The isomerization of methoxy radical: intramolecular hydrogen atom transfer mediated through acid catalysis

The catalytic ability of water, formic acid, and sulfuric acid to facilitate the isomerization of the CH(3)O radical to CH(2)OH has been studied. It is shown that the activation energies for isomerization are 30.2, 25.7, 4.2, and 2.3 kcal mol(-1), respectively, when the reaction is carried out in isolation and with water, formic acid, or sulfuric acid as a catalyst. The formation of a doubly hydrogen bonded transition state is central to lowering the activation energy and facilitating the intramolecular hydrogen atom transfer that is required for isomerization. The changes in the rate constant for the CH(3)O-to-CH(2)OH isomerization with acid catalysis have also been calculated at 298 K. The largest enhancement in the rate, by over 12 orders of magnitude, is with sulfuric acid. The results of the present study demonstrate the feasibility of acid catalysis of a gas-phase radical isomerization reaction that would otherwise be forbidden.     

Crossed molecular beam studies of the reactions of allyl radicals, C3H5(X2A2), with methylacetylene (CH3CCH(X1A1)), allene (H2CCCH2(X1A1)), and their isotopomers

The chemical dynamics of the reaction of allyl radicals, C(3)H(5)(X(2)A(2)), with two C(3)H(4) isomers, methylacetylene (CH(3)CCH(X(1)A(1))) and allene (H(2)CCCH(2)(X(1)A(1))) together with their (partially) deuterated counterparts, were unraveled under single-collision conditions at collision energies of about 125 kJ mol(-1) utilizing a crossed molecular beam setup. The experiments indicate that the reactions are indirect via complex formation and proceed via an addition of the allyl radical with its terminal carbon atom to the terminal carbon atom of the allene and of methylacetylene (alpha-carbon atom) to form the intermediates H(2)CCHCH(2)CH(2)CCH(2) and H(2)CCHCH(2)CHCCH(3), respectively. The lifetimes of these intermediates are similar to their rotational periods but too short for a complete energy randomization to occur. Experiments with D4-allene and D4-methylacetylene verify explicitly that the allyl group stays intact: no hydrogen emission was observed but only the release of deuterium atoms from the perdeuterated reactants. Further isotopic substitution experiments with D3-methylacetylene combined with the nonstatistical nature of the reaction suggest that the intermediates decompose via hydrogen atom elimination to 1,3,5-hexatriene, H(2)CCHCH(2)CHCCH(2), and 1-hexen-4-yne, H(2)CCHCH(2)CCCH(3), respectively, via tight exit transition states located about 10-15 kJ mol(-1) above the separated products. The overall reactions were found to be endoergic by 98 +/- 4 kJ mol(-1) and have characteristic threshold energies to reaction between 105 and 110 kJ mol(-1). Implications of these findings to combustion and interstellar chemistry are discussed.     

Clusters of hydrated methane sulfonic acid CH3SO3H.(H2O)n (n = 1-5): a theoretical study

Ab initio and density functional methods have been used to examine the structures and energetics of the hydrated clusters of methane sulfonic acid (MSA), CH3SO3H.(H2O)n (n = 1-5). For small clusters with one or two water molecules, the most stable clusters have strong cyclic hydrogen bonds between the proton of OH group in MSA and the water molecules. With three or more water molecules, the proton transfer from MSA to water becomes possible, forming ion-pair structures between CH3SO3- and H3O+ moieties. For MSA.(H2O)3, the energy difference between the most stable ion pair and neutral structures are less than 1 kJ/mol, thus coexistence of neutral and ion-pair isomers are expected. For larger clusters with four and five water molecules, the ion-pair isomers are more stable (>10 kJ/mol) than the neutral ones; thus, proton transfer takes place. The ion-pair clusters can have direct hydrogen bond between CH3SO3- and H3O+ or indirect one through water molecule. For MSA.(H2O)5, the energy difference between ion pairs with direct and indirect hydrogen bonds are less than 1 kJ/mol; namely, the charge separation and acid ionization is energetically possible. The calculated IR spectra of stable isomers of MSA.(H2O)n clusters clearly demonstrate the significant red shift of OH stretching of MSA and hydrogen-bonded OH stretching of water molecules as the size of cluster increases.     

