Bimolecular reactions of vibrationally excited molecules. Roaming atom mechanism at low kinetic energies

Quasiclassical trajectory calculations have been performed for the H + H'X(v) → X + HH' abstraction and H + H'X(v) → XH + H' (X = Cl, F) exchange reactions of the vibrationally excited diatomic reactant at a wide collision energy range extending to ultracold temperatures. Vibrational excitation of the reactant increases the abstraction cross sections significantly. If the vibrational excitation is larger than the height of the potential barrier for reaction, the reactive cross sections diverge at very low collision energies, similarly to capture reactions. The divergence is quenched by rotational excitation but returns if the reactant rotates fast. The thermal rate coefficients for vibrationally excited reactants are very large, approach or exceed the gas kinetic limit because of the capture-type divergence at low collision energies. The Arrhenius activation energies assume small negative values at and below room temperature, if the vibrational quantum number is larger than 1 for HCl and larger than 3 for HF. The exchange reaction also exhibits capture-type divergence, but the rate coefficients are larger. Comparisons are presented between classical and quantum mechanical results at low collision energies. At low collision energies the importance of the exchange reaction is enhanced by a roaming atom mechanism, namely, collisions leading to H atom exchange but bypassing the exchange barrier. Such collisions probably have a large role under ultracold conditions. The calculations indicate that for roaming to occur, long-range attractive interaction and small relative kinetic energy in the chemical reaction at the first encounter are necessary, which ensures that the partners can not leave the attractive well. Large orbital angular momentum of the primary products (equivalent to large rotational excitation in a unimolecular reaction) is favorable for roaming.     

A new method for the measurement of vibrational-state-selected ion-molecule reactions at thermal energies

A photoelectron-secondary-ion-coincidence method is described that allows us to determine the relative vibrational-energy-dependent cross sections for reactions of molecular ions with neutral atoms or molecules at thermal energies. Results for reactions of H₂⁺(ν) in vibrational states ν = 0–8 with H₂(H₃⁺), Ne(NeH⁺) and He(He⁺) are reported.     

Mutual capture of dipolar molecules at low and very low energies. II. Numerical study

The low-energy rate coefficients of capture of two identical dipolar polarizable rigid rotors in their lowest nonresonant (j(1) = 0 and j(2) = 0) and resonant (j(1) = 0, 1 and j(2) = 1, 0) states are calculated accurately within the close-coupling (CC) approach. The convergence of the quantum rate coefficients to their quantum-classical counterparts is studied. A comparison of the present accurate numerical with approximate analytical results (Nikitin, E. E.; Troe, J. J. Phys. Chem. A 2010, 114, 9762) indicates a good performance of the previous approach which was based on the interpolation between s-wave fly wheel quantal and all-wave classical adiabatic channel limits. The results obtained apply as well to the formation of transient molecular species in the encounter of two atoms at very low collision energy interacting via resonance dipole-dipole interaction.     

Calculation of the rates of atomic reactions at low energies

The theory of binary atomic reactions is discussed. The matrix Schrödinger equation in the space of electron channels is used for the wave functions of the relative motion of the nuclei; this gives the corresponding quasiclassical equation by retention of terms linear in . Criterion (10) applies for quasiclassical nuclear motion. The additional restriction (13) causes the quasiclassical matrix equation (9) to go over to the basic equation (15) in the method of the central parameter, which indicates the region of application of that method. An Algol-60 program has been drawn up for calculating the rates of atomic reactions via the central-parameter or quasiclassical methods.     

Rotational state dependence of ion-polar molecule reactions at very low temperature

The adiabatic rotational state method is used to investigate the rotational state dependence of the rate coefficients for ion-polar molecule reactions in the very low temperature regime characteristic of interstellar molecular clouds. Results obtained for the systems H ₃ ⁺ +HCl and H ₃ ⁺ +HCN indicate that all the methods based on the adiabatic separation of the rotational and radial motion of the collision complex — adiabatic capture centrifugal sudden approximation (ACCSA), statistical adiabatic channel model, classical adiabatic invariance method — agree very satisfactorily in the low temperature limit. Discrepancies observed between some of the published data would appear to arise from numerical inaccuracies rather than from any defect of the theory.     

