The H/D loss and CH3/CD3 loss reactions from energy selected ethanol isotopologue ions C2H5OH+, C2D5OD+, CD3CH2OH+, and CH3CD2OH+ have been studied by imaging threshold photoelectron photoion coincidence (iPEPICO) spectroscopy. In the lowest energy dissociation channel, the α-carbon loses a hydrogen or a deuterium atom. Asymmetry in the daughter ion time-of-flight (TOF) peaks, an ab initio study of the reaction rates, and shifts in the phenomenological onsets between isotopologues revealed that H/D loss is slow at its onset. Tunneling through a reverse barrier along the reaction coordinate was found to play a significant role. Modeling the data with an Eckart barrier suggests that H loss from light ethanol ions proceeds via a reverse barrier of 151 meV, which agrees very well with the ab initio result of 155 meV. The higher energy methyl loss channel appears at its thermochemical threshold, but the branching ratios for methyl and H loss as a function of the ion internal energy are not entirely consistent with statistical theory. The methyl-loss signal cannot completely outcompete the hydrogen atom loss process. The shape of the photoelectron spectrum as well as our calculations indicate that the lowest energy ethanol ion structure lies considerably below the reported IE of 10.48 eV. Franck–Condon factors are favorable for ionization to a metastable ion state, which can rearrange to a more stable equilibrium structure. Combining theoretical results with previous experimental ones yields a revised ethanol adiabatic ionization energy of 10.37 eV. This applies to all isotopologues, as the isotope effect on the ionization energy is not more than a few meV.

The dissociative photoionization of tetramethyltin (Me4Sn) and hexamethylditin (Me6Sn2) has been investigated by threshold photoelectron−photoion coincidence (TPEPICO). Ions are energy-selected, and their 0 K dissociation onsets are measured by monitoring the mass spectra as a function of ion internal energy. Me4Sn+ dissociates rapidly by methyl loss, with a 0 K onset of E0 = 9.382 ± 0.020 eV. The hexamethylditin ion dissociates slowly on the time scale of the experiment (i.e., during the 40 μs flight time to the detector) so that dissociation rate constants are measured as a function of the ion energy. RRKM and the simplified statistical adiabatic channel model (SSACM) are used to extrapolate the measured rate constants for methyl and Me3Sn• loss to their 0 K dissociation onsets, which were found to be 8.986 ± 0.050 and 9.153 ± 0.075 eV, respectively. Updated values for the heats of formation of the neutral Me4Sn and Me6Sn2 are used to derive the following 298.15 K gas-phase standard heats of formation, in kJ·mol−1: ΔfHmo(Me3Sn+,g) = 746.3 ± 2.9; ΔfHmo(Me5Sn2+,g) = 705.1 ± 7.5; ΔfHmo(Me3Sn•,g) = 116.6 ± 9.7; ΔfHmo(Me2Sn,g) = 123.0 ± 16.5; ΔfHmo(MeSn+,g) = 877.8 ± 16.4. These energetic values also lead to the following 298.15 K bond dissociation enthalpies, in kJ·mol−1: BDE(Me3Sn−Me) = 284.1 ± 9.9; BDE(Me3Sn−SnMe3) = 252.6 ± 14.8.

The dissociation of energy-selected acetic acid ions (CH3COOH+) has been investigated by Threshold Photoelectron–photoion Coincidence (TPEPICO) spectroscopy. The lowest energy dissociation pathway for CH3COOH+ is OH loss, and the 0 K onset (E0) of this process is measured to be 11.641 ± 0.008 eV. The reaction rate for this step is instantaneous on the time-scale of the experiment so that no rate theory is required to extract its onset. Because the heats of formation of acetic acid and the OH radical are known to an accuracy of ±0.5 kJ/mol, we can use the OH loss onset to obtain an accurate heat of formation of the acetyl ion, ΔfH°298 K[CH3CO+] = 658.5 ± 1.0 kJ/mol, which agrees to within 0.9 kJ/mol with an earlier determination based on CH3 loss from the acetone ion. At higher energies, the acetic acid ion loses CH3 to form the HOCO+ ion in competition with the lower energy OH-loss. Two versions of the statistical reaction rate theory failed to reproduce the branching ratios for these higher energy parallel reactions. Because of the magnitude of this disagreement, we conclude that the acetic ion may dissociate non-statistically at these higher energies.Threshold photoelectron photoion coincidence study of the dissociation dynamics of the acetic acid ion. Acetyl ion onset is 11.641 eV, but the reaction turns non-statistical at higher energies.

