Nuclear spin dependence of the reaction of H(3)+ with H2. I. Kinetics and modeling

The chemical reaction H(3)(+) + H(2) → H(2) + H(3)(+) is the simplest bimolecular reaction involving a polyatomic, yet is complex enough that exact quantum mechanical calculations to adequately model its dynamics are still unfeasible. In particular, the branching fractions for the "identity," "proton hop," and "hydrogen exchange" reaction pathways are unknown, and to date, experimental measurements of this process have been limited. In this work, the nuclear-spin-dependent steady-state kinetics of the H(3)(+) + H(2) reaction is examined in detail, and employed to generate models of the ortho:para ratio of H(3)(+) formed in plasmas of varying ortho:para H(2) ratios. One model is based entirely on nuclear spin statistics, and is appropriate for temperatures high enough to populate a large number of H(3)(+) rotational states. Efforts are made to include the influence of three-body collisions in this model by deriving nuclear spin product branching fractions for the H(5)(+) + H(2) reaction. Another model, based on rate coefficients calculated using a microcanonical statistical approach, is appropriate for lower-temperature plasmas in which energetic considerations begin to compete with the nuclear spin branching fractions. These models serve as a theoretical framework for interpreting the results of laboratory studies on the reaction of H(3)(+) with H(2).     

Unusual mechanism for H3(+) formation from ethane as obtained by femtosecond laser pulse ionization and quantum chemical calculations

The formation of H(3)(+) from saturated hydrocarbon molecules represents a prototype of a complex chemical process, involving the breaking and the making of chemical bonds. We present a combined theoretical and experimental investigation providing for the first time an understanding of the mechanism of H(3)(+) formation at the molecular level. The experimental approach involves femtosecond laser pulse ionization of ethane leading to H(3)(+) ions with kinetic energies on the order of 4 to 6.5 eV. The theoretical approach involves high-level quantum chemical calculation of the complete reaction path. The calculations confirm that the process takes place on the potential energy surface of the ethane dication. A surprising result of the theoretical investigation is, that the transition state of the process can be formally regarded as a H(2) molecule attached to a C(2)H(4)(2+) entity but IRC calculations show that it belongs to the reaction channel yielding C(2)H(3)(+) + H(3)(+). Experimentally measured kinetic energies of the correlated H(3)(+) and C(2)H(3)(+) ions confirm the reaction path suggested by theory.     

Action spectroscopy and temperature diagnostics of H+ 3 by chemical probing

Infrared absorption spectroscopy of few hundred H+(3) ions trapped in a 22-pole ion trap is presented using chemical probing as a sensitive detection technique down to the single ion level. By exciting selected overtone transitions of the (v(1)=0,v(2) (l)=3(1))<--(0,0(0)) vibrational band using an external cavity diode laser an accurate diagnostics measurement of the effective translational and rotational temperatures of the trapped ions was performed. The absolute accuracy of the measured transition frequencies was improved by a factor of four compared to previous plasma spectroscopy measurements using velocity modulation [Ventrudo et al., J. Chem. Phys. 100, 6263 (1994)]. The observed buffer gas cooling conditions in the ion trap indicate how to cool trapped H+(3) ions into the lowest ortho and para rotational states. Future experiments will utilize such an internally cold ion ensemble for state-selected dissociative recombination experiments at the heavy ion storage ring Test Storage Ring (TSR).     

Infrared spectra of C(3)H(3)(+)-N(2) dimers: identification of proton-bound c-C(3)H(3)(+)-N(2) and H(2)CCCH(+)-N(2) isomers.

