Hydrogen isotope exchange reaction rate in tritium and methane mixed gas

Hydrogen isotope exchange reaction rate in tritium and methane mixed gas, as induced by tritium decay and beta radiation, has been experimentally measured. Initially T₂ gas was filled to 40 kPa and 20 kPa of CH₄ gas was added. The mixed gas spectrum was analyzed periodically by laser Raman spectrometry. The first order HT and H₂ formation rates and T₂ and CH₄ decay rates by hydrogen isotope exchange reaction were observed between 2.9·10^–3 h^–1 and 4.8·10^–3 h^–1. Although the estimated hydrogen isotope exchange reaction rate was 1/20–1/10 slower than the rate of H₂+T₂ mixed gases, it was nearly equivalent to the ion formation rate by tritium beta radiation. This suggested that isotopic hydrogen radicals formed via ionization would disappear in the presence of methane.     

Exchange of mercury between mercuric ions and methylmercuric nitrate

The rate constant for the exchange between Hg²⁺ and CH₃Hg⁺ ions in a Cl-free 1NH₂SO₄ solution at 25°C was measured with²⁰³Hg as a radiotracer and separation of the two compounds by liquid extraction of the organic species into benzene from a NaCl- or NaI-solution.     

Fluorierte Elementorganica: Oxidative Flüssigphasen-Direktfluorierung. X. Organyloxyfluorphosphorane: Direktsynthese durch F2-Addition an Phosphinic-, Phosphonic- sowie Phosphoricsäureester(fluoride) und Thermolyseverhalten

Fluorinated Organoelements: Oxidative Liquid-Phase Direct Fluorination. X. Organyloxyfluorophosphoranes: Direct Synthesis by F₂-Addition to Phosphinic-, Phosphonic-, and Phosphoric-Acid Ester(fluorides) and Thermal Behaviour The phenoxyfluorophosphoranes (PhO)₂PF₂R ( 2a: R = CH₃, 2b: R = Ph) and (PhO)_3?nPF_n+2 ( 4a: n = 0, 4b: n = 1, 4c: n = 2) were obtained in reasonable yield by direct fluorination of the corresponding organyloxy(fluoro)phosphanes for the first time. Contrary to 4c the intramolecular ligand exchange can be frozen up in 4b. The up to now unknown thermally unstable alkoxy-substituted difluorides (AlkO)_3?PF₂R_n ( 6a: Alk = CH₃, R = Ph), n = 2; 6b: Alk = CH₂CF₃, R = Ph, n = 2; 6c: Alk = CH₃, R = Ph, n = 1; 6d: Alk = CH₃, n = 0) were isolated by low temperature F₂-addition in pure substance, too. Their thermal decomposition (scrambling, CH₃F-elimination) was cleared up for 6a as model substance and transferred to (CH₃O)₃PF₂ 6d Here the splitting of (CH₃)₂O under “Arbusov-conditions” is very surprising. The trigonal bipyramidal covalent structure of all organyloxyphosphoranes was confirmed by multinuclear ¹⁹F, ³¹P{¹H}, ¹³C{¹H}) NMR experiments.     

Methyl group exchange in methyllithiumlithium bromide solutions in diethyl ether

The 220 MHz ¹H NMR spectrum of an ether solution of CH₃Li and LiBr in 10–1 ratio has been examined as a function of temperature. At low temperature distinct resonances, assignable to Li₄(CH₃)₄ and Li₄(CH₃)₃Br, are seen. Methyl group exchange between the two tetramers is observed in the NMR spectra in the temperature interval ?32 to 0°. The exchange is shown to be much slower than the dissociation of Li₄(CH₃)₄ tetramer, measured in other work. It is proposed that the rate-determining step is dissociation of Li₄(CH₃)₃Br to form Li₂(CH₃)₂ and Li₂(CH₃)Br. The rate constant for dissociation, k₂, obeys the equation ln k₂ = 36.0?83303/T.     

Ion cyclotron resonance studies of some reactions of C+ ions

Product distributions and rate constants for the reaction of ground state C⁺ ions with O₂, NO, HCl, CO₂, H₂S, H₂O, HCN, NH₃, CH₄, H₂CO, CH₃OH, and CH₃NH₂ have been measured. Rate constants were obtained using ion cyclotron resonance trapped ion methods at JPL, and product distributions were obtained using a tandem (Dempster-ICR) mass spectrometer at the University of Utah. Rapid carbon isotope exchange has also been observed in C⁺-CO collisions.     

