MCSCF reaction-path energetics and thermal rate-constants for the reaction of3NH with H2

Summary The intrinsic reaction-path, reactants, transition state and products for the reaction of NH (^3–)+H₂ (¹ _g ⁺ ) NH₂ (²B₁)+H (²S) involving the lowest triplet electronic state of NH₃ were calculated using multi-configuration (MC) SCF methods. The calculated change of internal energy for the reaction of 11.0 kcal mol^–1 agrees with the experimental value within 2 kcal mol^–1. The barrier to reaction is 23.4 kcal mol^–1 high. The harmonic MCSCF reaction-path potential was calculated and canonical variational transition state theory calculations of the rate constants performed over a temperature range from 400 to 2500 K. The computed rate constants are generally two orders of magnitude smaller than those of the comparable reaction of OH with H₂, whereas those of the reverse reaction are by a factor of 20 larger than those of OH₂ with H.     

On the energy transfer at the radiolysis of CO-H2S gas mixture

The kinetics of hydrogen formation at various amounts of H₂S /1–60%/ in the radiolysis of CO–H₂S mixture has been studied. The ratio of the reaction rate constants for reactions CO^x+COproduct and CO^x+H₂SH+SH=CO, which amounts to 5×10^–2, has been estimated. The effective activation energy of hydrogen formation /E_eff/ has been determined at various amounts of H₂S in the temperature interval 323–573 K.     

A theoretical study of the H2SO4+H2O → HSO4 −+H3O+ reaction at the surface of aqueous aerosols

The surface region of sulfate aerosols (supercooled aqueous concentrated sulfuric acid solutions) is the likely site of a number of important heterogeneous reactions in various locations in the atmosphere, but the surface region ionic composition is not known. As a first step in exploring this issue, the first acid ionization reaction for sulfuric acid, H₂SO₄ + H₂O HSO₄^– + H₃O⁺, is studied via electronic structure calculations at the Hartree–Fock level on an H₂SO₄ molecule embedded in the surface region of a cluster containing 33 water molecules. An initial H₂SO₄ configuration is selected which could produce H₃O⁺ readily available for heterogeneous reactions, but which involves reduced solvation and is consistent with no dangling OH bonds for H₂SO₄. It is found that at 0 K and with zero-point energy included, the proton transfer is endothermic by 3.4 kcal/mol. This result is discussed in the context of reactions on sulfate aerosol surfaces and, further, more complex calculations.Contribution to the Jacopo Tomasi Honorary Issue     

Ab initio chemical kinetics for the unimolecular decomposition of Si2H5 radical and related reverse bimolecular reactions

For plasma enhanced and catalytic chemical vapor deposition (PECVD and Cat‐CVD) processes using small silanes as precursors, disilanyl radical (Si₂H₅) is a potential reactive intermediate involved in various chemical reactions. For modeling and optimization of homogeneous a‐Si:H film growth on large‐area substrates, we have investigated the kinetics and mechanisms for the thermal decomposition of Si₂H₅ producing smaller silicon hydrides including SiH, SiH₂, SiH_3, and Si₂H₄, and the related reverse reactions involving these species by using ab initio molecular‐orbital calculations. The results show that the lowest energy path is the production of SiH + SiH₄ that proceeds via a transition state with a barrier of 33.4 kcal/mol relative to Si₂H₅. Additionally, the dissociation energies for breaking the Si? Si and H? SiH₂ bonds were predicted to be 53.4 and 61.4 kcal/mol, respectively. To validate the predicted enthalpies of reaction, we have evaluated the enthalpies of formation for SiH, SiH₂, HSiSiH₂, and Si₂H₄(C_2h) at 0 K by using the isodesmic reactions, such as ²HSiSiH₂ + ¹C₂H₆→¹Si₂H₆ + ²HCCH₂ and ¹Si₂H₄(C_2h) + ¹C₂H₆ → ¹Si₂H₆ + ¹C₂H₄. The results of SiH (87.2 kcal/mol), SiH₂ (64.9 kcal/mol), HSiSiH₂ (98.0 kcal/mol), and Si₂H₄ (68.9 kcal/mol) agree reasonably well previous published data. Furthermore, the rate constants for the decomposition of Si₂H₅ and the related bimolecular reverse reactions have been predicted and tabulated for different T, P‐conditions with variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory by solving the master equation. The result indicates that the formation of SiH + SiH₄ product pair is most favored in the decomposition as well as in the bimolecular reactions of SiH₂ + SiH₃, HSiSiH₂ + H₂, and Si₂H₄(C_2h) + H under T, P‐conditions typically used in PECVD and Cat‐CVD. © 2013 Wiley Periodicals, Inc.     