Thermochemistry of N-N containing ions: a threshold photoelectron photoion coincidence spectroscopy study of ionized methyl- and tetramethylhydrazine

The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60-31.95 eV (for the MH ion) and 7.74-29.94 eV (for the TMH ion). Methylhydrazine ions (CH3NHNH2(+*)) have three low-energy dissociation channels: hydrogen atom loss to form CH2NHNH2(+) (m/z 45), loss of a methyl radical to form NHNH2(+) (m/z 31), and loss of methane to form the fragment ion m/z 30, N2H2(+*). Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold: that of methyl radical loss to form (CH3)2NNCH3(+) (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C3H8N2(+*). The experimental breakdown curves were modeled with Rice-Ramsperger-Kassel-Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (delta(f)H0) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E0) obtained from the fitting procedure: delta(f)H0[(CH3)2NNCH3(+)] = 833 +/- 5 kJ mol(-1), delta(f)H0 [C3H8N2(+*)] = 1064 +/- 5 kJ mol(-1), delta(f)H0[CH2NHNH2(+)] = 862 +/- 5 kJ mol(-1), delta(f)H0[NHNH2(+)] = 959 +/- 5 kJ mol(-1), and delta(f)H0[N2H2(+*)] = 1155 +/- 5 kJ mol(-1). The breakdown curves have been measured from threshold up to h nu approximately 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.     

尿酸质子转移反应的理论研究I——分子内质子转移研究

使用量子化学中的Hartree-Fock方法和密度泛函理论中的B3LYP方法，分别在3－21G^*和6－31G（d)水平上，计算了尿酸分子从三羰基异构体向三羟基异构体的转化。结果表明，转化过程经历了单羟基和双羟基异构体2种中间物和3种过渡态时的分子内质子转移（IPT），转移中的H原邻近的N，O和C原子形成了具有四元环结构的过渡态。随着IPT的进行，N－H键逐渐被削弱和断裂，O－H键则逐渐生成。3个反应的活化能分别为190.3kJ/mol,181.4kJ/mol和249.9kJ/mol(B3LYP/6-31G(d))。较高的活化能表明在室温下，无催化剂的IPT难以进行。     

On the formation of phenyldiacetylene (C6H5CCCCH) and D5-phenyldiacetylene (C6D5CCCCH) studied under single collision conditions

The crossed molecular beam reactions of the phenyl and D5-phenyl radical with diacetylene (C(4)H(2)) was studied under single collision conditions at a collision energy of 46 kJ mol(-1). The chemical dynamics were found to be indirect and initiated by an addition of the phenyl/D5-phenyl radical with its radical center to the C1-carbon atom of the diacetylene reactant. This process involved an entrance barrier of 4 kJ mol(-1) and lead to a long lived, bound doublet radical intermediate. The latter emitted a hydrogen atom directly or after a few isomerization steps via tight exit transition states placed 20-21 kJ mol(-1) above the separated phenyldiacetylene (C(6)H(5)CCCCH) plus atomic hydrogen products. The overall reaction was determined to be exoergic by about 49 ± 26 kJ mol(-1) and 44 ± 10 kJ mol(-1) as determined experimentally and computationally, thus representing a feasible pathway to the formation of the phenyldiacetylene molecule in combustion flames of hydrocarbon fuel.     

The OH* + CH3SH reaction: support for an addition-elimination mechanism from ab initio calculations

Several intermediates for the CH(3)SH + OH(*) --> CH(3)S(*) + H(2)O reaction were identified using MP2(full) 6-311+g(2df,p) ab initio calculations. An adduct, CH(3)S(H)OH(*), I, with electronic energy 13.63 kJ mol(-1) lower than the reactants, and a transition state, II(double dagger), located 5.14 kJ mol(-1) above I, are identified as the entrance channel for an addition-elimination reaction mechanism. After adding zero-point and thermal energies, DeltaH(r,298) ( degrees )(reactants --> I) = -4.85 kJ mol(-1) and DeltaH(298) (double dagger)(I --> II(double dagger)) = +0.10 kJ mol(-1), which indicates that the potential energy surface is broad and flat near the transition state. The calculated imaginary vibrational frequency of the transition state, 62i cm(-1), is also consistent with an addition-elimination mechanism. These calculations are consistent with experimental observations of the OH(*) + CH(3)SH reaction that favored an addition-elimination mechanism rather than direct hydrogen atom abstraction. An alternative reaction, CH(3)SH + OH(*) --> CH(3)SOH + H(*), with DeltaH(r,298) ( degrees ) = +56.94 kJ mol(-1) was also studied, leading to a determination of DeltaH(f,298) ( degrees )(CH(3)SOH) = -149.8 kJ mol(-1).     

