Influence of aggregation, defects, and contaminant oxygen on water dissociation at Cu(110) surface: a theoretical study

The DFT-PW91 slab model approach is employed to investigate the influence of aggregation, surface defects, and contaminant oxygen on water dissociation on Cu(110) at low temperatures. The dissociation barriers of water in various aggregate states are calculated in the range of 60-75 kJ/mol on the clean surfaces, in nice agreement with the experimentally determined values. It is revealed that the aggregation of water shows no propensity to reduce the activation barrier for the O-H bond breaking on Cu(110), at variance with the water chemistry on Ru(0001). The calculated activation energy on Cu(211) which is the most active stepped surface investigated is equal to the value on the (110) surface, indicating that the hydroxyl groups observed on Cu(110) at low temperatures may not stem from surface defects. The coadsorbed oxygen, whether as a "spectator" or a "participant," facilitates the water dissociation both kinetically and thermodynamically.     

The nature of the AgI...AgI interaction in different Ag(NH3)2 dimers embedded in supramolecular solids

An isolated silver(I) ammonia monomer, a dimer, and a novel dimer containing an intercalated water molecule have been embedded as guests in supramolecular frameworks, [Ag(NH3)2][(H2thpe)(H3thpe)].MeCN (1), [{Ag(NH3)2}2][(H2thpe)2]4.25 H2O (2), and [{Ag(NH3)2}-H2O-{Ag(NH3)2}][(H2thpe)(2)]benzene (3) (H3THPE=tris(hydroxyphenyl)ethane). The [{Ag(NH3)2}2]2+ dimer is not stable as an isolated entity, but is stabilized by hydrogen bonding in the supramolecular framework. The water-intercalated silver(I) ammonia dimer, which constitutes a novel species, is also subject by hydrogen bonding in concentrated solutions. The destabilization energy of the dimer relative to isolated monomers is calculated to be approximately 300 kJ mol(-1) by both perturbation methods and DFT theory. For the water-intercalated dimer it is calculated to be approximately 200 kJ mol(-1) according to the BSSE-corrected MP2 calculation. The different aggregate states show a dramatic variation of absorption and emission properties, in accordance with the concentration dependent red-shift observed in solutions. Natural-bond-orbital analysis shows that the disilver-ammonium-aquo "sandwich" cation in 3 is stabilized by interaction between the pi lone pair orbital on the oxygen atom of the water molecule and Ag(I)--N sigma antibonding molecular orbital.     

Role of the direct reaction H2S + SO2 in the homogeneous Claus reaction

Quantum chemical methods at the Gaussian-2 and -3 levels of theory have been used to investigate the reactions between H(2)S, SO(2), and S(2)O such as might occur in the front-end furnace of the Claus process. The direct reaction between H(2)S and SO(2) occurs via a 5-centered transition state with an initial barrier of approximately 135 kJ mol(-1) and an overall barrier of approximately 153 kJ mol(-1) to produce S(2)O and H(2)O. We indicate approximate values here because there are a number of isomers in the reaction pathway that have barriers slightly different from those quoted. The presence of a water molecule lowers this by approximately 60 kJ mol(-1), but the van der Waals complex required for catalysis by water is thermodynamically unfavorable under the conditions in the Claus reactor. The direct reaction between H(2)S and S(2)O can occur via two possible pathways; the analogous reaction to H(2)S + SO(2) has an initial barrier of approximately 117 kJ mol(-1) and an overall barrier of approximately 126 kJ mol(-1) producing S(3) and H(2)O, and a pathway with a 6-centred transition state has a barrier of approximately 111 kJ mol(-1), producing HSSSOH. Rate constants, including a QRRK analysis of intermediate stabilization, are reported for the kinetic scheme proposed here.     