Positron-nitrogen molecule scattering at low energies

It is demonstrated that time resolved laser spectroscopy can give atomic lifetime data with an accuracy of better than 0.5%. This is achieved by a single-photon counting delayed coincidence technique, using a mode-locked dye laser for the excitation. Natural radiative lifetimes are measured for the sodium and bismuth atoms.     

In-source CID of guanosine: Gas phase ion-molecule reactions

In-source collision induced dissociation was applied to access second generation ions of protonated guanosine. The in-source gas-phase behavior of [BH2]+-NH3 (m/z 135, C5H3N4O+) was investigated. Adduct formation and reactions with available solvent molecules (H2O and CH3OH) were demonstrated. Several addition/elimination sequences were observed for this particular ion and solvent molecules. Dissociation pathways for the newly formed ions were developed using a QqTOF mass spectrometer, permitting the assignment of elemental compositions of all product ions produced. Reaction schemes were suggested arising from the ring-opened intermediate of the protonated base moiety [BH2]+, obtained from fragmentation of guanosine. The mass spectral data revealed that the in-source CH3OH-reaction product underwent more complex fragmentations than the comparable ion following reaction with H2O. A rearrangement and a parallel radical dissociation pathway were discerned. Apart from the mass spectrometric evidence, the fragmentation schemes are supported by density functional theory calculations, in which the reaction of the ring-opened protonated guanine intermediate with CH3OH and a number of subsequent fragmentations were elaborated. Additionally, an in-source transition from the ring-opened intermediate of protonated guanine to the ring-opened intermediate of protonated xanthine was suggested. For comparison, a low-energy collision induced dissociation study of xanthosine was performed. Its dissociation pathways agreed with our assumption.     

Use of enzymatic catalysis with E.C. 1.2.3.2 xanthine oxidase for the kinetic determination of V(V) at low concentrations

An enzymatic kinetic method for the determination of V(V) is described, based on the activation of xanthine oxidase-catalysed NADH oxidation, in the presence of xanthine, dithiothreitol and Ag(+) ions. Under these conditions the activating power of V(V) was found to be enhanced. The linear relationship between relative enzyme activity and V(V) concentration allows the enzymatic determination of vanadium in the concentration range 20-500 ng ml , the maximum relative error being +/- 3%, and relative standard deviation less than +/- 2.2%. Possible interferences have been studied.     

Unimolecular fragmentation reactions of protonated and methyl-cationated n-octanones and N-nonanones. Sequential fragmentation reactions

Protonated and methyl-cationated n-octanonts and n-nonanones have been prepared by chemical ionization methods and the unimolecular fragmentation reactions occurring on the metastable ion time-scale have been exam ined. The [R₁R₂COR₃]⁺ (R₃ = H or CH₃) species fragment by elimination of R₃OH and by elimination of neutral alkenes. The elimination of (R₁ — H), where R₁ is the larger alkyl group of the original ketone, is particularly important. In addition, alkenyl ions are observed corresponding, nominally, to elimination of C₃H₇OR₃ from the ionized octanones and to elimination of C₃H₇OR₃ and C₄H₉OR₃ from the nonanones. These ions are shown to arise largely, if not completely, by sequential elimination of R₃OH plus an olefin (C₃H₆ or C₄H₈) from [R₁R₂COR₃]⁺. A comparison is made of the unimolecular fragmentation reactions occurring in the second field-free region and in the radio-frequency-only quadrupole of a hybrid BEQQ mass spectrometer.     

OOCO+ cation. II. Its role during the atmospheric ion-molecule reactions

For the charge transfer and vibrational and electronic deexcitations between O2/O2+ + CO+/CO, O/O+ + CO2+/CO2, and C/C+ + O3+/O3, multistep reaction pathways are discussed in light of the theoretical data of this and previous paper together with close comparison with the experimental observations. Our calculations show that these pathways involve both the long range and molecular region ranges of the potential energy surfaces of the electronic states of the stable isomers of OOCO+ and mostly those of the weakly bound charge transfer complex OOCO+. The couplings between these electronic states such as vibronic, Renner-Teller, Jahn-Teller, and spin orbit are viewed to play crucial roles here. Moreover, the initial orientation of the reactants, in the entrance channels, strongly influences the reaction mechanisms undertaken. We propose for the first time a mechanism for the widely experimentally studied spin-forbidden exothermic O+((4)S(u))+CO2(X (1)Sigmag+)-->O2+(X (2)Pi(g))+CO(X (1)Sigma+) reaction where the O turns around the OCO molecule.     