Mid-infrared photodissociation spectra of mass selected C(3)H(3)(+)-N(2) ionic complexes are obtained in the vicinity of the C-H stretch fundamentals (2970-3370 cm(-1)). The C(3)H(3)(+)-N(2) dimers are produced in an electron impact cluster ion source by supersonically expanding a gas mixture of allene, N(2), and Ar. Rovibrational analysis of the spectra demonstrates that (at least) two C(3)H(3)(+) isomers are produced in the employed ion source, namely the cyclopropenyl (c-C(3)H(3)(+)) and the propargyl (H(2)CCCH(+)) cations. This observation is the first spectroscopic detection of the important c-C(3)H(3)(+) ion in the gas phase. Both C(3)H(3)(+) cations form intermolecular proton bonds to the N(2) ligand with a linear -C-H...N-N configuration, leading to planar C(3)H(3)(+)-N(2) structures with C(2v) symmetry. The strongest absorption of the H(2)CCCH(+)-N(2) dimer in the spectral range investigated corresponds to the acetylenic C-H stretch fundamental (v(1) = 3139 cm(-1)), which experiences a large red shift upon N(2) complexation (Delta(v1) approximately -180 cm(-1)). For c-C(3)H(3)(+)-N(2), the strongly IR active degenerate antisymmetric stretch vibration (v4)) of c-C(3)H(3)(+) is split into two components upon complexation with N(2): v4)(a(1)) = 3094 cm(-1) and v4)(b(2)) = 3129 cm(-1). These values bracket the yet unknown v4) frequency of free c-C(3)H(3)(+) in the gas phase, which is estimated as 3125 +/- 4 cm(-1) by comparison with theoretical data. Analysis of the nuclear spin statistical weights and A rotational constants of H(2)CCCH(+)-N(2) and c-C(3)H(3)(+)-N(2) provide for the first time high-resolution spectroscopic evidence that H(2)CCCH(+) and c-C(3)H(3)(+) are planar ions with C(2v) and D(3h) symmetry, respectively. Ab initio calculations at the MP2(full)/6-311G(2df,2pd) level confirm the given assignments and predict intermolecular separations of R(e) = 2.1772 and 2.0916 A and binding energies of D(e) = 1227 and 1373 cm(-1) for the H-bound c-C(3)H(3)(+)-N(2) and H(2)CCCH(+)-N(2) dimers, respectively.     

Isotopic exchange processes in cold plasmas of H2/D2 mixtures

Isotope exchange in low pressure cold plasmas of H(2)/D(2) mixtures has been investigated by means of mass spectrometric measurements of neutrals and ions, and kinetic model calculations. The measurements, which include also electron temperatures and densities, were performed in a stainless steel hollow cathode reactor for three discharge pressures: 1, 2 and 8 Pa, and for mixture compositions ranging from 100% H(2) to 100% D(2). The data are analyzed in the light of the model calculations, which are in good global agreement with the experiments. Isotope selective effects are found both in the surface recombination and in the gas-phase ionic chemistry. The dissociation of the fuel gas molecules is followed by wall recycling, which regenerates H(2) and D(2) and produces HD. Atomic recombination at the wall is found to proceed through an Eley-Rideal mechanism, with a preference for reaction of the adsorbed atoms with gas phase D atoms. The best fit probabilities for Eley-Rideal abstraction with H and D are: γ(ER H) = 1.5 × 10(-3), γ(ER D) = 2.0 × 10(-3). Concerning ions, at 1 Pa the diatomic species H(2)(+), D(2)(+) and HD(+), formed directly by electron impact, prevail in the distributions, and at 8 Pa, the triatomic ions H(3)(+), H(2)D(+), HD(2)(+) and D(3)(+), produced primarily in reactions of diatomic ions with molecules, dominate the plasma composition. In this higher pressure regime, the formation of the mixed ions H(2)D(+) and HD(2)(+) is favoured in comparison with that of H(3)(+) and D(3)(+), as expected on statistical grounds. The model results predict a very small preference, undetectable within the precision of the measurements, for the generation of triatomic ions with a higher degree of deuteration, which is probably a residual influence at room temperature of the marked zero point energy effects (ZPE), relevant for deuterium fractionation in interstellar space. In contrast, ZPE effects are found to be decisive for the observed distribution of monoatomic ions H(+) and D(+), even at room temperature. The final H(+)/D(+) ratio is determined to a great extent by proton (and deuteron) exchange, which favours the enhancement of H(+) and the concomitant decrease of D(+).     

