Energy distribution among reaction products. H + NOCl,H + ICl

The infrared chemiluminescence technique has been used to obtain k(V′, R′, T′) (V′, R′, Tt? are product vibrational, rotational and translational energies) for the reactions (i) H + ClNO → HCl + NO (energy-release E_tot′ = 68.5 kcal mole^?1) and (ii) H + CII → HCl + I (E_tot′ = 55.8 kcal mole^?1). Reaction (i) exhibits inefficient conversion of energy-release into vibration in the new bond, characteristic of a light attacking atom reacting on a repulsive energy-surface. Reaction (ii) has a bimodal HCl product-energy-distribution suggesting that 18% of the reaction proceeds by direct attack at the Cl end of CII to yield low V′ and R′, and 82% by indirect reaction from the 1 end to give high V′ and R′.     

Association reaction between SiH3 and H2O2: a computational study of the reaction mechanism and kinetics

The association reaction between silyl radical (SiH₃) and H₂O₂ has been studied in detail using high-level composite ab initio CBS-QB3 and G4MP2 methods. The global hybrid meta-GGA M06 and M06-2X density functionals in conjunction with 6-311++G(d,p) basis set have also been applied. To understand the kinetics, variational transition-state theory calculation is performed on the first association step, and successive unimolecular reactions are subjected to Rice–Ramsperger–Kassel–Marcus calculations to predict the reaction rate constants and product branching ratios. The bimolecular rate constant for SiH₃–H₂O₂ association in the temperature range 250–600 K, k(T) = 6.89 × 10^?13 T ^?0.163exp(?0.22/RT) cm³ molecule^?1 s^?1 agrees well with the current literature. The OH production channel, which was experimentally found to be a minor one, is confirmed by the rate constants and branching ratios. Also, the correlation between our theoretical work and experimental literature is established. The production of SiO via secondary reactions is calculated to be one of the major reaction channels from highly stabilized adducts. The H-loss pathway, i.e., SiH₂(OH)₂ + H, is the major decomposition channel followed by secondary dissociation leading to SiO.     

Distribution of reaction products (theory). XI. H + F2

The availability for the first time of detailed rate constants k(V′, R′, T′) (where V′, R′ and T′ are product vibrational, rotational and translational excitation) for the highly exothermic reaction H + F₂ → HF(V′, R′) + F has prompted the 3D classical-trajectory study reported here. The potential-energy surface is found to be predominantly repulsive ($\text{A}$_⊥ ≈ 42%, R_⊥ ≈ 58%) corresponding to the rather low fractional conversion of reaction energy into vibration ((f′V) = 0.58 from experiment, and 0.56 from theory). In the homologous series of reactions H + X₂ (X  F, Cl, Br, I) the percentage of repulsive energy-release decreases for X  Cl, Br, I, but increases from X  F to Cl. It is shown that this cannot be due to charge in mass-combination, but can plausibly be explained by the anomolously short range of interaction between the separating X atoms in the case X  F. It is predicted that the more-forward scattered HF will be more rotationally excited. The form of the cross section function S_r(T) (where T is reagent translation) is analysed. In accordance with the expectation for a strongly exothermic reaction, it is found that S_r(T) rises more steeply than S_r(V) (where V is reagent vibrational energy). The effect on the product energy distribution conforms qualitatively to the “adiabatic” behaviour noted in previous work: ΔT → ΔT′ + ΔR′; ΔV → ΔV′. The explanation is to be found in reaction through more-compressed or more-extended intermediate configurations than are characteristic of room temperature reaction. We note the existence of an amplification effect: (ΔT′ + ΔR′)/ΔT ≈ 2, and ΔV′/ΔV ≈ 2.     