Reactions at very low temperatures: gas kinetics at a new frontier

Advances in experimental techniques, especially the development of the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method, allow many gas-phase molecular processes to be studied at very low temperatures. This Review focuses on the reactions of molecular and atomic radicals with neutral molecules. Rate constants for almost 50 such reactions have been measured at temperatures as low as 13 K by using the CRESU method. The surprising demonstration that so many reactions between electrically neutral species can be extremely rapid at these very low temperatures has excited interest both from theoreticians and from those seeking to understand the chemistry that gives rise to the 135 or so molecules that are present in low-temperature molecular clouds in the interstellar medium. Theoretical treatments of these reactions are based on the idea that a reaction occurs when the long-range potential between the reagent species brings them into close contact. The astrochemical context, theoretical studies, and the determination of the rate constants of these low-temperature reactions are critically discussed.     

Probing trapped ion energies via ion-molecule reaction kinetics: Quadrupole ion trap mass spectrometry

We present a detailed study of the energies of the ions stored in a quadrupole ion trap mass spectrometer (QITMS). Previous studies have shown that the rate constant, k, for the charge exchange reaction Ar⁺ N⁺ ₂ →, N⁺ ₂+Ar increases with increasing ion-molecule center-of-mass kinetic energy (K.E._cm). Thus, we have determined k for this chemical “thermometer” reaction at a variety of Ar and N₂ pressures and have assigned K.E._cm values as a function of the q₂ of the Ar⁺ ion both with and without He buffer gas present in the trap. The K.E._cm energies are found to lie within the range 0.11–0.34 eV over the variety of experimental conditions investigated. Quantitative “cooling” effects due to the presence of He buffer gas are reported, as are increases in K.E._cm due to an increase in the q₂ of the Ar⁺ ion. “Effective” temperatures of the Ar⁺ ions in He buffer are determined based on a Maxwell-Boltzmann distribution of ion energies. The resulting temperatures are found to lie within the range ≈ 1700–3300 K. We have also examined the K.E._cm, values arising from the chemical thermometer reaction of O⁺ ₂ with CH₄, as previous assignments of effective ion temperatures based on this reaction have been called into question.     

Termolecular ion-molecule reactions in Titan's atmosphere. IV. A search made at up to 1 micron in pure hydrocarbons

The results of a study of ion-molecule reactions occurring in pure methane, acetylene, ethylene, ethane, propyne, propene, propane, and diacetylene at pressures up to 40 microns of pressure are reported. A variety of experimental methods are used: The standard double resonance in an ICR, for determination of the precursor ions and the modulated double resonance ejection in an ICR, for the determination of the daughter ions. The FA-SIFT technique was used for validation and examination of termolecular reactions with rate coefficients that are less than 10(-26) cm(6) s(-1). An extensive database of reaction kinetics already exists for many of these reactions. The main point of this study was the determination of the accuracy of this database and to search for any missing reactions and reaction channels that may have been omitted from earlier investigations. A specific objective of this work was to extend the study to the highest pressures possible to find out if there were any important termolecular reaction channels occurring. A new approach was used here. In the pure hydrocarbon gases the mass spectra were followed as a function of the pressure changes of the gas. An initial guess was first made using the current literature as a source of the reaction kinetics that were expected. A model of the ion abundances was produced from the solution of the partial differential equations in terms of reaction rate coefficients and initial abundances. The experimental data was fitted to the model for all of the pressures by a least squares minimization to the reaction rate coefficients and initial abundances. The reaction rate coefficients obtained from the model were then compared to the literature values. Several new channels and reactions were discovered when the modeled fits were compared to the actual data. This is all explained in the text and the implications of these results are discussed for the Titan atmosphere.     

Endothermic character of toluene adsorption from supercritical carbon dioxide on activated carbon at low coverage

Recasens, P., Velo, E., Larrayoz, M.A. and Puiggené, J., 1993. Endothermic character of toluene adsorption from supercritical carbon dioxide on activated carbon at low coverage. Fluid Phase Equilibria, 90: 265-287.

Heat effects and volumetric properties are analyzed for the adsorption of toluene from supercritical carbon dioxide onto activated carbon at the limit of zero coverage, based on existing data for the system. Using values of the adsorption equilibrium constant at different temperatures as a function of fluid density, large, negative partial molar volumes for toluene in the fluid were obtained, which were previously unavailable.