Neutral and ion chemistry in low pressure dc plasmas of H2/N2 mixtures: routes for the efficient production of NH3 and NH4(+)

The chemistry in low pressure (0.8-8 Pa) plasmas of H(2) + 10% N(2) mixtures has been experimentally investigated in a hollow cathode dc reactor using electrical probes for the estimation of electron temperatures and densities, and mass spectrometry to determine the concentration of ions and stable neutral species. The analysis of the measurements by means of a kinetic model has allowed the identification of the main physicochemical mechanisms responsible for the observed distributions of neutrals and ions and for their evolution with discharge pressure. The chemistry of neutral species is dominated by the formation of appreciable amounts of NH(3) at the metallic walls of the reactor through the successive hydrogenation of atomic nitrogen and nitrogen containing radicals. Both Eley-Rideal and Langmuir-Hinshelwood mechanisms are needed in the chain of hydrogenation steps in order to account satisfactorily for the observed ammonia concentrations, which, in the steady state, are found to reach values ~30-70% of those of N(2). The ionic composition of the plasma, which is entirely due to gas-phase processes, is the result of a competition between direct electron impact dissociation, more relevant for high electron temperatures (lower pressures), and ion-molecule chemistry that prevails for the lower electron temperatures (higher pressures). At the lowest pressure, products from the protonation of the precursor molecules (H(3)(+), N(2)H(+) and NH(4)(+)) and others from direct ionization (H(2)(+) and NH(3)(+)) are found in comparable amounts. At the higher pressures, the ionic distribution is largely dominated by ammonium. It is found that collisions of H(3)(+), NH(3)(+) and N(2)H(+) with the minor neutral component NH(3) are to a great extent responsible for the final prevalence of NH(4)(+).     

Highly accurate quartic force fields, vibrational frequencies, and spectroscopic constants for cyclic and linear C3H3(+)

High levels of theory have been used to compute quartic force fields (QFFs) for the cyclic and linear forms of the C(3)H(3)(+) molecular cation, referred to as c-C(3)H(3)(+) and l-C(3)H(3)(+). Specifically, the singles and doubles coupled-cluster method that includes a perturbational estimate of connected triple excitations, CCSD(T), has been used in conjunction with extrapolation to the one-particle basis set limit, and corrections for scalar relativity and core correlation have been included. The QFFs have been used to compute highly accurate fundamental vibrational frequencies and other spectroscopic constants by use of both vibrational second-order perturbation theory and variational methods to solve the nuclear Schro?dinger equation. Agreement between our best computed fundamental vibrational frequencies and recent infrared photodissociation experiments is reasonable for most bands, but there are a few exceptions. Possible sources for the discrepancies are discussed. We determine the energy difference between the cyclic and linear forms of C(3)H(3)(+), obtaining 27.9 kcal/mol at 0 K, which should be the most reliable available. It is expected that the fundamental vibrational frequencies and spectroscopic constants presented here for c-C(3)H(3)(+) and l-C(3)H(3)(+) are the most reliable available for the free gas-phase species, and it is hoped that these will be useful in the assignment of future high-resolution laboratory experiments or astronomical observations.     

Calibration-quality adiabatic potential energy surfaces for H3(+) and its isotopologues

Calibration-quality ab initio adiabatic potential energy surfaces (PES) have been determined for all isotopologues of the molecular ion H(3)(+). The underlying Born-Oppenheimer electronic structure computations used optimized explicitly correlated shifted Gaussian functions. The surfaces include diagonal Born-Oppenheimer corrections computed from the accurate electronic wave functions. A fit to the 41,655 ab initio points is presented which gives a standard deviation better than 0.1 cm(-1) when restricted to the points up to 6000 cm(-1) above the first dissociation asymptote. Nuclear motion calculations utilizing this PES, called GLH3P, and an exact kinetic energy operator given in orthogonal internal coordinates are presented. The ro-vibrational transition frequencies for H(3)(+), H(2)D(+), and HD(2)(+) are compared with high resolution measurements. The most sophisticated and complete procedure employed to compute ro-vibrational energy levels, which makes explicit allowance for the inclusion of non-adiabatic effects, reproduces all the known ro-vibrational levels of the H(3)(+) isotopologues considered to better than 0.2 cm(-1). This represents a significant (order-of-magnitude) improvement compared to previous studies of transitions in the visible. Careful treatment of linear geometries is important for high frequency transitions and leads to new assignments for some of the previously observed lines. Prospects for further investigations of non-adiabatic effects in the H(3)(+) isotopologues are discussed. In short, the paper presents (a) an extremely accurate global potential energy surface of H(3)(+) resulting from high accuracy ab initio computations and global fit, (b) very accurate nuclear motion calculations of all available experimental line data up to 16,000 cm(-1), and (c) results suggest that we can predict accurately the lines of H(3)(+) towards dissociation and thus facilitate their experimental observation.     