A time-dependent wave packet study of the H4 four-center reaction

A quantum model based on the time-dependent initial state selected wave packet approach was developed to study the four-center (4C) reaction, A₂ + B₂ → 2AB, and the competing collision induced dissociation (CID), A₂ + B₂ → A + B₂ + A, as applied to the H₂(v₁) + H₂(v₂) system important in combustion. A reduced three-dimensional model of the reaction with the atoms constrained to an isosceles trapezium and a realistic global potential energy surface of Aguado et al. [J. Chem. Phys. 101 (1994) 2742], following Hernández and Clary [J. Chem. Phys. 104 (1996) 8413], was used. A method to analyse the reaction flux for 4C and CID reaction probabilities is presented. The initial A₂ vibrational excitation is not only more efficient than translational energy in facilitating the 4C and CID processes, it also reduces the threshold energy. Both the 4C and CID processes exhibit similar threshold energy behavior. For low vibrational excitation in the A₂ diatom, the 4C process is dominant; as the A₂ diatom becomes highly excited the CID process becomes more important at low collision energies with B₂, but as the collision energy increases the 4C process is favored again.     

Quantum-mechanical differential reaction cross sections for the F + H2(ν = 0) - FH(ν′= 2.3) + H reaction

Velocity scatterring angle intensity maps for the F + H₂(ν = 0): j = 0) $\text{\$}\text{?}$FH(ν′ = 2, 3: j′) + 11 reaction are predicted from quantum-mechanical J conserving, calculations. The extent of the shift in the angular distribution from backscattering at 1.8 kcal/mole to sideways scattering (intensity peak at 100°) at 3.0 kcal/mole is in quantitative agreement with recent crossed molecular beans experiments.     

Critical re-examination of the reaction Cl + H2 ⇌ HCl + H

Recent data on the system Cl + H₂

HCl + H indicate that the rate constant ratio k₁/k₂ is a factor of two smaller than the equilibrium constant K. Two earlier explanations of this discrepancy are shown to be compatible with recent experimental and theoretical work. As an alternative explanation we suggest that flow tube measurements, ostensibly of k₂, actually determine 2k₂, due to Cl + H recombination on the walls of the flow tube. After correcting for this factor of two, all kinetic studies on this system are reconciled.     

Dirhodium catalyzed intramolecular enantioselective C–H insertion reaction of N-cumyl-N-(2-p-anisylethyl)diazoacetamide: synthesis of (−)-Rolipram

Cumyl(2,2-dimethyl-benzyl) was used as an N-protecting group for intramolecular C–H insertion reaction of α-diazoacetamide. Excellent chemoselectivity (>98:2) in C–H insertion over the aromatic addition of N-cumyl-N-(2-p-anisylethyl)diazoacetamide was obtained with Rh₂[(4S)-MEOX)]₄ catalyst in moderate enantioselectivity (53% ee). The reaction was successfully applied in the synthesis of (−)-Rolipram in 15% total yield.     

Rapid alkenylation of quinoxalin-2(1H)-ones enabled by the sequential Mannich-type reaction and solar photocatalysis

Herein, a rapid alkenylation of quinoxalin-2(1H)-ones enabled by a combination of Mannich-type reaction and solar photocatalysis is demonstrated. A wide range of functional groups are compatible, affording the corresponding products in moderate-to-good yields. Control experiments illustrate that the in situ generated ¹O₂ plays a central role in this reaction. This green and efficient strategy provides a practical solution for the synthesis of potentially bioactive compounds that containing a 3,4-dihydroquinoxalin-2(1H)-one structure.     

The transition metal catalysed reaction between η5-C5H5Mo(CO)3I and isonitriles

The reaction betweeen (η⁵-C₅H₅Mo(CO)₃I and RNC is catalysed by [η⁵ -C₅H₅Mo(CO)₃]₂ and readily yields η⁵-C₅H₅Mo(CO)_3?n(RNC)_nI (n = 1–3). A free radical mechanism is consistent with experimental data.     

