), Mn(+), Cu(+) and Zn(+) are the most unreactive ions, due to the half and completely occupied valence electronic configurations. The order of reactivity is Ti(+) > V(+) > Sc(+) > Co(+) > Fe(+) approximately Ni(+) > Zn(+) > Cr(+) approximately Mn(+) approximately Cu(+).     

Relative proton transfer abilities of acids and alcohols in gas phase and solutions

The present paper applies a method, based on SCF CNDO MO charge densities and assumption of the validity of the virial theorem, for calculation of relative proton affinities of alcohols, and substituted and unsubstituted carboxylic acids in the gas phase. The paper also chalks out a path for calculation of their relative acidities in solution phase by utilising a solvation energy equation and binding energy data in the gas phase. The results obtained by the present method are mostly in agreement with ion cyclotron resonance mass spectrometric experimental studies. The method has also been applied to cover the cases of amines in the gas phase.     

Unexpected reactions of ferrocene acetal derived from tartaric acid with alkyllithium: competition between proton abstraction and nucleophilic attack

We wished to prepare planar chiral compounds by the lithiation of acetal 2-ferrocenyl-(4S,5S)-bis(methoxymethyl)-1,3-dioxolane (1) with butyllithium followed by the reaction with an electrophile. However, the desired products were not observed and two unexpected products, 1-ferrocenyl-1-pentanol (4) of the nucleophilic attack product and 2-ferrocenyl-4,5-dimethylene-1,3-dioxolane (5) of the proton abstraction product, were isolated. Because the nucleophilic attack on acetal carbon is rarely reported so far and both products 4 and 5 may have some potential uses in organic synthesis, these unexpected reactions are investigated in detail. The mechanisms of these reactions are discussed.     

Theoretical prediction of proton and electron affinities,gas phase basicities,and ionization energies of sulfinamides

Structural Chemistry - Sulfinamides, as an asymmetric synthesizer, especially in drug synthesis, play critical roles in organic chemistry. In this study, the gas phase ion energetics data including...     

N-NO bond dissociation energies of N-nitroso diphenylamine derivatives (or analogues) and their radical anions: implications for the effect of reductive electron transfer on N-NO bond activation and for the mechanisms of NO transfer to nitranions

The heterolytic and homolytic N-NO bond dissociation energies [i.e., deltaHhet(N-NO) and deltaHhomo(N-NO)] of 12 N-nitroso-diphenylamine derivatives (1-12) and two N-nitrosoindoles (13 and 14) in acetonitrile were determined by titration calorimetry and from a thermodynamic cycle, respectively. Comparison of these two sets of data indicates that homolysis of the N-NO bonds to generate NO* and nitrogen radical is energetically much more favorable (by 23.3-44.8 kcal/mol) than the corresponding heterolysis to generate a pair of ions, giving hints for the driving force and possible mechanism of NO-initiated chemical and biological transformations. The first (N-NO)-* bond dissociation energies [i.e., deltaH(N-NO)-* and deltaH'(N-NO)-*] of radical anions 1-*-14-* were also derived on the basis of appropriate cycles utilizing the experimentally measured deltaHhet(N-NO) and electrochemical data. Comparisons of these two quantities with those of the neutral N-NO bonds indicate a remarkable bond activation upon a possible one-electron transfer to the N-NO bonds, with an average bond-weakening effect of 48.8 +/- 0.3 kcal/mol for heterolysis and 22.3 +/- 0.3 kcal/mol for homolysis, respectively. The good to excellent linear correlations among the energetics of the related heterolytic processes [deltaHhet(N-NO), deltaH(N-NO)-*, and pKa(N-H)] and the related homolytic processes [deltaHhomo(N-NO), deltaH'(N-NO)-*, and BDE(N-H)] imply that the governing structural factors for these bond scissions are similar. Examples illustrating the use of such bond energetic data jointly with relevant redox potentials for analyzing various mechanistic possibilities for nitrosation of nitranions are presented.     

气相VO（∑＋）与CH3OH（1^A′）分子自旋禁阻反应C—H，O—H键活化机理的理论研究

采用密度泛函理论（DFT）B3LYP与cCsD方法研究了二重态和四重态势能面自旋禁阻反应VO（∑’）活化cH30H（1^A′）分子c—H,0—H键的微观机理．通过自旋一轨道耦合的计算讨论了势能面交叉点和可能的自旋翻转过程．在MEcP处,四重态和二重态问的旋轨耦合常数为131．14cm^-1．自旋多重度发生改变,从四重态系间穿越到二重态势能面形成中间体2^IM1,导致反应势能面的势垒明显降低．     

Dynamics of charge transfer states on metal surfaces: the competition between reactivity and quenching

The dynamics of excited states of adsorbates on surfaces caused by charge transfer is studied. Both negative and positive charge transfer processes are possible. In particular we are interested in positive charge transfer from a metal surface to molecular or atomic oxygen adsorbed on the surface. Once the negatively charged oxygen on the surface loses an electron it becomes chemically activated. The ability of this species to react depends on the quenching time or back transfer. The analysis of these processes is based on a set of diabatic potential energy surfaces each representing a different charged oxygen species. The dynamics is followed by solving the multichannel time-dependent Schr?dinger equation or Liouville von Neumann equation. Due to the nonadiabatic character of these reactions large isotope effects are predicted.     

Intramolecular proton transfer induced by divalent alkali earth metal cation in the gas state

Interactions between divalent alkali earth metal (DAEM) ions M (M?Be, Mg, Ca, Sr, Ba) and the second stable glycine conformer in the gas phase, which can transfer into the ground‐state glycine‐M²⁺ (except the glycine–Be²⁺) among each corresponding isomers when these divalent metal ions are bound, are studied at the hybrid three‐parameter B3LYP level with three different basis sets. Proton transfers from the hydroxyl to the amino nitrogen of the glycine without energy barriers have been first observed in the gas phase in these glycine–M²⁺ systems. The interaction between the glycine and these DAEM ions except beryllium and magnesium ion only create an amino hydrogen pointing to the original hydroxyl due to their weaker interaction relative to those divalent transition metal (DTM) ion‐bound glycine derivatives, being obviously different from that between the glycine and DTM ions, in which two amino hydrogens point to the original hydroxyl oxygen when these metal‐chelated glycine derivatives are produced. The interaction energy between the glycine and divalent magnesium would be the boundary of one or two amino hydrogens pointing to the hydrogyl oxygen, i.e., the ?170.3 kcal/mol of binding energy is a critical point. Similar intramolecular proton transfer has also been predicted for those DTM ion‐chelated glycine systems; however, that in the gas state has not been observed in the monovalent metal ion‐coordinated glycine systems. The binding energy between some monovalent TM ion and the glycine is similar to that of the glycine–Ba²⁺, which has the lowest binding strength among these DAEM–ion chelated glycine complexes. The difference among them only lies in the larger electrostatic and polarized effects in the latter, which favor the stability of the zwitterionic glycine form in the gas phase. According to these observations, we predict that the zwitterionic glycine would exist in the field of two positive charges in the gas phase. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 205–214, 2003     

Hydrogen bonding and dissociation effects on the gas phase proton transfer reactions of ozone

Recently, the proton affinity (PA) of ozone was experimentally determined by Cacace and Speranza [Science (1994) 265: 208] using a bracketing technique that involved the proton transfer (PT) reactions: O₃H⁺+B⇒O₃+BH⁺; for different Br?nsted bases B. These authors showed that the simple collision model is not adequate to describe PT. We now present a theoretical model reflecting this bracketing procedure by explicitly introducing H-bonding complexing, dissociation and PT contributions, to discuss the kinetic model that assumes that PT occurs through one elementary step. The methods used include semiempirical density functional theory and ab initio Hartree-Fock methods. The procedure is gauged by using estimated PA of ozone obtained from deprotonation reactions including the Br?nsted bases BNH₃, H₂O, HOCl, SO₂, CH₃F and Kr. The PA-obtained range was from 145.3 to 160.3 kcal/mol, in fair agreement with the experimental value of 148.0±3 kcal/mol. The model seems to be fairly independent of the reference bases used to evaluate the PA. H-bonding effects appear to be a determining factor to explain collision efficiencies. Received: 5 August 1997 / Accepted: 25 September 1997     

Ion-ion reactions in the gas phase: Proton transfer reactions of protonated pyridine with multiply charged oligonucleotide anions

Isolated triply and doubly charged anions of the single-stranded deoxynucleotide 5′-d(AAAA)-3′ were allowed to undergo ion-ion proton transfer reactions with protonated pyridine cations within a quadrupole ion trap mass spectrometer. Sufficiently high ion number densities and spatial overlap of the oppositely charged ion clouds could be achieved to yield readily measurable rates. Three general observations were made: (1) the ion-ion reaction rate constants were estimated to be 10^{? (7 ? 8)} cm³ ion^?1 s^?1; (2) the ion-ion reaction rates were found to be dependent on the reactant ion number density, which could be controlled by both the reactant ion number and the pseudopotential well depth, and (3) very little fragmentation, if any, was observed, as might normally be expected with highly exothermic proton transfer reactions.