Ab initio calculations of potential energy surface for reaction of nucleophilic addition of H− ion to methylacetylene

Within the framework of the Hartree-Fock-Roothaan Method, using double- basis sets 3-21++G and (6-31-H-G//3-21++G), the minimum energy paths (MEPs) have been calculated for reactions of nucleophilic addition of the hydride ion H^– to the methylacetylene molecule: CH₃-CCH+H^–[CH₃-CH=CH]^– (1) CH₃-CCH+H^–[CH₃-C=CH₂]^– (2). It has been established that the activation energy for reaction (2) is 7.02 kJ/mole lower than for reaction (1). An analysis has been made of the character of electron density distribution along the MEP of each reaction. It has been shown that distortion of geometry of the reactants plays an important role in intensifying the interaction of the frontier orbitals. The reasons for nonfulfillment of Markownikoff's rule for these reactions have been determined. The results from the calculations are compared with calculations reported in the literature for the related reaction of nucleophilic addition of the hydride ion H^– to the acetylene molecule: HCCH+H^–[CH₂=CH]^–.Translated from Teoreticheskaya i Éxperimental'naya Khimiya, Vol. 21, No. 3, pp. 303–309, May–June, 1985.     

Thermal isomerization of acetylnitrene: a quantum-chemical study

The electronic structure and pathways of thermal isomerization of formylnitrene and acetylnitrene were studied by the B3LYP/6-311G(d,p) density functional method and ab initio G2(MP2,SVP) computational procedure using the geometries obtained from B3LYP calculations. According to G2 calculations, both nitrenes have singlet ground states while the energies of the corresponding triplet states are 2.8 and 5.7 kcal mol^–1 higher. For acetylnitrene, the activation barrier to the nitrene isocyanate isomerization was estimated at 28.9 kcal mol^–1 (G2). Calculations revealed no pathway for single-step isomerization of nitrene into cyanate in both systems. The formation of methyl cyanate from isocyanate is thermodynamically unfavorable (E = 26.5 kcal mol^–1) and requires a high activation barrier (89.4 kcal mol^–1) should be overcome. Based on the results obtained, the pathways of transformation of nitrene formed in thermal decomposition of acetyl azide (Curtius rearrangement) were analyzed.     

Quantum chemical modeling of the photode-composition of the water dimer in chlorophyll complexes of magnesium and manganese

On the basis of quantum-chemical calculations in the MPNDO/3 approximation, a reaction has been proposed for the photodecomposition of water in complexes M·Chl_a...(H₂O)₂M·Chl_a+O₂+4H⁺+4e^– (M=Mg, Mn), which suggests an interpretation of the primary event of photosynthesis. Calculations of the photosynthetic evolution of oxygen according to the Joliot-Kok model in complexes M·Chl_a...(H₂O)₂ are in good agreement with the experimental data of the thermoluninescence of chlorophyll photosystem II.Kiev Polytechnic Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 33–38, March–April, 1993.     

Kinetics and mechanism in the decomposition of NH3 in a radio-frequency pulse discharge

Studies of time-resolved absorption spectra of transient species in the decomposition of NH₃ by an r.f. pulse discharge together with product analysis showed that the major radical formed was NH at concentrations of the order of 10^–6 mol dm^–3 (10⁵ molec. cm^–3). Possible mechanisms for the formation of the radical during the discharge and its decay following pulse cut-off were tested by computer simulation of the kinetic data. Following zero-order formation with rate coefficient 0.19±0.03 mol dm^–3 s^–1, the decay was second order in NH with rate coefficient 2.1±0.5×10⁹ mol^–1 dm³ s^–1 both for pure NH₃ and where NH₃/rare gas mixtures were investigated. The kinetic data are consistent with NH removal in a nonassociative radical-radical reaction proceeding via a short-lived collision complex, probably 2NH N₂H₂ N₂ + H₂.     

Conjugated radicals

Restricted open shell CNDO calculations have been carried out on butadiene anion radical (M_–), butadiene cation radical (M⁺), and butadiene dimer cation radical (M ₂ ⁺ ). Calculated transition energies are in agreement with the experimental data. Formation of M ₂ ⁺ and unsuccessful attempts to detect M ₂ ^– have been interpreted by the CNDO and extended Hückel calculations, in light of which the M + M⁺ M ₂ ⁺ process is energetically favourable while the M + M^– (M ₂ ^– ) process is connected with an energy loss. CNDO calculations support the assumed sandwich structure of M ₂ ⁺ .