A dynamic threshold in the F + H2 → HF + H reaction as determined from three-dimensional quasi-classical-reverse trajectory calculations

Quasi-classical dynamic threshold energies have been determined for two important reactive transitions in the F + H₂ reaction by performing extensive three-dimensional trajectory calculations for the corresponding reverse reactions. It is found that the energetic and dynamic thresholds for the reaction F + H₂(υ = 0,j) → HF(υ = 2,j = 6) + H are the same, whereas the latter threshold is approximately 0.08 eV greater than the former one for the reaction F + H₂(υ = 0,j) → HF(υ = 3,j=1) + H. These results are in good agreement with the corresponding semi-classical threshold results which are also reported. The relationship of these quasi-classical-reverse results to experimentally measured quantities is discussed.     

SIFDT study of the SO+2/H2 H-atom abstraction reaction

The exothermic H-atom abstraction reaction of SO⁺₂ with H₂ has been studied in a selected ion flow drift tube (SIFDT) over a range of center-of-mass energies from thermal (300 K) to about 0.12 eV. The measured rate coefficient at 300 K is 4.2 × 10⁻¹² cm³ s⁻¹ which is very much less than the Langevin capture rate. The increase in rate coefficient with ion kinetic energy gives a linear Arrhenius-type plot with a slope that indicates a barrier of ∼5 kJ mol⁻¹ exists on the potential surface. The H₂SO⁺₂ potential surface is also explored in an ab initio investigation using the G2 procedure. An (SO⁺₂.H₂)¹ transition state between reactants and products is identified, corresponding to the barrier found from experiments.     

Kinetics of HD exchange in the reaction of HBF2 with D2

The rate constant for the unusually rapid HD exchange reaction of D₂ with HBF₂ : D₂(g) + HBF₂(g) → DBF₂(g) + HD(g) has been measured (k₂(298K) = (7.42 ± 2.0) × 10^?23 cm³/molecule s). The activation energy for this reaction has been estimated to be 17.8 ± 1.2 kcal/mole. The mechanism probably involves a multicenter orbital interaction between D₂ and HBF₂.     

Iodine-catalyzed synthesis of 2-arylpyrazolo[5,1-b]quinazolin-9(3H)-one derivatives in ionic liquids via domino reaction

The domino reaction of (E)-2-amino-N′-(1-arylethylidene)benzohydrazide and triethyl orthoformate in ionic liquids catalyzed by 10 mol % iodine gave 3a,4-dihydro-2-arylpyrazolo[5,1-b]quinazolin-9(3H)-one derivatives unexpectedly. In the presence of K₂S₂O₈, it could be oxidized to aromatized products in good yields.     

Pentafluorophenyltrifluorosilane in the silicon Mannich reaction

The synthesis of α-C₆F₅-substituted amines by a three-component coupling reaction of C₆F₅SiF₃, aromatic aldehydes, and N-trimethylsilylamines has been elaborated. The optimized conditions include performing the reaction in dimethylformamide in the presence of stoichiometric amounts of lithium acetate.     

Homodimeric coupling-cyclization reaction of 2,3-allenamides

The PdCl₂/NaI/K₂CO₃-catalyzed highly stereoselective homodimeric coupling-cyclization reaction of 2,3-allenamides afforded Z-bis(furanimine) derivatives. The addition of K₂CO₃ is crucial for this reaction and a possible mechanism is proposed.     

Mechanism and reaction rate of the karl-fischer titration reaction: Part I. Potentiometric measurements

The reaction rate of the coulometric variant of the Karl-Fischer titration reaction (in which electrolytically generated triiodide is used as oxidant instead of iodine) has been measured in methanol. The reaction is first order in water, sulfur dioxide and triiodide, respectively. For pH<5 the reaction rate constant decreases logarithmically with decreasing pH. Addition of pyridine solely influences the pH (by fixing it to a value of about 6) and has no direct influence on the reaction rate. A linear relation exists between the reaction rate constant and the reciprocal value of the iodide concentration, from which we can calculate the individual reaction rates for the oxidation by iodine and triiodide, respectively. While the reaction rate constant for triiodide is relatively small (k₃≈350 l² mol^?2s^?1), the reaction rate constant for iodine is much larger (k₃≈1.5×10⁷ l² mol^?2 s^?1.     