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.     

Electron-rich radicals by neutralizationreionization mass spectrometry. Generation, dissociations and energetics of the hydrogen atom adduct to acetamide

Protonated acetamide exists as two planar conformers, the more stable anti-form (anti-1(+)) and the syn-form (syn-1(+)), DeltaG(degree) (298) (anti-->syn) = 10.8 kJ mol(-1). Collisional neutralization of 1(+) produces 1-hydroxy-1-amino-1-ethyl radicals (anti-1 and syn-1) which in part survive for 3.7 micros. The major dissociation of 1 is loss of the hydroxyl hydrogen atom (approximately 95%) which is accompanied by loss of one of the methyl hydrogen atoms (approximately 3%) and loss of the methyl group (approximately 2%). The most favorable dissociation of the OH bond is calculated to be only 34 kJ mol(1) endothermic but requires 88 kJ mol(-1) in the transition state. Other dissociations of 1, e.g., loss of one of the amide hydrogens, methyl hydrogens, and loss of ammonia are calculated to proceed through higher- energy transition states and are not kinetically competitive if proceeding from the ground doublet electronic state of 1. The unimolecular dissociation of 1 following collisional electron transfer is promoted by large Franck-Condon effects that result in 8090 kJ mol(-1) vibrational excitation in the radicals. Radicals 1 are calculated to exoergically abstract hydrogen atoms from acetamide in water, but not in the gas phase. The different reactivity is due to solvent effects that favor the products, (.)CH(2)CONH(2) and CH(3)CH(OH)NH(2), over the reactants.     

Low-temperature phase transition in [Mn(OS(CH3)2)6](ClO4)2 studied by single crystal X-ray diffraction, infrared absorption and Raman scattering spectroscopies

Single crystal X-ray diffraction studies of [Mn(OS(CH3)2)6](ClO4)2 have shown that the low temperature phase transition, detected by differential scanning calorimetry (DSC) at about 223 K, is associated with the crystal symmetry's reduction from an orthorhombic crystallographic system (Fdd2, No. 43) to a monoclinic one (Cc, No. 9). The analysis of the full width at half maximum of the bands connected with: δd(OClO)F2 and ρ(CH3) vibrational modes in the FT-IR and FT-RS spectra, respectively, registered in the function of temperature, proved that the reorientational motions of ClO4- anions and CH3 groups from (CH3)2SO ligands, began to slow down at temperatures below the phase transition at about 223K. Mean values of activation energy for ClO4- reorientation in the high temperature phase I and low temperature phase II are: Ea(I)≈14 kJ mol(-1) and Ea(II)≈10 kJ mol(-1), respectively. Analogous values for CH3 reorientation are: Ea(I)≈23 kJ mol(-1) and Ea(II)≈1 kJ mol(-1), respectively.     

Potential energy surface for activation of methane by Pt(+): a combined guided ion beam and DFT study

