Differential energy-loss spectra for CH4 + Ar collisions

In a crossed molecular-beam experiment differential time-of-flight spectra have been measured for CH₄ + Ar collisions at E = 93.1 meV. Ale energy-loss spectra, which show a remarkable transfer of rotational energy, are compared with coupled-states calculations based on a model potential previously derived.     

The structure of the H3O+ (hydronium) ion

Near Hartree-Fock level ab initio molecular orbital calculations on H₃O⁺ and a minimum energy structure with θ(HOH) = 112.5° and r(OH) = 0.963 Å and an inversion barrier of 1.9 kcal/mole. By comparing these results to calculations on NH₃ and H₂O, where precise experimental geometries are known, we estimate the “true” geometry of isolated H₃O⁺ to have a structure with θ(HOH) = 110-112°, r(OH) = 0.97–0.98 Å and an inversion barrier of 2–3 kcal/mole. Our prediction for the proton affinity of water is ≈ 170 kcal/mole, which is somewhat smaller than the currently accepted value.     

Vibrational relaxation of N2(A3Σu+,υ = 1, 2,3) by CH4 and CF4

N₂(A, υ = 0-3) produced by the Ar(³P_0,2) + N₂ reaction and detected by laser-induced fluorescence undergoes rapid, stepwise vibrational relaxation but slow electronic quenching with added CH₄ or CF₄. Rate constants, k_Q^υ, of 1.5, 3.1, and 5.0 × 10^?12 cm³ s^?1 are measured for Q = CH₄, υ = 1-3, and 0.47, 1.8, and 5.5 × 10^?12 cm³ s^?1 for Q = CF₄, υ = 1-3, with ≈±20% accuracy (1σ). Information is also obtained for the unrelaxed, relative υ populations.     

The crystal and molecular structure of Rh2(CH3CO2)4[HCON(CH3)]2—effect of ligands on metalmetal bonding

The structure of Rh₂(CH₃CO₂)₄(DMF)₂ {DMF = HCON(CH₃)₂} has been determined by single crystal X-ray methods. The compound crystallizes with eight formula units in a cell of dimensions: a = 29.438(7) Å, b = 7.978(2) Å, c = 20.279(5) Å, β = 113.20(4)°, V = 4377.5 ų, space group C2/c. The structure has been refined by full-matrix least-squares method to a final R = 0.030 for the 4156 observed data. Two Rh(II) atoms are linked by four acetate groups forming a dimeric unit, where the RhRh distance is 2.383(1) Å. The coordination sphere about each Rh atom is completed by a DMF molecule; the average RhO(DMF) distance is 2.296(3) Å.     

Threshold energies for production of CH2(3B1) and CH2(1A1) from ketene photolysis. The CH2(3B1) ↔ CH2(1A1) energy splitting

By measuring the relative CO quantum yields from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25° for CH₂CO(¹A₁) → CH₂(³B₁) + CO(¹Σ⁺) to be 75.7 ± 1.0 kcal/mole. This corresponds to a value of 90.7 ± 1.0 kcal/mole for ΔH_f298⁰[CH₂(³B₁)]. By measuring the relative ratio of CH₂(¹A₁)/CH₂(³B₁) from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25°C for CH₂CO(¹A₁) → CH₂(¹A₁) + CO(¹Σ⁺) to be 84.0 ± 0.6 kcal/mole. This corresponds to a value of 99.0 ± 0.6 kcal/mole for ΔH_f298⁰[CH₂(¹A₁)]. Thus a value for the CH₂(³B₁) ? CH₂(¹A₁) energy splitting of 8.3 ± 1 kcal/mole is determined, which agrees with three other recent independent experimental estimates and the most recent quantum theoretical calculations.     

Vibrational effects in proton and charge transfer in the H+2 + Ar system

Reaction and charge transfer of H⁺₂ + Ar to give ArH⁺ and Ar⁺ have been investigated as a function of H⁺₂ vibrational quantum state and kinetic energy (Ec.m.), using photoionization and guided beam ion optics. Resonance effects are important in charge transfer; proton and charge transfer are closely coupled for Ec.m. 3 eV.     

Dissociation rate measurements in SO2 + Ar mixtures behind incident shock waves

Dissociation rates of SO₂ in SO₂ + Ar mixtures at 6%, 11%, 15% and 20% of SO₂ were measured behind incident shock waves over a temperature range 4000–6000 K at initial pressures 1.0 to 2.5 Torr. The recorded laser schlieren signals exhibited two exponentials, the faster one due to vibrational relaxation and the slower one due to dissociation. The initial dissociation rate was calculated from the value of the density gradient at the point of intersection of the two exponentials. A least-squares analysis of the experimental data yielded the following empirical relations: k_SO2Ar = 3.34 × 10¹⁵ exp(?107.6 kcal mole^?1/RT) cm³/mole s, k_SO2SO2 = 5.02 × 10¹⁴ exp(?66.6 kcal mole^?1 kcal mole^?1/RT) cm³/mole s.     

