From Iron Pentacarbonyl to the Iron Ion by Imaging Photoelectron Photoion Coincidence

Synthesis and Characterization of Stable Iron Pentacarbonyl Radical Cation Salts

The open-shell iron pentacarbonyl cation [Fe(CO)5 ].+ was isolated by deelectronation, i.e., the single-electron oxidation of the parent neutral Fe(CO)5 using [phenazineF ].+ as the [Al(ORF )4 ]- and [F-{Al(ORF )3 }2 ]- salt (RF =C(CF3 )3 ; phenazineF =perfluoro-5,10-bis(perfluorophenyl)-5,10-dihydrophenazine). [Fe(CO)5 ].+ [Al(ORF )4 ]- was fully characterized (scXRD analysis, IR, NMR, EPR, 57 Fe spectroscopy, CV and SQUID magnetization study) and, apart from being a compound of fundamental interest, may serve as a precursor for low-valent iron coordination chemistry.

Dissociative Photoionization of Methyl Vinyl Ketone-Thermochemical Anchors and a Drifting Methyl Group

The dissociative photoionization of methyl vinyl ketone (MVK), an important intermediate in the atmospheric oxidation of isoprene, has been studied by photoelectron photoion coincidence spectroscopy. In the photon energy range of 9.5-13.8 eV, four main fragment ions were detected at m/z 55, 43, 42, and 27 aside from the parent ion at m/z 70. The m/z 55 fragment ion (C₂H₃CO⁺) is formed from ionized MVK by direct methyl loss, while breaking the C-C bond on the other side of the carbonyl group results in the acetyl cation (CH₃CO⁺, m/z 43) and the vinyl radical. The m/z 42 fragment ion is formed via a CO-loss from the molecular ion after a methyl shift. The lightest fragment ion, the vinyl cation (C₂H₃⁺ at m/z 27), is produced in two different reactions: acetyl radical loss from the molecular ion and CO-loss from C₂H₃CO⁺. Their contributions to the m/z 27 signal are quantified based on the acetyl and vinyl fragment thermochemical anchors and quantum chemical calculations. Based on the experimentally derived appearance energy of the m/z 43 fragment ion, a new, experimentally derived heat of formation is proposed herein for gaseous methyl vinyl ketone (Δ_fH_0K = -94.3 ± 4.8 kJ mol^-1; Δ_fH_298K = -110.5 ± 4.8 kJ mol^-1), together with cationic heats of formation and bond dissociation energies.

Dissociation kinetics of excited ions: PEPICO measurements of Os3(CO)12 - The 7-35 eV single ionization binding energy region

In this article, we study the photoinduced dissociation pathways of a metallocarbonyl, Os₃(CO)₁₂, in particular the consecutive loss of CO groups. To do so, we performed photoelectron-photoion coincidence (PEPICO) measurements in the single ionization binding energy region from 7 to 35 eV using 45-eV photons. Zero-energy ion appearance energies for the dissociation steps were extracted by modeling the PEPICO data using the statistical adiabatic channel model. Upon ionization to the excited ionic states above 13 eV binding energy, non-statistical behavior was observed and assigned to prompt CO loss. Double ionization was found to be dominated by the knockout process with an onset of 20.9 ± 0.4 eV. The oscillator strength is significantly larger for energies above 26.6 ± 0.4 eV, corresponding to one electron being ejected from the Os₃ center and one from the CO ligands. The cross section for double ionization was found to increase linearly up to 35 eV ionization energy, at which 40% of the generated ions are doubly charged.

Thermochemistry of the smallest QOOH radical from the roaming fragmentation of energy selected methyl hydroperoxide ions

PEPICO spectroscopy and quantum-chemical calculations, including BOMD simulations, reveal the importance of dynamic effects in methyl hydroperoxide dissociative photoionization.

