Selected ion flow tube study of the reactions between gas phase cations and CHCl2F, CHClF2, and CH2ClF

The branching ratios and rate coefficients have been measured at 298 K for the reactions between CHCl2F, CHClF2, and CH2ClF and the following cations (with recombination energies in the range 6.3-21.6 eV); H3O+, SFx+ (x = 1-5), CFy+ (y = 1-3), NO+, NO2+, O2+, Xe+, N2O+, O+, CO2+, Kr+, CO+, N+, N2+, Ar+, F+, and Ne+. The majority of the reactions proceed at the calculated collisional rate, but the reagent ions SF3+, NO+, NO2+, and SF2+ do not react. Surprisingly, although all of the observed product channels are calculated to be endothermic, H3O+ does react with CHCl2F. On thermochemical grounds, Xe+ appears to react with these molecules only when it is in its higher-energy 2P1/2 spin-orbit state. In general, most of the reactions form products by dissociative charge transfer, but some of the reactions of CH2ClF with the lower-energy cations produce the parent cation in significant abundance. The branching ratios produced in this study and by threshold photoelectron-photoion coincidence spectroscopy agree reasonably well over the energy range 11-22 eV. In about one-fifth of the large number of reactions studied, the branching ratios are in excellent agreement and appreciable energy resonance between an excited state and the ground state of the ionized neutral exists, suggesting that these reactions proceed exclusively by a long-range charge-transfer mechanism. Upper limits for the enthalpy of formation at 298 K of SF4Cl (-637 kJ mol-1), SClF (-28 kJ mol-1), and SHF (-7 kJ mol-1) are determined.     

Reactions of N+, N2+, and N3+ with NO from 300 to 1400 K

Rate constants have been measured from 300 to 1400 K in a selected ion flow tube (SIFT) and a high temperature flowing afterglow for the reactions of N+, N2+ and N3+ with NO. In all of the systems, the rate constants are substantially less than the collision rate constant. Comparing the high temperature results to kinetics studies as a function of translational energy show that all types of energy (translational, rotational, and vibrational) affect the reactivity approximately equally for all three ions. Branching ratios have also been measured at 300 and 500 K in a SIFT for the N+ and N3+ reactions. An increase in the N2+ product at the expense of NO+ nondissociative charge transfer product occurs at 500 K with N+. The branching ratios for the reaction of N3+ with NO have also been measured in the SIFT, showing that only nondissociative charge transfer giving NO+ occurs up to 500 K. The current results are discussed in the context of the many previous studies of these ions in the literature.     

Selected ion flow tube study of the ion-molecule reactions of monochloroethene, trichloroethene, and tetrachloroethene

Data for the rate coefficients and product cations of the reactions of a large number of atomic and small molecular cations with monochloroethene, trichloroethene, and tetrachloroethene in a selected ion flow tube at 298 K are reported. The recombination energy of the ions range from 6.27 (H3O(+)) through to 21.56 (Ne(+)) eV. Collisional rate coefficients are calculated by modified average dipole orientation theory and compared with experimental values. Thermochemistry and mass balance predict the most feasible neutral products. Together with previously reported results for the three isomers of dichloroethene ( Mikhailov, V. A. ; Parkes, M. A. ; Tuckett, R. P. ; Mayhew, C. A. J. Phys. Chem. A 2006, 110, 5760 ), the fragment ion branching ratios have been compared with those from threshold photoelectron photoion coincidence spectroscopy over the photon energy range of 9-22 eV to determine the importance or otherwise of long-range charge transfer. For ions with recombination energy in excess of the ionization energy of the chloroethene, charge transfer is energetically allowed. The similarity of the branching ratios from the two experiments suggest that long-range charge transfer is dominant. For ions with recombination energy less than the ionization energy, charge transfer is not allowed; chemical reaction can only occur following formation of an ion-molecule complex, where steric effects are more significant. The products that are now formed and their percentage yields are a complex interplay between the number and position of the chlorine atoms with respect to the C=C bond, where inductive and conjugation effects can be important.     

