From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF3SF5) by Double Velocity Map Imaging.

Trifluoromethylsulfur pentafluoride (CF3SF5) was valence threshold photoionized in a double imaging photoelectron photoion coincidence spectrometer using vacuum ultraviolet synchrotron radiation. In the 12.5-16.4 eV photon energy range, CF3+, SF5+, and SF3+ cations were observed in both room temperature (RT) and molecular beam (MB) experiments. Their fractional abundances exhibited differences beyond the sample temperature. Kinetic energy analysis of the fragment ions confirmed the difference in the dissociative photoionization mechanism. In the RT experiment, the CF3+ kinetic energies were extrapolated to a 11.84 ± 0.15 eV threshold, which was used in an ion cycle to determine the enthalpy of formation of CF3SF5 as ΔfH°298K(CF3SF5) = -1593 ± 16 kJ mol-1. We also updated the enthalpy of formation of the sulfur pentafluoride radical as ΔfH°298K(SF5) = -854 ± 7 kJ mol-1 and discuss the discrepancy between the CF3 ionization energy based on the Active Thermochemical Tables and the value anchored to the CF ionization energy. A computed reaction enthalpy network optimization resulted in ΔfH°298K(CF3SF5) = -1608 ± 20 kJ mol-1. Both values for ΔfH°298K(CF3SF5) agree with previous ab initio ones in contrast to the original, experimental determination. SF3+ is formed by F-transfer processes both in the RT and MB experiments. Although the same peaks were observed in both experiments, the lower SF3+ onset energy and the more slowly rising CF3+ kinetic energy release in the MB experiment revealed clustering and intracluster F-transfer reactions upon ionization. The monomer and dimer cation potential energy surfaces were explored to rationalize the observations. In the dimer cation, the observer CF3SF5 catalyzes fluorine transfer and promotes CF4 formation, which ultimately leads to the SF3+ fragment ion.

Advances in threshold photoelectron spectroscopy (TPES) and threshold photoelectron photoion coincidence (TPEPICO).

The history and evolution of molecular threshold photoelectron spectroscopy and threshold photoelectron photoion coincidence spectroscopy (TPEPICO) over the last fifty years are reviewed. Emphasis is placed on instrumentation and the extraction of dynamical information about energy selected ion dissociation, not on the detailed spectroscopy of certain molecules. Three important advances have expanded greatly the power of the technique, and permitted its implementation on modern synchrotron radiation beamlines. The use of velocity focusing of threshold electrons onto an imaging detector in the 1990s simultaneously improved the sensitivity and electron energy resolution, and also facilitated the subtraction of hot electron background in both threshold electron spectroscopy and TPEPICO studies. The development of multi-start multi-stop collection detectors for both electrons and ions in the 2000s permitted the use of the full intensity of modern synchrotron radiation thereby greatly improving the signal-to-noise ratio. Finally, recent developments involving imaging electrons in a range of energies as well as ions onto separate position-sensitive detectors has further improved the collection sensitivity so that low density samples found in a variety of studies can be investigated. As a result, photoelectron photoion coincidence spectroscopy is now well positioned to address a range of challenging problems that include the quantitative determination of compositions of isomer mixtures, and the detection and spectroscopy of free radicals produced in pyrolysis or discharge sources as well as in combustion studies.

Coincident velocity map image reconstruction illustrated by the single-photon valence photoionisation of CF3SF5.

Velocity map imaging offers high energy resolution and collection efficiency of the steady flux of photoelectrons and ions in continuous photoionisation experiments. In this proof-of-principle work, we show by the photoionisation of trifluoromethyl sulphur pentafluoride, CF3SF5, that the four-dimensional problem of reconstructing coincident velocity map images of electrons and ions of certain mass can be addressed by separating the energy distribution from the angular anisotropy. The energy spectrum is predominantly determined by the radial distribution of the image, whereas laboratory frame angular anisotropies are revealed based on the radial distribution of the image multiplied with a 2nd-degree Legendre polynomial. The reconstruction yields the energy correlation between the photoion and the photoelectron characteristic of the photoelectron spectrum and the kinetic energy release. The angular anisotropy ß-parameter maps of the photoelectrons and photoions are also obtained as 2D functions of the electron and ion kinetic energies. For photoionisation of CF3SF5, the energy correlation reveals suprastatistical kinetic energy release (KER) in CF3+ production in the ground cationic X[combining tilde]+ state, but statistical KER in the excited Ã+ and B[combining tilde]+ state bands. Although the photoelectron distribution is isotropic, the photoion anisotropy in the energy range of the X[combining tilde]+ state speaks for prompt dissociation after preferential ionisation of CF3SF5 molecules aligned with the polarisation vector of the synchrotron radiation. The angular dependence of the photoionisation cross section is confirmed by ab initio calculations for vertical ionisation.

