Hydrogen bonding and dissociation effects on the gas phase proton transfer reactions of ozone

Recently, the proton affinity (PA) of ozone was experimentally determined by Cacace and Speranza [Science (1994) 265: 208] using a bracketing technique that involved the proton transfer (PT) reactions: O₃H⁺+B⇒O₃+BH⁺; for different Br?nsted bases B. These authors showed that the simple collision model is not adequate to describe PT. We now present a theoretical model reflecting this bracketing procedure by explicitly introducing H-bonding complexing, dissociation and PT contributions, to discuss the kinetic model that assumes that PT occurs through one elementary step. The methods used include semiempirical density functional theory and ab initio Hartree-Fock methods. The procedure is gauged by using estimated PA of ozone obtained from deprotonation reactions including the Br?nsted bases BNH₃, H₂O, HOCl, SO₂, CH₃F and Kr. The PA-obtained range was from 145.3 to 160.3 kcal/mol, in fair agreement with the experimental value of 148.0±3 kcal/mol. The model seems to be fairly independent of the reference bases used to evaluate the PA. H-bonding effects appear to be a determining factor to explain collision efficiencies. Received: 5 August 1997 / Accepted: 25 September 1997     

“Inversion substitution” reactions with participation of molecular oxygen: ZH<Subscript>3</Subscript>I + O<Subscript>2</Subscript> → O<Subscript>2</Subscript>ZH<Subscript>3</Subscript> + I (Z = C,Si). The potential energy surface and rate constants

A new class of reactions of molecular oxygen O₂ + ZH₃I → O₂ZH₃ + I (Z = C, Si) proceeding by the mechanism of “inversion substitution” was investigated by quantum chemistry methods and the transition state theory (TST). The profiles of the potential energy surfaces (PES) along the reaction coordinate and the characteristics of transition states were calculated using the DFT approach with the B3LYP hybrid functional and the DZVP basis set. The characteristics of the transition states were then used for TST calculations of the rate constants for the direct and reverse “inversion substitution” reactions and their temperature dependences in the temperature interval 273–2000 K. The activation barriers to the substitution reactions under study were found to be substantially lower than the barriers to the abstraction reactions O₂ + ZH₃I → ZH₂I + HO₂ (by 16.3 kcal mol⁻¹ for Z = C and by 7.2 kcal mol⁻¹ for Z = Si). The results obtained show that the “inversion substitution” reactions dominate over the abstraction reactions in the interaction of molecular oxygen with carbon- and silicon-centered iodides as well as (probably) many other substrates. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1803–1807, September, 2008.     

Ab initio quantum chemical study of the formation, decomposition and isomerization of the formaldiminoxy radical (CH2NO): comparison of the Gaussian-2 and CASPT2 techniques in the calculation of potential energy surfaces

The reaction between triplet methylene and nitric oxide, producing the formaldiminoxy (CH₂NO) radical, and the subsequent decomposition and isomerization reactions of CH₂NO have been studied using ab␣initio quantum chemical techniques that include the Gaussian-2 (G2), CASSCF and CASPT2 methods. Stationary points on the potential energy surfaces were located at MP2/6-31G(d) and CASSCF/cc-pVDZ levels of theory, while the electronic energies were determined using G2, G2(MP2), QCISD(T)/cc-pVTZ, RCCSD(T)/cc-pVTZ and CASPT2/cc-pVTZ approaches. G2 is believed to be reliable at equilibrium geometries, but the determination of certain transition state geometries and energies requires a MCSCF-based approach. The calculations suggest that CH₂NO (²A^′) forms in a barrierless reaction and could readily decompose to H+HCNO. A subsequent abstraction reaction then results in H₂+CNO. No molecular elimination channel was found. An alternative pathway is the formation of CH₂ON, which readily isomerizes to CH₂NO. Received: 8 May 1998 / Accepted: 11 August / Published online: 9 October 1998     

The unimolecular chemistry of protonated glycine and the proton affinity of glycine: a computational model

The potential energy hypersurface of protonated glycine, GH⁺, has been investigated. The calculated G2(MP2) value for the proton affinity (PA) of glycine, PA _calc=895kJ mol⁻¹, is in good agreement with the experimental value which has been estimated to lie in the range 864kJ mol⁻¹ < PA _exp <891kJ mol⁻¹. Ab initio quantum chemical calculations of relevant parts of the potential energy surface of GH⁺ give a reaction model which is consistent with the observed mass spectrometric fragmentation pattern. The lowest energy unimolecular reactions of GH⁺ are two distinct processes: (1) loss of CO, which has a substantial barrier for the reverse reaction, and (2) loss of CO plus H₂O, which has no barrier for the reverse reaction. Received: 15 November 1996 / Accepted: 6 May 1997     