The dissociation dynamics of energy selected i-C3H7X (X = H, Cl, Br, and I) ions have been investigated by imaging photoelectron−photoion coincidence (iPEPICO) spectroscopy using synchrotron radiation from the X04DB VUV beamline in the Swiss Light Source of the Paul Scherrer Institut. The 0 K dissociation energy (E0) for i-C3H8 was determined to be 11.624 ± 0.002 eV. This leads to a 298 K isopropyl ion heat of formation of 805.9 ± 0.5 kJ mol−1. The ΔfH298K°(i-C3H7+) combined with the measured 0 K onsets for i-C3H7+ formation from isopropyl chloride (11.065 ± 0.004 eV), isopropyl bromide (10.454 ± 0.008 eV), and isopropyl iodide (9.812 ± 0.008 eV) yields the 298 K isopropyl chloride, bromide, and iodide heats of formation of −145.7 ± 0.7, −95.6 ± 0.9, and −38.5 ± 0.9 kJ mol−1, respectively. These values provide a significant correction to literature values and reduce the error limits. Finally, the new i-C3H7+ heat of formation leads to a predicted adiabatic ionization energy for the isopropyl radical of 7.430 ± 0.012 eV and a 298 K proton affinity for propene of 744.1 ± 0.8 kJ mol−1.

Threshold photoelectron photoion coincidence has been used to prepare selected internal energy distributions of nitrosobenzene ions [C6H5NO+]. Dissociation to C6H5+ + NO products was measured over a range of internal energies and rate constants from 103 to 107 s−1 and fitted with the statistical theory of unimolecular decay. A 0 K dissociative photoionization onset energy of 10.607 ± 0.020 eV was derived by using the simplified statistical adiabatic channel model. The thermochemical network of Active Thermochemical Tables (ATcT) was expanded to include phenyl and phenylium, as well as nitrosobenzene. The current ATcT heats of formation of these three species at 0 K (298.15 K) are 350.6 (337.3) ± 0.6, 1148.7 (1136.8) ± 1.0, and 215.6 (198.6) ± 1.5 kJ mol−1, respectively. The resulting adiabatic ionization energy of phenyl is 8.272 ± 0.010 eV. The new ATcT thermochemistry for phenyl entails a 0 K (298.15 K) C−H bond dissociation enthalpy of benzene of 465.9 (472.1) ± 0.6 kJ mol−1. Several related thermochemical quantities from ATcT, including the current enthalpies of formation of benzene, monohalobenzenes, and their ions, as well as interim ATcT values for the constituent atoms, are also given.

The ASMS conference on ion spectroscopy brought together at Asilomar on October 16–20, 2009 a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation “action spectroscopic” mode became active in the next decade. These approaches remained for many years the main source of information about ionization energies, electronic states, and electronic transitions of ions. In recent years, high-resolution laser techniques coupled with pulsed field ionization and sample cooling in molecular beams have provided high precision ionization energies and vibrational frequencies of small to intermediate sized molecules, including a number of radicals. More recently, optical parametric oscillator (OPO) IR lasers and free electron lasers have been developed and employed to record the IR spectra of molecular ions in either molecular beams or ion traps. These results, in combination with theoretical ab initio molecular orbital (MO) methods, are providing unprecedented structural and energetic information about gas-phase ions.

The dissociation dynamics of energy selected neopentane, t-butyl iodide, and t-butyl hydroperoxide ions have been investigated by threshold photoelectron−photoion coincidence (TPEPICO) spectrometry. Although the methyl loss reaction from neopentane ions producing the t-butyl ion is in competition with a lower energy methane loss channel, modeling these two channels with the statistical theory of unimolecular decay provides a 0 K dissociation onset for methyl loss of 10.564 ± 0.025 eV. This leads to a 298 K t-butyl ion heat of formation of 714.3 ± 2.5 kJ·mol−1, which is some 3 kJ·mol−1 higher than the previously accepted value. The ΔfH°298K(t-C4H9+) combined with the measured 0 K onsets for t-C4H9+ formation from t-butyl iodide (9.170 ± 0.007 eV) and from t-butyl hydroperoxide (9.904 ± 0.012 eV), yields 298 K t-butyl iodide and t-butyl hydroperoxide heats of formation of −68.5 ± 2.6 kJ·mol−1 and −233.2 ± 2.8 kJ·mol−1, respectively. Finally, the new t-C4H9+ heat of formation leads to a predicted adiabatic ionization energy for the t-butyl radical of 6.86 ± 0.20 eV, and a 298 K proton affinity for isobutene of 798.8 ± 2.5 kJ·mol−1. The predicted ionization energy exceeds all measured values by 0.10 eV.