Shining new light on the multifaceted dissociative photoionisation dynamics of CCl4.

Internal energy selected carbon tetrachloride cations have been prepared by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy using synchrotron vacuum ultraviolet radiation. The threshold photoelectron spectrum shows a newly observed vibrational progression corresponding to the ν2(e) scissors mode of CCl4(+) in the third, B̃(2)E band. Ab initio results on the first four doublet and lowest-lying quartet electronic states along the Cl3C(+)-Cl dissociation coordinate show the B̃ state to be strongly bound, and support its relative longevity. The X̃(2)T1 and Ã(2)T2 cationic states, on the other hand, are barely bound and dissociate promptly. The C̃(2)T2 state may intersystem cross to the quartet ã state, which dissociates to a triplet state of the CCl3(+) fragment ion. This path is unique among analogous MX4(+) (M = C, Si, Ge; X = F, Cl, Br) systems, among which several have been shown to have long-lived C̃ states, which decay by fluorescence. The breakdown diagram, recorded here for the first time for the complete valence photoionisation energy range of CCl4, is interpreted in the context of literature based and CBS-QB3, G4, and W1U computed dissociative photoionisation energies. No Cl2-loss channel is observed in association with the CCl2(+) or CCl(+) fragments below the 2 or 3 Cl-loss reaction energies, and Cl2 loss is unlikely to be a major channel above them. The breakdown diagram is modelled based on the calculated dissociative photoionisation onsets and assuming a statistical redistribution of the excess energy. The model indicates that dissociation is not impulsive at higher energies, and confirms that the C̃(2)T2 state of CCl4(+) forms triplet-state CCl3(+) fragments with some of the excess energy trapped as electronic excitation energy in CCl3(+).

The kinetics and product state distributions from gas-phase reactions of small atomic and molecular cations with C2H4, C2H3F, 1,1-C2H2F2, C2HF3 and C2F4.

The reactions of twenty one gas-phase cations with C2H3F, 1,1-C2H2F2, C2HF3 and C2F4 have been studied in a selected ion flow tube at 298 K. The cations are both atomic and molecular with recombination energies in the range 6-22 eV, and the kinetics and branching ratios into product ions are revealed for all the reactions. These data, together with that from an earlier study of reactions of C(x)F(y)(+) with these four fluorinated ethenes (J. Phys. Chem. A., 2012, 116, 8119), are compared with the reactions of these ions with C2H4, where available. Nearly all the reactions have a rate coefficient close to the collisional value calculated by either Langevin or modified average dipole orientation theories. The products of the reactions of N(+) and N2(+) with C2H4 are found to be anomalous, compared to their reactions with the four fluorinated ethenes. The branching ratios into product cations are compared with those from a high resolution (ca. 0.002 eV) photoionisation (hν = 10-22 eV) study of C2H3F, 1,1-C2H2F2, C2HF3 and C2F4 (Phys. Chem. Chem. Phys., 2012, 14, 3935) in order to gauge the importance of electron transfer in ion-molecule reactions. The higher the recombination energy of the cation, the better the agreement between the two sets of product branching ratios. Where there is disagreement at lower recombination energies, it appears that there is more fragmentation of the products in the photoionisation experiment compared to the ion-molecule reactions.

Vibrational and electronic excitations in fluorinated ethene cations from the ground up.