A guided-ion beam tandem mass spectrometer is used to study the reactions of Pt(+) with methane, PtCH(2)(+) with H(2) and D(2), and collision-induced dissociation of PtCH(4)(+) and PtCH(2)(+) with Xe. These studies experimentally probe the potential energy surface for the activation of methane by Pt(+). For the reaction of Pt(+) with methane, dehydrogenation to form PtCH(2)(+) + H(2) is exothermic, efficient, and the only process observed at low energies. PtH(+), formed in a simple C-H bond cleavage, dominates the product spectrum at high energies. The observation of a PtH(2)(+) product provides evidence that methane activation proceeds via a (H)(2)PtCH(2)(+) intermediate. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies in eV (kJ/mol) of D(0)(Pt(+)-H) = 2.81 +/- 0.05 (271 +/- 5), D(0)(Pt(+)-2H) = 6.00 +/- 0.12 (579 +/- 12), D(0)(Pt(+)-C) = 5.43 +/- 0.05 (524 +/- 5), D(0)(Pt(+)-CH) = 5.56 +/- 0.10 (536 +/- 10), and D(0)(Pt(+)-CH(3)) = 2.67 +/- 0.08 (258 +/- 8). D(0)(Pt(+)-CH(2)) = 4.80 +/- 0.03 eV (463 +/- 3 kJ/mol) is determined by measuring the forward and reverse reaction rates for Pt(+) + CH(4) right harpoon over left harpoon PtCH(2)(+) + H(2) at thermal energy. We find extensive hydrogen scrambling in the reaction of PtCH(2)(+) with D(2). Collision-induced dissociation (CID) of PtCH(4)(+), identified as the H-Pt(+)-CH(3) intermediate, with Xe reveals a bond energy of 1.77 +/- 0.08 eV (171 +/- 8 kJ/mol) relative to Pt(+) + CH(4). The experimental thermochemistry is favorably compared with density functional theory calculations (B3LYP using several basis sets), which also establish the electronic structures of these species and provide insight into the reaction mechanism. Results for the reaction of Pt(+) with methane are compared with those for the analogous palladium system and the differences in reactivity and mechanism are discussed.     

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.     

Methane activation on Pt and Pt4: a density functional theory study

The activation mechanisms of a methane molecule on a Pt atom (CH4-Pt) and on a Pt tetramer (CH4-Pt4) were investigated using density functional theory (B3LYP and PW91) calculations. The results from these two functionals are different mostly in predicting the reaction barrier, in particular for the CH4-Pt system. A new lower energy pathway was identified for the CH4 dehydrogenation on a Pt atom. In the new pathway, the PtCH2 + H2 products were formed via a transition state, in which the Pt atom forms a complex with carbene and both dissociated hydrogen atoms. We report here the first theoretical study of methane activation on a Pt4 cluster. Among the five single steps toward dehydrogenation, our results show that the rate-limiting step is the third step, that is, breaking the second C-H bond, which requires overcoming an energy barrier of 28 kcal/mol. On the other hand, the cleavage of the first C-H bond, that is, the first reaction step, requires overcoming an energy barrier of 4 kcal/mol.     

F+CH_3OH碰撞反应机机理和反应势能面

以MP2（full）/6-311 + +g(d,p)水平上详细研究了氟原子与甲醇抽氢反应的 多通道反应机理，得到了各条通道中涉及的驻点的构型和振动频率及其能量，给出 了两张完整的反应势能面，结果表明，氟原子从C原子上抽氢时有一条明显的最低 能量通道，而从氧原子上抽氢时要涉及多条分支通道和多个驻点构型，给出了各分 支通道势能面示意图，结果表明以形成五元环状过渡态通道为优势通道，计算得到 经途径1生成CH_2OH时反应放热170.62kJ/mol，经分支途径6生成CH_3O自由基时反 应放热119.4 kJ/mol，此结果与实验值一致。     

Entropy effects in the fragmentation of 1,1-dimethylhydrazine ions

The 1,1-dimethylhydrazine ion ((CH3)2NNH2+*) has two low-energy dissociation channels, the loss of a hydrogen atom to form the fragment ion m/z 59, (CH3)(CH2)NNH2+, and the loss of a methyl radical to form the fragment ion m/z 45, the methylhydrazyl cation, CH3NNH2+. The dissociation of the 1,1-dimethylhydrazine ion has been investigated using threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy, in the photon energy range 8.25-31 eV, and tandem mass spectrometry. Theoretical breakdown curves have been obtained from a variational transition state theory (VTST) modeling of the two reaction channels and compared to those obtained from experiment. Seven transition states have been found at the B3-LYP/6-31+G(d) level of theory for the methyl radical loss channel in the internal energy range of 2.32-3.56 eV. The methyl loss channel transition states are found at R(N-C) = 4.265, 4.065, 3.965, 3.165, 2.765, 2.665, and 2.565 A over this internal energy range. Three transition states have been found for the hydrogen atom loss channel: R(H-C) = 2.298, 2.198, and 2.098 A. The DeltaS++(45) value, at an internal energy of 2.32 eV and a bond distance of R(N-C) = 4.265 A, is 65 J K-1 mol-1. As the internal energy increases to 3.56 eV the variational transition state moves to lower R value so that at R(N-C) = 2.565 A, the DeltaS++ decreases to 29 J K-1 mol-1. For the hydrogen atom loss channel the variation in DeltaS++ is less than that for the methyl loss channel. To obtain agreement with the experimental breakdown curves, DeltaS++(59) = 26-16 J K-1 mol-1 over the studied internal energy range. The 0 K enthalpies of formation (DeltafH0) for the two fragment ions m/z 45 and m/z 59 have been calculated from the 0 K activation energies (E0) obtained from the fitting procedure: DeltafH0[CH3NNH2+] = 906 +/- 6 kJ mol-1 and DeltafH0[(CH3)(CH2)NNH2+] = 822 +/- 7 kJ mol-1. The calculated G3 values are DeltafH0[CH3NNH2+] = 911 kJ mol-1 and DeltafH0[(CH3)(CH2)NNH2+] = 825 kJ mol-1. In addition to the two low-energy dissociation products, 21 other fragment ions have been observed in the dissociation of the 1,1-dimethylhydrazine ion as the photon energy was increased. Their appearance energies are reported.     