Photoelectron spectroscopy and thermochemistry of tert-butylisocyanide-substituted cobalt tricarbonyl nitrosyl

A new organometallic complex, Co(CO)2NOtBuNC, was synthesized and investigated by photoelectron spectroscopy (PES) and threshold photoelectron photoion coincidence (TPEPICO) spectrometry in order to determine its ionization energy as well as the bond energies in the ionic forms. The assignment of the nine peaks in the PES was based on Kohn-Sham molecular orbital energies, and an adiabatic ionization energy of 7.30 +/- 0.05 eV was determined. In the TPEPICO experiment, the following 0 K onsets were determined for the various fragment ions: CoCONOtBuNC+ (8.17 +/- 0.05 eV); CoNOtBuNC+ (9.01 +/- 0.05 eV); and CotBuNC+ (10.42 +/- 0.05 eV). Because the photon source did not extend above 14 eV, we could not observe the bare Co+ ion in the experiment. The heat of formation of the CotBuNC+ ion was estimated by ab initio and DFT calculations of the CoL+ + tBuNC --> CotBuNC+ + L (L = CO, NO, NH3, H2O, PMe3) substitution enthalpies.     

Electron-impact ionization of CCl4 and CCl2F2

Absolute partial and total cross sections for electron-impact ionization of CCl4 and CCl2F2 are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for production of CCl3(+), CCl2(+), CCl+, C+, Cl2(+), and CCl3(2+) from CCl4; and for production of CCl(2)F+, CClF2(+), CClF(+), (CCl+ + CF2(+)), Cl+, CF+, F+, and C+ from CCl2F2. Data are also reported for formation of (CCl2(+),Cl+) and (CCl+, Cl+) ion pairs from CCl4. The total cross section for each target is obtained as the sum of the partial cross sections. The overall uncertainty in the absolute cross sections for most of the singly charged ions is +/- 5-7 %. The present partial cross sections for lighter fragment ions are found to be considerably greater than had been previously reported but the most recent total cross section measurements agree well with those reported here. Neither the binary-encounter-Bethe theory nor the Deutsch-Mark theory reproduces the experimental cross sections correctly for both targets.     

Isomeric effects in the gas-phase reactions of dichloroethene, C2H2CL2, with a series of cations

A study of the reactions of a series of gas-phase cations (NH(4)(+), H(3)O(+), SF(3)(+), CF(3)(+), CF(+), SF(5)(+), SF(2)(+), SF(+), CF(2)(+), SF(4)(+), O(2)(+), Xe(+), N(2)O(+), CO(2)(+), Kr(+), CO(+), N(+), N(2)(+), Ar(+), F(+), and Ne(+)) with the three structural isomers of dichloroethene, i.e., 1,1-C(2)H(2)Cl(2), cis-1,2-C(2)H(2)Cl(2), and trans-1,2-C(2)H(2)Cl(2) is reported. The recombination energy (RE) of these ions spans the range of 4.7-21.6 eV. Reaction rate coefficients and product branching ratios have been measured at 298 K in a selected ion flow tube (SIFT). Collisional rate coefficients are calculated by modified average dipole orientation (MADO) theory and compared with experimental data. Thermochemistry and mass balance have been used to predict the most feasible neutral products. Threshold photoelectron-photoion coincidence spectra have also been obtained for the three isomers of C(2)H(2)Cl(2) with photon energies in the range of 10-23 eV. The fragment ion branching ratios have been compared with those of the flow tube study to determine the importance of long-range charge transfer. A strong influence of the isomeric structure of dichloroethene on the products of ion-molecule reactions has been observed for H(3)O(+), CF(3)(+), and CF(+). For 1,1-C(2)H(2)Cl(2) the reaction with H(3)O(+) proceeds at the collisional rate with the only ionic product being 1,1-C(2)H(2)Cl(2)H(+). However, the same reaction yields two more ionic products in the case of cis-1,2- and trans-1,2-C(2)H(2)Cl(2), but only proceeds with 14% and 18% efficiency, respectively. The CF(3)(+) reaction proceeds with 56-80% efficiency, the only ionic product for 1,1-C(2)H(2)Cl(2) being C(2)H(2)Cl(+) formed via Cl(-) abstraction, whereas the only ionic product for both 1,2-isomers is CHCl(2)(+) corresponding to a breaking of the C=C double bond. Less profound isomeric effects, but still resulting in different products for 1,1- and 1,2-C(2)H(2)Cl(2) isomers, have been found in the reactions of SF(+), CO(2)(+), CO(+), N(2)(+), and Ar(+). Although these five ions have REs above the ionization energy (IE) of any of the C(2)H(2)Cl(2) isomers, and hence the threshold for long-range charge transfer, the results suggest that the formation of a collision complex at short range between these ions and C(2)H(2)Cl(2) is responsible for the observed effects.     