Valence threshold photoelectron spectra of four fluorinated ethenes; C2H3F, 1,1-C2H2F2, C2HF3, and C2F4 were recorded at the Swiss Light Source with 0.002 eV resolution. The adiabatic ionization energies were found to be 10.364 ± 0.007, 10.303 ± 0.005, 10.138 ± 0.007, and 10.110 ± 0.009 eV, respectively. The electronic ground state of each cation shows well-resolved multi-component vibrational progressions, the dominant transitions being in the C=C stretching mode. Density functional theory based Franck-Condon simulations are used to model the vibrational structure and assign the spectra, sometimes revising previous assignments. An additional vibrational progression in the first photoelectron band of 1,1-C2H2F2 indicates that the ground electronic state of the molecular ion is no longer planar. It is shown that ab initio vibrational frequencies together with the observed vibrational spacings do not always suffice to assign the spectra. In addition to symmetry rules governing the transitions, it is often essential to consider the associated Franck-Condon factors explicitly. Ionization to higher lying excited valence electronic states were also recorded by threshold ionization up to 23 eV photon energy. Equation-of-motion coupled cluster with single and double substitutions for ionization potential (EOM-IP-CCSD/cc-pVTZ) calculations confirmed historic electronic state assignments, and untangled the ever more congested spectra with increasing F-substitution. Previous attempts at illuminating the intriguing dissociative photoionization mechanism of fluorinated ethenes are reconsidered in view of new computational and experimental results. We show how non-statistical F-atom loss from C2H3F(+) is decoupled from the ground state dissociation dynamics in the energy range of its C̃ state. Both the statistical and the non-statistical dissociation processes are mediated by a plethora of conical intersections.

Dissociation dynamics of fluorinated ethene cations: from time bombs on a molecular level to double-regime dissociators.

The dissociative photoionization mechanism of internal energy selected C(2)H(3)F(+), 1,1-C(2)H(2)F(2)(+), C(2)HF(3)(+) and C(2)F(4)(+) cations has been studied in the 13-20 eV photon energy range using imaging photoelectron photoion coincidence spectroscopy. Five predominant channels have been found; HF loss, statistical and non-statistical F loss, cleavage of the C-C bond post H or F-atom migration, and cleavage of the C=C bond. By modelling the breakdown diagrams and ion time-of-flight distributions using statistical theory, experimental 0 K appearance energies, E(0), of the daughter ions have been determined. Both C(2)H(3)F(+) and 1,1-C(2)H(2)F(2)(+) are veritable time bombs with respect to dissociation via HF loss, where slow dissociation over a reverse barrier is followed by an explosion with large kinetic energy release. The first dissociative ionization pathway for C(2)HF(3) and C(2)F(4) involves an atom migration across the C=C bond, giving CF-CHF(2)(+) and CF-CF(3)(+), respectively, which then dissociate to form CHF(2)(+), CF(+) and CF(3)(+). The nature of the F-loss pathway has been found to be bimodal for C(2)H(3)F and 1,1-C(2)H(2)F(2), switching from statistical to non-statistical behaviour as the photon energy increases. The dissociative ionization of C(2)F(4) is found to be comprised of two regimes. At low internal energies, CF(+), CF(3)(+) and CF(2)(+) are formed in statistical processes. At high internal energies, a long-lived excited electronic state is formed, which loses an F atom in a non-statistical process and undergoes statistical redistribution of energy among the nuclear degrees of freedom. This is followed by a subsequent dissociation. In other words only the ground electronic state phase space stays inaccessible. The accurate E(0) of CF(3)(+) and CF(+) formation from C(2)F(4) together with the now well established Δ(f)H(o) of C(2)F(4) yield self-consistent enthalpies of formation for the CF(3), CF, CF(3)(+) and CF(+) species.

A halomethane thermochemical network from iPEPICO experiments and quantum chemical calculations.

Internal energy selected halomethane cations CH(3)Cl(+), CH(2)Cl(2)(+), CHCl(3)(+), CH(3)F(+), CH(2)F(2)(+), CHClF(2)(+), and CBrClF(2)(+) were prepared by vacuum ultraviolet photoionization, and their lowest energy dissociation channel studied using imaging photoelectron photoion coincidence spectroscopy (iPEPICO). This channel involves hydrogen atom loss for CH(3)F(+), CH(2)F(2)(+), and CH(3)Cl(+), chlorine atom loss for CH(2)Cl(2)(+), CHCl(3)(+), and CHClF(2)(+), and bromine atom loss for CBrClF(2)(+). Accurate 0 K appearance energies, in conjunction with ab initio isodesmic and halogen exchange reaction energies, establish a thermochemical network, which is optimized to update and confirm the enthalpies of formation of the sample molecules and their dissociative photoionization products. The ground electronic states of CHCl(3)(+), CHClF(2)(+), and CBrClF(2)(+) do not confirm to the deep well assumption, and the experimental breakdown curve deviates from the deep-well model at low energies. Breakdown curve analysis of such shallow well systems supplies a satisfactorily succinct route to the adiabatic ionization energy of the parent molecule, particularly if the threshold photoelectron spectrum is not resolved and a purely computational route is unfeasible. The ionization energies have been found to be 11.47 ± 0.01 eV, 12.30 ± 0.02 eV, and 11.23 ± 0.03 eV for CHCl(3), CHClF(2), and CBrClF(2), respectively. The updated 0 K enthalpies of formation, Δ(f)H(o)(0K)(g) for the ions CH(2)F(+), CHF(2)(+), CHCl(2)(+), CCl(3)(+), CCl(2)F(+), and CClF(2)(+) have been derived to be 844.4 ± 2.1, 601.6 ± 2.7, 890.3 ± 2.2, 849.8 ± 3.2, 701.2 ± 3.3, and 552.2 ± 3.4 kJ mol(-1), respectively. The Δ(f)H(o)(0K)(g) values for the neutrals CCl(4), CBrClF(2), CClF(3), CCl(2)F(2), and CCl(3)F and have been determined to be -94.0 ± 3.2, -446.6 ± 2.7, -702.1 ± 3.5, -487.8 ± 3.4, and -285.2 ± 3.2 kJ mol(-1), respectively.