Mechanistic study of the deamination reaction of guanine: a computational study

The mechanism for the deamination of guanine with H(2)O, OH(-), H(2)O/OH(-) and for GuaH(+) with H(2)O has been investigated using ab initio calculations. Optimized geometries of the reactants, transition states, intermediates, and products were determined at RHF/6-31G(d), MP2/6-31G(d), B3LYP/6-31G(d), and B3LYP/6-31+G(d) levels of theory. Energies were also determined at G3MP2, G3MP2B3, G4MP2, and CBS-QB3 levels of theory. Intrinsic reaction coordinate (IRC) calculations were performed to characterize the transition states on the potential energy surface. Thermodynamic properties (ΔE, ΔH, and ΔG), activation energies, enthalpies, and Gibbs free energies of activation were also calculated for each reaction investigated. All pathways yield an initial tetrahedral intermediate and an intermediate in the last step that dissociates to products via a 1,3-proton shift. At the G3MP2 level of theory, deamination with OH(-) was found to have an activation energy barrier of 155 kJ mol(-1) compared to 187 kJ mol(-1) for the reaction with H(2)O and 243 kJ mol(-1) for GuaH(+) with H(2)O. The lowest overall activation energy, 144 kJ mol(-1) at the G3MP2 level, was obtained for the deamination of guanine with H(2)O/OH(-). Due to a lack of experimental results for guanine deamination, a comparison is made with those of cytosine, whose deamination reaction parallels that of guanine.     

Structure and reactivity of V(2)O(5): bulk solid, nanosized clusters, species supported on silica and alumina, cluster cations and anions

Vanadyl bond dissociation energies are calculated by density functional theory (DFT). While the hybrid (B3LYP) functional results are close to the available reference data, gradient corrected functionals (BP86, PBE) yield large errors (about 50 to 100 kJ mol(-1)), but reproduce trends correctly. PBE calculations on a V(20)O(62)H(24) cluster model for the (001) surface of V(2)O(5) crystals virtually reproduce periodic slab calculations. The low bond dissociation energy (formation of oxygen surface defect) of 113 kJ mol(-1)(B3LYP) is due to substantial structure relaxations leading to formation of V-O-V bonds between the V(2)O(5) layers of the crystal. This relaxation cannot occur in polyhedral (V(2)O(5))(n) clusters and also not for V(2)O(5) species supported on silica or alumina (represented by cage-type models) for which bond dissociation energies of 250-300 kJ mol(-1) are calculated. The OV(OCH(3))(3) molecule and its dimer are also considered. Radical cations V(2)O(5)(+) and V(4)O(10)(+) have very low bond dissociation energies (22 and 14 kJ mol(-1), respectively), while the corresponding radical anions have higher dissociation energies (about 330 kJ mol(-1)) than the neutral clusters. The bond dissociation energies of the closed shell V(3)O(7)(+) cation (165 kJ mol(-1)) and the closed shell V(3)O(8)(-) anion (283 kJ mol(-1)) are closest to the values of the neutral clusters. This makes them suitable for gas phase studies which aim at comparisons with V(2)O(5) species on supporting oxides.     

Enthalpies of formation, bond dissociation energies and reaction paths for the decomposition of model biofuels: ethyl propanoate and methyl butanoate

The complete basis set method CBS-QB3 has been used to study the thermochemistry and kinetics of the esters ethyl propanoate (EP) and methyl butanoate (MB) to evaluate initiation reactions and intermediate products from unimolecular decomposition reactions. Using isodesmic and isogeitonic equations and atomization energies, we have estimated chemically accurate enthalpies of formation and bond dissociation energies for the esters and species derived from them. In addition it is shown that controversial literature values may be resolved by adopting, for the acetate radical, CH3C(O)O(.-), DeltaH(o)(f)298.15K) = -197.8 kJ mol(-1) and for the trans-hydrocarboxyl radical, C(.-)(O)OH, -181.6 +/- 2.9 kJ mol(-1). For EP, the lowest energy decomposition path encounters an energy barrier of approximately 210 kJ mol(-1) (approximately 50 kcal mol(-1)), which proceeds through a six-membered ring transition state (retro-ene reaction) via transfer of the primary methyl H atom from the ethyl group to the carbonyl oxygen, while cleaving the carbon-ether oxygen to form ethene and propanoic acid. On the other hand, the lowest energy path for MB has a barrier of approximately 285 kJ mol(-1), producing ethene. Other routes leading to the formation of aldehydes, alcohols, ketene, and propene are also discussed. Most of these intramolecular hydrogen transfers have energy barriers lower than that needed for homolytic bond fission (the lowest of which is 353 kJ mol(-1) for the C(alpha)-C(beta) bond in MB). Propene formation is a much higher energy demanding process, 402 kJ mol(-1), and it should be competitive with some C-C, C-O, and C-H bond cleavage processes.     

The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer-Tropsch synthesis: a quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study

The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H(2) at 303 K, whereas the presence of H(2) induced CO dissociation at higher temperatures leading to the formation of CH(4) and H(2)O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO(2) formation according to C(ads) + 2O(ads)→ CO(2(g)). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol(-1), whereas it increased to 110 kJ mol(-1) on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol(-1), which increased to 225 kJ mol(-1) in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites.     

Mechanistic studies on the reversible binding of nitric oxide to metmyoglobin.

The ferriheme protein metmyoglobin (metMb) in buffer solution at physiological pH 7.4 reversibly binds the biomessenger molecule nitric oxide to yield the nitrosyl adduct (metMb(NO)). The kinetics of the association and dissociation processes were investigated by both laser flash photolysis and stopped-flow kinetics techniques at ambient and high pressure, in three laboratories using several different sources of metMb. The activation parameters DeltaH, DeltaS, and DeltaV were calculated from the kinetic effects of varying temperature and hydrostatic pressure. For the "on" reaction of metMb plus NO, reasonable agreement was found between the various techniques with DeltaH(on), DeltaS(on), and DeltaV(on) determined to have the respective values approximately 65 kJ mol(-1), approximately 60 J mol(-1) K(-1), and approximately 20 cm(3) mol(-1). The large and positive DeltaS and DeltaV values are consistent with the operation of a limiting dissociative ligand substitution mechanism whereby dissociation of the H(2)O occupying the sixth distal coordination site of metMb must precede formation of the Fe-NO bond. While the activation enthalpies of the "off" reaction displayed reasonable agreement between the various techniques (ranging from 68 to 83 kJ mol(-1)), poorer agreement was found for the DeltaS(off) values. For this reason, the kinetics for the "off" reaction were determined more directly via NO trapping experiments, which gave the respective activation parameters DeltaH(off) = 76 kJ mol(-1), DeltaS(off) = approximately 41 J mol(-1) K(-1), and DeltaV(off) = 20 cm(3) mol(-1)), again consistent with a limiting dissociative mechanism. These results are discussed in reference to other investigations of the reactions of NO with both model systems and metalloproteins.     

Calorimetric and computational study of thiacyclohexane 1-oxide and thiacyclohexane 1,1-dioxide (thiane sulfoxide and thiane sulfone). Enthalpies of formation and the energy of the S=O bond

A rotating-bomb combustion calorimeter specifically designed for the study of sulfur-containing compounds [J. Chem. Thermodyn. 1999, 31, 635] has been used for the determination of the enthalpy of formation of thiane sulfone, 4, Delta(f)H(o) m(g) = -394.8 +/- 1.5 kJ x mol(-1). This value stands in stark contrast with the enthalpy of formation reported for thiane itself, Delta(f)H(o) m(g) = -63.5 +/- 1.0 kJ x mol(-1), and gives evidence of the increased electronegativity of the sulfur atom in the sulfonyl group, which leads to significantly stronger C-SO2 bonds. Given the known enthalpy of formation of atomic oxygen in the gas phase, Delta(f)H(o) m(O,g) = +249.18 kJ x mol(-1), and the reported bond dissociation energy for the S=O bond in alkyl sulfones, BDE(S=O) = +470.0 kJ x mol(-1), it was possible to estimate the enthalpy of formation of thiane sulfoxide, 5, a hygroscopic compound not easy to use in experimental calorimetric measurements, Delta(f)H(o) m(5) = -174.0 kJ x mol(-1). The experimental enthalpy of formation of both 4 and 5 were closely reproduced by theoretical calculations at the G2(MP2)+ level, Delta(f)H(o) m(4) = -395.0 kJ x mol(-1) and Delta(f)H(o) m(5) = -178.0 kJ x mol(-1). Finally, calculated G2(MP2)+ values for the bond dissociation energy of the S=O bond in cyclic sulfoxide 5 and sulfone 4 are +363.7 and +466.2 kJ x mol(-1), respectively.     

Nitrosation, thiols, and hemoglobin: energetics and kinetics

Nitrosothiols are powerful vasodilators. Although the mechanism of their formation near neutral pH is an area of intense research, neither the energetics nor the kinetics of this reaction or of subsequent reactions have been addressed. The following considerations may help to guide experiments. (1) The standard Gibbs energy for the homolysis reaction RSNO → RS(?) + NO(?)(aq) is +110 ± 5 kJ mol(-1). (2) The electrode potential of the RSNO, H(+)/RSH, NO(?)(aq) couple is -0.20 ± 0.06 V at pH 7. (3) Thiol nitrosation by NO(2)(-) is favorable by 37 ± 5 kJ mol(-1) at pH 7. (4) N(2)O(3) is not involved in in vivo nitrosation mechanisms for thermodynamic--its formation from NO(2)(-) costs 59 kJ mol(-1)--or kinetic--the reaction being second-order in NO(2)(-)--reasons. (5) Hemoglobin (Hb) cannot catalyze formation of N(2)O(3), be it via the intermediacy of the reaction of Hb[FeNO(2)](2+) with NO(?) (+81 kJ mol(-1)) or reaction of Hb[FeNO](3+) with NO(2)(-) (+88 kJ mol(-1)). (6) Energetically and kinetically viable are nitrosations that involve HNO(2) or NO(?) in the presence of an electron acceptor with an electrode potential higher than -0.20 V. These considerations are derived from existing thermochemical and kinetics data.     

Hydrogen-atom transfer from transition metal hydroperoxides, hydrogen peroxide, and alkyl hydroperoxides to superoxo and oxo metal complexes

Superoxochromium(III) complexes L(H2O)CrOO2+ (L = (H2O)4 and 1,4,8,11-tetraazacyclotetradecane) oxidize hydroperoxo complexes of rhodium and cobalt in an apparent hydrogen-atom transfer process, i.e., L(H2O)CrOO2+ + L(H2O)RhOOH2+ --> L(H2O)CrOOH2+ + L(H2O)RhOO2+. All of the measured rate constants fall in a narrow range, 17-135 M-1 s-1. These values are about 2.5-3.0 times smaller in D2O, where the hydroperoxo hydrogen is replaced by deuterium, and coordinated molecules of water by D2O. The failure of the back reaction to take place in the available concentration range places the O-H bond dissociation energy in RhOO-H2+ at or=80 kJ/mol) in the driving force for the two types of reactions. A chromyl ion, CrIVaqO2+, oxidizes L(H2O)RhOOH2+ and the cobalt analogs to the corresponding superoxo complexes. The rate constants are approximately 102-fold larger than those for the oxidation by CraqOO2+. The oxidation of tert-BuOOH by CrIVaqO2+ has k = 160 M-1 s-1 and exhibits an isotope effect kBuOOH/kBuOOD = 12. Hydrogen atom transfer from H2O2 to CraqOO2+ is slow, k approximately 10-3 M-1 s-1.     

碘和氧修饰银(110)表面对甲醇吸附的影响: 密度 泛函理论的计算研究

用密度泛函理论(DFT)的B3-LYP方法和原子簇模型研究了碘和修饰银(110)表面对甲醇吸附的影响。结果表明,甲醇分子在干净的银表面吸附很弱甚至不吸附,但在氧或碘修饰过的银表面上,由于预吸附导致吸附能的增加而变得容易吸附。并进一步采用目前较新的映像电荷模型计算验证了在甲醇部分氧化制甲醛反应中氧或碘对银催化剂表面修饰的本质是电荷修饰这一推论,为实验中如何筛选修饰提供了良好的判据。     

Mechanisms for the deamination reaction of cytosine with H2O/OH(-) and 2H2O/OH(-): a computational study

Mechanisms for the deamination reaction of cytosine with H 2O/OH (-) and 2H 2O/OH (-) to produce uracil were investigated using ab initio calculations. Optimized geometries of reactants, transition states, intermediates, and products were determined at MP2 and B3LYP using the 6-31G(d) basis set and at B3LYP/6-31+G(d) levels of theory. Single point energies were also determined at MP2/G3MP2Large and G3MP2 levels of theory. Thermodynamic properties (Delta E, Delta H, and Delta G), activation energies, enthalpies, and free energies of activation were calculated for each reaction pathway investigated. Intrinsic reaction coordinate (IRC) analysis was performed to characterize the transition states on the potential energy surface. Seven pathways for the deamination reaction were found. All pathways produce an initial tetrahedral intermediate followed by several conformational changes. The final intermediate for all pathways dissociates to product via a 1-3 proton shift. The activation energy for the rate-determining step, the formation of the tetrahedral intermediate for pathway D, the only pathway that can lead to uracil, is 115.3 kJ mol (-1) at the G3MP2 level of theory, in excellent agreement with the experimental value (117 +/- 4 kJ mol (-1)).     

Temperature‐programmed reduction of silver(I) oxide using a titania‐supported silver catalyst under a H2 atmosphere

Reduction kinetics of silver(I) oxide using a titania‐supported silver catalyst was analyzed using temperature‐programmed reduction (TPR) with hydrogen as a reducing gas. Ag₂O reduction to Ag was observed in all samples as a single reduction step occurring at two reduction peaks. Observation of these reduction peaks indicates the existence of different lattice oxygen species, that is, surface and bulk, which are, respectively, attributed to surficial and pore‐deposited Ag₂O aggregates. The powdered samples exhibited high reducibility with average final oxidation states ranging from 0 to +0.18. The apparent activation energies for Ag₂O reduction to Ag metal were 73.35 and 81.71 kJ/mol for surficial and pore‐deposited Ag₂O aggregates, respectively. In this study, a unimolecular decay model was reported to accurately describe the reduction mechanism of Ag/TiO₂ catalysts. Hence, this would also infer that the catalyst reduction is rate‐limited by the nucleation of Ag metal instead of the topochemical reaction and the diffusion of hydrogen and oxygen molecules.     

The wetting-dewetting transition of monolayer water on a hydrophobic metal surface observed by surface-state resonant second-harmonic generation

The isothermal adsorption and desorption of monolayer water on a Ag(110) surface in the temperature range of 130-137 K were characterized by monitoring second-harmonic (SH) generation from the silver surface. The SH intensity resonantly enhanced by the silver surface-state transition is highly sensitive to the amount of silver surface area covered by water and allows the observation of an abrupt change in the adsorption/desorption behavior at 133.5 K. At temperatures below 133.5 K water wets the Ag surface in a two-dimensional structure with a measured desorption energy of 25.0 (+/-3.3) kJ/mol. At temperatures greater than 133.5 K water desorbs from three-dimensional clusters with a measured desorption energy of 48.3 (+/-2.2) kJ/mol, in agreement with temperature-programmed desorption measurements. This wetting-dewetting transition of water adsorbed on the silver surface at 133.5 K is supported by classical nucleation theory calculations.     

硝基甲烷异构化反应势能面的ab initio研究

在B3LYP/6-311+ +G(2d,2p)水平上,优化得到硝基甲烷CH3NO2的10种异构体和23个异构化反应过渡态,并用G2MP2方法进行了单点能计算.根据计算得到的G2MP2相对能量,探讨了CH3NO2势能面上异构化反应的微观机理.研究表明,反应初始阶段的CH3NO2异构化过程具有较高的能垒,其中CH3NO2的两个主要异构化反应通道,即CH3NO2→CH3ONO和CH3NO2→CH2N(O)OH的活化能分别为270.3和267.8 kJ/mol,均高于CH3NO2的C-N键离解能.因而,从动力学角度考虑, CH3NO2的异构化反应较为不利.     

In-flight kinetic measurements of the aerosol growth of carbon nanotubes by electrical mobility classification

We describe an on-the-fly kinetic study of gas-phase growth of multiwalled carbon nanotubes. The methodology employs electrical mobility classification of the CNT, which enables a direct measure of CNT length distribution in an aerosol reactor. The specific experiment employs two mobility classification steps. In the first step we mobility classify the catalyst particle, in this case Ni, created by pulsed laser ablation, to generate a stream of monodisperse particles. This then determined the diameter of the CNT, when a hydrocarbon/H2 mix is added in a heated aerosol reactor. A second electrical mobility classification step allows us to determine the length distribution of the CNTs. We found that CNT growth from ethylene required the addition of small amounts of water vapor, whereas growth from acetylene did not. We show that acetylene, which always has small amounts of acetone present when purchased, can provide the oxygen source to prevent catalyst coking. By varying the temperature of the growth, we were able to extract Arrhenius growth parameters. We found an activation energy for growth approximately 80 kJ mol(-1) from both acetylene and ethylene, which is considerably lower than previous works for substrate-grown CNTs (E(a) = 110-150 kJ mol(-1)). Furthermore, we observed that our aerosol CNT growth rates were about 2 orders of magnitude higher than those for substrate-grown CNTs. The dominant growth mechanism of CNT previously proposed is based upon bulk diffusion of carbon through nickel particles. However, on the basis of the lower activation energy found in this work, we proposed that the possible mechanism of gas-phase growth of CNT is correlated with both surface (E(a) = 29 kJ mol(-1)) and bulk diffusion (E(a) = 145 kJ mol(-1)) of carbon on nickel aerosol particles. Finally, the experimental approach described in this work should be amenable to other nanowire systems grown in the aerosol phase.     