Dissociative scattering of hyperthermal energy CF3+ ions from modified surfaces

Dissociative scattering of CF3+ ions in collision with a self-assembled monolayer surface of fluorinated alkyl thiol on a gold 111 crystal has been studied at low ion kinetic energies (from 29 to 159 eV) using a custom built tandem mass spectrometer with a rotatable second stage energy analyzer and mass spectrometer detectors. Energy and intensity distributions of the scattered fragment ions were measured as a function of the fragment ion mass and scattering angle. Inelastically scattered CF3+ ions were not observed even at the lowest energy studied here. All fragment ions, CF2+, CF+, F+, and C+, were observed at all energies studied with the relative intensity of the highest energy pathway, C+, increasing and that of the lowest energy pathway, CF2+, decreasing with collision energy. Also, the dissociation efficiency of CF3+ decreased significantly as the collision energy was increased to 159 eV. Energy distributions of all fragment ions from the alkyl thiol surface showed two distinct components, one corresponding to the loss of nearly all of the kinetic energy and scattered over a broad angular range while the other corresponding to smaller kinetic energy losses and scattered closer to the surface parallel. The latter process is due to delayed dissociation of collisionally excited ions after they have passed the collision region as excited parent ions. A similar study performed at 74 eV using a LiF coated surface on a titanium substrate resulted only in one process for all fragment ions; corresponding to the delayed dissociation process. The intensity maxima for these fragmentation processes were shifted farther away from the surface parallel compared to the thiol surface. A new mechanism is proposed for the delayed dissociation process as proceeding via projectile ions' neutralization to long-lived highly excited Rydberg state(s), reionization by the potential field between the collision region and entrance to the energy analyzer, and subsequent dissociation several microseconds after collisional excitation. A kinematic analysis of experimental data plotted as velocity Newton diagrams demonstrates that the delayed dissociation process results from the collisions of the ion with the bulk surface; i.e., the self-assembled monolayer surface acts as a bulk surface. A similar analysis for the highly inelastic collision processes shows that these are due to stronger collisions with a fraction of the thiol molecular chain, varying in length (mass) with the ion energy.     

Ion chemistry of NOO+

The kinetics for the reactions of NOO+ ions with neutral molecules having ionization potentials (IPs) from 9.27 to 15.58 eV was measured in a selected ion flow tube at 298 K. The NOO+ ions are produced from the reaction of N3+ + O2 and have been reacted with the following: NO, C6F6, CS2, CF3I, C3F6, OCS, C2H6, Xe, SO2, O3, N2O, CO2, Kr, CO, D2, and N2. Numerous types of reactions were observed with the various neutral reagents, including production of NO+ (which may involve loss of an O from the ion or addition of O to the neutral reactant, although the two channels could not be distinguished here), charge transfer, isomerization of NOO+ to ONO+, and hydride abstraction. High level theoretical calculations of the structures and energetics of the various isomers, electronic states, and transition states of NOO and NOO+ were performed to better understand the observed reactivity. All neutral species with an IP< or =11.18 eV were observed to react with NOO+ in part by charge transfer. Detailed calculations showed that the recommended adiabatic and vertical IPs of NOO are 10.4 and 11.7 eV, respectively, at the MRCISDQ/AVQZ level of theory. The observed experimental limit for charge transfer of 11.18 eV agreed well with the energetics of the final products obtained from theory if dissociation of the neutral metastable product occurred, i.e., the products were X+ +[O(3P) + NO(2Pi)], where [O(3P)+NO(2Pi)] formed via dissociation of metastable NOO. Charge exchange with neutral reagent X would, therefore, be exothermic if IP(X)<[IPad(NOO)-DeltaE(O+NO)-NOO]= approximately 11.1 eV, where IPad(NOO) is the adiabatic IP. The potential energy surface for the reaction of NOO+ with C2H6 was also calculated, indicating that two pathways for formation of HNO2 + C2H5 (+) exist.     

