•1-Iodoalkanes dissociatively photoionize to produce 2-alkyl cations.•Appearance energies from propyl to heptyl iodide show a shift to lower energies.•In heptyl iodide, alkyl-chain fragmentation happens shortly after the iodine loss.•Isodesmic reaction calculations allow the derivation of thermochemical onsets.•Reverse barriers are derived from the experimental data and the calculations.Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy has been used to determine 0 K appearance energies for the unimolecular dissociation reactions of several energy selected straight chain alkyl iodide cations 1-CnH2n+1I+ → CnH2n+1+ + I, (n = 3–7). The 0 K appearance energy of iodine atom loss, yielding in fact the 2-alkyl radical cation up to n = 6, was determined to be 9.836 ± 0.010, 9.752 ± 0.010, 9.721 ± 0.010, 9.684 ± 0.010 and 9.688 ± 0.015 eV in 1-C3H7I, 1-C4H9I, 1-C5H11I, 1-C6H13I, and 1-C7H15I, respectively. In 1-iodohexane and the smaller molecules, these correspond to the transition state along the 1-iodoalkane cation → 2-iodoalkane cation reaction path, and can be used in conjunction with isodesmic reaction energies to determine the reverse barriers to dissociative photoionization. The small kinetic shift is indicative of little H tunneling during isomerization. Directly computed reverse barriers show that run-of-the-mill computational approaches are of limited use when applied to open shell systems containing period 5 elements. Hindered rotors were found to play a minor role in the internal energy distribution and the dissociation rate constants.

The dissociative photoionization of internal energy selected diethyl ether ions was investigated by imaging photoelectron photoion coincidence spectroscopy. In a large, 5 eV energy range Et2O+ cations decay by two parallel and three sequential dissociative photoionization channels, which can be modeled well using statistical theory. The 0 K appearance energies of the CH3CHOCH2CH3+ (H-loss, m/z = 73) and CH3CH2O═CH2+ (methyl-loss, m/z = 59) fragment ions were determined to be 10.419 ± 0.015 and 10.484 ± 0.008 eV, respectively. The reemergence of the hydrogen-loss ion above 11 eV is attributed to transition-state (TS) switching, in which the second, outer TS is rate-determining at high internal energies. At 11.81 ± 0.05 eV, a secondary fragment of the CH3CHOCH2CH3+ (m/z = 73) ion, protonated acetaldehyde, CH3CH═OH+ (m/z = 45) appears. On the basis of the known thermochemical onset of this fragment, a reverse barrier of 325 meV was found. Two more sequential dissociation reactions were examined, namely, ethylene and formaldehyde losses from the methyl-loss daughter ion. The 0 K appearance energies of 11.85 ± 0.07 and 12.20 ± 0.08 eV, respectively, indicate no reverse barrier in these processes. The statistical model of the dissociative photoionization can also be used to predict the fractional ion abundances in threshold photoionization at large temperatures, which could be of use in, for example, combustion diagnostics.

Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy on isolated water molecules and water dimers establishes a new route to determining the water proton affinity (PA) with unprecedented accuracy. A floating thermochemical cycle constructed from the OH+ and H3O+ appearance energies and three other spectroscopic values establishes the water PA as 683.22 ± 0.25 kJ mol−1 at 0 K, which converts to 688.81 ± 0.25 kJ mol−1 at room temperature. The experimental results are corroborated by a hierarchy of coupled-cluster calculations up to pentuple excitations and septuple-ζ basis set. Combined with diagonal Born–Oppenheimer and Dirac–Coulomb–Gaunt relativistic corrections, they provide the best theoretical estimate for both the hydronium ion's geometry and a water PA of 683.5 ± 0.4 kJ mol−1 and 689.1 ± 0.4 kJ mol−1 at 0 K and 298.15 K, respectively.