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Zusammenfassung Restricted open shell CNDO-Rechnungen für das Butadien-Radikalanion (M^–), das Butadien-Radikalkation (M⁺) und das dimere Butadien-Radikalkation (M ₂ ⁺ ) wurden durchgeführt. Die berechneten Übergangsenergien stimmen mit den experimentellen Daten überein. Die Bildung von M ₂ ⁺ einerseits und die erfolglosen Versuche, das dimere Radikalanion M ₂ ^– herzustellen, andererseits wurden mittels CNDO- und EHT-Berechnungen gedeutet. Es zeigte sich hierbei, daß der Prozeß M + M⁺ M ₂ ⁺ energetisch möglich sein, die Reaktion M + M ₂ ^– M ₂ ^– dagegen mit einem Energieverlust ablaufen sollte. Die CNDO-Rechnungen sprechen für die Annahme einer Sandwich-Struktur von M ₂ ⁺ .

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Résumé On a effectué des calculs avec restricted open shell d'après la méthode CNDO sur l'anion radical du butadiène (M^–) sur le cation radical du butadiene (M⁺) et sur le cation radical du dimère du butadiène (M ₂ ⁺ ). Les énergies de transition calculées sont en accord avec les données expérimentales. La formation de M ₂ ⁺ et les efforts sans succès à prouver la présence de M ₂ ⁺ ont été interprétés par la méthode de CNDO et par les calculs étendus d'après Hückel. De ce point de vue le procès M + M⁺ M ₂ ⁺ est énergétiquement favorable pendant que le procès M + M^– M ₂ ^– est accompagné d'une perte d'énergie. Les calculs d'après la méthode de CNDO soutiennent la structure supposée deM ₂ ⁺ .

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Part XI: ársky, P., Hobza, P., Zahradnik, R.: Collect. Czechoslov. Chem. Commun., in press.     

Pure rotational spectrum of PH3 in the 10–100 cm−1 range: Collisional broadening coefficients by H2 and He

The H₂- and He-broadened widths have been measured at room temperature for seven pure rotational lines of phosphine in the spectral range 10–80 cm^–1. The retrieval of linewidths values was carried out, on the assumption of identical linewidths for theJ+1K-components within a given multipletJ+1J. The results obtained are in good agreement with the previous determination reported for the 10 line, in the microwave range.     

Anomalies of the partition coefficient and properties of crystals in the region of low concentrations of an isomorphous admixture. 1. Unusual decomposition of solid solutions of the NaCl—CdCl2 system

Solid solutions NaCl—CdCl₂ were studied in an interval of CdCl₂ concentrations of 0.05—3 mol.% by Raman spectroscopy. The molecular form Na₂CdCl₄ decomposes: Na₂CdCl₄ 2Na⁺ + Cd²⁺ + 2Cl₂ ^– + 2e^–; 2Na + 2e^– 2Na⁰. Free sodium atoms form color centers of crystal (F centers) in the region of jumpwise changing the partition coefficient of CdCl₂ in NaCl (K jump).     

Kinetik und Mechanismus der Diazotierung, 15. Mitt.: Kinetik der anorganischen Startreaktion der Diazotierung

Zusammenfassung Es werden die Geschwindigkeitskoeffizienten der Primärreaktion der Diazotierung, nämlich der Bildung des Nitrosoacidiumions aus salpetriger Säure und Hydroxoniumion: HNO₂+H₃O⁺H₂O·NO⁺+H₂O bei verschiedenen Temperaturen bestimmt. Die thermodynamischen Aktivierungsgrößen dieser Urreaktion werden daraus errechnet.Die Resultate werden aus der Kinetik der Diazotierung des Anilins in Nitritpufferlösungen und in schwach sauren ungepufferten Lösungen geschöpft, bei welchen nur die beiden anorganischen Urreaktionen HNO₂+H₃O+H₂O·NO⁺+H₂O H₂O·NO⁺+NO_2–N₂O₃+H₂O kinetisch in Erscheinung treten.Originalvortrag vonHermann Schmid beim XVII. Internat. Kongr. Reine u. Angew. Chem. (München, 2. Sept. 1959).     

The influence of solvation on the calculated activation energy for the reaction CH3F+F−

The influence of the solvent on the activation energy of the synchronous, concerted, narcissistic reaction: CH₃F + F^–¦CH₃F₂¦^–FCH₃+F^–, has been investigated by means of CNDO/2 method. The reaction path has been fully followed in vacuo, and at a suitable point only, of reaction coordinate, in solution. The geometries of H₂O, F^–, CH₃F, and ¦CH₃F₂¦^–, in vacuo and in different hydrated cages, have been optimized.