Ligand Effects on the Mechanisms of Thermal Bond Activation in the Gas‐Phase Reactions NiX+/CH4→Ni(CH3)+/HX (X=H,CH3, OH,F). Short Communication

The thermal ion‐molecule reactions NiX⁺+CH₄→Ni(CH₃)⁺+HX (X=H, CH₃, OH, F) have been studied by mass spectrometric methods, and the experimental data are complemented by density functional theory (DFT)‐based computations. With regard to mechanistic aspects, a rather coherent picture emerges such that, for none of the systems studied, oxidative addition/reductive elimination pathways are involved. Rather, the energetically most favored variant corresponds to a σ‐complex‐assisted metathesis (σ‐CAM). For X=H and CH₃, the ligand exchange follows a ‘two‐state reactivity (TSR)’ scenario such that, in the course of the thermal reaction, a twofold spin inversion, i.e., triplet→singlet→triplet, is involved. This TSR feature bypasses the energetically high‐lying transition state of the adiabatic ground‐state triplet surface. In contrast, for X=F, the exothermic ligand exchange proceeds adiabatically on the triplet ground state, and some arguments are proposed to account for the different behavior of NiX⁺/Ni(CH₃)⁺ (X=H, CH₃) vs. NiF⁺. While the couple Ni(OH)⁺/CH₄ does not undergo a thermal ligand switch, the DFT computations suggest a potential‐energy surface that is mechanistically comparable to the NiF⁺/CH₄ system. Obviously, the ligands X act as a mechanistic distributor to switch between single vs. two‐state reactivity patterns.     

Deposition of Diamondlike Carbon Film and Mass Spectrometry Measurement in CH4/N2 RF Plasma

The deposition of diamondlike carbon (DLC) film and the measurements of ionic species by means of mass spectrometry were carried out in a CH₄/N₂ RF (13.56 MHz) plasma at 0.1 Torr. The film deposition rate greatly depended on both CH₄/N₂ composition ratio and RF power input. It was decreased monotonically as CH₄ content decreased in the plasma and then rapidly diminished to negligible amounts at a critical CH₄ content, which became large for higher RF power. The rate increased with increasing RF power, reaching a maximum value in 40% CH₄ plasma. The predominant ionic products in CH₄/N₂ plasma were NH⁺ ₄ and CH₄N⁺ ions, which were produced by reactions of hydrocarbon ions, such as CH⁺ ₃, CH⁺ ₂, CH⁺ ₅, and C₂H⁺ ₅ with NH₃ molecules in the plasma. It was speculated that the production of NH⁺ ₄ ion induced the decrease of C₂H⁺ ₅ ion density in the plasma, which caused a reduction in higher hydrocarbon ions densities and, accordingly, in film deposition rate. The N⁺ ₂ ion sputtering also plays a major role in a reduction of film deposition rate for relatively large RF powers. The incorporation of nitrogen atoms into the bonding network of the DLC film deposited was greatly suppressed at present gas pressure conditions.     

Reaction of excited iodine atoms with methyl iodide: Rate constant determinations

The rate constant for the reaction I(²P_1/2) + CH₃I → I₂ + CH₃ has been reevaluated taking into account both collisional deactivation of excited iodine atoms and loss of I₂ by I₂ + CH₃ → I + CH₃I. The reevaluation is based upon data obtained (R. T. Meyer), J. Chem. Phys., 46 , 4146 (1967) from the flash photolysis of CH₃I using time-resolved mass spectrometry to measure the rate of I₂ formation. Computer simulations of the complete kinetic system and a closed-form solution of a simplified set of the differential equations yielded a value of 6(± 4) × 10⁶ 1./mole-sec for the excited iodine atom reaction in the temperature region of 316 to 447 K. A slight temperature dependence was observed, but an activation energy could not be evaluated quantitatively due to the small temperature range studied. An upper limit for the collisional deactivation of I(²P_1/2) with CH₃I was also determined (2.4 × 10⁷ 1./mole-sec).     