The crystal and molecular structures of the compounds [(η5-C5H5)Fe(CO)2]2(CH2)n, where 0 n = 3 and 4

The crystal stuctures of [(η⁵-C₅H₅)Fe(CO)₂]₂(CH₂)_n, n = 3 and 4, have been determined.[(η⁵-C₅H₅)Fe(CO)₂]₂(CH₂)₃: a = 21.20, b = 10.39, c = 7.88Å, β = 101.6°, U = 1699ų, C2/c, Z = 4, R = 0.059, 1036 observed data.[(η⁵-C₅H₅)Fe(CO)₂]₂(CH₂)₄: a = 7.63, b = 10.54, c = 21.87Å, β = 96.4°, U= 1748ų, P2₁/c, Z = 4, R = 0.051, 1418 observed data.In each compound the iron atoms are joined by simple chains of sigma bonded CH₂ groups. Bond lengths are similar in both: mean Fe-CO 1.75, C-O 1.15, FeC(cp) 2.11, FeCH₂ 2.08, (cp)CC(cp) 1.41, CH₂CH₂ 1.55Å. The (CH₂)₃ compound retains a 2-fold axis of symmetry in the crystal. The (CH₂)₄ compound has no imposed symmetry, but closely approximates centrosymmetry. The effects of molecular symmetry on the IR spectrum between 2100 and 1900 cm^-1 are discussed. The¹³C and₁H (270 MHz) NMR spectra in solution are shown to be consistent with the structures found crystallographically.     

Computation of the complexation energies of BH3NH3 and BH3PH3

Extended basis set computations on SCF and CEPA level were performed for BH₃NH₃ and BH₃PH₃ to determine the complexation energy ΔE and the equilibrium distance r(BX) between the “heavy” atoms. Our CEPA results (SCF in parentheses): ΔE(BH₃NH) = ?27(?21.3) kcal/mol, ΔE(BH₃PH₃) = ?17(?11.8) kcal/mol, r(BN) = 1.65(1.68) Å, r(BP) = 1.95(1.99) Å indicate a marked influence of electron correlation on these properties.     

Synthesis and crystal structure of (ThCu3)(Mn3+2Mn4+2)O12, a new ferrimagnetic perovskite-like compound

Single crystals of [ThCu₃](Mn³⁺₂Mn⁴⁺₂]O₁₂, a ferrimagnetic perovskite-like compound, have been synthesized by hydrothermal conditions at 600°C and 2 kbar. They have been found to be cubic, of space group Im3, with a = 7.359 Å, and isostructural with [NaMn₃](Mn³⁺₂Mn⁴⁺₂)O₁₂. The crystal structure has been refined by single-crystal X-ray diffraction data. The Th⁴⁺ cations are surrounded by slightly distorted icosahedra; the ThO distance is 2.556 Å. The Cu²⁺ cations are also surrounded by 12 oxygens, which are arranged as three mutually perpendicular rectangles of different size, the smallest and the largest of which are almost squares. The three sets of CuO distances are 1.973, 2.800, and 3.238 Å. The octahedral MnO distance is 1.950 Å. A test based on neutron diffraction powder data indicated that the square sites are occupied by only the Cu²⁺ cations.     

Carbon monoxide molecules in an argon matrix: empirical evaluation of the Ar·Ar,C·Ar and O·Ar potential parameters

A potential force field has been evaluated for the calculation of the properties of the solid CO-Ar system. The CO·Ar potential energy has been expressed as a sum of the C·Ar and O·Ar interatomic interactions. The (6-exp) Buckingham form of the atom—atom potential, ? = ?Ar^?6 + B exp (?αr), has been used (r is the interatomic distance). The values of the A, B and α numerical parameters for the C·Ar and O·Ar potential have been obtained from those for the C·C, O·O, and Ar·Ar potentials using known combining rules. These values are the following: A_C·Ar = 3379 kJ/mol A⁶, B_C·Ar = 3.12 × 10⁵ kJ/mol, α_C·Ar = 3.493 A^?1, A_O·Ar = 2737 kJ/mol A⁶, B_O·Ar = 3.28 × 10⁵ kJ/mol, α_O·Ar = 3.706 A^?1. The three parameters of the Ar·Ar potential function (A_Ar·Ar = 6554 kJ/mol A⁶, B_Ar·Ar = 3.27 × 10⁵ kJ/mol, α_Ar·Ar = 3.305 A^?1) have been fitted to a set of experimental data for the Ar crystal (zero-temperature lattice spacing and energy, and the value of the isothermal compressibility). The CO·Ar potential surface has been calculated showing the most favourable position of an Ar atom near the CO molecule and the orientational dependence of the CO·Ar interactions. The CO·Ar separation distance at the potential minimum and the depth of the potential well are equal to 3.63 A and ?1.321 kJ/mol, respectively. Comparison has been made of the derived Ar·Ar and Co·Ar potential functions with other such functions available in the literature.     