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Zusammenfassung Der Einfluß des Lösungsmittels auf die Aktivierungsenergie der synchronverlaufenden Reaktionen CH₃F+F^–¦CH₃F₂¦FCH₃+ F^–, wurde mit der CNDO/2 Methode geprüft. Der Reaktions-Ablauf wurde in Vakuum vollkommen und in der Lösung an einem geeigneten Punkt der Reaktionskoordinate durchgeführt. Die Geometrien von H₂O, F^–, CH₃F, und ¦CH₃F₂¦^– im Vakuum und in verschiedenen hydrierten Käfigen wurden optimalisiert.

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Résumé L'influence du solvent sur l'énergie d'activation de la réaction synchronique, concertée, narcissistique: CH₃F+ F^–¦CH₃F₂¦^–FCH₃+F^– a été examinée par la méthode CNDO/2. On a suivie l'entier chemin de réaction in vacuo, alors qu'on a observé un seul point convenable de la coordonée de réaction en solution. On a optimize les geometries de H₂O, F^–, CH₃F, et ¦CH₃F₂¦^– in vacuo et en differentes cages hydratés.

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Financial aid from Italian Centro Nazionale delle Ricerche is gratefully acknowledged.     

Structure,vibrational spectra,and energetic stability of LnX<Stack><Subscript>4</Subscript><Superscript>−</Superscript></Stack> ions (Ln=La,Lu; X=F,Cl, Br,I)

Molecular structure and properties of La and Lu tetrahalide ions LnX ₄ ^– ) are studied by the configuration interaction singles-and-doubles method augmented with quadruple excitation correction (CISD+Q) and by the fourth-order Möller-Plesset perturbation theory with account for single, double, triple, and quadruple excitations (SDTQ-MP4). The atomic inner shells are described by Stevens relativistic effective core potentials. Valence basis sets are augmented with diffuse s-, p-, and polarization d-, f-, and g-functions. The equilibrium configuration of nuclei in LnX ₄ ^– ions was found to be tetrahedral. The equilibrium internuclear distances, quadratic force constants, vibrational frequencies, and IR intensities of LnX ₄ ^– ions are compared with the corresponding parameters of La and Lu trihalide molecules (LnX₃), calculated within the same approximations. Regularities in the behavior of molecular parameters on going along the LnF ₄ ^– LnCl ₄ ^– LnBr ₄ ^– LnI ₄ ^– series and from La compounds to Lu compounds are revealed. Heights of the energy barriers to the LnX ₄ ^– inversion through the square planar structures (T _d D _4h T _d ) are evaluated: 100–110 and 130–150 kJ/mol for LaX ₄ ^– and LuX ₄ ^– , respectively. Enthalpies of dissociation reactions LnX ₄ ^– LnX₃+X^– are calculated and the results obtained are compared with the available experimental data.Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 3, 2005, pp. 218–228.Original Russian Text Copyright © 2005 by Solomonik, Smirnov, Mileyev.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Error of Atomic Charges Derived from Electrostatic Potential

The purpose of this article is to show that CHELP, CHELPG, and Merz and Kollman undergo error for the charge on atoms of HCOO^– (H₂O) _n for n = 1 6. We also demonstrate that the CHELP, CHELPG, and Merz and Kollman show error for the tendency toward change in the charges on carbons for CH₃NH⁺ ₃ (CH₃)₂NH⁺ ₂ (CH₃)₃NH⁺ (CH₃)₄N⁺.     

Heteronuclear diatomic transition-metal cluster ions in the gas phase: Reactivity and thermochemistry of AgFe+

The gas-phase chemistry of AgFe⁺ was studied by using Fourier transform ion cyclotron resonance mass spectrometry. AgFe⁺ is unreactive with alkanes but reacts with cyclic and linear (C4–C8) alkenes. The primary reactions are dominated by dehydrogenation and condensation. In addition, cluster splitting is observed in the reaction of AgFe⁺ with benzene. Secondary reactions generally involve cluster splitting with the loss of Ag, although AgFeC₅H ₆ ⁺ is observed to dehydrogenate cyclopentene to yield AgFeC₁₀H ₁₂ ⁺ . Ion-molecule reactions, collision-induced dissociation, and photodissociation experiments were used to determine the bond energiesD°(Fe⁺–Ag)=53±7 kcal/mol andD°(Ag⁺–Fe)=46±7 kcal/mol. These values in turn were used to calculateH _f (AgFe⁺)=296±7 kcal/mol andIP(AgFe)=6.5±0.3 eV. Related chemical and physical properties of CuFe⁺ are presented for comparison.     