Selected ion flow tube study of the gas-phase reactions of CF+, CF2+, CF3+, and C2F4+ with C2H4, C2H3F, CH2CF2, and C2HF3.

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.

Vacuum-UV negative photoion spectroscopy of CH(3)F, CH(3)Cl and CH(3)Br.

Using tunable vacuum-UV radiation from a synchrotron, negative ions are detected by quadrupolar mass spectrometry following photoexcitation of three gaseous halogenated methanes CH(3)X (X = F, Cl, Br). The anions X(-), H(-), CX(-), CHX(-) and CH(2)X(-) are observed, and their ion yields recorded in the range 8-35 eV. The anions show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically described as AB + hnu--> A(-) + B(+) (+ neutrals). Absolute cross sections for ion-pair formation are obtained by calibrating the signal intensities with those of F(-) from both SF(6) and CF(4). The cross sections for formation of X(-) + CH(3)(+) are much greater than for formation of CH(2)X(-) + H(+). In common with many quadrupoles, the spectra of m/z 1 (H(-)) anions show contributions from all anions, and only for CH(3)Br is it possible to perform the necessary subtraction to obtain the true H(-) spectrum. The anion cross sections are normalised to vacuum-UV absorption cross sections to obtain quantum yields for their production. The appearance energies of X(-) and CH(2)X(-) are used to calculate upper limits to 298 K bond dissociation energies for D(o)(H(3)C-X) and D(o)(XH(2)C-H) which are consistent with literature values. The spectra suggest that most of the anions are formed indirectly by crossing of Rydberg states of the parent molecule onto an ion-pair continuum. The one exception is the lowest-energy peak of F(-) from CH(3)F at 13.4 eV, where its width and lack of structure suggest it may correspond to a direct ion-pair transition.

Vacuum-ultraviolet negative photoion spectroscopy of SF5Cl.

With use of vacuum-UV radiation from a synchrotron, gas-phase negative ions are detected by mass spectrometry following photoexcitation of SF(5)Cl. F(-), Cl(-), and SF(5)(-) are observed, and their ion yields recorded in the range 8-30 eV. F(-) and Cl(-) show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically written AB + h nu --> C(-) + D(+) (+ neutral(s)). F(-) is the strongest signal, and absolute cross sections are determined by calibrating the signal intensity with that of F(-) from SF(6) and CF(4). Resonances are observed and assigned to transitions to Rydberg states of SF(5)Cl. The Cl(-) signal is much weaker, despite the S-Cl bond being significantly weaker than the S-F bond. Appearance energies for F(-) and Cl(-) of 12.7 +/- 0.2 and 10.6 +/- 0.2 eV are determined. The spectra suggest that these ions form indirectly by crossing of Rydberg states of SF(5)Cl onto an ion-pair continuum.

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.

Selected ion flow tube cation-molecule reaction studies and threshold photoelectron photoion coincidence spectroscopy of cyclic-C5F8.