The 0 K dissociative ionization onsets of C2H3X → C2H3+ + X (X = Cl, I) are measured by threshold photoelectron-photoion coincidence spectroscopy. The heats of formation of C2H3Cl (ΔHf,0K0 = 30.2 ± 3.2 kJ mol−1 and ΔHf,298K0 = 22.6 ± 3.2 kJ mol−1) and C2H3I (ΔHf,0K0 = 140.2 ± 3.2 kJ mol−1 and ΔHf,298K0 = 131.2 ± 3.2 kJ mol−1) and C−X bond dissociation enthalpies as well as those of their ions are determined. The data help resolve a longstanding discrepancy among experimental values of the vinyl chloride heat of formation, which now agrees with the latest theoretical determination. The reported vinyl iodide heat of formation is the first reliable experimental determination. Additionally, the adiabatic ionization energy of C2H3I (9.32 ± 0.01 eV) is measured by threshold photoelectron spectroscopy.

Single-particle kinetic studies of the reaction between oleic acid and O3 have been conducted on two different types of core particles: polystyrene latex (PSL) and silica. Oleic acid was found to adsorb to both particle types in multilayer islands that resulted in an adsorbed layer of a total volume estimated to be less than one monolayer. The rate of the surface reaction between surface-adsorbed oleic acid and O3 has been shown for the first time to be influenced by the composition of the aerosol substrate in a mixed organic/inorganic particle. A Langmuir−Hinshelwood mechanism was applied to the observed dependence of the pseudo-first-order rate constant with [O3], and the resulting fit parameters for the ozone partition coefficient (KO3) and maximum first order rate constant (k1,max) suggest that the reaction proceeded faster on the less polar PSL core at lower [O3] due to the increased residence time of O3 on the PSL surface, but the reaction was ultimately more efficient on the silica surface at high [O3]. Values for the uptake coefficient, γoleic, for reaction of oleic acid on PSL spheres decrease from 2.5 × 10−5 to 1 × 10−5 with increasing [O3] from 4 to 25 ppm and overlap at high [O3] with the estimated values for γoleic on silica, which decrease from 1.6 × 10−5 to 1.3 × 10−5. The relationship between γoleic and the more common expression for γO3 is discussed.

We report on the development of a new temperature controlled inlet system for the study of gas phase unimolecular reaction dynamics using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. Temperatures in the range of 220–400 K can be achieved, with a deviation of less than 5 K over the course of a 48 h experiment. Iodine loss from energy selected 1-butyl iodide (1-C4H9I) ions was studied at four temperatures, 220, 275, 298 and 400 K. The fractional ion abundances, in the form of breakdown diagrams are presented. Particular attention is paid to the slopes of the molecular and daughter ion abundances in crossover region of the breakdown curve since they are governed solely by the internal energy distribution, P(E), of the neutral precursor. P(E) is a function of both the temperature and vibrational frequencies of the neutral precursor. A detailed discussion regarding the transposition of the neutral thermal energy distribution to the ionic manifold is presented. From the four experimental measurements, the E0 for the production of the 2-C4H9+ ion was determined to be 9.738 ± 0.015 eV.

Aerosol mass spectrometry has become an essential tool in monitoring tropospheric aerosols. Various approaches have been developed for analyzing particles that range in size from 10 nm to 10 μm in diameter, and which consist of salts, soot, crustal matter, metals, and organic molecules, often mixed together. This wide variety of particle types has generated an equally wide variety of ionization sources, which include electron impact, laser ionization, laser desorption, chemical ionization, and electron capture ionization. Some instruments are capable of single particle analysis, while others require the collection of an ensemble of particles to obtain sufficient sample for analysis. Most instruments have been designed to ionize and analyze particular classes of compounds (e.g. salts, soot, or organics). This review provides a very broad overview of the aerosol mass spectrometry field and serves as an introduction to the many papers in this issue that deal with details about specific instruments.