Electron capture in charge-tagged peptides. Evidence for the role of excited electronic states

Electron capture dissociation (ECD) was studied with doubly charged dipeptide ions that were tagged with fixed-charge tris-(2,4,6-trimethoxyphenyl)phosphonium-methylenecarboxamido (TMPP-ac) groups. Dipeptides GK, KG, AK, KA, and GR were each selectively tagged with one TMPP-ac group at the N-terminal amino group while the other charge was introduced by protonation at the lysine or arginine side-chain groups to give (TMPP-ac-peptide + H)(2+) ions by electrospray ionization. Doubly tagged peptide derivatives were also prepared from GK, KG, AK, and KA in which the fixed-charge TMPP-ac groups were attached to the N-terminal and lysine side-chain amino groups to give (TMPP-ac-peptide-ac-TMPP)(2+) dications by electrospray. ECD of (TMPP-ac-peptide + H)(2+) resulted in 72% to 84% conversion to singly charged dissociation products while no intact charge-reduced (TMPP-ac-dipeptide + H)(+) ions were detected. The dissociations involved loss of H, formation of (TMPP + H)(+), and N-C(alpha) bond cleavages giving TMPP-CH(2)CONH(2)(+) (c(0)) and c(1) fragments. In contrast, ECD of (TMPP-ac-peptide-ac-TMPP)(2+) resulted in 31% to 40% conversion to dissociation products due to loss of neutral TMPP molecules and 2,4,6-trimethoxyphenyl radicals. No peptide backbone cleavages were observed for the doubly tagged peptide ions. Ab initio and density functional theory calculations for (Ph(3)P-ac-GK + H)(2+) and (H(3)P-ac-GK + H)(2+) analogs indicated that the doubly charged ions contained the lysine side-chain NH(3)(+) group internally solvated by the COOH group. The distance between the charge-carrying phosphonium and ammonium atoms was calculated to be 13.1-13.2 A in the most stable dication conformers. The intrinsic recombination energies of the TMPP(+)-ac and (GK + H)(+) moieties, 2.7 and 3.15 eV, respectively, indicated that upon electron capture the ground electronic states of the (TMPP-ac-peptide + H)(+*) ions retained the charge in the TMPP group. Ground electronic state (TMPP-ac-GK + H)(+*) ions were calculated to spontaneously isomerize by lysine H-atom transfer to the COOH group to form dihydroxycarbinyl radical intermediates with the retention of the charged TMPP group. These can trigger cleavages of the adjacent N-C(alpha) bonds to give rise to the c(1) fragment ions. However, the calculated transition-state energies for GK and GGK models suggested that the ground-state potential energy surface was not favorable for the formation of the abundant c(0) fragment ions. This pointed to the involvement of excited electronic states according to the Utah-Washington mechanism of ECD.     

Existence of an exceptional reaction pathway for H3(+) formation observed in collision-induced dissociation of methane ions at 1000 eV

Dissociation of CH(4)(+) ions at 1000 eV induced by collision with Ar atoms was investigated by measuring the kinetic energies of the ionized fragments. At small scattering angles, including zero, H(+), H(2)(+), H(3)(+), CH(3)(+), CH(2)(+), CH(+), and C(+) fragments were observed. The attractive part of the potential in the CH(4)(+)-Ar collision system played an important role in the formation of the ionized fragments. Rainbow scattering, leading to a large scattering cross section, was shown to be responsible for the increased formation of H(3)(+). It is proposed that on collision-induced dissociation of CH(4)(+), its three hydrogen atoms, which form a triangle, simultaneously react and move together to form H(3)(+).     