The product ion branching ratios and rate coefficients have been measured using a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C5F8 with several atomic and molecular cations. The majority of reactions occur at the collisional rate calculated by the modified average dipole orientation theory, with the exception of H2O+ for which the reaction efficiency is only 55%. Apart from H2O+ and N+, the similarity of the product ion branching ratios determined from threshold photoelectron photoion coincidence (TPEPICO) and ion-molecule data suggests that long-range electron transfer is the dominant mechanism for reactions involving ions with recombination energies between 12 and 17 eV. For N+, the product ion branching ratios are very different to those produced by photoionisation; this result may be explained if some of the N-atom products are formed electronically excited. The onset of an ionisation signal of c-C5F8 measured by TPEPICO spectroscopy occurs at 12.25 +/- 0.05 eV. This is much higher than the value of the first adiabatic ionisation energy determined from electron ionisation (11.24 +/- 0.10 eV), He (I) photoionisation (11.30 +/- 0.05 eV), and an independent high resolution threshold photoelectron spectrum (11.237 +/- 0.002 eV). The ground electronic state of c-C5F8+ has very weak intensity under threshold electron conditions. The TPEPICO spectrum of c-C5F8 recorded from 12-23 eV shows detection of the parent ion and the daughter ions C4F6+ and C5F7+, with their appearance energies increasing in this order. Ion yield curves and branching ratios have been determined. Using Gaussian 03, the enthalpy of formation of c-C5F8 at 298 K has been determined to be -1495 kJ mol(-1).

Threshold photoelectron photoion coincidence spectroscopy and selected ion flow tube cation-molecule reaction studies of cyclic-C4F8.

Using tunable vacuum-UV radiation from a synchrotron, the threshold photoelectron and threshold photoelectron photoion coincidence (TPEPICO) spectra of cyclic-C4F8 in the range 11-25 eV have been recorded. The parent ion is observed very weakly at threshold, 11.60 eV, and is most likely to have cyclic geometry. Ion yield curves and branching ratios have been determined for five fragments. Above threshold, the first ion observed is C3F5+, at slightly higher energy C2F4+, then successively CF+, CF2+ and CF3+ are formed. The dominant ions are C3F5+ and C2F4+, with the data suggesting the presence of a barrier in the exit channel to production of C3F5+ whilst no barrier to production of C2F4+. In complementary experiments, the product branching ratios and rate coefficients have been measured in a selected ion flow tube (SIFT) at 298 K for the bimolecular reactions of cyclic-C4F8 with a large number of atomic and small molecular cations. Below the energy where charge transfer becomes energetically allowed, only one of the ions, CF2+, reacts. Above this energy, all but one of the remaining ions react. Experimental rate coefficients are consistently greater than the collisional values calculated from modified average dipole orientation theory. The inclusion of an additional ion-quadrupole interaction has allowed better agreement to be achieved. With the exception of N+, a comparison of the fragment ion branching ratios from the TPEPICO and SIFT data suggest that long-range charge transfer is the dominate mechanism for reactions of ions with recombination energy between 12.9 and 15.8 eV. For all other ions, either short-range charge transfer or a chemical reaction, involving cleavage and making of new bond(s), is the dominant mechanism.

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.

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.

Threshold photoelectron-photoion coincidence spectroscopy study of ChCl2F+, CHClF2+ and CH2ClF+: steric influence of the chlorine, fluorine and hydrogen atoms.

The threshold photoelectron spectrum and threshold photoelectron-photoion coincidence spectra of CHCl2F, CHClF2 and CH2ClF are reported in the range 11.3-24.8 eV. Tunable photoionizing radiation with a resolution of 0.3 nm is provided from a synchrotron source with a vacuum-UV monochromator. The coincidence spectra are recorded continuously as a function of photon energy, allowing yields of the fragment ions to be obtained. Energetic comparisons suggest that the major products of the titled molecules dissociate in a similar manner at low photon energy, with the parent and first fragment ion, corresponding to cleavage of the weakest bond, appearing at their thermochemical thresholds. The second major ion, corresponding to cleavage of the second weakest bond, is formed ca. 1 eV higher than its predicted threshold, this disparity implying state-selected dissociation. CHCl2F and CHClF2 fragment in a similar manner at higher photon energies, with minor ions formed by the cleavage of three bonds possessing lower appearance energies than fragment ions formed by the cleavage of two bonds. CH2ClF displays the more expected behaviour, namely sequential bond cleavage as the photon energy increases. These observations can be rationalised in terms of the height of the barrier on the exit channel, as determined by the steric bulk of the leaving group. For the three titled molecules, mean translational kinetic energy releases have also been measured into the channels involving C-F or C-Cl bond fission. These data infer that impulsive dissociations occur at lower energy, with a trend towards statistical behaviour with increasing photon energy. Competition between statistical and impulsive processes is observed, for example C-Cl vs. C-F bond cleavage in CHCl2F+ and CHClF2+.