Different van der Waals radii for organic and inorganic halogen atoms: a significant improvement in IMOMM performance

The discrepancies between X-ray and integrated molecular orbital molecular mechanics computed geometries for Os(H)₂Cl₂(PⁱPr₃)₂ and Ir(H)₂Cl(P^tBu₂ Ph)₂ are explained by the inadequacy of the default molecular mechanics van der Waals radii for halogen elements. A simple procedure is proposed for the calculation of corrected van der Waals radii, and the application of the corrected radius for chloride is shown to improve substantially the results for the systems under test. Received: 25 February 1997 / Accepted: 7 April 1997     

New applications of the retention index concept in gas and high performance liquid chromatography

New methods for the precalculation of GC retention indices (RIs) are discussed. The first is based on a new modification of the correlation equation log RI = a logT_b + bA + c and is recommended for low boiling compounds of the general type R-X with known boiling points (T_b) analyzed on polymer sorbent Porapack Q. The second method permits one to predict RIs of products of organic reactions A + B → C +… with the correlation ΔRI = a ΔE + b (a < 0,|ρ| > 0.9), where ΔRI = RI_C– RI_A– RI_B and ΔE = E_C– E_A– E_B– are the differences in the internal molecular energies of reagents and products of organic reactions which are estimated by molecular dynamics methods. In the final section new possibilities of the use of RIs in reversed phase HPLC, namely for the determination of the number of hydroxyl groups in phenols, are illustrated. Received: 9 November 1998 / Revised: 15 April 1999 / Accepted: 30 April 1999     

Second-order quantum similarity measures from intracule and extracule densities

The calculation of quantum similarity measures from second-order density functions contracted to intracule and extracule densities obtained at the Hartree-Fock level is presented and applied to a series of atoms, (He, Li, Be, and Ne), isoelectronic molecules (C₂H₂, HCN, CNH, CO, and N₂), and model hydrogen-transfer processes (H₂/H⁺, H₂/Hot, H₂/H⁻). Second-order quantum similarity measures and indices are found to be suitable measures for quantitatively analyzing electron-pair density reorganizations in atoms, molecules, and chemical processes. For the molecular series, a comparative analysis between the topology of pairwise similarity functions as computed from one-electron, intracule, and extracule densities is carried out and the assignment of each particular local similarity maximum to a molecular alignment discussed. In the comparative study of the three hydrogen-transfer reactions considered, second-order quantum similarity indices are found to be more sensitive than first-order indices for analyzing the electron-density reorganization between the reactant complex and the transition state, thus providing additional insights for a better understanding of the mechanistic aspects of each process. Received: 7 July 1997 / Accepted: 29 October 1997     

On the Apparent Compensation Effect Found for Two Consecutive Reactions

A critical analysis of the use of an overall single rate reaction equation instead of the true rate equation corresponding to a complex process consisting in two consecutive reactions is presented. In accordance with this approximation, often used in the kinetic analysis of the system in which several reactions take place, the overall process is described by the apparent activation parameters (the apparent activation energy, E _ap, and the apparent pre-exponential factor, A _ap) and the apparent conversion function. The theoretical isotherms (α=α(t), where a is the conversion degree and t is the time) have been simulated for a system in which two consecutive reactions occur. In this case, the apparent activation parameters depends on: (a) the considered range of the temperature; (b) the temperature, for a given conversion degree. It is shown that the apparent activation parameters are corrrelated by the compensation effect relationship: lnA _ap=α*+β*E _ap where α* and β* are the parameters of the linear regression. The possibility of using the apparent kinetic parameters to predict the isotherms α=α(t) for temperatures lower than those for which these parameters were evaluated, is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the Apparent Compensation Effect Observed for Two Consecutive Reactions

A critical analysis is presented of the use of an overall single rate reaction equation instead of the true rate equation corresponding to a complex process consisting of two consecutive reactions. In accordance with this approximation, which is often used in the kinetic analysis of systems in which several reactions take place, the overall process is described by apparent activation parameters (the apparent activation energy E_ap and the apparent pre-exponential factor A_ap) and an apparent conversion function. The theoretical isotherms (α=α(t), where α is the conversion degree and t is time) were simulated for a system in which two consecutive reactions occur. In this case, the apparent activation parameters depend on (a) the considered range of temperature; and (b) the temperature for a given conversion degree. It is shown that the apparent activation parameters are correlated by the compensation effect relationship: ln A_ap = α^* + β^*E_ap where α^* and β^* are the linear regression parameters. The possibility of using the apparent kinetic parameters to predict the isotherms α=α(t) for temperatures lower than those for which these parameters were evaluated is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electrochemical reactions of La(Ni,Cr)O3 and La(Ni,Fe)O3 in acidic aqueous solutions