The hydroxyl radical reaction rate constant and atmospheric transformation products of 2-butanol and 2-pentanol

The relative rate technique has been used to measure the hydroxyl radical (OH) reaction rate constant of +2-butanol (2BU, CH₃CH₂CH(OH)CH₃) and 2-pentanol (2PE, CH₃CH₂CH₂CH(OH)CH₃). 2BU and 2PE react with OH yielding bimolecular rate constants of (8.1±2.0)×10⁻¹² cm³molecule⁻¹s⁻¹ and (11.9±3.0)×10⁻¹² cm³molecule⁻¹s⁻¹, respectively, at 297±3 K and 1 atmosphere total pressure. Both 2BU and 2PE OH rate constants reported here are in agreement with previously reported values [1–4]. In order to more clearly define these alcohols' atmospheric reaction mechanisms, an investigation into the OH+alcohol reaction products was also conducted. The OH+2BU reaction products and yields observed were: methyl ethyl ketone (MEK, (60±2)%, CH₃CH₂C((DOUBLEBOND)O)CH₃) and acetaldehyde ((29±4)% HC((DOUBLEBOND)O)CH₃). The OH+2PE reaction products and yields observed were: 2-pentanone (2PO, (41±4)%, CH₃C((DOUBLEBOND)O)CH₂CH₂CH₃), propionaldehyde ((14±2)% HC((DOUBLEBOND)O)CH₂CH₃), and acetaldehyde ((40±4)%, HC((DOUBLEBOND)O)CH₃). The alcohols' reaction mechanisms are discussed in light of current understanding of oxygenated hydrocarbon atmospheric chemistry. Labeled (¹⁸O) 2BU/OH reactions were conducted to investigate 2BU's atmospheric transformation mechanism details. The findings reported here can be related to other structurally similar alcohols and may impact regulatory tools such as ground level ozone-forming potential calculations (incremental reactivity) [5]. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 745–752, 1998     

Palladium(II)-catalysed H/D allylic exchange in alkenes: An intermediacy of palladium(IV)?

Evidence is presented that the dimeric π-allylic species [(η³-allyl)PdCl]₂ is not intermediate in the Li₂Pd₂Cl₆-catalysed allylic H/D exchange in alkenes. Neither H/D exchange in α-methylstyrene, nor enrichment of [(η³-2-PhC₃H₄)PdCl]₂, was observed when the latter complex was incubated at 100°C in D₃CCOOD either in the presence or in the absence of PhC(CH₃)?CH₂, respectively. The kinetics of H/D exchange in α-methylstyrene catalysed by Li₂Pd₂Cl₆ were studied in some detail. The exchange proceeds at highest rates when reduction of palladium(II) takes place and is much slower in the presence of 1,4-benzoquinone as a palladium reoxidant. The exchange rate is directly proportional to the alkene and catalyst concentrations and independent of the reoxidant concentration. It is suggested that the palladium(II)-catalysed exchange involves an intermediate hydrid-allyl species where palladium has a formal oxidation state of IV.     

Über das fluktuierende Verhalten von triphenylmethylsubstituierten Cyclopentadienyl-Metallverbindungen

The fluxional behaviour of triphenylmethyl substituted cyclopentadienyl metal compounds ¹H-NMR. spectroscopy has been used to study the influence of the triphenylmethyl substituent on the metallotropic rearrangement in C₅H₄(CPh₃)Si(CH₃)₃ and C₅H₄ (CPh₃)Sn(CH₃)₃. The most likely mechanism corresponds to a degenerate metal exchange between two neighbouring ring positions. The AA'XX' spectrum of the cyclopentadienyl ring protons in C₅H₄(CPh₃)Sn(CH₃)₃ has been analysed under rapid exchange conditions. The free energy of activation for the sigmatropic [1,5]-Sn shift has been measured by comparison with computer simulated spectra for slow exchange.     

Ab initio calculations of activation energy of the reaction of hydrogen exchange on strongly acidic centers

Ab initio calculations of fragments of the potential energy surfaces of hydrogen exchange reactions between H₂, CH₄, and alanine molecules and the H₃O⁺ ion were performed by the restricted Hartree-Fock method, at the second-order Møller-Plesset level of perturbation theory, and by the method of coupled clusters using the 6–31G^* and aug-cc-pVDZ basis sets. The one-center synchronous mechanism of hydrogen exchange reaction was studied and the activation energies and structures of transition states were determined. It was found that the geometric parameters of the H₂ and CH₄ molecules in the transition states are close to those of the H₃ ⁺ and CH₅ ⁺ ions. The higher the proton affinity of the reacting molecule in the reaction studied the lower the activiation energy of hydrogen exchange. The one-center mechanism studied can be used to describe the high-temperature solid-state catalytic isotope exchange (HSCIE) reaction. The results ofab initio calculations of synchronous hydrogen exchange between the H₃O⁺ ion and hydrogen atoms in different positions of the alanine molecule are in good agreement with experimental data on the regioselectivity and stereoselectivity of the HSCIE reaction with spillover-tritium.     

