The isomerization of dinitrogen tetroxide: O2N-NO2 --> ONO-NO2

The N2O4 isomerization in gas phase has an energy barrier of 31 kcal mol-1 at 298 K. This energy barrier may be reduced due to the interaction of the N2O4 isomers with water or nitric acid clusters adsorbed on surfaces. The Gibbs free energy barrier for this reaction in water medium is estimated to be reduced to 21.1 kcal mol-1 by using the ab initio calculations and the polarizable continuum model (PCM). By using the transition state theory (TST), this model estimates that the N2O4 isomerization may be as fast as 2.0 x 10(-3) s-1 in aqueous phase at room temperature, which confirms the Finlayson-Pitts model for the heterogeneous hydrolysis of NO2 on surfaces. The activation energy of the N2O4 isomerization is about 21 kcal mol-1. The rate coefficient for this reaction is considerably fast, 1.2 x 10(-2) s-1, in aqueous phase at T = 373 K.     

Thermochemistry of methyl and ethyl nitro, RNO2, and nitrite, RONO, organic compounds

Computational quantum theory is employed to determine the thermochemical properties of n-alkyl nitro and nitrite compounds: methyl and ethyl nitrites, CH3ONO and C2H5ONO, plus nitromethane and nitroethane, CH3NO2 and C2H5NO2, at 298.15 K using multilevel G3, CBS-QB3, and CBS-APNO composite methods employing both atomization and isodesmic reaction analysis. Structures and enthalpies of the corresponding aci-tautomers are also determined. The enthalpies of formation for the most stable conformers of methyl and ethyl nitrites at 298 K are determined to be -15.64 +/- 0.10 kcal mol-1 (-65.44 +/- 0.42 kJ mol-1) and -23.58 +/- 0.12 kcal mol-1 (-98.32 +/- 0.58 kJ mol-1), respectively. DeltafHo(298 K) of nitroalkanes are correspondingly evaluated at -17.67 +/- 0.27 kcal mol-1 (-74.1 +/- 1.12 kJ mol-1) and -25.06 +/- 0.07 kcal mol-1 (-121.2 +/- 0.29 kJ mol-1) for CH3NO2 and C2H5NO2. Enthalpies of formation for the aci-tautomers are calculated as -3.45 +/- 0.44 kcal mol-1 (-14.43 +/- 0.11 kJ mol-1) for aci-nitromethane and -14.25 +/- 0.44 kcal mol-1 (-59.95 +/- 1.84 kJ mol-1) for the aci-nitroethane isomers, respectively. Data are evaluated against experimental and computational values in the literature with recommendations. A set of thermal correction parameters to atomic (H, C, N, O) enthalpies at 0 K is developed, to enable a direct calculation of species enthalpy of formation at 298.15 K, using atomization reaction and computation outputs.     

Ab initio studies of ClO(x) reactions: prediction of the rate constants of ClO+NO2 for the forward and reverse processes.

The potential-energy surface for the reaction of ClO with NO2 has been constructed at the CCSD(T)/6-311+G(3df)//B3LYP/6-311+G(3df) level of theory. Six ClNO3 isomers are located; these are ClONO2, pc-ClOONO, pt-ClOONO, OClNO2, pt-OClONO, pc-OClONO, with predicted energies relative to the reactants of -25.6, -0.5, 1.0, 1.9, 12.2 and 13.6 kcal mol-1, and heats of formation at 0 K of 7.8, 32.9, 34.4, 35.5, 45.6 and 47.0 kcal mol-1, respectively. Isomerizations among them are also discussed. The rate constants for the low-energy pathways have been computed by statistical theory calculations. For the association reaction producing exclusively ClONO2, the predicted low- and high-pressure-limit rate constants in N2 for the temperature range of 200-600 K can be represented by: (N2)=3.19 x 10-17 T-5.54 exp(-384 K/T) cm6 molecule-2 s-1 and =3.33 x 10-7 T-1.48 exp(-18 K/T) cm3 molecule-1 s-1. The predicted low- and high-pressure-limit rate constants for the decomposition of ClONO2 in N2 at 200-600 K can be expressed, respectively, by =6.08 x 1013 T-6.54 exp(-13813 K/T) cm3 molecule-1 s-1 and =4.59 x 1023 T-2.43 exp(-13437 K/T) s-1. The predicted values compare satisfactorily with available experimental data. The reverse Cl+NO3 reaction was found to be independent of the pressure, giving exclusively ClO+NO2; the predicted rate constant can be expressed as k(Cl+NO3)=1.19 x 10-9 T-0.60 exp(58 K/T) cm3 molecule-1 s-1..     