Structure and electrical conductivity of La0.84Sr0.16MnO3

X-Ray diffraction, density, and electrical conductivity measurements were performed on the perovskite-like mixed oxide La_0.84Sr_0.16MnO₃. A rhombohedral crystalline structure with lattice parameters a = 3.893 Å and α = 90°29′16″ was assigned to the powder prepared by standard ceramic technique. Its theoretical density is therefore 6.576 g/cm³, while the experimental density was determined as 6.48 g/cm³. The conductivity measured at 1000°C is 133 Ω^?1 cm^?1. The temperature dependence of the conductivity indicates that the charge carriers are small polarons. The activation energy of the mobility is 9.6 kJ/mole.     

1,2-Eliminations from (CH3)2NH+CH2CH3 and (CH3)2NH 2+ : Guided dissociations

1,2-Eliminations are a varied and extensive set of dissociations of ions in the gas phase. To understand better such dissociations, elimination of CH₂=CH₂ and CH₃CH₃ from (CH₃)₂NH⁺CH₂CH₃ (1) and of CH₄ from (CH₃)₂NH₂⁺ are characterized by quantum chemical calculations. Stretching of the CN bond to ethyl is followed by shift of an H from methyl to the bridging position in ethyl and then to N to reach (CH₃)₂NH₂⁺ + CH₂=CH₂ from 1. CH₃CH₃ elimination by H-transfer to C₂H₅⁺ to form CH₃NH⁺=CH₂ + CH₃CH₃ also takes place. (CH₃)₂NH₂⁺ eliminates methane by CN bond extension followed by β-H-transfer to give CH₂=NH⁺ + CH₄. Low-energy reactions resembling complex-mediated 1,2-eliminations occur and constitute a hitherto largely unrecognized type of reaction. As in many complex-mediated reactions, these reactions transfer H between incipient fragments. They are distinguished from complex-mediated processes by the fragments not being able to rotate freely relative to each other near the transition state for reaction, as they do in complexes. Most 1,2-eliminations are ion-neutral complex-mediated, occur by the just described lower energy reactions, have 1,1-like transition states, or utilize highly asynchronous 1,2 transition states. All of these avoid synchronized 1,2-transition states that would violate conservation of orbital symmetry.     

Kinetics of the reactions OH (v = O) + NH3 →; H2O + NH2 and OH(v = O) + O3 → HO2 + O2 AT 298°K

The rate constants for the reactions OH(X₂Π, ν = O) + NH₃ → ^k1 H₂O + NH₂ and OH(X²Π, ν = O) + O₃^k2 → HO₂ + O₂ were measured at 298°K by the flash photolysis resonance fluorescence technique. The values of the rate constants thus obtained are K₁ = (4.1 ± 0.6) × 10^?14 and k₂ = (6.5 ± 1.0) × 10^?14 in units of cm³ molecule ^?1 sec¹. The results are discussed in terms of understanding the dynamics of the perturbed stratosphere.     

Thermodynamic and structural investigations of chlorides in the systems KCl/MgCl2 and KCl/MnCl2

By means of a galvanic cell, emf values were measured for the solid-state reactionsnKCl+ MCl₂ = K_nMCl_n+2 for all existing compounds in the pseudobinary systems withM = Mg and Mn. ThusΔG^r values could be calculated and, from their linear temperature dependence in the range 550–730 K, reaction entropies could be determined. EnthalpiesΔH^r were calculated using the Gibbs-Helmholtz relation; they are compared with values found by solution calorimetry at room temperature. The magnitude of the entropy term for the free enthalpy of the formation reactions is discussed for the different compounds. For the modifications ofKMCl₃ the lattice parameters for the cubic, tetragonal, and one of the orthorhombic phases were determined by X-ray photographs at varying temperatures. By DSC measurements the transition enthalpy for the tetragonal to cubic transition of KMnCl₃ at 659 K was found to be 0.20–0.4 kJ · mole^?1, compared to 4.6 kJ · mole for the transition of the stable room-temperature modification with the NH₄CdCl₃ structure to the metastable GdFeO₃ structure.     