Ab initio and microcalorimetric investigations of alkene adsorption on phosphotungstic acid

The adsorption of ethene, propene, 1-butene, trans-2-butene, and isobutene on phosphotungstic acid has been characterized by density functional theory (DFT) calculations and microcalorimetric experiments. The DFT-calculated chemisorption energies to form the corresponding alkoxides for ethene, propene, 1-butene, trans-2-butene, and isobutene were -86.8, -90.3, -102.6, -79.9, and -91.4 kJ mol(-1), respectively (for their most-favorable binding modes). The relative chemisorption energies to form the alkoxides are dictated by the strength of interaction of the acidic proton with the carbon atom of the double bond that becomes protonated. The activation barrier for chemisorption was greatest for alkenes with primary (1 degrees) carbenium-like transition states followed by secondary (2 degrees) and tertiary (3 degrees) transition states. The adsorption enthalpy established from microcalorimetric experiments with propene and isobutene was approximately -100 kJ mol(-1), which is close to the DFT-calculated values. Chemisorption of ethene on phosphotungstic acid during microcalorimetric experiments was minimal, presumably because of the large activation barrier associated with a 1 degrees carbenium-like transition state. The results from this study are compared with those in the literature for the adsorption of alkenes on zeolites, which have a similar adsorption mechanism. Our results suggest that alkene adsorption is stronger on phosphotungstic acid than on zeolites, as supported by the more exothermic chemisorption energies. Additionally, activation barriers for alkene adsorption are lower over phosphotungstic acid than over zeolites.     

PAH formation under single collision conditions: reaction of phenyl radical and 1,3-butadiene to form 1,4-dihydronaphthalene

The crossed beam reactions of the phenyl radical (C(6)H(5), X(2)A(1)) with 1,3-butadiene (C(4)H(6), X(1)A(g)) and D6-1,3-butadiene (C(4)D(6), X(1)A(g)) as well as of the D5-phenyl radical (C(6)D(5), X(2)A(1)) with 2,3-D2-1,3-butadiene and 1,1,4,4-D4-1,3-butadiene were carried out under single collision conditions at collision energies of about 55 kJ mol(-1). Experimentally, the bicyclic 1,4-dihydronaphthalene molecule was identified as a major product of this reaction (58 ± 15%) with the 1-phenyl-1,3-butadiene contributing 34 ± 10%. The reaction is initiated by a barrierless addition of the phenyl radical to the terminal carbon atom of the 1,3-butadiene (C1/C4) to form a bound intermediate; the latter underwent hydrogen elimination from the terminal CH(2) group of the 1,3-butadiene molecule leading to 1-phenyl-trans-1,3-butadiene through a submerged barrier. The dominant product, 1,4-dihydronaphthalene, is formed via an isomerization of the adduct by ring closure and emission of the hydrogen atom from the phenyl moiety at the bridging carbon atom through a tight exit transition state located about 31 kJ mol(-1) above the separated products. The hydrogen atom was found to leave the decomposing complex almost parallel to the total angular momentum vector and perpendicularly to the rotation plane of the decomposing intermediate. The defacto barrierless formation of the 1,4-dihydronaphthalene molecule involving a single collision between a phenyl radical and 1,3-butadiene represents an important step in the formation of polycyclic aromatic hydrocarbons (PAHs) and their partially hydrogenated counterparts in combustion and interstellar chemistry.     