Products, rate constants and mechanisms of gas-phase reactions of CX(3)(+), CX(2)(+), CX(+) (X = F and/or Cl) and Cl(+) with 1,1,1- and 1,1,2-trichlorotrifluoroethane.

The gas-phase ion chemistry of 1,1,1- and 1,1,2-trichlorotrifluoroethane was investigated with an ion trap mass spectrometer. Following electron ionization both compounds (M) fragment to [M - Cl](+), CX(3)(+), CX(2)(+), CX(+) (X = F and/or Cl) and Cl(+). The reactivity of each of these fragments towards their neutral precursors was studied to obtain product and kinetic data. Whereas [M - Cl](+), CCl(3)(+) and CCl(2)F(+) cations are unreactive under the experimental conditions used, all other species react via halide abstraction to give [M - Cl](+) and, to a far lesser extent, [M - F](+). In addition, CX(2)(+) ions form CClX(2)(+) in a process which formally amounts to chlorine atom abstraction, but more likely involves chloride ion abstraction followed by charge transfer. CX(+) ions also form minor amounts of CX(3)(+) product ions, possibly via chloride abstraction followed by or concerted with dihalocarbene elimination from the (incipient) [M - Cl](+) ion. Trivalent carbenium ions are less reactive than divalent species, which in turn are less reactive than the monovalent ions (reaction efficiencies are given in parentheses): CF(3)(+)(0.70) < CF(2)(+)(0.78) < CF(+)(0.96). More interestingly, within each family of ions reactivity increases with the number of fluorine substituents (e.g. CF(2)(+) > CFCl(+) > CCl(2)(+) and CF(+) > CCl(+)), i.e. reactivity increases with the ion thermochemical stability, as measured by available standard free enthalpies of formation. Evaluation of the energetics involved shows that reactions are largely driven by the stability of the neutrals more than of the ions. Finally, the products observed in the reaction of Cl(+) are attributed to ionization of the neutral via charge transfer and fragmentation.     

The N3+ reactivity in ionized gases containing sulfur, nitrogen, and carbon oxides.

The N(3)(+) reactivity with SO(2), N(2)O, CO(2), and CO is studied by mass spectrometric techniques under a wide range of pressures from 10(-7) to 10(-4) Torr. The kinetics, reaction mechanism, and role of vibrationally excited ions are investigated by experimental and theoretical methods. Key distinguishing features of the N(3) (+) reactivity are evidenced by comparison to N(+) and N(2)(+) ions, which mainly undergo charge-exchange reactions. The N(+) transfer to SO(2) prompts formation of NO(+) ions and neutral oxides NO and SO. The N(+) transfer to N(2)O also leads to NO(+) ions by a process not allowed by spin conservation rules. In both cases no reaction intermediate is detected, whereas CO(2) and CO are captured to form the very stable NCO(2) (+) and NCO(+) ions. NCO(2)(+) ions are characterized for the first time as strongly bound triplet ions of NOCO and ONCO connectivity. DFT and CCSD(T) computations have been carried out to investigate the structural and energetic features of the NCO(2) (+) species and their formation process.     

Electronic structure of linear thiophenolate-bridged heteronuclear complexes [LFeMFeL](n)(+) (M = Cr, Co, Fe; n = 1-3): a combination of kinetic exchange interaction and electron delocalization