Electron ionization of acetylene

Relative partial ionization cross sections and precursor specific relative partial ionization cross sections for fragment ions formed by electron ionization of C2H2 have been measured using time-of-flight mass spectrometry coupled with a 2D ion-ion coincidence technique. We report data for the formation of H+, H+2, C2+, C+/C2+ 2, CH+/C2H+2, CH+2, C+2, and C2H+ relative to the formation of C2H+2, as a function of ionizing electron energy from 30-200 eV. While excellent agreement is found between our data and one set of previously published absolute partial ionization cross sections, some discrepancies exist between the results presented here and two other recent determinations of these absolute partial ionization cross sections. We attribute these differences to the loss of some translationally energetic fragment ions in these earlier studies. Our relative precursor-specific partial ionization cross sections enable us, for the first time, to quantify the contribution to the yield of each fragment ion from single, double, and triple ionization. Analysis shows that at 50 eV double ionization contributes 2% to the total ion yield, increasing to over 10% at an ionizing energy of 100 eV. From our ion-ion coincidence data, we have derived branching ratios for charge separating dissociations of the acetylene dication. Comparison of our data to recent ab initio/RRKM calculations suggest that close to the double ionization potential C2H2+2 dissociates predominantly on the ground triplet potential energy surface (3Sigma*g) with a much smaller contribution from dissociation via the lowest singlet potential energy surface (1Delta g). Measurements of the kinetic energy released in the fragmentation reactions of C2H2+2 have been used to obtain precursor state energies for the formation of product ion pairs, and are shown to be in good agreement with available experimental data and with theory.     

Branching ratio deviations from statistical behavior in core photoionization

Accurate calculations of carbon 1s photoionization cross sections have been performed at the density functional level with the B-spline linear combination of atomic orbitals approach. The molecules considered are FC[triple bond]CH, FC[triple bond]CCH3, FC[triple bond]CCN, F2C=CH2, CF3COOCH2CH3, and C3H5O. The variation of the branching ratios relative to inequivalent C 1s ionizations have been evaluated from threshold to about 100 eV photoelectron kinetic energy. Large deviations from the statistical ratios are observed at low energies, which remain often significant several tens of eV above threshold. The importance of taking into account core branching ratios for peak deconvolution and quantitative analysis, as well as an additional tool for structural information, is pointed out. Strong shape resonant effects are found to largely cancel in branching ratios. Their nature and variation along the series is analyzed in the framework of excitations into sigma* valence orbitals.     

Bond-forming reactions of dications with molecules: a computational and experimental study of the mechanisms for the formation of HCF2+ from CF3(2+) and H2

The QCISD and QCISD(T) quantum chemical methods have been used to characterize the energetics of various possible mechanisms for the formation of HCF2+ from the bond-forming reaction of CF3(2+) with H2. The stationary points on four different pathways leading to the product combinations HCF2+ + H+ + F and HCF2+ + HF+ have been calculated. All four pathways begin with the formation of a collision complex [H2-CF3]2+, followed by an internal hydrogen atom migration to give HC(FH)F2(2+). In two of the mechanisms, immediate charge separation of HC(FH)F2(2+) via loss of either HF+ or a proton, followed by loss of an F atom, yields the experimentally observed bond-forming product HCF2+. For the other two mechanisms, internal hydrogen rearrangement of HC(FH)F2(2+) to give C(FH)2F(2+), followed by charge separation, yields the product CF2H+. This product can then overcome a 2.04 eV barrier to rearrange to the HCF2+ isomer, which is 1.80 eV more stable. All four calculated mechanisms are in agreement with the isotope effects and collision energy dependencies of the product ion cross sections that have been previously observed experimentally following collisions between CF3(2+) and H2/D2. We find that in this open-shell system, CCSD(T) and QCISD(T) T1-diagnostic values of up to 0.04 are acceptable. A series of angularly resolved crossed-beam scattering experiments on collisions of CF3(2+) with D2 have also been performed. These experiments show two distinct channels leading to the formation of DCF2+. One channel appears to correspond to the pathway leading to the ground state 1DCF2+ + D+ + F product asymptote and the other to the 3DCF2+ + D+ + F product asymptote, which is 5.76 eV higher in energy. The experimental kinetic energy releases for these channels, 7.55 and 1.55 eV respectively, have been determined from the velocities of the DCF2+ product ion and are in agreement with the reaction mechanisms calculated quantum chemically. We suggest that both of these observed experimental channels are governed by the reaction mechanism we calculate in which charge separation occurs first by loss of a proton, without further hydrogen atom rearrangement, followed by loss of an F atom to give the final products 1DCF2+ + D+ + F or 3DCF2+ + D+ + F.     