Internal energy selected dimethyl disulfide and dimethyl diselenide cations were prepared by vacuum ultraviolet threshold photoionization in Imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy experiments. XH-, CH3- and CHnX-loss reactions (n = 2–4, X = S, Se) were observed in both samples with varying branching ratios. SH loss from dimethyl disulfide, DMDS, and SeH loss from dimethyl diselenide were both found to be slow at threshold, and proceed through a tight transition state. By modeling the breakdown diagram and the ion time-of-flight distributions to extract unimolecular dissociation rates to account for kinetic shifts, we obtained a new, significantly revised 0 K SH-loss CH3SCH2+ appearance energy. At slightly higher energies, CHnX+ (n = 2–4) fragments are observed, still in the metastable energy range of the parent ion. Later, CH3-loss outcompetes the lower energy channels and becomes dominant. At yet higher energies, the CH3-loss fragment ion, probably CH3X2+, forms CHX+ by H2X abstraction. The newly obtained 0 K appearance energies are used in the ion cycle to discuss the heats of formation of CH3SCH2+, CH3S2+, CH2S+, C2H5Se+, and CH3Se2+.

Photoionization mass spectrometry is a powerful method for time- and space-resolved chemical analysis of reactants, intermediates, and products. Tunable synchrotron light enables isomer separation based on unique photoionization spectra; however, mixtures of three or more isomers can be difficult to resolve by this method. In this work we demonstrate, by measuring the photoelectron spectrum corresponding to each cation m/z ratio, that imaging photoelectron photoion coincidence spectroscopy provides a more detailed spectral fingerprint than photoionization spectra. This method offers increased selectivity for analyzing gas-phase mixtures of many components. Mixtures of two C4H6 and four C5H8 unsaturated hydrocarbons were analyzed, and the components could easily be identified based on mass-selected threshold photoelectron spectra (scanning the photon energy) or photoelectron velocity map images (PEVMIs) at a single or a few fixed photon energies. The PEVMI approach is more highly multiplexed, trading somewhat lower spectral resolution for faster data acquisition and photoelectron angular distributions.Keywords: imaging photoelectron photoion coincidence; mass-selected TPES; photoionization; threshold photoelectron spectroscopy; velocity map imaging;

Metal–cyclopentadienyl bond dissociation energies (BDEs) were measured for seven metallocene ions (Cp2M+, Cp = η5-cyclopentadienyl = c-C5H5, M = Ti, V, Cr, Mn, Fe, Co, Ni) using threshold collision-induced dissociation (TCID) performed in a guided ion beam tandem mass spectrometer. For all seven room temperature metallocene ions, the dominant dissociation pathway is simple Cp loss from the metal. Traces of other fragment ions were also detected, such as C10H10+, C10H8+, C8H8+, C3H3+, H2M+, C3H3M+, C6H6M+, and C7H6M+, depending on the metal center. Statistical modeling of the Cp-loss TCID experimental data, including consideration of energy distributions, multiple collisions, and kinetic shifts, allow the extraction of 0 K [CpM+– Cp] BDEs. These are found to be 4.85 ± 0.15, 4.02 ± 0.14, 4.22 ± 0.13, 3.51 ± 0.12, 4.26 ± 0.15, 4.57 ± 0.15, and 3.37 ± 0.12 eV for Cp2Ti+, Cp2V+, Cp2Cr+, Cp2Mn+, Cp2Fe+, Cp2Co+, and Cp2Ni+, respectively. The measured BDE trend is largely in line with arguments based on a simple molecular orbital picture, with the exception of the anomalous case of titanocene, most likely attributable to its bent structure. The new results presented here are compared to previous literature values and are found to provide a more complete and accurate set of thermochemistry.