The electronic properties of the isostructural series of heterotrinuclear thiophenolate-bridged complexes of the general formula [LFeMFeL](n)(+) with M = Cr, Co and Fe where L represents the trianionic form of the ligand 1,4,7-tris(4-tertbutyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, synthesized and investigated by a number of experimental techniques in the previous work(1), are subjected now to a theoretical analysis. The low-lying electronic excitations in these compounds are described within a minimal model supported by experiment and quantum chemistry calculations. It was found indeed that various experimental data concerning the magnetism and electron delocalization in the lowest states of all seven compounds are completely reproduced within a model which includes the electron transfer between magnetic orbitals at different metal centers and the electron repulsion in these orbitals (the Hubbard model). Moreover, due to the trigonal symmetry of the complexes, only the electron transfer between nondegenerate orbital, a(1), originating from the t(2g) shell of each metal ion in a pseudo-octahedral coordination, is relevant for the lowest states. An essential feature resulting from quantum chemistry calculations, allowing to explain the unusual magnetic properties of these compounds, is the surprisingly large value and, especially, the negative sign of the electron transfer between terminal iron ions, beta'. According to their electronic properties the series of complexes can be divided as follows: (1). The complexes [LFeFeFeL](3+) and [LFeCrFeL](3+) show localized valences in the ground electronic configuration. The strong antiferromagnetic exchange interaction and the resulting spin 1/2 of the ground-state arise from large values of the transfer parameters. (2). In the complex [LFeCrFeL](+), due to a higher energy of the magnetic orbital on the central Cr ion than on the terminal Fe ones, the spin 3/2 and the single unpaired a(1) electron are almost localized at the chromium center in the ground state. (3). The complex [LFeCoFeL](3+) has one ground electronic configuration in which two unpaired electrons are localized at terminal iron ions. The ground-state spin S = 1 arises from a kinetic mechanism involving the electron transfer between terminal iron ions as one of the steps. Such a mechanism, leading to a strong ferromagnetic interaction between distant spins, apparently has not been discussed before. (4). The complex [LFeFeFeL](2+) is characterized by both spin and charge degrees of freedom in the ground manifold. The stabilization of the total spin zero or one of the itinerant electrons depends on beta', i.e., corresponds to the observed S = 1 for its negative sign. This behavior does not fit into the double exchange model. (5). In [LFeCrFeL](2+) the delocalization of two itinerant holes in a(1) orbitals takes place over the magnetic core of chromium ion. Although the origin of the ground-state spin S = 2 is the spin dependent delocalization, the spectrum of the low-lying electronic states is again not of a double exchange type. (6). Finally, the complex [LFeCoFeL](2+) has the ground configuration corresponding to the electron delocalization between terminal iron atoms. The estimated magnitude of the corresponding electron transfer is smaller than the relaxation energy of the nuclear distortions induced by the electron localization at one of the centers, leading to vibronic valence trapping observed in this compound.     

Reactive processes in gas phase Na(+)-iso-C3H7Cl collisions: experimental guided-ion-beam and ab initio studies of the reactions on the ground singlet potential surface of the system up to 12.00 eV

Reactive processes, taking place when sodium ions collide with neutral iso-C(3)H(7)Cl molecules in the 0.02-12.00 eV range of energies in the center of mass frame, have been studied using an octopole radiofrequency guided-ion-beam apparatus developed in our laboratory. A dehydrohalogenation reaction channel leading to Na(C(3)H(6))(+) formation has been observed up to 1.00 eV while another process producing NaHCl(+) continues up to 4.00 eV. Furthermore, C(3)H(7)(+) formation resulting from decomposition of the reactants, ion-molecule adducts, has also been observed as well as its decomposition into C(2)H(3)(+) on increasing collision energy. Cross-section energy dependences for all these reactions have been obtained in absolute units. The ab initio electronic structure calculations have been done at the MP2 level for the colliding system ground singlet potential surface, giving information on the reactive surface main topological features. From the surface reactants side to the products' one, different potential wells and barriers have been characterized and their connectivity along the reaction evolution has been established using the intrinsic-reaction-coordinate method, thus interpreting the dynamical evolution of the reactants' collision complex to products. Experimental results demonstrate that NaHCl(+) can be produced via different channels. Reaction rate constants at 308.2 K for both dehydrohalogenation reactions have been calculated from measured excitation functions. It has been also confirmed that the reactants adduct decomposition giving C(3)H(7)(+) and NaCl takes place on the same potential surface. A qualitative interpretation of the experimental results in terms of ab initio calculations is also given.     