Kinetic studies of atmospherically relevant silicon chemistry. Part III: Reactions of Si+ and SiO+ with O3, and Si+ with O2

Silicon ions are generated in the Earth's upper atmosphere by hyperthermal collisions of material ablated from incoming meteoroids with atmospheric molecules, and from charge transfer of silicon-bearing neutral species with major atmospheric ions. Reported Si(+) number density vs. height profiles show a sharp decrease below 95 km, which has been commonly attributed to the fast reaction with H(2)O. Here we report rate coefficients and branching ratios of the reactions of Si(+) and SiO(+) with O(3), measured using a flow tube with a laser ablation source and detection of ions by quadrupole mass spectrometry. The results obtained are (2σ uncertainty): k(Si(+) + O(3), 298 K) = (6.5 ± 2.1) × 10(-10) cm(3) molecule(-1) s(-1), with three product channels (branching ratios): SiO(+) + O(2) (0.52 ± 0.24), SiO + O(2)(+) (0.48 ± 0.24), and SiO(2)(+) + O (<0.1); k(SiO(+) + O(3), 298 K) = (6 ± 4) × 10(-10) cm(3) molecule(-1) s(-1), where the major products (branching ratio ≥ 0.95) are SiO(2) + O(2)(+). Reactions (1) and (2) therefore have the unusual ability to neutralise silicon directly, as well as forming molecular ions which can undergo dissociative recombination with electrons. These reactions, along with the recently reported reaction between Si(+) and O(2)((1)Δ(g)), largely explain the disappearance of Si(+) below 95 km in the atmosphere, relative to other major meteoric ions such as Fe(+) and Mg(+). The rate coefficient of the Si(+) + O(2) + He reaction was measured to be k(298 K) = (9.0±1.3) × 10(-30) cm(6) molecule(-2) s(-1), in agreement with previous measurements. The SiO(2)(+) species produced from this reaction, which could be vibrationally excited, is observed to charge transfer at a relatively slow rate with O(2), with a rate constant of k(298 K) = (1.5 ± 1.0) × 10(-13) cm(3) molecule(-1) s(-1).     

Kinetics of the O + HCNO reaction

The kinetics of the O + HCNO reaction were investigated by a relative rate technique using infrared diode laser absorption spectroscopy. Laser photolysis (355 nm) of NO2 was used to produce O atoms, followed by O atom reactions with CS2, NO2, and HCNO, and infrared detection of OCS product from the O + CS2 reaction. Analysis of the experiment data yields a rate constant of k1= (9.84 +/- 3.52) x 10-12 exp[(-195 +/- 120)/T)] (cm3 molecule-1 s-1) over the temperature range 298-375 K, with a value of k1 = (5.32 +/- 0.40) x 10-12 cm3 molecule-1 s-1 at 298 K. Infrared detection of product species indicates that CO producing channels, probably CO + NO + H, dominate the reaction.     

Chloryl nitrate: a novel product of the OClO + NO3 + M recombination

The products of the reaction of OClO with NO3 were investigated between 220 and 298 K using a flow reactor and infrared, visible, and ultraviolet analysis. At temperatures below 250 K new infrared and ultraviolet absorption features were observed and assigned to the novel compound chloryl nitrate (O2ClONO2). Additionally, ClO and NO2 were observed as reaction products, indicating the existence of a second reaction channel. O2ClONO2 formation predominates at temperatures below 230 K. The reaction rate constant at 220 K is estimated to be on the order of 10(-14) cm3 molecule-1 s-1 in 1-5 Torr of helium. These observations suggest that O2ClONO2 may exist in the terrestrial stratosphere.     