Surface catalyzed reaction of Hg + Cl2

An investigation of the reaction of mercury atoms with molecular chlorine was performed in heated reaction vessels constructed of Inconel, quartz, stainless steel and Teflon-coated stainless steel. The reaction was shown to proceed as a surface catalyzed reaction stoichiometrically producing (HgCl₂)_n.     

Selective detection of the tolyl cation among other [C7H7]+ isomers by ion/molecule reaction with dimethyl ether

The ion/molecule reaction of the tolyl cation with dimethyl ether has been investigated using triple quadrupole mass spectrometry. Three isomers with [C₇H₇]⁺ composition, the 3-tolyl, benzyl, and tropylium cations, were individually selected and reacted with dimethyl ether at a pressure of 1 mtorr in the second quadrupole (Q₂) collision cell. Only the tolyl ion reacted to yield a methoxylated product ion peak at m/z 122. This reaction product having m/z 122 is postulated to be identical in structure with the molecular ion of 3-methyl anisole, as supported by thermochemical data and the similarity of the collision induced dissociation (CID) daughter ion mass spectra of the product ion and the molecular ion of authentic 3-methyl anisole. The daughter ion mass spectra of the three [C₇H₇]⁺ isomers during CID, by using a triple quadrupole mass spectrometer, are nearly identical; on the other hand, the analytical approach based on the ion/molecule reaction with dimethyl ether clearly exhibits distinct gas-phase chemistry reflecting structural differences among the isomers. Sot     

The reaction between solid lead and chlorine gas

When lead reacts with a chlorine atmosphere the reaction rate shows a logarithmic rate law followed by a parabolic one. Both reaction rates can be explained by assuming that electron holes in the reaction product are rate-determining. In the beginning, the reaction of holes with lead (I) ions on lead ion lattice sites in PbCl₂ is the rate-determining step; later, the transport of holes by diffusion through the growing PbCl₂ layer becomes rate-determining.An oxide layer on the lead before chlorination retards the reaction, while, on the contrary, traces of oxygen in the chlorine gas accelerate the reaction. Both phenomena can be explained by assuming that a mixed compound PbO_1?xCl_2x is formed. It is concluded that the presence of impurities in the system may influence the reaction rate drastically.     

The reaction of metalloporphyrins with nitrogen dioxide

The octaethylporphyrin(OEP) complexes of iron(III) chloride, iron(III) acetate, thallium(III) hydroxide, zinc(II), and cobalt(II) and the mesoporphyrin IX dimethyl ester (MPDME) complexes of zinc(II) and iron(III) chloride were reacted with a 20:1 ratio of NO₂ to metalloporphyrin in CH₂Cl₂. The +3 metalloporphyrins gave products which had a nitromethyl group in each of the four meso positions of the porphyrin ring and a chloride ion bound to the metal atom. The products of +2 metalloporphyrin reaction had a nitro group bound in each of the meso positions. The spectral and electrochemical properties of some of the products were measured. ³⁶Cl labelled OEPFeCl was reacted with NO₂ in CH₂Cl₂. The product, meso-tetranitromethyl OEPFeCl, had 17% of the original activity which indicates that the chloride ion bound to the iron is exchanged with chloride ions formed in the reaction. The nitromethylation reaction appears to involve initially the displacement of chloride from iron(III) by NO₂ and solvent attack on the bound NO₂. The meso-nitration of the +2 metalloporphyrin by NO₂ has been proposed to proceed by a π-cation radical mechanism (E.C. Johnson and D. Dolphin, TetrahedronLetters 2197 (1976).