Voltammetry of immobilised microcrystalline perovskites La(Ni,Cr)O₃ and La(Ni,Fe)O₃ revealed that these oxides yield three types of reactions in acidic aqueous solutions (0.1 M HClO₄): irreversible oxidative dissolution of Cr-rich oxides, irreversible reductive dissolution of Fe-rich oxides, and a quasi-reversible reaction most likely related to alteration of the valencies of Cr, Fe and/or Ni in the solid state. The samples of La(Cr_{1− x} Ni _x )O₃ with x = 0.3 and 0.5 especially showed limited cycling stability that is particularly surprising in the very strongly acidic solution. Received: 4 January 1999 / Accepted: 16 February 1999     

Variational transition state theory without the minimum-energy path

In this paper we propose a method for carrying out variational transition state theory calculations without first obtaining a converged minimum-energy path (MEP). We illustrate the method in two ways, first of all by employing an unconverged MEP and secondly by using a dynamically optimized distinguished reaction path. Preliminary tests of the algorithm for the reactions OH+H₂→H₂O+H and C₂H₅→C₂H₄+H are very encouraging. Received: 22 January 1997 / Accepted: 11 March 1997     

The reactivity of ketyl and alkyl radicals in reactions with carbonyl compounds

A parabolic model of bimolecular radical reactions was used for analysis of the hydrogen transfer reactions of ketyl radicals: >C·OH+R¹COR²→>C=O+R¹R²C·OH. The parameters describing the reactivity of the reagents were calculated from the experimental data. The parameters that characterize the reactions of ketyl and alkyl radicals as hydrogen donors with olefins and with carbonyl compounds were obtained: >C·OH+R¹CH=CH₂→>C=O+R¹C·HCH₃; >R¹CH=CH₂+R²C·HCH₂R³→R²C·HCH₃+R²CH=CHR³. These parameters were used to calculate the activation energies of these transformations. The kinetic parameters of reactions of hydrogen abstraction by free radicals and molecules (adelhydes, ketones, and quinones) from the C−H and O−H bonds were compared. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2178–2184, November, 1998.     

Structure and stabilities of (HA1NH)n (n = 2–4)

Ab initio molecular electronic structure methods have been used to study the relative stability of the planar inorganic ring (HAlNH)_n (n = 2–4) during homodesmotic and monomer polymerization reactions. Optimized geometries, frequencies and energies through restricted Hartree-Fock/6-31G* are reported, and energies at the self-consistent field optimized geometries including M?ller-Plesset fourth perturbation theory with single, double and quadruple excitation (MP4SDQ) corrections are also reported for both reactions. Homodesmotic reactions with MP4SDQ −28.5 kcal/mol for (AlN)₂, 1.9 kcal/mol for (AlN)₃ and −0.97 kcal/mol for (AlN)₄. On analysing a π-molecular orbitals diagram, only one, three and three strongly bonding π-molecular orbitals exist for the planar four-, six- and eight-membered AlN rings, respectively. Received: 9 March 1998 / Accepted: 19 March 1998 / Published online: 23 June 1998     

Contracted Gaussian-type basis functions revisited II. Atoms Na through Ar

We report five minimal-type contracted Gaussian-type function (CGTF) basis sets of the second-row atoms, Na – Ar, for molecular applications. Three of the present CGTF sets are revised versions of those given by Huzinaga and co-workers and the other two are newly developed for more accurate calculations. Practical utility and improved reliability of the present basis sets, augmented by polarization functions, are confirmed by test calculations on the P atom and P₂ molecule both at the self-consistent field (SCF) and configuration interaction (CI) levels. Received: 10 February 1997 / Accepted: 23 April 1997     

Theoretical studies on the structure of M+BF−4 ion pairs M = Li+, NH+4: the role of electrostatics and electron correlation

A systematic investigation of the M⁺BF₄ ⁻ (M = Li or NH₄) ion-pair conformers has been carried out using an electrostatic docking model based on the molecular electrostatic potential topography of the free anion. This method provides a guideline for the subsequent ab initio molecular orbital calculations at the Hartree-Fock (HF) and second-order M?ller-Plesset perturbation theory (MP2) levels. It has been demonstrated that the model presented here yields more than 75% of the HF interaction energy when Li⁺ is the cation involved and more than 90% for the case of NH₄ ⁺. Inclusion of MP2 correlation in the HF-optimized geometries leads to stationary point geometries with different numbers of imaginary frequencies and in some places where the energies of two adjacent conformers are very close, the energy rank order is altered. The HF lowest-energy minima for the Li⁺BF₄ ⁻ and NH₄ ⁺BF₄ ⁻ show a bidentate and tridentate coordinating cation, respectively, whereas at the MP2 level, this ordering is reversed. Received: 9 September 1997 / Accepted: 5 November 1997     