Selected Ion Flow Tube Study of the Gas-Phase Reactions of CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+) with C(2)H(4), C(2)H(3)F, CH(2)CF(2), and C(2)HF(3)

We study how the degree of fluorine substitution for hydrogen atoms in ethene affects its reactivity in the gas phase. The reactions of a series of small fluorocarbon cations (CF(+), CF(2)(+), CF(3)(+), and C(2)F(4)(+)) with ethene (C(2)H(4)), monofluoroethene (C(2)H(3)F), 1,1-difluoroethene (CH(2)CF(2)), and trifluoroethene (C(2)HF(3)) have been studied in a selected ion flow tube. Rate coefficients and product cations with their branching ratios were determined at 298 K. Because the recombination energy of CF(2)(+) exceeds the ionization energy of all four substituted ethenes, the reactions of this ion produce predominantly the products of nondissociative charge transfer. With their lower recombination energies, charge transfer in the reactions of CF(+), CF(3)(+), and C(2)F(4)(+) is always endothermic, so products can only be produced by reactions in which bonds form and break within a complex. The trends observed in the results of the reactions of CF(+) and CF(3)(+) may partially be explained by the changing value of the dipole moment of the three fluoroethenes, where the cation preferentially attacks the more nucleophilic part of the molecule. Reactions of CF(3)(+) and C(2)F(4)(+) are significantly slower than those of CF(+) and CF(2)(+), with adducts being formed with the former cations. The reactions of C(2)F(4)(+) with the four neutral titled molecules are complex, giving a range of products. All can be characterized by a common first step in the mechanism in which a four-carbon chain intermediate is formed. Thereafter, arrow-pushing mechanisms as used by organic chemists can explain a number of the different products. Using the stationary electron convention, an upper limit for Δ(f)H°(298)(C(3)F(2)H(3)(+), with structure CF(2)═CH-CH(2)(+)) of 628 kJ mol(-1) and a lower limit for Δ(f)H°(298)(C(2)F(2)H(+), with structure CF(2)═CH(+)) of 845 kJ mol(-1) are determined.     

Gas-phase positive ion chemistry of 1-bromo-1-chloro-2,2,2-trifluoroethane (halothane) upon electron ionization within an ion trap mass spectrometer

The positive ion chemistry occurring within an ion trap mass spectrometer upon electron ionization of 1-bromo-1-chloro-2,2,2-trifluoroethane, the important anaesthetic halothane, has been mapped by means of collision-induced decomposition and ion/molecule self-reaction experiments. Ionized halothane (M+*) reacts with neutral halothane to form the ionized olefin [ClBrC=CF2]+*. via HF elimination. Among the ionic fragments, [M-Br]+ and [M-F]+ react with halothane via chloride abstraction while [M-Cl]+ is unreactive under the same experimental conditions. Substituted methyl cations CHFX+ and CF2X+ (X = F, Cl, Br) undergo halide transfer processes, their reactivity being highest for X = F. Ionized carbenes CXY+ (X,Y = F,F; H,Br; H,Cl; H,F) react with halothane to form CClXY+ and CBrXY+, whereas CF+ inserts into the C-Cl bond to form CF3+ and CClF2+. Finally, Br+ and Cl+ react with halothane by charge transfer. Collision-induced dissociation experiments disclosed interesting rearrangements involved in the dissociations of +CHX-CF3 ions (X = Br, Cl), which undergo fluorine migration and elimination of CF2, as already observed for +CCl2-CF3 in a previous investigation.     