Stereochemical nonrigidity of (O—Ge)-chelate bis[(2-oxo-1-hexahydroazepinyl)methyl]dichlorogermane: substantiation of the dissociative mechanism of chelating ligand exchange

Stereochemical nonrigidity of the hexacoordinated (O—Ge)-chelate bis(2-oxo-1-hexahydroazepinylmethyl)dichlorogermane in CDCl₃ was studied by dynamic NMR. The activation parameters of the intramolecular rearrangement at the coordination center are G ^# ₂₉₈ = 12.3±0.2 kcal mol^–1, H ^# = 16.9±0.2 kcal mol^–1, and S ^# = 15.3±0.7 cal mol^–1 K^–1. The dissociative mechanism of ligand exchange involving the cleavage of the OGe coordination bond is discussed based on the positive entropy of activation.     

Kinetics and mechanism of the decomposition of CS2, OCS,and SCl2 in a radiofrequency pulse discharge

The identification of transient species in r.f. discharges and measurement of rate coefficients for their reactions contributes to the understanding of the complex mechanisms in r.f. plasma chemistry. Using kinetic spectroscopy in conjunction with a short-duration r.f. pulse to investigate the decomposition of CS₂, OCS, and SCl₂ at low pressure, it has been shown that the predominant primary dissociation steps are CS₂CS+S, OCSCO+S(¹D), and SCl₂S+Cl+Cl. With OCS the most important subsequent steps involved the formation and removal of S₂: S(¹D)+OCSCO+S₂(a¹), S₂(a¹)+MS₂(X³)+M, (13), and 2S₂+MS₄+M(15). Taking the previously published value of k₁₂, computer simulation gave the rate coefficient values k₁₃=6.4±2.4×10⁸ mol^–1 dm³ s^–1 and k₁₅=1.8±0.5×10¹³ mol^–2 dm⁶ s^–1 at 295±3 K.     

Calculs non empiriques sur une réaction SN2 typique

Résumé On a calculé la structure électronique de l'état initial et de l'état de transition de la réaction de substitution F^–+CH₃FFCH₃+F^– par la méthode des orbitales moléculaires SCF dans l'approximation gaussienne. On trouve que l'énergie de l'état de transition est inférieure à la somme des énergies des produits séparés CH₃F et F^–. Cependant, cette contribution négative à l'énergie d'activation est contrebalancée par la variation des énergies de solvatation.

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Ab initio calculations on a typical SN₂ ReactionElectronic structure of methyl fluoride and of the transition state (FCH₃F)^–

The electronic structure of the initial and transition states of the substitution reaction F^–+CH₃F FCH₃+F^– have been calculated by the SCF molecular orbital method in the Gaussian approximation. It is found that the energy of the transition state is lower than the sum of the energies of the separated systems CH₃F and C^–. However, this negative contribution to the activation energy is balanced by the variation of solvatation energies.

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Zusammenfassung Die Elektronenstruktur des Ausgangs- und des Übergangszustandes der Substitutionsreaktion F^–+CH₃FFCH₃+F^– wurden mit einer SCF-MO-Methode mit Gaußfunktionen berechnet. Die Energie des Übergangszustandes ist niedriger als die Summe der Energien der separierten Systeme CH₃F und F^–. Dieser negative Beitrag zu der Aktivierungsenergie wird allerdings ausgeglichen durch eine Änderung der Solvatationsenergie.

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Remerciements. Nous remercions bien sincèrement M. A. Rassat (CEN, Grenoble) de nous avoir suggéré ce problème et M. Ph. Millie (ENS, Paris) de nous avoir apporté des éléments de discussion constructifs.     

Theoretical study of the H 5 + system

Ab initio calculations have been carried out for the ground state of H ₅ ⁺ in order to predict its equilibrium geometry, binding energy, enthalpy of formation, and the features of the H₂ · H ₃ ⁺ interaction at large and intermediate intermolecular distances. The extended basis set of Gaussian functions was carefully optimized to describe the various kinds of intermolecular interactions. Electron correlation was accounted for by means of CI calculations. Different from previous studies we find a D _2d equilibrium geometry with D _e = 7.4 kcal/mol and H ₃₀₀ ⁰ –8.7 kcal/mol. The potential surface turns out to be extremely shallow in the vicinity of the D _2d structure which results in a great mobility of the central nucleus at room temperature.