H-Atom abstraction from CH(3)NHNH(2) by NO(2): CCSD(T)/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) and CCSD(T)/6-311+G(2df,p)//CCSD/6-31+G(d,p) calculations

Stationary points of paths for H atom abstraction from CH(3)NHNH(2) (monomethylhydrazine) by NO(2) were characterized via CCSD(T)/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) and CCSD(T)/6-311+G(2df,p)//CCSD/6-31+G(d,p) calculations. Five transition states connecting CH(3)NHNH(2)-NO(2) complexes to a manifold that includes CH(3)NHNH-HONO, CH(3)NNH(2)-HONO, CH(3)NNH(2)-HNO(2), and CH(3)NHNH-HNO(2) complexes were identified. Transition states that connect CH(3)NHNH-HONO, CH(3)NNH(2)-HONO, CH(3)NNH(2)-HNO(2), and CH(3)NHNH-HNO(2) complexes to each other via H atom exchange and/or hindered internal rotation were also identified. The high point in the minimum energy path from the CH(3)NHNH(2) + NO(2) reactant asymptote to the manifold of HONO-containing product states is a transition state 8.6 kcal/mol above the reactant asymptote. From a kinetics standpoint, this value is considerably higher than the 5.9 kcal/mol value that was estimated for it based on theoretical results for H atom abstraction from NH(3) by NO(2).     

Gas-phase acidities and O-H bond dissociation enthalpies of phenol, 3-methylphenol, 2,4,6-trimethylphenol, and ethanoic acid

Energy-resolved, competitive threshold collision-induced dissociation (TCID) methods are used to measure the gas-phase acidities of phenol, 3-methylphenol, 2,4,6-trimethylphenol, and ethanoic acid relative to hydrogen cyanide, hydrogen sulfide, and the hydroperoxyl radical using guided ion beam tandem mass spectrometry. The gas-phase acidities of Delta(acid)H298(C6H5OH) = 1456 +/- 4 kJ/mol, Delta(acid)H298(3-CH3C6H4OH) = 1457 +/- 5 kJ/mol, Delta(acid)H298(2,4,6-(CH3)3C6H2OH) = 1456 +/- 4 kJ/mol, and Delta(acid)H298(CH3COOH) = 1457 +/- 6 kJ/mol are determined. The O-H bond dissociation enthalpy of D298(C6H5O-H) = 361 +/- 4 kJ/mol is derived using the previously published experimental electron affinity for C6H5O, and thermochemical values for the other species are reported. A comparison of the new TCID values with both experimental and theoretical values from the literature is presented.     

Density functional theory analysis of the reaction pathway for methane oxidation to acetic acid catalyzed by Pd2+ in sulfuric acid

Density functional theory has been used to investigate the thermodynamics and activation barriers associated with the direct oxidation of methane to acetic acid catalyzed by Pd2+ cation in concentrated sulfuric acid. Pd2+ cations in such solutions are ligated by two bisulfate anions and by one or two molecules of sulfuric acid. Methane oxidation is initiated by the addition of CH4 across one of the Pd-O bonds of a bisulfate ligand to form Pd(HSO4)(CH3)(H2SO4)2. The latter species will react with CO to produce Pd(HSO4)(CH3CO)(H2SO4)2. The most likely path to the final products is found to be via oxidation of Pd(HSO4)(CH3)(H2SO4)2 and Pd(HSO4)(CH3CO)(H2SO4)2 to form Pd(eta2-HSO4)(HSO4)2(CH3)(H2SO4) and Pd(eta2-HSO4)(HSO4)2(CH3CO)(H2SO4), respectively. CH3HSO4 or CH3COHSO4 is then produced by reductive elimination from the latter two species, and CH(3)COOH is then formed by hydrolysis of CH3COHSO4. The loss of Pd2+ from solution to form Pd(0) or Pd-black is predicted to occur via reduction with CO. This process is offset, though, by reoxidation of palladium by either H2SO4 or O2.