Measured Appearance Energies Research Articles

Photoionization, Structures, and Energetics of Na-Doped Formic Acid-Water Clusters.

The influence of formic acid on water cluster aggregation has been investigated experimentally by mass spectrometry and tunable UV laser ionization applied to Na‐doped clusters formed in the supersonic expansion of water vapors seeded with formic acid (FA) as well as theoretically using high level quantum chemistry methods. The mass spectra of Na−FA(H2O)n clusters show an enlarging of mass distribution toward heavier clusters with respect to the Na−(H2O)n clusters, suggesting similar mass distribution in neutral clusters and an influence of formic acid in water aggregation. Density functional theory and coupled‐cluster type (DLPNO‐CCSD(T)) calculations have been used to calculate structures and energetics of neutral and ionized Na−FA(H2O)n as well as neutral FA(H2O)n. Na‐doped clusters are characterized by very stable geometries. The theoretical adiabatic ionization potential values match pretty well the measured appearance energies and the calculated first six electronic excited states show Rydberg‐type characters, indicating possible autoionization contributions in the mass spectra. Finally, theoretical calculations on neutral FA(H2O)n clusters show the possibility of similarly stable structures in small clusters containing up to n=4–5 water molecules, where FA interacts significantly with waters. This suggests that FA can compete with water molecules in the starting stage of the aggregation process, by forming stable nucleation seed.

Insights into 2-Chloropyrimidine fragmentation through a thermochemical analysis of the ionic fragments

In the present work we have studied the photoinduced ion chemistry of the 2Cl-pyrimidine molecule in the energy region 9−14 eV. The theoretical gas phase enthalpies of formation of the main fragments calculated using the G3B3 and G2 ab initio methods are compared to the experimental values, derived by the measured appearance energy of the fragments. This approach provides new insights into both the geometric structure of the ionic fragments and the basic mechanisms governing the molecular fragmentation.

Fragmentation reactions of Si2Cl6+[rad] in the gas phase—A quantum-chemical and mass-spectrometric assessment

The gas-phase ion chemistry of the hexachlorodisilane radical cation Si2Cl6+ has been reinvestigated by a combined electron ionization (EI) mass spectrometry (MS) and high-level quantum chemistry approach. The computational investigation of isomerization and fragmentation pathways of [Si2Cln]+() (n=1–6) species reveals (a) the coexistence of structures of the type Cl3Si–SiClm+1+() and chlorobridged SiCl4–SiClm+() isomers for m=1,2, and (b) a wealth of nearly isoenthalpic structures[Si2Cln]+() for each n with n=2–4, which can result from degenerate rearrangements. This structural diversity appears to be a general feature of the disilicon (radical) cations with low chlorine content. The ionization energy of Si2Cl6 determined by EI-MS (10.1±0.2stat±0.2systeV) with argon (15.759±0.001eV) used as reference, is slightly lower than previously reported experimental values, but in good agreement with the CBS-QB3 (9.87eV) and W1U (9.97eV) adiabatic ionization energy. Measured appearance energies for the lowest two fragmentation channels, which afford SiCl2+ and SiCl3+, are well in line with CBS-QB3 values and with the literature value determined for SiCl3+ in threshold photoelectron–photoion coincidence experiments. Using the experimental heats of formation (ΔfH°298) for SiCl4 and Cl· as anchor points, W1U theory was employed to derive ΔfH°298 values for Si2Cln+() with n=0–6 and for SiClm+() with m=1–4 based on the lowest energy isomer identified in our quantum chemical investigation of reaction pathways for all species investigated. W1U heats of formations obtained for select neutrals lead to a more consistent description of kinetic aspects previously inferred from metastable ion mass spectra.