The metastable formation of di-ethylchloronium ions from ethylchloride dimer ions in a seeded molecular beam

Cluster ions of ethylchloride and their dissociation products have been produced in a supersonic expansion of ethylchloride seeded in Ar and energy selected by the threshold photoelectron photoion coincidence (TPEPICO) method. The peak widths of the ion time of flight distribution indicate that all of the clusters are produced by dissociative photoionization of higher order clusters. Thus, trimer ions dissociate to form dimer ions and an ethylchloride monomer. This dimer ion was found to be metastable with respect to the formation of the di-ethylchloronium ion and a chlorine atom. The measured dissociation rate as a function of the dimer ion internal energy was compared to the calculated rates based on the statistical RRKM/QET theory. Good agreement was found when the dimer adiabatic IP was assumed to be 10.2 eV. The Cl loss from the ethylchloride dimer ion is associated with a reverse activation energy of about 0.32 eV.     

Interaction of low-energy ions and atoms of light elements with a fluorinated carbon molecular lattice

The mechanism of interaction of low-energy atoms and ions of light elements (H, H+, He, Li, the kinetic energy of the particles 2-40 eV) with C6H6, C6F12, C60, and C60F48 molecules was studied by ab initio MD simulations and quantum-chemical calculations. It was shown that starting from 6 A from the carbon skeleton for the "C6H6 + proton" and "C60 + proton" systems, the electronic charge transfer from the aromatic molecule to H+ occurs with a probability close to 1. The process transforms the H+ to a hydrogen atom and the neutral C6H6 and C60 molecules to cation radicals. The mechanism of interaction of low-energy protons with C6F12 and C60F48 molecules has a substantially different character and can be considered qualitatively as the interaction between a neutral molecule and a point charge. The Coulomb perturbation of the system arising from the interaction of the uncompensated proton charge with the Mulliken charges of fluorine atoms results in an inversion of the energies of the electronic states localized on the proton and on the C6F12 and C60F48 molecules and makes the electronic charge transfer energetically unfavorable. On the different levels of theory, the barriers of the proton penetration for the C6F12 and C60F48 molecules are from two to four times lower than those for the corresponding parent systems (C6H6 and C60). The penetration barriers of the He atom and Li+ ion depend mainly on the effective radii of the bombarding particles. The theoretical penetration and escaped barriers for the "Li+ + C60" process qualitatively explain the experimental conditions of synthesis of the Li@C60 complex.     

Gas-phase ion/molecule reactions in C5F8

Gas-phase ion-molecule reactions in octafluorocyclopentene (C5F8) were studied with a pulsed electron beam mass spectrometer. When a few Torr of major gas, CH4, Ar, or N2, containing approximately 10 mTorr C5F8 was ionized by 2 keV electrons, C5F8+, C5F7+, C4F6+, C4F5+, and C3F3+ were formed as major fragment ions. The interaction between those ions and C5F8 is found to be a weak electrostatic interaction. The cation...C5F8 bonding energies are around 10 kcal/mol, which were reproduced well by (U)B3LYP/6-311+G(d) calculations. The proton affinity of C5F8 (=148.6 kcal/mol by B3LYP/6-311+G(d)) was found to be smaller than that of C2H4 (=162.8 kcal/mol). In the negative mode of operation, the intense signal of C5F8- was observed during the electron pulse. This indicates that C5F8 has a positive electron affinity (1.27 eV by (U)B3LYP/6-311+G(d)). The C5F8- ion was quickly converted to a complex C10F16-. This complex did not react further with C5F8 down to 170 K. The theoretical calculation revealed that a C5F7-F-...C5F8 interaction mode in (C5F8)2- was converted to a C5F7*...C5F9- one via fluoride-ion transfer. The F- ion was found to form a strong covalent bond with C5F8, but the interaction in F-(C5F8)- - -C5F8 is a weak electrostatic interaction due to the charge dispersal in F-(C5F8). The halide ions except F- interact with C5F8 only weakly. Thermochemical stabilities for the cluster ions I-(CH3I)n (n = 1, 2) were also determined.     