A rate constant for CH2(3B1) + CH3

Triplet methylene, CH₂(³B₁), and methyl radicals were produced by flash photolysis of a mixture of ketene and azomethane. A computer fit of the product ratios, using the known rate constants for CH₂ + CH₂, and CH₃ + CH₃, requires a rate constant of 5.0 × 10^?11 cm³ molecule^?1s^?1 for the reaction CH₂ + CH₃ ? C₂H₄ + H.     

Adducts of Niobium (V) and Tantalum (V) Halides. XV. Ligand Exchange of the Adducts with Some Phosphoryl Compounds

The ligand exchange MX₅·L + *L?MX₅·*L + L for the octahedral adducts MX₅·L, in an inert solvent (CH₂Cl₂ or CHCl₃) with neutral ligands, proceeds via a dissociative D mechanism when M = Nb, X = Cl and L = phosphoryl compound. A dissociative interchange I_d mechanism is suggested when M = Nb or Ta, and X = F. A first order rate law and positive values for ΔS* (+4 to +14 cal K^?1 mol^?1) are observed for the exchanges on the pentachloride adducts. However, a second order rate law and large negative values for ΔS* (-15 to -24 cal K^?1 mol^?1) are found for the intermolecular neutral ligand exchange (measured by ¹H-NMR.) and for the intramolecular fluorine exchange (measured by ¹⁹F-NMR.) reactions on the pentafluoride adducts. The fluorine exchange is 2 to 5 times faster than the ligand exchange. The exchanges, on the pentachloride and on the pentafluoride adducts, are slowed down with increasing donor strength of the phosphoryl compound.     

Electrophilic hydrogen-deuterium exchange of some deactivated pyrroles

The rate coefficients and partial rate factors for the hydrogen-deuterium exchange of some 1-substituted pyrroles (1-substituents = -COCH₃, - COC₆H₅, -SO₂CH₃, -SO₂CF₃, -N(CH₃)₃, -NH(CH₃)₂, -SO₂C₆H₅) in deuterated trifluoroacetic acid have been determined. In all cases the rate of exchange is faster at the 2-position. Similarly, the hydrogen-deuterium exchange of some pyrroles with electron withdrawing substituents in the 2-position indicate a relative reactivity of 4→ 5→ 3-position with the selectivity being greatest for the more electron withdrawing groups. A nitro group in the 3-position of pyrrole shows a relative reactivity of 5→2→4-position for the hydrogen-deuterium exchange in deuterated trifluoroacetic acid. A linear correlation is observed for the log of the rate coefficient of the hydrogen-deuterium exchange at the 4-position of some 2-substituted pyrroles and the difference in the calculated energy of formation of the pyrrole and the corresponding 4-deuterated cation.     

Dynamics of the CH3 + OH reaction

We study dynamics of the CH₃ + OH reaction over the temperature range of 300–2500 K using a quasiclassical method for the potential energy composed of explicit forms of short‐range and long‐range interactions. The explicit potential energy used in the study gives minimum energy paths on potential energy surfaces showing barrier heights, channel energies, and van der Waals well, which are consistent with ab initio calculations. Approximately, 20% of CH₃ + OH collisions undergo OH dissociation in a direct‐mode mechanism on a subpicosecond scale (<50 fs) with the rate coefficient as high as ～10^?10 cm³ molecule^?1 s^?1. Less than 10% leads to the formation of excited intermediates CH₃OH? with excess vibrational energies in CO and OH bonds. CH₃OH? stabilizes to CH₃OH, redissociates back to reactants, or forms one of various products after intramolecular energy redistribution via bond dissociation and formation on the time scale of 50–200 fs. The principal product is ¹CH₂ (k being ～10^?11), whereas ks for CH₂OH, CH₂O, and CH₃O are ～10^?12. The minor products are HCOH and CH₄ (k～10^?13). The total rate coefficient for CH₃ + OH → CH₃OH? → products is ～10^?11 and is weakly dependent on temperature. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 455–466, 2011     

