Matthew J. Simpson

Vacuum-UV negative photoion spectroscopy of CF3Cl, CF3Br, and CF3I

Using synchrotron radiation, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of trifluorochloromethane (CF(3)Cl), trifluorobromomethane (CF(3)Br), and trifluoroiodomethane (CF(3)I). The anions F(-), X(-), F(2)(-), FX(-), CF(-), CF(2)(-), and CF(3)(-) are observed from all three molecules, where X = Cl, Br, or I, and their ion yields recorded in the range of 8-35 eV. With the exception of Br(-) and I(-), the anions observed show a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Dissociative electron attachment, following photoionization of CF(3)Br and CF(3)I as the source of low-energy electrons, is shown to dominate the observed Br(-) and I(-) signals, respectively. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of F(-) from both SF(6) and CF(4). These anion cross sections are normalized to vacuum-UV absorption cross sections, where available, and the resulting quantum yields are reported. Anion appearance energies are used to calculate upper limits to 298 K bond dissociation energies for D(o)(CF(3)-X), which are consistent with literature values. We report new data for D(o)(CF(2)I(+)-F) < or = 2.7+/-0.2 eV and Delta(f)H(o)(298)(CF(2)I(+)) < or = (598+/-22) kJ mol(-1). No ion-pair formation is observed below the ionization energy of the parent molecule for CF(3)Cl and CF(3)Br, and only weak signals (in both I(-) and F(-)) are detected for CF(3)I. These observations suggest that neutral photodissociation is the dominant exit channel to Rydberg state photoexcitation at these lower energies.

Vacuum-UV negative photoion spectroscopy of CH3F, CH3Cl and CH3Br

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-UV negative photoion spectroscopy of SF5CF3

Ion pair formation, generically described as AB-->A(+)+B(-), from vacuum-UV photoexcitation of trifluoromethyl sulfur pentafluoride, SF(5)CF(3), has been studied by anion mass spectrometry using synchrotron radiation in the photon energy range of 10-35 eV. The anions F(-), F(2)(-), and SF(x)(-) (x=1-5) are observed. With the exception of SF(5)(-), the anions observed show a linear dependence of signal with pressure, showing that they arise from ion pair formation. SF(5)(-) arises from dissociative electron attachment, following photoionization of SF(5)CF(3) as the source of low-energy electrons. Cross sections for anion production are put on to an absolute scale by calibration of the signal strengths with those of F(-) from both SF(6) and CF(4). Quantum yields for anion production from SF(5)CF(3), spanning the range of 10(-7)-10(-4), are obtained using vacuum-UV absorption cross sections. Unlike SF(6) and CF(4), the quantum yield for F(-) production from SF(5)CF(3) increases above the onset of photoionization.

Vacuum-UV negative photoion spectroscopy of gas-phase polyatomic molecules

This review describes recent experiments to detect anions following vacuum-UV photoexcitation of gas-phase polyatomic molecules. Using synchrotron radiation in the range 10–35 eV at a resolution down to 0.02 eV, negative ions formed are detected by mass spectrometry. The molecules studied in detail include CF4, SF6 and CH4; the CF3X series where X = Cl, Br, I; the CH3Y series where Y = F, Cl, Br and SF5Z where Z = CF3, Cl. Spectra and raw data only are reported for other members of the CH x F y , CH x Cl y including CCl4, and CF x Cl y series where (x + y) = 4; and saturated and unsaturated members of the C m H n and C m F n series up to m = 3. Anions detected range from atomic species such as H−, F− and Cl− through to heavier polyatomics such as , and CH2Cl−. The majority of anions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation, generically written as ABC + hν →D− + E+ + neutral(s). In a few cases, the anion signal increases much more rapidly than a linear dependence with pressure, suggesting that anions now form via a multi-step process, such as dissociative electron attachment. Cross-sections for ion-pair formation can be put on to an absolute scale by calibrating the signal strength with those of F− from SF6 and CF4, although there are difficulties associated with the determination of H− cross-sections from hydrogen-containing molecules unless this anion is dominant. Following normalisation to total vacuum-UV absorption cross-sections (where data are available), quantum yields for anion production are obtained. Cross-sections in the range ca. 10−23–10−19 cm2, and quantum yields in the range ca. 10−6–10−3 are reported. This review describes the two ion-pair mechanisms of indirect and direct formation and their differing characteristics, and the properties needed for anion formation by dissociative electron attachment. From this huge quantity of data, attempts are made to rationalise the circumstances needed for favourable formation of anions, and which anions have the largest cross-section for their formation. Since most anions form indirectly via predissociation of an initially excited Rydberg state of the parent molecule by an ion-pair continuum, it appears that the dynamics of this curve crossing is the dominant process which determines which anions are formed preferentially. The thermochemistry of the different exit channels and the microscopic properties of the anion formed do not appear to be especially significant. Finally, for the reaction ABC + hν → A − + BC+, the appearance energy of A− can be used to determine an upper limit to the bond dissociation energy of AB (to A + BC), or an upper limit to that of ABC+ (to A + BC+). Where known, the data are in excellent agreement with literature values.

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 CH4

Using synchrotron radiation in the range 12–35 eV, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of methane. Ion yields for H−, CH− and are recorded, the spectra of CH− and for the first time. All ions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of F− from SF6 and CF4. Following normalisation to total vacuum-UV absorption cross sections, quantum yields for anion production are reported. There is a major discrepancy in the H− cross section with an earlier measurement, which remains unresolved. The anions arise from both direct and indirect ion-pair mechanisms. For a generic polyatomic molecule AB, the former is defined as AB → A− + B+ (+neutrals), the latter as the predissociative crossing of an initially-excited Rydberg state of AB by an ion-pair state. In a separate experiment, the threshold photoelectron spectrum of the second valence band of CH4, ionisation to 2A1 at 22.4 eV, is recorded with an instrumental resolution of 0.004 eV; many of the Rydberg states observed in indirect ion-pair formation converge to this state. The widths of the peaks are lifetime limited, increasing with increasing v in the ν1 (a1) vibrational ladder. They are the first direct measurement of an upper value to the dissociation rate of these levels into fragment ions.

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.

Threshold photoelectron photoion coincidence spectroscopy of trichloroethene and tetrachloroethene

The threshold photoelectron, the threshold photoelectron photoion coincidence and ion breakdown spectra of trichloroethene and tetrachloroethene have been recorded from 9–22 eV. Comparisons with the equivalent data for the three dichloroethene molecules and theoretical calculations highlight the nature of the orbitals involved during photoionisation in this energy range. The ground electronic state of ( ) is bound, with excited valence states dissociating to ( ) and C2HCl+ ( ). Appearance energies suggest that C2HCl+ forms from by loss of two chlorine atoms, whereas forms from by loss of a Cl2 molecule. The translational kinetic energy release into ( ) + Cl is determined as a function of energy. In both cases, the fraction of the available energy released into translational energy of the two products decreases as the photon energy increases.

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.