Low-energy paths for the unimolecular decomposition of CH3OH: a G2M/statistical theory study

The potential energy surface (PES) of the CH3OH system has been characterized by ab initio molecular orbital theory calculations at the G2M level of theory. The mechanisms for the decomposition of CH3OH and the related bimolecular reactions, CH3 + OH and 1CH2 + H2O, have been elucidated. The rate constants for these processes have been calculated using variational RRKM theory and compared with available experimental data. The total decomposition rate constants of CH3OH at the high- and low-pressure limits can be represented by k infinity = 1.56 x 10(16) exp(-44,310/T) s-1 and kAr0 = 1.60 x 10(36) T-12.2 exp(-48,140/T) cm3 molecule-1 s-1, respectively, covering the temperature range 1000-3000 K, in reasonable agreement with the experimental values. Our results indicate that the product branching ratios are strongly pressure dependent, with the production of CH3 + OH and 1CH2 + H2O dominant under high (P > 10(3) Torr) and low (P < 1 atm) pressures, respectively. For the bimolecular reaction of CH3 and OH, the total rate constant and the yields of 1CH2 + H2O and H2 + HCOH at lower pressures (P < 5 Torr) could be reasonably accounted for by the theory. For the reaction of 1CH2 with H2O, both the yield of CH3 + OH and the total rate constant could also be satisfactorily predicted theoretically. The production of 3CH2 + H2O by the singlet to triplet surface crossing, predicted to occur at 4.3 kcal mol-1 above the H2C...OH2 van der Waals complex (which lies 82.7 kcal mol-1 above CH3OH), was neglected in our calculations.     

Hydrogen atom transfer from iron(II)-tris[2,2'-bi(tetrahydropyrimidine)] to TEMPO: a negative enthalpy of activation predicted by the Marcus equation

The transfer of a hydrogen atom from iron(II)-tris[2,2'-bi(tetrahydropyrimidine)], [FeII(H2bip)3]2+, to the stable nitroxide, TEMPO, was studied by stopped-flow UV-vis spectrophotometry. The products are the deprotonated iron(III) complex [FeIII(H2bip)2(Hbip)]2+ and the hydroxylamine, TEMPO-H. This reaction can also be referred to as proton-coupled electron transfer (PCET). The equilibrium constant for the reaction is close to 1; thus, the reaction can be driven in either direction. The rate constants for the forward and reverse reactions at 298 K are k1 = 260 +/- 30 M-1 s-1 and k-1 = 150 +/- 20 M-1 s-1. Interestingly, the rate constant for the forward reaction decreases as reaction temperature is increased, implying a negative activation enthalpy: DeltaH1 = -2.7 +/- 0.4 kcal mol-1, DeltaS1 = -57 +/- 8 cal mol-1 K-1. Marcus theory predicts this unusual temperature dependence on the basis of independently measured self-exchange rate constants and equilibrium constants: DeltaHcalcd = -3.5 +/- 0.5 kcal mol-1, DeltaScalcd = -42 +/- 10 cal mol-1 K-1. This result illustrates the value of the Marcus approach for these types of reactions. The dominant contributor to the negative activation enthalpy is the favorable enthalpy of reaction, DeltaH1 degrees = -9.4 +/- 0.6 kcal mol-1, rather than the small negative activation enthalpy for the H-atom self-exchange between the iron complexes.     

Unimolecular reactions of vibrationally excited CF2ClCHFCH3 and CF2ClCHFCD3: evidence for the 1,2-FCl interchange pathway

Chemically activated CF2ClCHFCH3 and CF2ClCHFCD3 molecules were prepared with 94 kcal mol-1 of vibrational energy by the recombination of CF2ClCHF and CH3(CD3) radicals at room temperature. The unimolecular reaction pathways were 2,3-FH(FD) elimination, 1,2-ClF interchange and 1,2-ClH elimination; the interchange produces CF3CHClCH3(CF3CHClCD3) with 105 kcal mol-1 of vibrational energy. Rate constants for CF2ClCHFCH3 [CF2ClCHFCD3] were (3.1+/-0.4)x10(6) s-1 [(1.0+/-0.1)x10(6) s-1] for 2,3-FH [FD] loss, (1.5+/-0.2)x10(6) s-1 [(8.3+/-0.9)x10(5) s-1] for 1,2-ClF interchange, and (8.2+/-1.0)x10(5) s-1 [(5.3+/-0.6)x10(5) s-1] for 1,2-ClH [DCl] loss. These correspond to branching fractions of 0.55+/-0.06 [0.43+/-0.04] for 2,3-FH [FD] loss, 0.29+/-0.03 [0.35+/-0.04] for 1,2-ClF interchange, and 0.16+/-0.02 [0.22+/-0.02] for 1,2-ClH [ClD] loss. Kinetic-isotope effects were 3.0+/-0.6 for 2,3-FH [FD] loss, 1.6+/-0.3 for 1,2-ClH loss, and 1.8+/-0.4 for 1,2-ClF interchange. The CF3CHClCH3 (CF3CHClCD3) molecules formed by 1,2-FCl interchange react by loss of HCl [DCl] with rate constants of (5.6+/-0.9)x10(7) s-1 [(2.1+/-0.4)x10(7)] s-1 for an isotope effect of 2.7+/-0.4. Density functional theory was employed to calculate vibrational frequencies and moments of inertia for the molecules and for the transition-state structures. These results were used with RRKM theory to assign threshold energies from comparison of computed and experimental unimolecular rate constants. The threshold energy for ClF interchange is 57.5 kcal mol-1, and those for HF and HCl channels are 2-5 kcal mol-1 higher. Experiments with vibrationally excited CF2ClCF2CF3, CF2ClCF2CF2Cl, and CF2ClCF2Cl, which did not show evidence for ClF interchange, also are reported.     