Cross sections and ion kinetic energy analysis for the electron impact ionization of acetylene

Using a Nier-type electron impact ion source in combination with a double focusing two sector field mass spectrometer, partial cross sections for electron impact ionization of acetylene are measured for electron energies up to 1000 eV. Discrimination factors for ions are determined using the deflection field method in combination with a three-dimensional ion trajectory simulation of ions produced in the ion source. Analysis of the ion yield curves obtained by scanning the deflectors allows the assignment of ions with the same mass-to-charge ratio to specific production channels on the basis of their different kinetic energy distributions. This analysis also allows to determine, besides kinetic energy distributions of fragment ions, partial cross sections differential in kinetic energy. Moreover a charge separation reaction, the Coulomb explosion of the doubly charged parent ions C2H2++ into the fragment ions C2H+ and H+, is investigated and its mean kinetic energy release (KER=3.88 eV) is deduced.     

Dissociative photoionization of mono-, di- and trimethylamine studied by a combined threshold photoelectron photoion coincidence spectroscopy and computational approach

Energy selected mono-, di- and trimethylamine ions were prepared by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Below 13 eV, the main dissociative photoionization path of these molecules is hydrogen atom loss. The ion time-of-flight (TOF) distributions and breakdown diagrams for H loss are analyzed in terms of the statistical RRKM theory, which includes tunneling. Experimental evidence, supported by quantum chemical calculations, indicates that the reverse barrier along the H loss potential energy curve for monomethylamine is 1.8 +/- 0.6 kJ mol(-1). Accurate dissociation onset energies are derived from the TOF simulation, and from this analysis we conclude that Delta(f)H degrees (298K)[CH(2)NH(2)(+)] = 750.4 +/- 1.3 kJ mol(-1) and Delta(f)H degrees (298K)[CH(2)NH(CH(3))(+)] = 710.9 +/- 2.8 kJ mol(-1). Quantum chemical calculations at the G3, G3B3, CBS-APNO and W1U levels are extensively used to support the experimental data. The comparison between experimental and ab initio isodesmic reaction heats also suggests that Delta(f)H degrees (298K)[N(CH(3))(3)] = -27.2 +/- 2 kJ mol(-1), and that the dimethylamine ionization energy is 8.32 +/- 0.03 eV, both of which are in slight disagreement with previous experimental values. Above 13 eV photon energy, additional dissociation channels appear besides the H atom loss, such as a sequential C(2)H(4) loss from trimethylamine for which a dissociation mechanism is proposed.     

Photofragmentation of the K-shell excited perfluorocyclobutane: anisotropies in the fragments and breakdown pathways

Total ion yield spectrum of perfluorocyclobutane (c-C(4)F(8)) has been measured in the C and F K-shell excitation regions. The peak assignments are presented based on angle-resolved photofragment ion mass spectrometry. The peaks at 291.34 and 688.5 eV are found to come from the transitions from the C 1s and the F 1s to the lowest unoccupied b(2)σ(CF)* orbital, respectively. A photoelectron-photoion-photoion coincidence spectrum is acquired at 700.1 eV for clarifying the breakdown pathways of c-C(4)F(8)(2+). Two series of the pathways are identified; fission of F atom(s) followed by charge separation and elimination of CF(2) or CF(3) followed by charge separation.     

Two-channel dissociation of chemically and thermally activated n-butylbenzene cations (C10H14+)

The charge-transfer reaction O(2)(+) + n-butylbenzene (C(10)H(14)) --> O(2) + C(10)H(14)(+) was studied in a turbulent ion flow tube at temperatures between 423 and 548 K and pressures between 15 and 250 Torr in the buffer gases He and N(2). Under chemical activation conditions stabilization vs dissociation ratios S/D of vibrationally highly excited C(10)H(14)(+)* as well as branching ratios of the fragments C(7)H(7)(+) (m/z = 91) vs C(7)H(8)(+) (m/z = 92) of the dissociation of C(10)H(14)(+)* were measured. Under thermal activation conditions, the rate constant of the dominating dissociation channel 92 was measured at 498 and 523 K. Employing information on the specific rate constants k(E) of the two channels 91 and 92 and on collisional energy transfer rates from the literature, the measured S/D curves and branching ratios 91/92 could be modeled well. It is demonstrated that the charge transfer occurs approximately equally through resonant transfer and complex-forming transfer. The thermal dissociation experiments provide a high precision value of the energy barrier for the channel 92, being 1.14 (+/-0.02) eV.