Rearrangement and predissociation processes in negative molecular ions of nitrobenzenes

The reactions of resonant electron capture by the molecules of benzene nitroderivatives has been studied in the gas phase. Some fragment negative ions were found to be unstable with respect to electron autodetachment. This circumstance has been used for the determination of their structure. In particular, it has been established that the low measured appearance energy of neutral component of [M-H](-) ion beam is a result of isomerization of nitrobenzenes' molecular ion, leading to the 2-nitrosophenol structure with the subsequent formation of the phenoxide anion in the autodetaching state. The effective yield curves of some types of fragment ions demonstrate fine vibrational structures, testifying the predissociation mechanism of ion formation. For all detected ions, the absolute cross sections of formation have been measured.

A new broadly tunable (7.4–10.2 eV) laser based VUV light source and its first application to aerosol mass spectrometry

A laser based vacuum ultraviolet (VUV) light source using resonance enhanced four wave difference mixing in xenon gas was developed for near threshold ionization of organics in atmospheric aerosol particles. The source delivers high intensity pulses of VUV light (in the range of 10 10 to 10 13 photons/pulse depending on wavelength, 5 ns FWHM) with a continuously tunable wavelength from 122 nm (10.2 eV) to 168 nm (7.4 eV). The setup allows for tight (<1 mm 2) and precise focusing (μrad pointing angle adjustability), attributes required for single particle detection. The generated VUV is separated from the pump wavelengths by a custom monochromator which ensures high spectral purity and minimizes absorptive losses. The performance of the source was characterized using organic molecules in the gas phase and optimal working conditions are reported. In the gas phase measurements, photoionization efficiency (PIE) curves were collected for seven different organic species with ionization energies spanning the full wavelength range of the VUV source. The measured appearance energies are very close to the literature values of the ionization energies for all seven species. The effectiveness of the source for single particle studies was demonstrated by analysis of individual caffeine aerosols vaporized by a pulsed CO 2 laser in an ion trap mass spectrometer. Mass spectra from single particles down to 300 nm in diameter were collected. Excellent signal to noise characteristics for these small particles give a caffeine detection limit of 8 × 10 5 molecules which is equivalent to a single 75 nm aerosol, or approximately 1.5% of a 300 nm particle. The appearance energy of caffeine originating from the aerosol was also measured and found to be 7.91 ± 0.05 eV, in good agreement with literature values.

On the Determination of Monomer Dissociation Energies of Small Water Clusters from Photoionization Experiments

Recently, monomer dissociation energies of neutral water clusters were estimated via a thermodynamic cycle that utilized the measured appearance energies of vacuum ultraviolet (VUV) photoionized water clusters and the previously reported dissociation energies of protonated water clusters. The thermodynamic cycle incorrectly assumed that the energy difference between the (H2O)n+ --> (H2O)n-1H+ + OH* asymptotes (the relaxation energy) was zero. We show that these relaxation energies are large and cannot be neglected in the analysis. Thus, the neutral water cluster monomer dissociation energies cannot be directly determined from the measured ionization potentials because they are themselves involved in the appropriate thermodynamic cycle.

Energy-Resolved Photoionization of Alkylperoxy Radicals and the Stability of Their Cations