Theoretical study of the fragmentation pathways of norbornane in its doubly ionized ground state

The potential energy surface of norbornane in its dicationic singlet ground state has been investigated in detail using density functional theory along with the nonlocal hybrid and gradient-corrected Becke three-parameter Lee-Yang-Parr functional (B3LYP) and the cc-pVDZ basis set. For the sake of more quantitative insight into the chemical reactions induced by double ionization of norbornane, this study was supplemented by a calculation of basic thermodynamic state functions coupled to a focal point analysis of energy differences obtained using correlation treatments and basis sets of improving quality, enabling an extrapolation of these energy differences at the CCSD(T) level in the limit of an asymptotically complete (cc-pV infinity Z) basis set. Our results demonstrate the likelihood of an ultrafast intramolecular rearrangement of the saturated hydrocarbon cage after a sudden removal of two electrons into a kinetically metastable five-membered cyclic C5H8+-CH+-CH3 intermediate, prior to a Coulomb explosion into C5H7+=CH2 and CH3+ fragments, which might explain a tremendous rise of electron-impact (e, 2e) ionization cross sections at electron binding energies around the double-ionization threshold. The first step is straightforward and strongly exothermic (DeltaH298 = -114.0 kcal mol-1). The second step is also exothermic (DeltaH298 = -10.2 kcal mol-1) but requires an activation enthalpy (DeltaH298) of 39.7 kcal/mol. The various factors governing the structure of this intermediate, such as electrostatic interactions, inductive effects, cyclic strains, and methylenic hyperconjugation interactions, are discussed in detail.     

土卫六大气中H2和N+反应的动力学研究

依据Titan大气的压强和温度条件计算了N++H2→NH++H反应的热化学函数. 结果发现, 该反应是一个吸热反应, 在Titan的低温环境中不具有反应自发性. 运用量子化学理论计算研究了反应的动力学性质, 发现该反应在300 K温度下的反应速率k=4.16×10−10 cm3•mol−1•s−1, 在实验室温度下(298.15 K)的反应势垒是109.847 kJ•mol−1. 依据Titan电离层气压温度条件计算了90 Pa压强条件下1 K到5000 K温度范围内(极低温度和极高温度)的 反应活化能和反应速率, 研究发现低温下该反应的反应速率非常低, 而且, 随着温度的降低, 反应速率急剧降低. 理论计算值和文献中的实验值也符合得较好, 理论计算数据可以为星际分子的模拟实验提供一定的参考.     

Mechanism of the hydration of carbon dioxide: direct participation of H2O versus microsolvation