Threshold photoelectron photoion coincidence spectroscopy was used to study a series of cobalt–organic complexes with phosphine and phosphine analogue ligands: PMe3Co(CO)2NO, PEt3Co(CO)2NO, AsMe3Co(CO)2NO, and SbMe3Co(CO)2NO. The two lowest energy dissociative photoionization channels were sequential carbonyl losses in all four cases. Nitrosyl loss was also observed as a minor channel from the molecular ion and as a major competitive dissociation from the first (carbonyl loss) daughter ion. Further sequential CO and NO losses lead to the LCo+ (L = PMe3, PEt3, AsMe3, SbMe3) ions, which, similarly to an earlier threshold collision-induced dissociation (TCID) mass spectrometry study on the phosphine complexes,(1) exhibited parallel ethene loss and methane loss dissociation reactions, although the bare metal ion was not observed. Unimolecular statistical rate theory (RRKM) calculations were performed to model the first two carbonyl loss channels and relate cobalt–carbonyl bond energy trends to the electron donor and acceptor properties of the phosphine analogue ligands. Co–CO bond energies of 0.90 ± 0.09, 0.84 ± 0.08, 1.13 ± 0.08, and 1.15 ± 0.09 eV were obtained in LCo(CO)2NO+ (L = PMe3, PEt3, AsMe3, SbMe3, respectively) and 0.82 ± 0.11, 0.74 ± 0.11, 0.95 ± 0.10, and 0.94 ± 0.09 in the first daughter ions, respectively.

Internal energy selected bromofluoromethane cations were prepared and their internal energy dependent fragmentation pathways were recorded by imaging photoelectron photoion coincidence spectroscopy (iPEPICO). The first dissociation reaction is bromine atom loss, which is followed by fluorine atom loss in CF3Br and CF2Br2 at higher energies. Accurate 0 K appearance energies have been obtained for these processes, which are complemented by ab initio isodesmic reaction energy calculations. A thermochemical network is set up to obtain updated heats of formation of the samples and their dissociative photoionization products. Several computational methods have been benchmarked against the well-known interhalogen heats of formation. As a corollary, we stumbled upon an assignment issue for the ClF heat of formation leading to a 5.7 kJ mol–1 error, resolved some time ago, but still lacking closure because of outdated compilations. Our CF3+ appearance energy from CF3Br confirms the measurements of Asher and Ruscic ( J. Chem. Phys. 1997, 106, 210) and Garcia et al. ( J. Phys. Chem. A 2001, 105, 8296) as opposed to the most recent result of Clay et al. ( J. Phys. Chem. A 2005, 109, 1541). The ionization energy of CF3 is determined to be 9.02–9.08 eV on the basis of a previous CF3–Br neutral bond energy and the CF3 heat of formation, respectively. We also show that the breakdown diagram of CFBr3+, a weakly bound parent ion, can be used to obtain the accurate adiabatic ionization energy of the neutral of 10.625 ± 0.010 eV. The updated 298 K enthalpies of formation ΔfHo(g) for CF3Br, CF2Br2, CFBr3, and CBr4 are reported to be −647.0 ± 3.5, −361.0 ± 7.4, −111.6 ± 7.7, and 113.7 ± 4 kJ mol–1, respectively.

Metallocene ions (Cp2M+, M = Cr, Co, Ni) were studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) to investigate the mechanism, energetics, and kinetics of the ionic dissociation processes. The examined energy-selected Cp2M+ ions fragment by losing the neutral cyclopentadienyl ligand. In addition, CH and C2H2 losses appear as minor channels, while the cobaltocene ion also loses an H atom. A possible isomerization pathway has also been observed for Cp2Ni+, yielding a complex with pentafulvalene (C10H8) with a loss of H2. In order to determine the 0 K appearance energies for the CpM+ fragment ions, the asymmetric time-of-flight peak shapes and the breakdown diagrams of the energy-selected metallocene ions were modeled by both the rigid activated complex (RAC) Rice−Ramsperger−Kassel−Marcus (RRKM) theory and the simplified statistical adiabatic channel model (SSACM). The following appearance energies were obtained with SSACM, which is more reliable for loose transition states: 10.57 ± 0.14, 11.01 ± 0.13, and 10.18 ± 0.13 eV for M = Cr, Co, and Ni, respectively. These values combined with the corresponding adiabatic ionization energies yield M−Cp bond dissociation energies in Cp2M+ ions of 5.04 ± 0.16, 5.77 ± 0.15, and 3.96 ± 0.15 eV. Density functional calculations at the B3LYP/6-311G(d,p) level of theory were used to determine the structures of these complexes and to provide parameters necessary for the analysis of the experimental data. The trends in the M−Cp bond energies can be related to the electronic structures of the metallocene ions based on a simple molecular orbital picture.

The dissociative photoionization onsets of Br and I loss reactions were measured for C2H5Br and C2H5I, respectively, by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy to establish the heats of formation of these two basic ethyl halides. The appearance energy of ethyl cation from ethyl bromide was found to be 1074.2 ± 0.8 kJ mol−1, and that from ethyl iodide was found to be 1016.4 ± 0.8 kJ mol−1. The heats of formation of ethyl bromide and ethyl iodide are interconnected through the ethyl cation. In establishing the thermochemistry of the ethyl halides, the ethyl cation heat of formation was concluded to be 915.5 ± 1.3 kJ mol−1 on the basis of a recent value for ethyl radical heat of formation and the well-established ionization energy and on the basis of ab initio isodesmic calculations using recent PEPICO data. Using this anchor, we obtained the following heats of formation: ΔfH0K°[EtBr] = −40.8 ± 1.5 kJ mol−1 and ΔfH0K°[EtI] = 6.3 ± 1.5 kJ mol−1. These results are more consistent with the higher EtBr heats of formation values in the literature, contrary to recent findings. For ethyl iodide, the latest calorimetric value does not agree within the claimed accuracy.

The unimolecular dissociation of 1,1-dimethylhydrazine ions was studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO). Time-of-flight distributions and breakdown curves were recorded in the photon energy range of 9.5−10.4 eV. The 0 K appearance energies of the fragment ions were extracted by modeling the experimental data with rigid activated complex (RAC-) RRKM theory. It was found that the data could be well-reproduced with a single TS for each dissociation channel if two different H-loss channels were assumed, one corresponding to a C−H and the other to a N−H bond dissociation. Once the appearance energies were established, heats of formation of the fragment ions could be derived. The heat of formation of the neutral molecule was computed by applying composite ab initio methods (G3, CBS-APNO, W1U) on a series of isodesmic reactions between methyl hydrazines and methyl amines.

The dissociative photoionization of four compounds, SCl2, S2Cl2, SOCl2, and SO2Cl2, were measured with threshold and imaging photoelectron photoion coincidence spectrometry (TPEPICO and iPEPICO). In all systems, the molecular ion loses a chlorine atom in a fast dissociation. The 0 K appearance energies of the first chlorine-loss fragment ions were determined to be 12.252 ± 0.012 eV, 11.205 ± 0.003 eV, 11.709 ± 0.003 eV, and 12.505 ± 0.003 eV, respectively. SCl2 was measured on the laboratory-based TPEPICO instrument, in which the second Cl-loss dissociation could not be observed within the available photon energy. For S2Cl2+ and SOCl2+, the appearance energy of the fragment ion after two chlorine-loss dissociations were determined to be 13.32 ± 0.02 eV and 14.88 ± 0.02 eV, respectively. On the basis of the analysis of the breakdown curves, it was concluded that assuming three-dimensional translational degrees of freedom yields a more reliable statistical model of the product energy distributions. The literature heat of formation of the neutral precursor molecule thionyl chloride, SOCl2 does not agree with our results based on the SO+ cation and is revised by more than 10 kJ mol−1 to −198.2 ± 2.4 kJ mol−1. A particularly broad Franck−Condon gap with vanishingly small threshold electron signal in the photon energy range for the second Cl-loss reaction in SO2Cl2+ is discussed with regard to the mechanism of threshold ionization.

The electronic structure of organometallic complexes with the phosphine analogue ligands trimethylarsine and trimethylstibine was studied. Four new complexes, CpMn(CO)2AsMe3, CpMn(CO)2SbMe3, Co(CO)2NOAsMe3, and Co(CO)2NOSbMe3, were synthesized, and their He I and He II photoelectron spectra were recorded. The first ionization energies, which correspond to ionization from mainly metal d orbitals, are 6.83, 6.83, 7.58, and 7.69 eV, respectively. These numbers correspond to an approximately 1 eV uniform destabilization of the metal d orbitals, with respect to the parent carbonyls. The lone-pair orbital stabilization, with respect to the free ligands, was significant: 1.00, 0.94, 1.70, and 1.34 eV. The trends observed in these ionization energies were explained by the σ- and π-donor and π-acceptor properties of the ligands, on the basis of the changes in the hybridization of lone-pair orbitals.

Threshold photoelectron photoion coincidence spectroscopy is used to study the dissociation of energy-selected X(CH3)3+ ions (X = As, Sb, Bi) by methyl loss, the only process observed up to 2 eV above the ionization energy. The ion time-of-flight distributions and the breakdown diagrams are analyzed in terms of the statistical RRKM theory to obtain accurate ionic dissociation energies. These experiments complement previous studies on analogous trimethyl compounds of the N group where X = N and P. However, trimethylamine was observed to lose only an H atom, whereas trimethylphosphine was shown to lose methyl radical, H atom, and, to a lesser extent, methane in parallel dissociation reactions. Both kinetic and thermodynamic arguments are needed to explain these trends. The methyl radical loss has two channels: either a H transfer to the central atom, followed by CH3 loss, or a direct homolytic bond cleavage. However, the H transfer channel is blocked in trimethylamine by an H loss channel with an earlier onset, and, thus, the methyl loss is not observed. Bond energies are defined based on ab initio reaction energies and show that the main thermodynamic reason behind the trends in the energetics is the significantly weakening C═X double bond in the ion in the N → As direction. The first adiabatic ionization energies of Sb(CH3)3 and Bi(CH3)3 have also been measured by ultraviolet photoelectron spectroscopy to be 8.02 ± 0.05 and 8.08 ± 0.05 eV, respectively.

Imaging photoelectron photoion coincidence (iPEPICO) spectroscopy on isolated water molecules and water dimers establishes a new route to determining the water proton affinity (PA) with unprecedented accuracy. A floating thermochemical cycle constructed from the OH+ and H3O+ appearance energies and three other spectroscopic values establishes the water PA as 683.22 ± 0.25 kJ mol−1 at 0 K, which converts to 688.81 ± 0.25 kJ mol−1 at room temperature. The experimental results are corroborated by a hierarchy of coupled-cluster calculations up to pentuple excitations and septuple-ζ basis set. Combined with diagonal Born–Oppenheimer and Dirac–Coulomb–Gaunt relativistic corrections, they provide the best theoretical estimate for both the hydronium ion's geometry and a water PA of 683.5 ± 0.4 kJ mol−1 and 689.1 ± 0.4 kJ mol−1 at 0 K and 298.15 K, respectively.

The fragmentation processes of internal energy selected acetic acid anhydride cations, Ac2O+, were investigated by imaging photoelectron photoion coincidence (iPEPICO) spectroscopy. The first dissociation channel leads to the formation of CH3C(O)OCO+ (m/z = 87) by a CH3-loss. The 0 K appearance energy (E0) was determined to be 10.289 ± 0.010 eV, in excellent agreement with the G4-calculated 10.28 eV transition state (TS) energy. Based on the thermochemical onset of CH3C(O)OCO+, a reverse barrier of 40 kJ mol−1 was found. The second dissociation channel leads to the formation of the acetyl cation, CH3CO+ (m/z = 43). The appearance of trace amounts of acetone in the mass spectra, statistical modeling of the branching ratios, and quantum chemical calculations point to the existence of a post-transition-state bifurcation on the potential energy surface and a single TS leading to multiple products. That is, at higher excess energies, the CH3-group may swerve back along an orbiting pathway to form the acetone cation by CO2-loss instead of leaving directly. The acetone cation thus formed is then energetic enough to lose a methyl group and yield the acetyl cation at a phenomenological E0 = 10.316 ± 0.015 eV. The acetyl cation, which dominates the breakdown diagram up to 16 eV photon energy, is also formed by sequential CO2-loss from the CH3C(O)OCO+ intermediate at E0 = 10.53 ± 0.03 eV. The CH3+ (m/z = 15) fragment ion appears above 13 eV photon energy. This species can be produced directly from the parent ion or via two sequential dissociation channels: by acetyl radical loss from the acetone cation or CO-loss from the acetyl cation.