Atom and ion chemistry in low pressure hydrogen dc plasmas

The chemical composition of a low-pressure hydrogen dc plasma produced in a hollow cathode discharge has been measured and modeled. The concentrations of H atoms and of H(+), H(2)(+) and H(3)(+) ions were determined with a combination of optical spectroscopic and mass spectrometric techniques, over the range of pressures (p approximately 0.008-0.2 m bar) investigated. The results were rationalized with the help of a zero-order kinetic model. A comparatively high fraction ( approximately 0.1+/-0.05) of H atoms, indicative of a relatively small wall recombination, was observed. Low ionization degrees (<10(-4)) were obtained in all cases. In general, the ionic composition of the plasma was found to be dominated by H(3)(+), except at the lowest pressures, where H(2)(+) was the major ion. The key physicochemical processes determining the plasma composition were identified from the comparison of experimental and model results, and are discussed in the paper.     

Kinetic studies of atmospherically relevant silicon chemistry. Part III: Reactions of Si+ and SiO+ with O3, and Si+ with O2

Silicon ions are generated in the Earth's upper atmosphere by hyperthermal collisions of material ablated from incoming meteoroids with atmospheric molecules, and from charge transfer of silicon-bearing neutral species with major atmospheric ions. Reported Si(+) number density vs. height profiles show a sharp decrease below 95 km, which has been commonly attributed to the fast reaction with H(2)O. Here we report rate coefficients and branching ratios of the reactions of Si(+) and SiO(+) with O(3), measured using a flow tube with a laser ablation source and detection of ions by quadrupole mass spectrometry. The results obtained are (2σ uncertainty): k(Si(+) + O(3), 298 K) = (6.5 ± 2.1) × 10(-10) cm(3) molecule(-1) s(-1), with three product channels (branching ratios): SiO(+) + O(2) (0.52 ± 0.24), SiO + O(2)(+) (0.48 ± 0.24), and SiO(2)(+) + O (<0.1); k(SiO(+) + O(3), 298 K) = (6 ± 4) × 10(-10) cm(3) molecule(-1) s(-1), where the major products (branching ratio ≥ 0.95) are SiO(2) + O(2)(+). Reactions (1) and (2) therefore have the unusual ability to neutralise silicon directly, as well as forming molecular ions which can undergo dissociative recombination with electrons. These reactions, along with the recently reported reaction between Si(+) and O(2)((1)Δ(g)), largely explain the disappearance of Si(+) below 95 km in the atmosphere, relative to other major meteoric ions such as Fe(+) and Mg(+). The rate coefficient of the Si(+) + O(2) + He reaction was measured to be k(298 K) = (9.0±1.3) × 10(-30) cm(6) molecule(-2) s(-1), in agreement with previous measurements. The SiO(2)(+) species produced from this reaction, which could be vibrationally excited, is observed to charge transfer at a relatively slow rate with O(2), with a rate constant of k(298 K) = (1.5 ± 1.0) × 10(-13) cm(3) molecule(-1) s(-1).     

Gas-phase ion-molecule reactions of metal-carbide cations MCn+ (M=Y and La; n=2, 4, and 6) with benzene and cyclohexane investigated by FTICR mass spectrometry and DFT calculations

Yttrium- and lanthanum-carbide cluster cations YC(n)(+) and LaC(n)(+) (n = 2, 4, and 6) are generated by laser ablation of carbonaceous material containing Y(2)O(3) or La(2)O(3). YC(2)(+), YC(4)(+), LaC(2)(+), LaC(4)(+), and LaC(6)(+) are selected to undergo gas-phase ion-molecule reactions with benzene and cyclohexane. The FTICR mass spectrometry study shows that the reactions of YC(2)(+) and LaC(2)(+) with benzene produce three main series of cluster ions. They are in the form of M(C(6)H(4))(C(6)H(6))(n)(+), M(C(8)H(4))(C(6)H(6))(n)(+), and M(C(8)H(6))(C(6)H(6))(m)(+) (M = Y and La; n = 0-3; m = 0-2). For YC(4)(+), LaC(4)(+), and LaC(6)(+), benzene addition products in the form of MC(n)(C(6)H(6))(m)(+) (M = Y and La; n = 4, 6; m = 1, 2) are observed. In the reaction with cyclohexane, all the metal-carbide cluster ions are observed to form metal-benzene complexes M(C(6)H(6))(n)(+) (M = Y and La; n= 1-3). Collision-induced-dissociation experiments were performed on the major reaction product ions, and the different levels of energy required for the fragmentation suggest that both covalent bonding and weak electrostatic interaction exist in these organometallic complexes. Several major product ions were calculated using DFT theory, and their ground-state geometries and energies were obtained.     