Electrochemical reduction of dichlorodifluoromethane at a Nafion® solid polymer electrolyte cell

The electrochemical reduction of dichlorodifluoromethane (CFC-12) at Ag, Pd, Cu and Au electrodes (which were deposited on Nafion® 117 (H⁺ form) membranes, by reduction with NaBH₄ (10% w/v) solution) was studied. The products of the reduction were CHClF₂, CH₂F₂, CH₃F and CH₄, as well as small amounts of dimers, CF₂=CF₂ and CHF₂CHF₂. The silver electrode gave the highest current efficiency (CE) and reduction rate. The rate of reduction at the silver electrode was almost 10–160 times higher than that measured for the other electrodes, under the same conditions. Selectivity of CH₄ production increased for all metals with increasing negative potential, except for CHClF₂ where it decreased. For the other products, a maximum in the selectivity–potential curve appeared. This fact led us to the conclusion that the reduction proceeds by the following mechanism: CCl₂F₂→CHClF₂→CH₂F₂→CH₃F→CH₄. The rate of reduction of CFC-12 and the product distribution also depend on the pH of the solution, which is in contact with the membrane. The rate of reduction at the silver electrode was about 4000 times higher at pH 14 than at pH 1. The cation of the supporting electrolyte was also important: the rate of reduction was lowered in the order K⁺>Na⁺>Li⁺, and this was attributed to the size of the cations, which influenced the structure of the double layer.     

A combined ab initio quantum mechanical and molecular mechanical method for carrying out simulations on complex molecular systems: Applications to the CH3Cl + Cl− exchange reaction and gas phase protonation of polyethers

We present an approach to couple ab initio quantum mechanical geometry optimiuzations with molecular mechanical optimizations, with the added capability to carry out molecular dynamics simulations of the systems to earch for new local minima. The approach is applied to the aqueous solution CH₃Cl + Cl^? exchange reaction and the gas phase protonation of polyethers.     

Exchange bias in Ni4 single-molecule magnets

The syntheses and physical properties are reported for three single-molecule magnets (SMMs) with the composition [Ni(hmp)(ROH)Cl]₄, where R is CH₃ (complex 1), CH₂CH₃ (complex 2) or CH₂CH₂C(CH₃)₃ (complex 3) and hmp⁻ is the monoanion of 2-hydroxymethylpyridine. The core of each complex is a distorted cube formed by four Ni^II ions and four alkoxide hmp⁻ oxygen atoms at alternating corners. Ferromagnetic exchange interactions give a S=4 ground state. Single crystal high-frequency EPR spectra clearly indicate that each of the complexes has a S=4 ground state and that there is negative magnetoanisotropy, where D is negative for the axial zero-field splitting DŜ_z². Magnetization versus magnetic field measurements made on single crystals with a micro-SQUID magnetometer indicate these Ni₄ complexes are SMMs. Exchange bias is seen in the magnetization hysteresis loops for complexes 1 and 2.     

LIGAND-EXCHANGE REACTION BETWEEN TRIS(1,10-PHENANTHROLINE)METAL(II) AND TRIS(4,7-DIPHENYL-1,10-PHENANTHROLINE)METAL(II) IONS

Abstract

The ligand exchange reaction between [M(phen)₃]²⁺ and [M(DIP)₃]²⁺ (where M is the same and M = Fe^II or Ni^II, phen = 1,10-phenanthroline, DIP = 4,7-diphenyl-1,10-phenanthroline) has been investigated by reversed phase ion-paired chromatography (RP-IPC). The effect of pH and solvent on the ligand-exchange reaction is studied by monitoring the variation in chromatograms with time after mixing. The results have shown that the ligand exchange reaction between [M(phen)₃]²⁺ and [M(DIP)₃]²⁺ takes place in the pH range of 3–8 and the rate of reaction for nickel(II) complexes is about two times slower than that for iron(II) complexes. Experiments on the effect of various solvents on the ligand-exchange reaction have revealed that the rate of reaction is enhanced by the solvent in the following order: (CH₃)₂CO > CHCl₃ ≥ CH₂Cl₂ > CH₃CN > CH₃OH. Elemental analysis and UV-visible spectroscopy confirmed that the products obtained from the ligand-exchange reaction are mixed-ligand complexes containing phen and DIP ligands, i.e., [M(phen)₂(DIP)]²⁺ and [M(phen)(DIP)₂]²⁺.