The photoionization of alkylperoxy radicals has been investigated using a newly developed experimental apparatus that combines the tunability of the vacuum ultraviolet radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory with time-resolved mass spectrometry. Methylperoxy (CH(3)OO) and ethylperoxy (C(2)H(5)OO) radicals are produced by the reaction of pulsed, photolytically produced alkyl radicals with molecular oxygen, and the mass spectrum of the reacting mixture is monitored in time by using synchrotron-photoionization with a double-focusing mass spectrometer. The kinetics of product formation is used to confirm the origins and assignments of ionized species. The photoionization efficiency curve for CH(3)OO has been measured, and an adiabatic ionization energy of (10.33 +/- 0.05) eV was determined with the aid of Franck-Condon spectral simulations, including ionization to the lowest triplet and singlet cation states. Using the appearance energy of CH(3)(+) from CH(3)OO, an enthalpy of formation for CH(3)OO of Delta(f) (CH(3)OO) = (22.4 +/- 5) kJ mol(-1) is derived. The enthalpy of formation of CH(3)OO(+) is derived as Delta(f) = (1019 +/- 7) kJ mol(-1) and the CH(3)(+)-OO bond energy as (CH(3)(+) - O(2)) = (80 +/- 7) kJ mol(-1). The C(2)H(5)OO(+) signal is not detectable; however, the time profile of the ethyl cation signal suggests its formation from dissociative ionization of C(2)H(5)OO. Electronic structure calculations suggest that hyperconjugation reduces the stability of the ethylperoxy cation, making the C(2)H(5)OO(+) ground state only slightly bound with respect to the ground-state products, C(2)H(5)(+) and O(2). The value of the measured appearance energy of C(2)H(5)(+) is consistent with dissociative ionization of C(2)H(5)OO via the Franck-Condon favored ionization to the ã (1)A' state of C(2)H(5)OO(+).

Hidden rearrangement processes in short-lived negative molecular ions

The study of resonant electron capture by nitrobenzene molecules showed that some fragmentary negative ions are unstable toward electron autodetachment. The measured appearance energy of the neutral component of an [M — H]− ion beam does not agree with the energetics of direct dissociation in a molecular ion. The estimation calculations show that the low appearance energy of [M — H]0 neutral components is caused by isomerization of a molecular ion of nitrobenzene to the 2-nitrobenzene structure followed by the formation of a phenoxide ion in the autodetachment state.

Ionization energies of argon clusters: A combined experimental and theoretical study

We have measured appearance energies of Ar(n)+, n<or=30, by electron impact of gas phase clusters. Quantum-chemical calculations have been performed to determine the adiabatic and vertical ionization energies of argon clusters up to n=4 and 6, respectively. The experimental appearance energy of the dimer ion approaches, under suitable cluster source conditions, the adiabatic ionization energy. The agreement with values obtained by photoionization and threshold photoelectron-photoion coincidence (TPEPICO) spectra demonstrates that autoionizing Rydberg states are accessible by electron impact. Appearance energies of larger clusters, though, exceed the TPEPICO values by about 0.5 eV.

How do dimethyl oxalate ions CH 3O–C(O)–C(O)–OCH 3·+ break in half? Loss of CH 3· + CO 2 Versus CH 3O–CO ·

The interesting unimolecular dissociation chemistry of dimethyl oxalate (DMO) ions, CH 3O–C(O)–C(O)–OCH 3 ·+, has been studied by vacuum ultraviolet photoionization and tandem mass spectrometry based experiments. The measured appearance energy (AE) for the generation of CH 3O–CO + (10.5 eV) is not compatible with a simple bond cleavage involving the cogeneration of the radical CH 3O–CO · whose calculated AE is 11 kcal/mol higher. However, because the CH 3O–CO · radical is thermodynamically less stable than its dissociation products CH 3 · and CO 2, by 19 kcal/mol, a two-step dissociation of ionized DMO into CH 3O–CO + + CH 3 · + CO 2 is energetically feasible. Collision induced dissociative ionization experiments clearly show that low energy DMO ions dissociate into CH 3 · + CO 2 without the intermediacy of CH 3O–CO ·. Experiments using a charged collision chamber further indicate that CO 2 is released first, followed by loss of CH 3 · and not vice versa and a mechanism is proposed. The measured AE, which we assign to the two-step process, is 8 kcal/mol higher than the calculated value. This could be due to a competitive shift caused by a prominent low energy decarbonylation reaction yielding the hydrogen bridged radical cation CH 2O · · · H · · · OC–OCH 3 ·+. However, from metastable ion observations and AE measurements on deuterium labeled DMO ions, it follows that there is no competitive shift and that the elevated AE for the two-step process corresponds to the barrier for the first step, loss of CO 2. Finally, neutralization–reionization experiments on ionized DMO and CH 3O–CO + provide evidence for the existence of CH 3O–CO · as a kinetically stable radical.