Redox reactions of nitrite ions on the surface of colloidal magnetite particles coated with chondroitin sulfate

Colloidal magnetite particles coated with chondroitin sulfate (abbreviated as Fe/CS) were prepared under conditions of varying Fe(II) fractions at a fixed Fe concentration and a given concentration of CS. The average size of the magnetite core region was estimated as 7 nm from transmittance electron microscopy measurements, while the size of the Fe/CS particles ranged 155–175 nm, as estimated using Rayleigh scattering measurements by reference to a control size derived from the dynamic light scattering. The reaction of various Fe/CS with NO⁻ ₂ in aqueous solutions was determined by fluorometry using 2,3-diaminonaphthalene as a probe and by gas chromatography–mass spectrometry. The concentrations of NO⁻ ₂ in the reaction mixtures decreased in the presence of Fe/CS to a greater extent under Ar compared with aerobic conditions. The reactivity of Fe/CS toward NO⁻ ₂ under aerobic conditions increased with decreasing the size of Fe/CS particles or with increasing content of Fe(III) in the Fe/CS solutions, but was independent of the Fe(II) fraction in the preparation process. While CS molecules had no influence on the NO⁻ ₂ decomposition, those coated with the magnetite core may prevent the diffusion of NO⁻ ₂ to be adsorbed on the core surfaces. NO⁻ ₂ was concluded to undergo redox reactions with Fe(II) and Fe(III) located on the core surface of magnetite crystalline structures of Fe/CS. Received: 12 April 2000/Accepted: 9 August 2000     

Non-traditional pathways of extraterrestrial formation of organic compounds

The mechanisms of solid-phase reactions that have been experimentally and theoretically studied during recent decades and can be directly related to the formation of organic substances in space and their delivery to Earth and to the problems of prebiotic evolution are considered. Among these mechanisms are molecular tunneling (hypothesis of the cold prehistory of life), polycondensation of solid monomers by shock waves (problem of the delivery of organic substances to Earth by meteorites), thermal and thermal-wave explosions. and oscillations of temperature and radical concentrations in small cold particles under radiation exposure, mechanochemical explosions and autowave propagation of chemical reactions due to the positive feedback between fragile destruction of solids and reactions at freshly formed surfaces. Written on the basis of the report at the International Conference “Chemical Physics at the Threshold of XXI Century,” April 16–19, 1996, Moscow. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 409–417, March, 1997.     

Gas-phase reactions of CF3 and CF2 with atomic and molecular fluorine: Their significance in plasma etching

Reaction rate coefficients have been measured at 295 K for both CF₃ and CF₂ with atomic and molecular fluorine. The reaction between CF₃ and F was studied over a gas number density range of (2.4–23)×10¹⁶ cm^–3 with helium as the bath gas. The measured rate coefficient increased from (1.1–1.7)×10^–11 cm³ s^–1 as the gas number density increased over this range. In contrast to this relatively small change in rate coefficient with gas number density, the rate coefficient for CF₂+F increased from (0.4–2.3)×10^–12 cm³ s^–1 as the helium gas number density increased from (3.4–28.4)×10¹⁶ cm^–3. Even for the highest bath gas number density employed, the rate coefficient was still more than an order of magnitude lower than earlier measurements of this coefficient performed at comparable gas number densities.Both these association reactions are examined from the standpoint of the Gorin model for association of radicals and use is made of unimolecular dissociation theory to examine the expected dependence on gas number density. The calculations reveal that CF₃+F can be explained satisfactorily in these terms but CF₂+F is not well described by the simple Gorin model for association.CF₃ was found to react with molecular fluorine with a rate coefficient of (7±2)×10^–14 cm³ s^–1 whereas only an upper limit of 2×10^–15 cm³ s^–1 could be placed on the rate coefficient for the reaction between CF₂ and F₂. The values obtained for this set of reactions mean that the reaction between CF₃ and F will play an important role in plasmas containing CF₄. The high rate coefficient will mean that, under certain conditions, this particular reaction will control the amount of CF₄ consumed. On the other hand, the much lower rate coefficient for reactions between CF₂ and F means that CF₂ will attain much higher concentrations than CF₃ in plasmas where these combination reactions are dominant.     

The reactions of perfluorinated α- and β-alkoxyradicals with hydrogen chloride, chlorine and fluorine

The kinetics of the reactions of the macroradicals R_f′OCF₂ (I) and R_f′OCF₂CF₂ (II) with HCl, Cl₂ and F₂ have been studied in the liquid phase, being R_f′ a poly(oxydifluoromethylene-oxytetrafluoroethylene) chain with average molecular weight of about 10⁴ Da. Radical (I) showed a higher reactivity compared to radical (II) with all the three radical transfer agents.In case of HCl the activation energy for the reaction: