Experimental and theoretical studies of small-angle electron scattering by the water molecule

The total (elastic plus inelastic) intensities of 51 keV electrons scattered by water molecules have been measured over a range of 1 ≦ K = (4π/λ) sin(θ/2) ≦ 12 Å^?1. A computer program, ELIC, has been written for calculating the total intensities of electrons scattered by free molecules. The intensities can be calculated with self-consistent field and configuration interaction wavefunctions. The theoretical intensities based on a CI wavefunction are in good agreement with the observed intensities.     

A theoretical analysis of the reaction between hydrogen atoms and isocyanic acid

Using BAC-MP4 potential-surface parameters, supplemented by an MP2 normal-mode analysis at one transition state, and statistical theoretical methods, we have computed thermal rate coefficients for the reactions, and Over the entire temperature range considered, 300 K < T < 3300 K, reaction (2) is the dominant product channel. The theoretical predictions are in excellent agreement with the experimental results available for k₂ and k_?1, the rate coefficient for the reverse of reaction (1). Modified Arrhenius expressions are given for k₁, k_?1, and k₂. In addition, we identify and discuss a weakness in utilizing a Hartree-Fock normal-mode analysis in the prediction of k₂. The present result for k₂ is much smaller than that used in the initial modeling of the RAPRENO_x process. The implications of this are discussed.     

TPD studies of hydrogen sorption on platinum powder catalysts modified with adsorbed gold

It has been shown that hydrogen determined from the TPD curves of platinum saturated with hydrogen sorbed above room temperature is chiefly absorbed and not adsorbed hydrogen; this absorbed hydrogen has no effect on the charging curve.

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Spin uncoupling in ethylene activation by palladium and platinum atoms

Spin‐dependent effects in complex formation reactions of the ethylene molecule with palladium and platinum atoms were studied by electron correlation calculations with account of spin–orbit coupling. Simple correlation diagrams illustrating spin‐uncoupling mechanisms were obtained, showing that the low spin state of the transition‐metal atom or the transition‐metal atom complex is always more reactive than are the high spin states because of the involvement of the triplet excited molecule in the chemical activation. Spin–orbit coupling calculations of the reaction between a platinum atom and ethylene explain the high‐spin Pt(³D) reactivity as due to an effective spin flip at the stage of the weak triplet complex formation. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 581–596, 1999     

Catalytic oxidation of hydrogen on platinum

Using the thermochemical approach to interpret the kinetics of heterogeneous reactions and the mechanism of congruent dissociative decomposition of solids developed in the 1980s and (re)analyzing the experimental data available in the literature over the last 90 years, a novel mechanism for the catalytic oxidation of H₂ by PtO₂ is proposed. In place of the conventional Langmuir–Hinshelwood and Eley–Rideal adsorption reaction mechanisms, our model is based on the reactions: PtO₂(s) + 2H₂ ? Pt(g) + 2H₂O and Pt(g) + O₂ ? PtO₂(g) → PtO₂(s). The first reaction determines the kinetics of H₂ oxidation and the second determines the kinetics of restoration of the PtO₂ layer. Thermochemical consideration of kinetic features of this model enables (for first time in the history of this reaction) the enthalpy and equilibrium constants for H₂ oxidation on platinum to be calculated. The results are in good agreement with experimental data. In addition, the proposed mechanism explains the origin of the surface-retexturing effect, the impact of autocatalysis, the influence of H₂O vapor on oxidation rate, and the three-fold variation of the Arrhenius E parameter with temperature. This all convincingly demonstrates the value of the thermochemical approach in interpreting heterogeneous reactions.     

Low-temperature oxidation of hydrogen on platinum

The interaction of oxygen adsorbed on platinum with hydrogen at low temperatures has been studied. The molecular surface oxygen is shown to be involved in the reaction with hydrogen. The steady-state rate of hydrogen oxidation has been measured at 170–350 K. The activation energy is 3 kcal/mol.

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Matrix-isolation infrared spectroscopic and theoretical studies on reactions of laser-ablated germanium atoms with water molecules

Laser-ablated germanium atoms have been codeposited at 4 K with water molecules in excess argon. Adduct and insertion products, such as Ge(H2O), HGeOH, HGeO, H2GeO, GeOH, Ge(OH)2, HGeOGeH, and HGeGeO, have been formed in the present experiments and characterized by using infrared spectroscopy on the basis of the results of the isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, change of reagent concentration and laser energy, and comparison with theoretical predictions. Density functional theory calculations have been performed on these molecules and the corresponding transition states. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these molecules from the matrix infrared spectra. Plausible reaction mechanisms have been proposed to account for the formation of these molecules.     

Reaction of hydrogen atoms with propyne at high temperatures: an experimental and theoretical study

The kinetics of the reaction of hydrogen atoms with propyne (pC3H4) was experimentally studied in a shock tube at temperatures ranging from 1200 to 1400 K and pressures between 1.3 and 4.0 bar with Ar as the bath gas. The hydrogen atoms (initial mole fraction 0.5-2.0 ppm) were produced by pyrolysis of C2H5I and monitored by atomic resonance absorption spectrometry under pseudo-first-order conditions with respect to propyne (initial mole fraction 5-20 ppm). From the hydrogen atom time profiles, overall rate coefficients k(ov) identical with -([pC3H4][H])(-1) x d[H]/dt for the reaction H + pC3H4 --> products ( not equal H) were deduced; the following temperature dependence was obtained: kov = 1.2 x 10(-10) exp(-2270 K/T) cm(3) s(-1) with an estimated uncertainty of +/-20%. A pressure dependence was not observed. The results are analyzed in terms of statistical rate theory with molecular and transition state data from quantum chemical calculations. Geometries were optimized using density functional theory at the B3LYP/6-31G(d) level, and single-point energies were computed at the QCISD(T)/cc-pVTZ level of theory. It is confirmed that the reaction proceeds via an addition-elimination mechanism to yield C2H2 + CH3 and via a parallel direct abstraction to give C3H3 + H2. Furthermore, it is shown that a hydrogen atom catalyzed isomerization channel to allene (aC3H4), H + pC3H4 --> aC3H4 + H, is also important. Kinetic parameters to describe the channel branching of these reactions are deduced.     

High-temperature capture of molecular hydrogen by cerium dioxide

Conclusions The high-temperature activated chemisorption of hydrogen on reduced cerium dioxide, i.e., CeO_1.85, has been discovered. Appreciable desorption of the chemisorbed hydrogen occurs only at 350°C and is completed at 700°C. This type of strong chemisorption of hydrogen can be classified as the high-temperature capture of molecular hydrogen by cerium dioxide.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2194–2196, October, 1979.     

Host-guest hydrogen atom transfer induced by electron capture

1,n-Alkanediammonium cations in noncovalent complexes with two dibenzo-18-crown-6-ether (DBCE) ligands undergo an unusual intramolecular tandem hydrogen atom and proton transfer to the crown ether ligand upon charge reduction by electron capture. Deuterium labeling established that both migrating hydrogens originated from the ammonium groups. The double hydrogen transfer was found to depend on the length of the alkane chain connecting the ammonium groups. Ab initio calculations provided structures for select alkanediammonium·dibenzo-18-crown-6-ether complexes and dissociation products. This first observation of an intra-complex hydrogen transfer is explained by the unusual electronic properties of the complexes and the substantial hydrogen atom affinity of the aromatic rings in the crown ligand.     

Theoretical study and atoms in molecule analysis of hydrogen bonded clusters of ammonia and isocyanic acid

Abstract  

Ab initio and density functional calculations were used to analyze the interaction between a molecule of the isocyanic acid with 1 up to 4 molecules of ammonia at the B3LYP/6-311++G(d,p) and MP2/6-311++G(d,p) computational levels. The cooperative effect is increased with the increasing size of studied clusters. Red shifts of the H–N stretching frequency for complexes involving the isocyanic acid as an H-donor were predicted. Atom in molecules was used to analyze cooperative effects on topological parameters.     

Combined theoretical and FTIR spectroscopic studies on hydrogen adsorption on the zeolites Na-FER and K-FER

The interaction between molecular hydrogen and the alkali-metal-exchanged zeolites Na-FER and K-FER at a low temperature was investigated by combining variable-temperature infrared spectroscopy and theoretical calculations by using a periodic DFT model. The experimentally determined values of standard adsorption enthalpy, DeltaH degrees , were -6.0 (+/-0.8) and -3.5 (+/-0.8) kJ mol(-1) for Na-FER and K-FER, respectively. These results were found to be in agreement with corresponding DeltaH degrees values obtained from calculations on the periodic model. Two types of alkali-metal cation sites in FER were found: channel intersection sites and channel wall sites. Calculations showed a similar interaction energy for both site types, and similar structures of adsorption complexes. Up to two dihydrogen molecules can be physisorbed on the alkali-metal cation located on the intersection of two channels, while only one H2 molecule is physisorbed on the cation at the channel wall site. The adsorption enthalpies of H2 on alkali-metal-exchanged FER are significantly smaller than those found previously for the MFI-type zeolites Na-ZSM-5 and K-ZSM-5, which is likely due to a difference in the alkali-metal cation coordination in the two zeolite frameworks.     

Liquid-phase hydrogen abstraction from chlorinated ethanes by chlorine atoms

The hydrogen abstraction from the chlorinated ethanes by chlorine atoms has been investigated in the liquid phase. Rate constants relative to that for hydrogen abstraction from chloroform have been measured between 267° and 333°K using a competition technique. The results are compared with gas-phase data.     