Numerical integration of the differential equation that expresses the isobaric temperature dependence of the equilibrium constant, coupled with parameter optimization, enabled us to estimate the differential enthalpy of toluene adsorption onto the surface from the ideal gas at the same pressure and temperature, in addition to the enthalpy of transfer from the fluid to the surface. This is found to be large and positive near the critical conditions. Using the thermodynamic analysis of Kelley and Chimowitz, our results show that in terms of the enthalpy of transfer, the isothermal adsorption from a supercritical fluid is an endothermic process, thus explaining the retrograde behavior experimentally observed for the regeneration of carbon with supercritical CO₂ at conditions not far from the solvent's critical point.     

On the role of dynamical barriers in barrierless reactions at low energies: S(1D) + H2

Reaction probabilities as a function of total angular momentum (opacity functions) and the resulting reaction cross sections for the collision of open shell S((1)D) atoms with para-hydrogen have been calculated in the kinetic energy range 0.09-10 meV (1-120 K). The quantum mechanical hyperspherical reactive scattering method and quasi-classical trajectory and statistical quasi-classical trajectory approaches were used. Two different ab initio potential energy surfaces (PESs) have been considered. The widely used reproducing kernel Hilbert space (RKHS) PES by Ho et al. [T.-S. Ho, T. Hollebeek, H. Rabitz, S. D. Chao, R. T. Skodje, A. S. Zyubin, and A. M. Mebel, J. Chem. Phys 116, 4124 (2002)] and the recently published accurate double many-body expansion (DMBE)/complete basis set (CBS) PES by Song and Varandas [Y. Z. Song and A. J. C. Varandas, J. Chem. Phys. 130, 134317 (2009)]. The calculations at low collision energies reveal very different dynamical behaviors on the two PESs. The reactivity on the RKHS PES is found to be considerably larger than that on the DMBE/CBS PES as a result of larger reaction probabilities at low total (here also orbital) angular momentum values and to opacity functions which extend to significantly larger total angular momentum values. The observed differences have their origin in two major distinct topographic features. Although both PESs are essentially barrierless for equilibrium H-H distances, when the H-H bond is compressed the DMBE/CBS PES gives rise to a dynamical barrier which limits the reactivity of the system. This barrier is completely absent in the RHKS PES. In addition, the latter PES exhibits a van der Walls well in the entrance channel which reduces the height of the centrifugal barrier and is able to support resonances. As a result, a significant larger cross section is found on this PES, with marked oscillations attributable to shape resonances and/or to the opening of partial wave contributions. The comparison of the results on both PESs is illustrative of the wealth of the dynamics at low collision energy. It is also illuminating about the difficulties encountered in modeling an all-purpose global potential energy surface.     

Quantitative analysis by (p,X) and (p, γ) reactions at low energies

Production of atomic X-rays and nuclear γ-rays by bombardment with 0–3 MeV protons of thick targets is described. In the case of low-Z atomic X-rays, the absorption in the target is very large, while in the case ofK X-rays from high-Z atoms or for γ-rays, this phenomenon is negligible. Both of these reactions can be used for analysis of elements from F to U, and the sensitivities and the accuracies of the determinations are discussed. A table is given showing the γ-ray energies observed in 11 substances and the limits of sensitivity.     

Probing trapped ion energies via ion-molecule reaction kinetics: Fourier transform ion cyclotron resonance mass spectrometry

The kinetic energy-dependent Ar⁺+ N₂ ion-molecule reaction has been used as a chemical “thermometer” to determine the kinetic energy of ions produced by electron ionization and trapped by using a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. The rate constant for this reaction obtained on the FTICR mass spectrometer was compared to previous work, which allowed a kinetic energy estimate to be made. In addition, the effects of varying parameters such as trapping voltage and pressure on ion kinetic energy were investigated. No evidence of the differing reactivity of higher energy electronic states of Ar⁺, such as ²P_1/2, was observed and the results of a model of this system are presented that support this observation. Pressure studies revealed that with an average of as few as 13 ion-molecule collisions, Ar⁺ ions are collisionally relaxed to an extent unaffected by additional collisions. Based on recent variable temperature selected ion flow drift tube measurements, FTICR ion energies are estimated to be slightly above thermal.