Effect of subsurface oxygen on the reactivity of the Ag(111) surface

Periodic, self-consistent, density functional theory calculations have been performed to demonstrate that subsurface oxygen (O(sb)) dramatically increases the reactivity of the Ag(111) surface. O(sb) greatly facilitates the dissociation of H2, O2, and NO and enhances the binding of H, C, N, O, O2, CO, NO, C2H2, and C2H4 on the Ag(111) surface. This effect originates from an O(sb)-induced upshift of the d-band center of the Ag surface and becomes more pronounced at higher O(sb) coverage. Our findings point to the important role that near-surface impurities, such as O(sb), can play in determining the thermochemistry and kinetics of elementary steps catalyzed by transition metal surfaces.     

Adsorption and dissociation of NH3 on clean and hydroxylated TiO2 rutile (110) surfaces: a computational study

The adsorption and dissociation of NH(3) on the clean and hydroxylated TiO(2) rutile (110) surfaces have been investigated by the first-principles calculations. The monodentate adsorbates such as H(3)N-Ti(a), H(2)N-Ti(a), N-Ti(a), H(2)N-O(a), HN-O(a), N-O(a) and H-O(a), as well as the bidentate adsorbate, Ti-N-Ti(a) can be formed on the clean surface. It is found that the hydroxyl group enhances the adsorption of certain adsorbates on the five-fold-coordinated Ti atoms (5c-Ti), namely H(2)N-Ti(a), HN-Ti(a), N-Ti(a) and Ti-N-Ti(a). In addition, the adsorption energy increases as the number of hydroxyl groups increases. On the contrary, the opposite effect is found for those on the two-fold-coordinated O atoms (2c-O). The enhanced adsorption of NH(x) (x = 1-2) on the 5c-Ti is due to the large electronegativity of the OH group, increasing the acidity of the Ti center. This also contributes to diminish the adsorption of NH(x) (x = 1-2) on the two-fold-coordinated O atoms (2c-O) decreasing its basicity. According to potential energy profile, the NH(3) dissociation on the TiO(2) surface is endothermic and the hydroxyl group is found to lower the energetics of H(2)N-Ti(a)+H-O(a) and HN-Ti(a)+2{H-O(a)}, but slightly raise the energetic of Ti-N-Ti(a)+3{H-O(a)} compare to those on the clean surface. However, the dissociation of NH(3) is found to occur on the hydroxylated surface with an overall endothermic by 31.8 kcal/mol and requires a barrier of 37.5 kcal/mol. A comparison of NH(3) on anatase surface has been discussed. The detailed electronic analysis is also carried out to gain insights into the interaction nature between adsorbate and surface.     

Spin-forbidden deprotonation of aqueous nitroxyl (HNO)

The first mechanistic study of a spin-forbidden proton-transfer reaction in aqueous solution is reported. Laser flash photolysis of alkaline trioxodinitrate (N(2)O(3)(2)(-), Angeli's anion) is used to generate a nitroxyl anion in its excited singlet state ((1)NO(-)). Through rapid partitioning between protonation by water and electronic relaxation, (1)NO(-) produces (1)HNO (ground state, yield 96%) and (3)NO(-) (ground state, yield 4%), which comprise a unique conjugate acid-base couple with different ground-state multiplicities. Using the large difference between reactivities of (1)HNO and (3)NO(-) in the peroxynitrite-forming reaction with (3)O(2), the kinetics of spin-forbidden deprotonation reaction (1)HNO + OH(-) --> (3)NO(-) + H(2)O is investigated in H(2)O and D(2)O. Consistent with proton transfer, this reaction exhibits primary kinetic hydrogen isotope effect k(H)/k(D) = 3.1 at 298 K, which is found to be temperature-dependent. Arrhenius pre-exponential factors and activation energies of the second-order rate constant are found to be: log(A, M(-)(1) s(-)(1)) = 10.0 +/- 0.2 and E(a) = 30.0 +/- 1.1 kJ/mol for proton transfer and log(A, M(-)(1) s(-)(1)) = 10.4 +/- 0.1 and E(a) = 35.1 +/- 0.7 kJ/mol for deuteron transfer. Collectively, these data are interpreted to show that the nuclear reorganization requirements arising from the spin prohibition necessitate significant activation before spin change can take place, but the spin change itself must occur extremely rapidly. It is concluded that a synergy between the spin prohibition and the reaction energetics creates an intersystem barrier and is responsible for slowness of the spin-forbidden deprotonation of (1)HNO by OH(-); the spin prohibition alone plays a minor role.