Experimental and theoretical studies on the nature of weak nonbonded interactions between divalent selenium and halogen atoms

To investigate the nature of weak nonbonded selenium...halogen interactions (Se...X interactions; X = F, Cl, and Br), three types of model compounds [2-(CH(2)X)C(6)H(4)SeY (1-3), 3-(CH(2)X)-2-C(10)H(6)SeY (4-6), and 2-XC(6)H(4)CH(2)SeY (7-9); Y = CN, Cl, Br, SeAr, and Me] were synthesized, and their (77)Se NMR spectroscopic behaviors were analyzed in CDCl(3). The gradual upfield shifts of (77)Se NMR absorptions observed for series 1-3 and 4-6 suggested that the strength of Se...X interaction decreases in the order of Se...F > Se...Cl > Se...Br. The quantum chemical calculations at the B3LYP/631H level using the polarizable continuum model (PCM) revealed that the most stable conformer for 1-3 is the one with an intramolecular short Se...X atomic contact in CHCl(3) (epsilon = 4.9) and also that the n(X) --> sigma(Se-Y) orbital interaction (E(Se...X)) can reasonably explain the order of strength for the Se...X interactions. On the other hand, the (77)Se NMR absorptions observed for series 7-9 did not shift significantly from the reference compounds (C(6)H(5)CH(2)SeY), indicating the absence of the Se...X interaction for 7-9 presumably due to attenuation of basicity for the halogen atom that is substituted directly to the aromatic ring. These observations suggested that the n(X) --> sigma(Se-Y) orbital interaction is a dominant factor for formation of weak Se...X interactions. Electron correlation was also suggested to be important for the stability.     

Matrix isolation infrared spectroscopic and density functional theoretical studies on the reactions of lanthanum atoms with acetylene

Laser-ablated lanthanum atoms have been codeposited at 4 K with acetylene in excess argon. Products, La(C 2H 2), LaCCH 2, HLaCCH, and La 2(C 2H 2), have been formed in the present experiments and characterized using infrared spectroscopy on the basis of the results of the isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, the change of reagent concentration and laser energy, and the comparison with theoretical predictions. Density functional theory calculations have been performed on these molecules. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these molecules from the matrix infrared spectra. Plausible reaction mechanisms have been proposed to account for the formation of these molecules.     

Shock‐tube studies on the reactions of dimethyl ether with oxygen and hydrogen atoms

The reactions of dimethyl ether (CH₃OCH₃, DME) with O(³P) and H atoms have been studied at high temperatures by using a shock tube apparatus coupled with atomic resonance absorption spectroscopy (ARAS). The rate coefficients for the reactions CH₃OCH₃ + O(³P) → CH₃OCH₂ + OH (1) and CH₃OCH₃ + H → CH₃OCH₂ + H₂ (2) were experimentally determined from the decay of O(³P) and H atoms as: These results show that DME can react with O(³P) atoms more easily than with H atoms. By combining these results with the previous lower temperature data, we obtained the following modified Arrhenius expressions applied over the wide temperature range between 300 and 1500 K: Both reactions of DME are faster than those of ethane, because the dissociation energy of the C? H bond in DME is smaller. Furthermore, the rate coefficients for reactions ( 1 ) and ( 2 ) were calculated with the transition‐state theory (TST). Structural parameters and vibrational frequencies of the reactants and the transition states required for the TST calculation were obtained from the MP2(full)/6‐31G(d) ab initio molecular orbital (MO) calculation. The energy barrier, E^?₀, was adjusted until the TST rate coefficient most closely matched the observed one. The fitting results of E(1) = 23 kJ mol^?1 and E(2) = 34 kJ mol^?1 were in agreement with the G2 energy barriers, within the expected uncertainty, demonstrating that the experimentally determined rate coefficients were theoretically valid. © 2006 Wiley Periodicals, Inc. 39: 97–108, 2007     

Polarization and bonding of the intrinsic characteristic contours of hydrogen and fluorine atoms of forming a hydrogen fluoride molecule based on an ab initio study

The spatial changing feature of the shapes and sizes of the system consisted of one hydrogen atom and one fluorine atom of forming a hydrogen fluoride molecule is investigated. We give formalism of the potential acting on an electron in a molecule and derive its concrete expression in Hartree-Fock self-consistent molecular orbital theory including configuration interaction. The program of calculating the potential acting on an electron in a molecule is programmed and compiled in the framework of the MELD program package. We formulate briefly the approach of the molecular intrinsic characteristic contour (MICC) which is defined in terms of the classical turning points of electronic motion. The MICC for a molecular system is intrinsic and can be calculated by means of an ab initio CI method. Then, the polarization and bonding features of the intrinsic characteristic contours of hydrogen and fluorine atoms forming a hydrogen fluoride molecule are presented and discussed from ab initio calculations. Furthermore, electron density distribution as an added dimension has been demonstrated on the changing MICC and thus the vivid polarization and bonding features for a chemical process have been shown. It seems that at the early stage (internuclear distance Ind=5.0-20.0 a.u.) the fluorine atom gives more enthusiastic with the sensitive and expanded polarization to welcome coupling with the hydrogen atom while the latter has little response even "shy" with shrinking a bit its size at the beginning of putting the two atoms into a system and it is only around the critical point, the contact point (Ind=4.73 a.u.), that both of them stretch their hands and arms to meet and then fuse together.