Reactivity of 4Fe+(CO)n=0-2 + O2: oxidation of CO by O2 at an isolated metal atom

The kinetics of ⁴Fe⁺(CO)_n=0-2 + O₂ are measured under thermal conditions from 300-600 K using a selected-ion flow tube apparatus. Both the bare metal and n = 2 cations are inert to reaction over this temperature range, but ⁴Fe⁺(CO) reacts rapidly (k = 3.2 ± 0.8 × 10^-10 cm³ s^-1 at 300 K, 52% of the collisional rate coefficient) to form FeO⁺ + CO₂. This is an example of the oxidation of CO by O₂ occurring entirely on a single non-noble metal atom. The reaction of the bare metal reaction is known to be endothermic, such that this result is expected; however, the n = 2 reaction has highly exothermic product channels available, such that the lack of reaction is surprising in light of the n = 1 reactivity. Stationary points along all three reaction coordinates are calculated using the TPSSh hybrid functional. These surfaces show that the n = 1 reaction is an example of two-state reactivity; the reaction proceeds initially on the sextet surface over a submerged barrier to a structure with an O-O bond distance longer than that in O₂, but must cross to the quartet surface in order to proceed over a second submerged barrier to rearrange to form CO₂. The n = 2 reaction does not proceed because, on all spin surfaces, the transition state corresponding to O-O separation is at higher energy than the separated reactants. The difference between the n = 1 and n = 2 reactions is not a result of steric effects, but rather because the O₂ is more strongly bound to Fe in the entrance well of the n = 1 case, and that energy is available to overcome the rate-limiting barrier to O-O cleavage. Experimental verification of some of these details are provided by guided ion beam tandem mass spectrometry results. The kinetic energy dependence of the n = 1 reaction shows evidence for a curve crossing and yields relevant thermochemistry for competing reaction channels.

Reactivity from excited state (4)FeO(+) + CO sampled through reaction of ground state (4)FeCO(+) + N2O

The kinetics of the FeCO(+) + N2O reaction have been studied at thermal energies (300-600 K) using a variable temperature selected ion flow tube apparatus. Rate constants and product branching fractions are reported. The reaction is modestly inefficient, proceeding with a rate constant of 6.2 × 10(-11) cm(3) s(-1) at 300 K, with a small negative temperature dependence, declining to 4.4 × 10(-11) cm(3) s(-1) at 600 K. Both Fe(+) and FeO(+) products are observed, with a constant branching ratio of approximately 40:60 at all temperatures. Calculation of the stationary points along the reaction coordinate shows that only the ground state quartet surface is initially sampled resulting in N2 elimination; a submerged barrier along this portion of the surface dictates the magnitude and temperature dependence of the total rate constant. The product branching fractions are determined by the behavior of the remaining (4)OFeCO(+) fragment, and this behavior is compared to that found in the reaction of FeO(+) + CO, which initially forms (6)OFeCO(+). Thermodynamic and kinetic arguments are used to show that the spin-forbidden surface crossing in this region is efficient, proceeding with an average rate constant of greater than 10(12) s(-1).

Real-time observation of ultrafast electron injection at graphene-Zn porphyrin interfaces

We report on the ultrafast interfacial electron transfer (ET) between zinc(II) porphyrin (ZnTMPyP) and negatively charged graphene carboxylate (GC) using state-of-the-art femtosecond laser spectroscopy with broadband capabilities. The steady-state interaction between GC and ZnTMPyP results in a red-shifted absorption spectrum, providing a clear indication for the binding affinity between ZnTMPyP and GC via electrostatic and π-π stacking interactions. Ultrafast transient absorption (TA) spectra in the absence and presence of three different GC concentrations reveal (i) the ultrafast formation of singlet excited ZnTMPyP*, which partially relaxes into a long-lived triplet state, and (ii) ET from the singlet excited ZnTMPyP* to GC, forming ZnTMPyP˙(+) and GC˙(-), as indicated by a spectral feature at 650-750 nm, which is attributed to a ZnTMPyP radical cation resulting from the ET process.

Shining new light on the multifaceted dissociative photoionisation dynamics of CCl4

Statisticality restored: high internal energy CCl₄⁺ dissociates mostly according to statistical theory, and an intersystem crossing path precludes fluorescence.