Nuclear spin dependence of the reaction of H(3)+ with H2. II. Experimental measurements

The nuclear spin dependence of the chemical reaction H(3)(+)+ H(2) → H(2)?+ H(3)(+) has been studied in a hollow cathode plasma cell. Multipass infrared direct absorption spectroscopy has been employed to monitor the populations of several low-energy rotational levels of ortho- and para-H(3)(+) (o-H(3)(+) and p-H(3)(+)) in hydrogenic plasmas of varying para-H(2) (p-H(2)) enrichment. The ratio of the rates of the proton hop (k(H)) and hydrogen exchange (k(E)) reactions α ≡ k(H)/k(E) is inferred from the observed p-H(3)(+) fraction as a function of p-H(2) fraction using steady-state chemical models. Measurements have been performed both in uncooled (T(kin) ～ 350 K) and in liquid-nitrogen-cooled (T(kin) ～ 135 K) plasmas, marking the first time this reaction has been studied at low temperature. The value of α has been found to decrease from 1.6 ± 0.1 at 350 K to 0.5 ± 0.1 at 135 K.     

Low temperature kinetics and energetics of the electron and hole traps in irradiated TiO2 nanoparticles as revealed by EPR spectroscopy

We have monitored exclusively the dynamics of photogenerated charge carriers trapping in deep traps and trapped electron-hole recombination in UV irradiated anatase TiO2 powders by electron paramagnetic resonance (EPR) spectroscopy at 10 K. The results reveal that the strategy of using low temperatures contributes to the stabilization of the charged pair states for hours by reducing the rate of electron-hole recombination processes. Since only the localized states such as holes trapped at oxygen anions and electrons trapped at coordinatively unsaturated cations are accessible to EPR spectroscopy, the time-dependent population and depopulation of these EPR signals reflect the kinetics and energetics of these trap states. The data support a model of sequential accumulation of deep trap site populations in which the initial fast direct trapping into a deep trap site is followed by slower carrier trap-to-trap hopping until a deep trap is encountered for both photogenerated electrons and holes. Effective modeling of the subsequent decay of trapped-holes is achieved by employing a first-order kinetics, whereas the decay of either surface- or inner-trapped electrons has both a fast and a slow component. The fast component is attributed to a trapped-electron and a free-hole recombination, and the slow component is attributed to trapped electron-hole recombination. The activation energies for the process of diffusion of trapped electrons from their Ti3+ trapping sites are estimated.     

Evaluation of a novel approach for peptide sequencing: laser-induced acoustic desorption combined with P(OCH(3))(2)(+) chemical ionization and collision-activated dissociation in a Fourier transform ion cyclotron resonance mass spectrometer

A novel mass spectrometric method has been developed for obtaining sequence information on small peptides. The peptides are desorbed as intact neutral molecules into a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) by means of laser-induced acoustic desorption (LIAD). Reactions of the neutral peptides with the dimethoxyphosphenium ion, P(OCH(3))(2)(+), occur predominantly by addition of the peptide to P(OCH(3))(2)(+) followed by the loss of two methanol molecules, thus yielding product ions with the composition (peptide + P - 2H)(+). Upon sustained off-resonance irradiation for collision-activated dissociation (SORI-CAD), the (peptide + P - 2H)(+) ions undergo successive losses of CO and NHCHR or H(2)O, CO, and NHCHR to yield sequence-related fragment ions in addition to the regular a(n)- and b(n)-type ions. Under the same conditions, SORI-CAD of the analogous protonated peptides predominantly yields the regular a(n)- and b(n)-type ions. The mechanisms of the reactions of peptides with P(OCH(3))(2)(+) and the dissociation of the (peptide + P - 2H)(+) ions were examined by using model peptides and molecular orbital calculations.