Relative dissociation energies of singly and doubly charged fullerene ions, C nz+ , for n = 52 to 70

Relative activation energies for dissociation of fullerene ions C n z+ into C n−2 z+ + C 2 have been determined for z = 1 and 2, covering the size range 52 ≤ n ≤ 70. These data are derived from measured appearance energies of fragment ions, obtained by electron impact ionization of C 70, with the help of the finite heat bath theory. The present results are compared with experimental and theoretical values for C 2 loss reported in the literature.

Cross-sections for the electron impact ionization of ND x ( x =1−3)

Absolute cross-sections for the electron impact ionization and dissociative ionization of the ND 3 molecule and the ND 2 and ND free radicals were measured from threshold to 200 eV using the fast neutral beam technique. The deuterated rather than protonated targets were used in this work to facilitate a better separation of the various product ions from a given parent in our apparatus. A common feature of all three targets studied in this work is a dominant parent ionization cross-section with absolute values (at 70 eV) of 1.21 × 10 −16 cm 2 (ND 3 +), 1.44 × 10 −16 cm 2 (ND 2 +) and 1.45 × 10 −16 cm 2 (ND +). Dissociative ionization processes for all three targets are less significant, with a single dissociative process dominating in each case, the removal of a single D atom (ND x + e − → ND x−l + + D + 2e −). The cross-section for this dominant dissociative ionization channel decreases from 1.03 × 10 −16 cm 2 for ND 2 + ND 3 to 0.85 × 10 −16 cm 2 for ND + ND 2 to 0.40 × 10 −16 cm 2 for N + ND . The measured appearance energies for the various fragment ions indicate that all fragment ions, except for D +, are formed with little excess kinetic energy. Our experimental data for ND 3 are compared with existing experimental and theoretical data for NH 3.

Unimolecular dissociation of singly- and multiply-charged fullerene ions

The formation of fragment ions from C60 due to electron impact ionization leading to a bimodal fragmentation pattern with a minimum around the cluster size of 30 is investigated with help of mass spectrometric techniques. Based on a detailed analysis of the energetics and kinetics of the singly- and multiply-charged fragment ions by two-sector field mass spectrometry (MIKE-scans) we can conclude that on the one hand some of the dissociation reactions proceed via evaporation of an intact C6 + or C4 + unit (and not via the sequential loss of smaller molecular carbon units). On the other hand, measured appearance energies and the direct observation of sequential C2-loss reactions confirm that the sequential loss of neutral C2 units is the dominant production mechanism for Cn z+ fragment ions with sizes 40 < n < 60.

Measured appearance energies of fragment ions produced by electron impact on

We have used mass spectrometric techniques to determine the appearance energies for the formation of small, singly charged fragment ions produced by controlled electron impact on under single collision conditions. The measured appearance energies range from 79 eV to 91 eV and show, within the experimental error bar of eV, little variation with the cluster size. This result is distinctly different from the behaviour of the previously measured appearance energies for the large fragment ions to and indicates that the small fragment ions are the result of a different fragmentation pathway.

Electron impact ionization of C60and C70: production and properties of parent and fragment ions studied with a two-sector field mass spectrometer

Abstract Electron impact ionization and dissociation studies of C60 and C70 carried out in our laboratory over the past three years have revealed many exciting and novel features of this new class of molecules (clusters). The most salient results are summarized in this paper, including first a description of the crossed beams apparatus and the various mass spectrometric (two-sector field) techniques used. This is followed by a discussion of the production of parent and fragment ions (with up to eight charges) via electron impact ionization of C60 and C70 including results on mass spectral fragmentation patterns and measured and calculated absolute ionization cross-section functions. The experimental results concerning the energetics of produced parent and fragment ions are presented in the next section and measured appearance energies and breakdown curves are analysed in the frame of various theoretical concepts (Rrkm, Fhbt) thereby allowing us to derive the corresponding binding energies. The last section is devoted to (i) the different aspects of the stability of singly- and multiply-charged fullerene ions (i.e. the quantitative study of various unimolecular decay channels such as monomer evaporation, sequential decay series, charge separation reactions, etc.) and, based on the measured properties of these decay reactions, to (ii) a discussion of the possible decay mechanisms.