Thermochemical parameters of carbonic acid and the stationary points on the neutral hydration pathways of carbon dioxide, CO 2 + nH 2O --> H 2CO 3 + ( n - 1)H 2O, with n = 1, 2, 3, and 4, were calculated using geometries optimized at the MP2/aug-cc-pVTZ level. Coupled-cluster theory (CCSD(T)) energies were extrapolated to the complete basis set limit in most cases and then used to evaluate heats of formation. A high energy barrier of approximately 50 kcal/mol was predicted for the addition of one water molecule to CO 2 ( n = 1). This barrier is lowered in cyclic H-bonded systems of CO 2 with water dimer and water trimer in which preassociation complexes are formed with binding energies of approximately 7 and 15 kcal/mol, respectively. For n = 2, a trimeric six-member cyclic transition state has an energy barrier of approximately 33 (gas phase) and a free energy barrier of approximately 31 (in a continuum solvent model of water at 298 K) kcal/mol, relative to the precomplex. For n = 3, two reactive pathways are possible with the first having all three water molecules involved in hydrogen transfer via an eight-member cycle, and in the second, the third water molecule is not directly involved in the hydrogen transfer but solvates the n = 2 transition state. In the gas phase, the two transition states have comparable energies of approximately 15 kcal/mol relative to separated reactants. The first path is favored over in aqueous solution by approximately 5 kcal/mol in free energy due to the formation of a structure resembling a (HCO 3 (-)/H 3OH 2O (+)) ion pair. Bulk solvation reduces the free energy barrier of the first path by approximately 10 kcal/mol for a free energy barrier of approximately 22 kcal/mol for the (CO 2 + 3H 2O) aq reaction. For n = 4, the transition state, in which a three-water chain takes part in the hydrogen transfer while the fourth water microsolvates the cluster, is energetically more favored than transition states incorporating two or four active water molecules. An energy barrier of approximately 20 (gas phase) and a free energy barrier of approximately 19 (in water) kcal/mol were derived for the CO 2 + 4H 2O reaction, and again formation of an ion pair is important. The calculated results confirm the crucial role of direct participation of three water molecules ( n = 3) in the eight-member cyclic TS for the CO 2 hydration reaction. Carbonic acid and its water complexes are consistently higher in energy (by approximately 6-7 kcal/mol) than the corresponding CO 2 complexes and can undergo more facile water-assisted dehydration processes.     

Stepwise hydration of ionized aromatics. Energies, structures of the hydrated benzene cation, and the mechanism of deprotonation reactions

The stepwise binding energies (DeltaHdegree(n-1,n)) of 1-8 water molecules to benzene(.+) [Bz(.+)(H2O)n] were determined by equilibrium measurements using an ion mobility cell. The stepwise hydration energies, DeltaHdegree(n-1,n), are nearly constant at 8.5 +/- 1 kcal mol-1 from n = 1-6. Calculations show that in the n = 1-4 clusters, the benzene(.+) ion retains over 90% of the charge, and it is extremely solvated, that is, hydrogen bonded to an (H2O)n cluster. The binding energies and entropies are larger in the n = 7 and 8 clusters, suggesting cyclic or cage-like water structures. The concentration of the n = 3 cluster is always small, suggesting that deprotonation depletes this ion, consistent with the thermochemistry since associative deprotonation Bz(.+)(H2O)(n-1) + H2O-->C6H5. + (H2O)nH+ is thermoneutral or exothermic for n > or = 4. Associative intracluster proton transfer Bz(.+)(H2O)(n+1) + H2O-->C6H5.(H2O)nH+ would also be exothermic for n > or = 4, but lack of H/D exchange with D2O shows that the proton remains on C6H6(.+) in the observed Bz(.+)(H2O)n clusters. This suggests a barrier to intracluster proton transfer, and as a result, the [Bz(.+)(H2O)n]* activated complexes either undergo dissociative proton transfer, resulting in deprotonation and generation of (H2O)nH+, or become stabilized. The rate constant for the deprotonation reaction shows a uniquely large negative temperature coefficient of K = cT(-67+/-4) (or activation energy of -34+/- 1 kcal mol-1), caused by a multibody mechanism in which five or more components need to be assembled for the reaction.     

Enthalpies of formation, bond dissociation energies, and molecular structures of the n-aldehydes (acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal) and their radicals