The kinetics of the reaction between NH2 and propylene studied by laser resonance absorption

The reaction kinetics of NH₂ with propylene is studied by flash photolysis in the temperature range 300-500K. The NH₂ radicals are detected by resonance absorption, using a cw single mode dye laser. This method allows the detection of very small radical concentrations in a wide range of experimental conditions. The reaction of NH₂ with propylene is fairly slow and seems to correspond to the addition process. The Arrhenius expression obtained is (E in kcal/mole):k(NH₂ + C₃H₆) = 2.9 × 10⁸ exp[-4.3(± 0.2)[RT]M^?1s^?1.     

The enthalpies of formation of CH3Mn(CO)5 and of CH3Re(CO)5, and the strengths of the CH3Mn and CH3Re bonds

From measurements of the heats of iodination of CH₃Mn(CO)₅ and CH₃Re(CO)₅ at elevated temperatures using the ‘drop’ microcalorimeter method, values were determined for the standard enthalpies of formation at 25° of the crystalline compounds: ΔH^o_f[CH₃Mn(CO)₅, c] = ?189.0 ± 2 kcal mol^?1 (?790.8 ± 8 kJ mol^?1), ΔH^o_f[Ch₃Re(CO)₅,c] = ?198.0 ± kcal mol^?1 (?828.4 ± 8 kJ mo^?1). In conjunction with available enthalpies of sublimation, and with literature values for the dissociation energies of MnMn and ReRe bonds in Mn₂(CO)₁₀ and Re₂(CO)₁₀, values are derived for the dissociation energies: D(CH₃Mn(CO)₅) = 27.9 ± 2.3 or 30.9 ± 2.3 kcal mol^?1 and D(CH₃Re(CO)₅) = 53.2 ± 2.5 kcal mol^?1. In general, irrespective of the value accepted for D(MM) in M₂(CO)₁₀, the present results require that, D(CH₃Mn) = $\text{1}\text{2}$D(MnMn) + 18.5 kcal mol^?1 and D(CH₃Re) = $\text{1}\text{2}$D(ReRe) + 30.8 kcal mol^?1.     

Crystal structures and magnetic properties of BaTiF5 and CaTiF5

Single crystals of BaTiF₅ and CaTiF₅ were obtained by the Czochralski and Bridgman techniques, respectively. The crystal structures were determined by X-ray diffraction; BaTiF₅: $\text{14}\text{m}\text{, a = 15.091(5)}$Å, c = 7.670(3)Å; CaTiF₅: $\text{I2}\text{c}\text{, a = 9.080(4)}$Å, b = 6.614Å, c = 7.696(3)Å, β = 115.16(3)°. Both structures are characterized by the presence of either branched or straight chains of TiF₆ octahedra. BaTiF₅ contains the unusual dimeric unit (Ti₂F₁₀)^4?. Magnetic susceptibility measurements were performed on both compounds in the temperature range 4.2 to 300 K, however, no evidence for magnetic interactions between the Ti³⁺ moments were observed.     

Molecular beam chemiluminescence. VIII: Pressure dependence and kinetics of Sm + (N2O,O3, F2, Cl2) and Yb + (O3, F2, Cl2) reaction. Dissociation energies of the diatomic reaction products

The CL spectra of the title reactions and their pressure dependences have been studied over the 5 × 10^?6 ? 5 × 10^?3 torr range in a beam-gas experiment. In the Sm + N₂O, O₃ and Yb + O₃ reactions simple bimolecular formation of the short lived (radiative lifetime τ_R < 3 × 10^?6 s) MO* emitters dominates the entire pressure range. In the other systems Sm + (F₂, Cl₂), Yb + (F₂, Cl₂) the CL spectra are strongly pressure dependent, indicating extensive energy transfer from long-lived intermediates. Reaction mechanisms are suggested. The quantum yields Φ, obtained by calibrating relative quantum yields with Dickson and Zare's absolute value for Sm + N₂O [Chem. Phys. 7 (1975) 367], range from Φ = 2.3% (for Sm + F₂, the most efficient reaction) down to Φ = 0.005% for Yb + Cl₂. The following lower limit estimates were obtained for the product dissociation energies from the short wavelength CL cutoffs: D⁰₀(SmF) ? 121.3 ± 2.4 kcal/mole, D⁰₀(SmCl) ? ? 100 ± 3 kcal/mole, D⁰₀(YbO) ? 94.2 ± 1.5 kcal/moie, D⁰₀(YbF) ? 123.7 ± 2.3 kcal/mole.