Appearance Energies of C60 Fragment Ions Revisited

Following up on a serious discrepancy (factor of 2) in reported appearance energies for fragment ions produced by electron impact ionization of C60, we have measured here the appearance energies as a function of electron current using a crossed beam apparatus dedicated to ionization cross section measurements. Thus, it was possible to conclude that the low-energy peaks in the electron ionization curves of Baba et al. (which have been used to deduce the appearance energies) are very likely caused by consecutive ionization and excitation processes due to the high electron currents employed in these measurements. From this it follows that the true appearance energies are the high appearance energies reported earlier by Märk and co-workers (and Anderson and co-workers) which were determined at low ionizing electron currents to prevent secondary processes in the ion source. Moreover, we discuss here the crucial role of the kinetic shift in deriving activation energies and other thermochemical properties from measured appearance energies.

Dissociative electron attachment to OCS

The formation of S − and C − ions from electron impact on OCS has been investigated by using a crossed electron beam-molecular beam collision geometry. Measured appearance energies and peak energies for the formation of S − and C − ions are reported. These data were then used to identify the various channels of dissociation by employing Hess's law. It is shown that OCS, although isoelectronic in the valence shell, does not conform in its attachment properties to CO 2 and CS 2.

Ionization and Fragmentation of C60: An Electron Impact Ionization Study

Ionization efficiency curves of both singly and doubly charged fullerene ions (C[sub 60[minus]2n][sup m+], n = 1-5, m = 1 and 2) were determined for the first time, employing electron impact ionization. Appearance energies of all these fragment ions were derived from these data. Relative abundances of the fragment ions were computed from the measured ion intensities and compared with those available in the literature. The results of the present investigation are compared with the data available in the literature on multiply charged and fragment ions. Based on the measured appearance energies of the fragment ions, ionization energies for the formation of doubly charged fragment ions are derived. Enthalpies of formation of C[sub 60[minus]2n] (fragment) fullerenes are derived for the first time. The enthalpy data of fullerenes derived in the present work support the loss of C[sub 2n] as the mechanism of fragmentation. 102 refs., 15 figs., 10 tabs.

The energetics of resonant dissociative electron attachment to molecules of five-membered heterocyclic compounds

The thermochemistry of resonant dissociative electron attachment processes for furan, thiophene, selenophene, and pyrrole molecules has been studied. The structures of the dissociation products originating from negative molecular ions at energies ranging from 2 to 6 eV have been established using the measured appearance energies of fragment ions and the known thermodynamic functions of radical and molecular dissociation products. Heats of formation and electron affinities for some radicals and molecules have been assessed by calculations and estimated experimentally. It has been concluded that the majority of the fragment ions are formedvia rearrangement processes in molecular or fragment ions.

O-protonated sulfine CH 2=S=OH +. Its generation from ionized dimethylsulfoxide by delayed dissociation

Mass spectrometry based experiments show that dimethylsulfoxide (DMSO) radical cations, CH 3S(=O)CH +. 3, dissociating with rate constants < 10 5 s −1 form not CH 3S=O +, 1, but rather the more stable isomer CH 2=S=OH +, 2, via the aci-tautomer CH 2=S(OH)CH +. 3. It is proposed that the previously measured appearance energy for loss of CH 3 from DMSO corresponds to 2, leading to Δ H f( 2)= 736 kJ/mol. Ab initio calculations indicate that Δ H f( 1) may need to be revised upwards to about 815 kJ/mol. Previous labelling results on DMSO are in excellent agreement with the formation of 2.