Aldehydes are important intermediates and products in a variety of combustion and gas-phase oxidation processes, such as in low-temperature combustion, in the atmosphere, and in interstellar media. Despite their importance, the enthalpies of formation and bond dissociation energies (BDEs) for the aldehydes are not accurately known. We have determined enthalpies of formation for acetaldehyde, propanal, and butanal from thermodynamic cycles, using experimentally measured reaction and formation enthalpies. All enthalpy values used for reference molecules and reactions were first verified to be accurate to within around 1 kcal mol-1 using high-level ab initio calculations. Enthalpies of formation were found to be -39.72 +/- 0.16 kcal mol-1 for acetaldehyde, -45.18 +/- 1.1 kcal mol-1 for propanal, and -49.27 +/- 0.16 kcal mol-1 for butanal. Enthalpies of formation for these three aldehydes, as well as for pentanal, hexanal, and heptanal, were calculated using the G3, G3B3, and CBS-APNO theoretical methods, in conjunction with bond-isodesmic work reactions. On the basis of the results of our thermodynamic cycles, theoretical calculations using isodesmic work reactions, and existing experimental measurements, we suggest that the best available formation enthalpies for the aldehydes acetaldehyde, propanal, butanal, pentanal, hexanal, and heptanal are -39.72, -45.18, -50.0, -54.61, -59.37, and -64.2 kcal mol-1, respectively. Our calculations also identify that the literature enthalpy of formation of crotonaldehyde is in error by as much as 1 kcal mol-1, and we suggest a value of -25.1 kcal mol-1, which we calculate using isodesmic work reactions. Bond energies for each of the bonds in the aldehydes up to pentanal were calculated at the CBS-APNO level. Analysis of the BDEs reveals the R-CH(2)CH=O to be the weakest bond in all aldehydes larger than acetaldehyde, due to formation of the resonantly stabilized vinoxy radical (vinyloxy radical/formyl methyl radical). It is proposed that the vinoxy radical as well as the more commonly considered formyl and acetyl radicals are important products of aldehyde combustion and oxidation, and the reaction pathways of the vinoxy, formyl, and acetyl radicals are discussed. Group additivity values for the carbon-oxygen-hydrogen groups common to the aldehydes are also determined. Internal rotor profiles and electrostatic potential surfaces are used to study the dipole induced dipole-dipole interaction in the synperiplanar conformation of propanal. It is proposed that the loss of this dipole-dipole interaction in RC(.-)HCH(2)CH=O radicals causes a ca. 1-2 kcal mol-1 decrease in the aldehyde C-H and C-C bond energies corresponding to RC(.-)HCH(2)CH=O radical formation.     

Hydrolysis, hydrosulfidolysis, and aminolysis of imido(methyl)rhenium complexes

The tris(imido)methylrhenium complex CH3Re(NAd)3 (1a, Ad = 1-adamantyl) reacts with H2O to give CH3Re(NAd)2O (2a) and AdNH2. The resulting di(imido)oxo species can further react with another molecule of H2O to generate CH3Re(NAd)O2 (3a). The kinetics of these reactions have been studied by means of 1H NMR and UV-vis spectroscopies. The second-order rate constant for the reaction of 1a with H2O at 298 K in C6H6 is 3.3 L mol-1 s-1, which is much larger than the value 1 x 10(-4) L mol-1 s-1 obtained for the reaction between CH3Re(NAr)3 (1b, Ar = 2,6-diisopropylphenyl) and H2O in CH3CN at 313 K. Both 1a and 1b react with H2S to produce the rhenium(VII) sulfide, (CH3Re(NR)2)2(mu-S)2 (4a, R = Ad; 4b, R = Ar), with second-order rate constants of 17 and 1.6 x 10(-4) L mol-1 s-1 in C6H6 and CH3CN, respectively. Complex 4b has been structurally characterized. The crystal data are as follows: space group C2/c, a = 30.4831 (19) A, b = 10.9766 (7) A, c = 18.1645 (11) A, beta = 108.268(1) degrees, V = 5771.5 (6) A3, Z = 4. The reaction between CH3Re(NAr)2O (2b) and H2S also yields the dinuclear compound 4b. Unlike 1b, 1a reacts with aniline derivatives to give mixed imido rhenium complexes.     

Kinetics of silicate exchange in alkaline aluminosilicate solutions

In strongly alkaline aqueous KOH solutions containing SiIV in large excess over AlIII, the kinetics of exchange of monomeric silicate with small acyclic aluminosilicate solute species is much more rapid than with either cyclic aluminosilicates or any all-silicate anions. Selective inversion recovery 29Si NMR studies of homogeneous solutions of stoichiometric composition 3.0 mol kg-1 of SiO2, 0.1 mol kg-1 of Al2O3, and 8.0 mol kg-1 of K2O in 60-75% D2O gave rate constants of 2.0 +/- 0.2 kg mol-1 s-1 and 17 +/- 4 s-1 for the forward and reverse reactions of monomeric silicate with (HO)3AlOSiOn(OH)(3-n)(n+1)- (n = 2 or 3) at 0 degree C. These rate constants are more than 10(4)-fold faster than those extrapolated from 60 to 90 degrees C for comparable reactions of silicate anions. The greater lability of acyclic aluminate centers relative to silicate is ascribed partly to the availability of HO- groups for condensation reactions on Al and mainly to the ease of expansion of the coordination number of AlIII beyond 4. The latter attribute is diminished when AlIII is constrained to be tetrahedral in cyclic structures. With respect to the mechanism of formation of zeolites from alkaline aqueous media, it is suggested that small, labile AlOSi units add rapidly to growing zeolitic structures "on demand", whereas the more kinetically inert cage or ring structures cannot. This would explain why a silicate or aluminosilicate structure that is dominant among solute species at equilibrium in the presence of a particular cation may bear little or no geometric relation to the zeolitic framework promoted kinetically by that same cation.     

Active Thermochemical Tables: accurate enthalpy of formation of hydroperoxyl radical, HO2

Through the use of the Active Thermochemical Tables approach, the best currently available enthalpy of formation of HO2 has been obtained as delta(f)H(o)298 (HO2) = 2.94 +/- 0.06 kcal mol(-1) (3.64 +/- 0.06 kcal mol(-1) at 0 K). The related enthalpy of formation of the positive ion, HO2+, within the stationary electron convention is delta(f)H(o)298 (HO2+) = 264.71 +/- 0.14 kcal mol(-1) (265.41 +/- 0.14 kcal mol(-1) at 0 K), while that for the negative ion, HO2- (within the same convention), is delta(f)H(o)298 (HO2-) = -21.86 +/- 0.11 kcal mol(-1) (-21.22 +/- 0.11 kcal mol(-1) at 0 K). The related proton affinity of molecular oxygen is PA298(O2) = 100.98 +/- 0.14 kcal mol(-1) (99.81 +/- 0.14 kcal mol(-1) at 0 K), while the gas-phase acidity of H2O2 is delta(acid)G(o)298 (H2O2) = 369.08 +/- 0.11 kcal mol(-1), with the corresponding enthalpy of deprotonation of H2O2 of delta(acid)H(o)298 (H2O2) = 376.27 +/- 0.11 kcal mol(-1) (375.02 +/- 0.11 kcal mol(-1) at 0 K). In addition, a further improved enthalpy of formation of OH is briefly outlined, delta(f)H(o)298 (OH) = 8.93 +/- 0.03 kcal mol(-1) (8.87 +/- 0.03 kcal mol(-1) at 0 K), together with new and more accurate enthalpies of formation of NO, delta(f)H(o)298 (NO) = 21.76 +/- 0.02 kcal mol(-1) (21.64 +/- 0.02 kcal mol(-1) at 0 K) and NO2, delta(f)H(o)298 (NO2) = 8.12 +/- 0.02 kcal mol(-1) (8.79 +/- 0.02 kcal mol(-1) at 0 K), as well as H(2)O(2) in the gas phase, delta(f)H(o)298 (H2O2) = -32.45 +/- 0.04 kcal mol(-1) (-31.01 +/- 0.04 kcal mol(-1) at 0 K). The new thermochemistry of HO2, together with other arguments given in the present work, suggests that the previous equilibrium constant for NO + HO2 --> OH + NO2 was underestimated by a factor of approximately 2, implicating that the OH + NO2 rate was overestimated by the same factor. This point is experimentally explored in the companion paper of Srinivasan et al. (next paper in this issue).     

Solvent effects on the oxidation of RuIV=O to O=RuVI=O by MnO4-. hydrogen-atom versus oxygen-atom transfer