Recent advances in threshold photoelectron photoion coincidence (TPEPICO) make possible the analysis of several parallel and sequential dissociations of energy selected ions. The use of velocity focusing optics for the simultaneous collection of threshold and energetic electrons not only improves the resolution, but also permits subtraction of coincidences associated with “hot” electrons, thereby yielding TPEPICO data with no contamination from “hot” electrons. The data analysis takes into account the thermal energy distribution of the sample and uses statistical theory rate constants and energy partitioning in dissociation reactions to model the time of flight distributions and the breakdown diagram. Examples include CH2BrCl and P(C2H5)3. Of particular interest is the ability to extract error limits for rate constants and dissociation energies.

Threshold photoelectron–photoion coincidence (TPEPICO) spectroscopy has been used to investigate the unimolecular chemistry of gas-phase methyl 2-methyl butanoate ions [CH3CH2CH(CH3)COOCH3·+]. This ester ion isomerizes to a lower energy distonic ion [CH2CH2CH(CH3)COHOCH3·+] prior to dissociating by the loss of C2H4. The asymmetric time of flight distributions, which arise from the slow rate of dissociation at low ion energies, provide information about the ion dissociation rates. By modeling these rates with assumed k(E) functions, the thermal energy distribution for room temperature sample, and the analyzer function for threshold electrons, it was possible to extract the dissociative photoionization threshold for methyl 2-methyl butanoate which at 0 K is 9.80 ± 0.01 eV as well as the dissociation barrier of the distonic ion of 0.86 ± 0.01 eV. By combining these with an estimated heat of formation of methyl 2-methyl butanoate, we derive a 0 K heat of formation of the distonic ion CH2CH2CH(CH3)COHOCH3·+ of 101.0 ± 2.0 kcal/mol. The product ion is the enol of methyl propionate, CH3CHCOHOCH3·+, which has a derived heat of formation at 0 K of 106.0 ± 2.0 kcal/mol.

The ASMS conference on ion spectroscopy brought together at Asilomar on October 16–20, 2009 a large group of mass spectrometrists working in the area of ion spectroscopy. In this introduction to the field, we provide a brief history, its current state, and where it is going. Ion spectroscopy of intermediate size molecules began with photoelectron spectroscopy in the 1960s, while electronic spectroscopy of ions using the photodissociation “action spectroscopic” mode became active in the next decade. These approaches remained for many years the main source of information about ionization energies, electronic states, and electronic transitions of ions. In recent years, high-resolution laser techniques coupled with pulsed field ionization and sample cooling in molecular beams have provided high precision ionization energies and vibrational frequencies of small to intermediate sized molecules, including a number of radicals. More recently, optical parametric oscillator (OPO) IR lasers and free electron lasers have been developed and employed to record the IR spectra of molecular ions in either molecular beams or ion traps. These results, in combination with theoretical ab initio molecular orbital (MO) methods, are providing unprecedented structural and energetic information about gas-phase ions.The optical spectroscopy of ions is reviewed with special emphasis on recent advances with Free Electron IR lasers that permit MS/MS study of conformational isomers.Download high-res image (88KB)Download full-size image

The H/D loss and CH3/CD3 loss reactions from energy selected ethanol isotopologue ions C2H5OH+, C2D5OD+, CD3CH2OH+, and CH3CD2OH+ have been studied by imaging threshold photoelectron photoion coincidence (iPEPICO) spectroscopy. In the lowest energy dissociation channel, the α-carbon loses a hydrogen or a deuterium atom. Asymmetry in the daughter ion time-of-flight (TOF) peaks, an ab initio study of the reaction rates, and shifts in the phenomenological onsets between isotopologues revealed that H/D loss is slow at its onset. Tunneling through a reverse barrier along the reaction coordinate was found to play a significant role. Modeling the data with an Eckart barrier suggests that H loss from light ethanol ions proceeds via a reverse barrier of 151 meV, which agrees very well with the ab initio result of 155 meV. The higher energy methyl loss channel appears at its thermochemical threshold, but the branching ratios for methyl and H loss as a function of the ion internal energy are not entirely consistent with statistical theory. The methyl-loss signal cannot completely outcompete the hydrogen atom loss process. The shape of the photoelectron spectrum as well as our calculations indicate that the lowest energy ethanol ion structure lies considerably below the reported IE of 10.48 eV. Franck–Condon factors are favorable for ionization to a metastable ion state, which can rearrange to a more stable equilibrium structure. Combining theoretical results with previous experimental ones yields a revised ethanol adiabatic ionization energy of 10.37 eV. This applies to all isotopologues, as the isotope effect on the ionization energy is not more than a few meV.