The kinetics of the oxidation of trans-[RuIV(tmc)(O)(solv)]2+ to trans-[RuVI(tmc)(O)2]2+ (tmc is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, a tetradentate macrocyclic tertiary amine ligand; solv = H2O or CH3CN) by MnO4- have been studied in aqueous solutions and in acetonitrile. In aqueous solutions the rate law is -d[MnO4]/dt = kH2O[RuIV][MnO4-] = (kx + (ky)/(Ka)[H+])[RuIV][MnO4-], kx = (1.49 +/- 0.09) x 101 M-1 s-1 and ky = (5.72 +/- 0.29) x 104 M-1 s-1 at 298.0 K and I = 0.1 M. The terms kx and ky are proposed to be the rate constants for the oxidation of RuIV by MnO4- and HMnO4, respectively, and Ka is the acid dissociation constant of HMnO4. At [H+] = I = 0.1 M, DeltaH and DeltaS are (9.6 +/- 0.6) kcal mol-1 and -(18 +/- 2) cal mol-1 K-1, respectively. The reaction is much slower in D2O, and the deuterium isotope effects are kx/kxD = 3.5 +/- 0.1 and ky/kyD = 5.0 +/- 0.3. The reaction is also noticeably slower in H218O, and the oxygen isotope effect is kH216O/kH218O = 1.30 +/- 0.07. 18O-labeled studies indicate that the oxygen atom gained by RuIV comes from water and not from KMnO4. These results are consistent with a mechanism that involves initial rate-limiting hydrogen-atom abstraction by MnO4- from coordinated water on RuIV. In acetonitrile the rate law is -d[MnO4-]/dt = kCH3CN[RuIV][MnO4-], kCH3CN = 1.95 +/- 0.08 M-1 s-1 at 298.0 K and I = 0.1 M. DeltaH and DeltaS are (12.0 +/- 0.3) kcal mol-1 and -(17 +/- 1) cal mol-1 K-1, respectively. 18O-labeled studies show that in this case the oxygen atom gained by RuIV comes from MnO4-, consistent with an oxygen-atom transfer mechanism.     

·OH自由基与CH3CN反应机理及动力学

在CBS-QB3水平上研究了CH3CN 和·OH反应的势能面, 其中包括两个中间体和9个反应过渡态. 分别给出了各主要物质的稳定构型、相对能量及各反应路径的能垒. 根据计算的CBS-QB3势能面, 探讨了CH3CN+·OH反应机理. 计算结果表明, 生成产物P1(·CH2CN+H2O)的反应路径在整个反应体系中占主要地位. 运用过渡态理论对产物通道P1(·CH2CN+H2O)的速率常数k1(cm3·molecule-1·s-1)进行了计算. 预测了k1(cm3·molecule-1·s-1)在250-3000 K温度范围内的速率常数表达式为k1(250-3000 K)=2.06×10^-20T^3.045exp(-780.00/T). 通过与已有的实验值进行对比得出, 在实验所测定的250-320 K 范围内, 计算得到的k1的数值与已有的实验值比较吻合. 由初始反应物生成产物P1 (·CH2CN+H2O)只需要克服一个14.2 kJ·mol-1的能垒. 而产物·CH2CN+H2O生成后要重新回到初始反应物CH3CN+·OH, 则需要克服一个高达111.2 kJ·mol-1的能垒,这就表明一旦产物P1生成后就很难再回到初始反应物.     

Does Silica Surface Catalyse Peptide Bond Formation? New Insights from First‐Principles Calculations

The role that silica surface could have played in prebiotic chemistry as a catalyst for peptide bond formation has been addressed at the B3LYP/6-31+G(d,p) level for a model reaction involving glycine and ammonia on a silica cluster mimicking an isolated terminal silanol group present at the silica surface. Hydrogen-bond complexation between glycine and the silanol is followed by the formation of the mixed surface anhydride Si(surf)-O-C(=O)-R, which has been suggested in the literature to activate the C=O bond towards nucleophilic attack by a second glycine molecule, here simulated by the simpler NH3 molecule. However, B3LYP/6-31+G(d,p) calculations show that formation of the surface mixed anhydride Si(surf)-O-C(=O)-R is disfavoured (delta(r)G298 approximately 6 kcal mol(-1)), and that the surface bond only moderately lowers the free-energy barrier of the nucleophilic attack responsible for peptide bond formation (deltaG298(double dagger) approximately 48 kcal mol(-1)) in comparison with the uncatalysed reaction (deltaG298(double dagger) approximately 52 kcal mol(-1)). A further decrease of the free-energy barrier of peptide bond formation (deltaG298(double dagger) approximately 41 kcal mol(-1)) is achieved by a single water molecule close to the reaction centre acting as a proton-transfer helper in the activated complex. A possible role of strained silica surface defects on the formation of the surface mixed anhydride Si(surf)-O-C(=O)-R has also been addressed.