Binding Energies and Isomerization in Metallocene Ions from Threshold Photoelectron Photoion Coincidence Spectroscopy

Ferrocene/ferrocenium, cobaltocene/cobaltocenium and nickelocene/nickelocenium: from gas phase ionization energy to one-electron reduction potential in solvated medium

We propose a theoretical procedure for accurate determination of reduction potentials for three metallocene couples, Cp₂M⁺/Cp₂M, where M = Fe, Co and Ni. This procedure first computes the gas phase ionization energy (IE) using the explicitly correlated CCSD(T)-F12 method and includes the zero-point energy correction, core-valence electronic correlation, and relativistic and spin-orbit coupling effects. By means of Born-Haber thermochemical cycle, the one-electron reduction potential is obtained as the sum of the gas phase IE and the corresponding Gibbs free energies of solvation (ΔG_solv) for both the neutral and cationic species. Among the three solvent models (PCM, SMD and uESE) investigated here, it turns out that only the SMD model (computed at the DFT level) gives the best estimation of the value for "ΔG_solv(cation) - ΔG_solv(neutral)" and thus, combining with the accurate IE values, the theoretical protocol is capable of yielding reliable values (in V) for , and . These predictions compare favorably with the available experimental data (in V): , , and . We show that our theoretical procedure is reliable for accurate reduction potential predictions of Cp₂Fe⁺/Cp₂Fe, Cp₂Co⁺/Cp₂Co and Cp₂Ni⁺/Cp₂Ni redox couples in aqueous and non-aqueous media; the maximum absolute deviation is as small as ≈120 mV, which outperforms those of the existing theoretical methods.

Ionization of Decamethylmanganocene: Insights from the DFT-Assisted Laser Spectroscopy

Metallocenes represent one of the most important classes of organometallics with wide prospects for practical use in various fields of chemistry, materials science, molecular electronics, and biomedicine. Many applications of these metal complexes are based on their ability to form molecular ions. We report the first results concerning the changes in the molecular and electronic structure of decamethylmanganocene, Cp*₂Mn, upon ionization provided by the high-resolution mass-analyzed threshold ionization (MATI) spectroscopy supported by DFT calculations. The precise ionization energy of Cp*₂Mn is determined as 5.349 ± 0.001 eV. The DFT modeling of the MATI spectrum shows that the main structural deformations accompanying the detachment of an electron consist in the elongation of the Mn-C bonds and a change in the Me out-of-plane bending angles. Surprisingly, the DFT calculations predict that most of the reduction in electron density (ED) upon ionization is associated with the hydrogen atoms of the substituents, despite the metal character of the ionized orbital. However, the ED difference isosurfaces reveal a complex mechanism of the charge redistribution involving also the carbon atoms of the molecule.

Mechanochemistry of Cationic Cobaltocenium Mechanophore

Recent research on the mechanochemistry of metallocene mechanophores has shed light on the force-responsiveness of these thermally and chemically stable organometallic compounds. In this work, we report a combination of experimental and computational studies on the mechanochemistry of main-chain cobaltocenium-containing polymers. Ester derivatives of the cationic cobaltocenium, though isoelectronic to neutral ferrocene, are unstable in the nonmechanical control experimental conditions that were accommodated by their ferrocene analogs. Replacing the electron withdrawing C-ester linkages with electron-donating C-alkyls conferred the necessary stability and enabled the mechanochemistry of the cobaltocenium to be assessed. Despite their high bond dissociation energy, cobaltocenium mechanophores are found to be selective sites of main chain scission under sonomechanical activation. Computational CoGEF calculations suggest that the presence of a counterion to cobaltocenium plays a vital role by promoting a peeling mechanism of dissociation in conjunction with the initial slipping.

Modeling Gas-Phase Unimolecular Dissociation for Bond Dissociation Energies: Comparison of Statistical Rate Models within RRKM Theory

The Rice-Ramsperger-Kassel-Marcus (RRKM) theory provides a simple yet powerful rate theory for calculating microcanonical rate constants. In particular, it has found widespread use in combination with gas-phase kinetic experiments of unimolecular dissociations to extract experimental bond dissociation energies (BDEs). We have previously found several discrepancies between the computed BDE values and the respective experimental ones, obtained with our empirical rate model, named L-CID. To investigate the reliability of our rate model, we conducted a theoretical analysis and comparison of the performance of conventional rate models and L-CID within the RRKM framework. Using the previously published microcanonical rate data as well as reaction cross-section data, we show that the BDE values obtained with the L-CID model agree with the ones from the other rate models within the expected uncertainty bounds. Based on this agreement, we discuss the possible rationalization of the good performance of the L-CID model.

Stress-responsive properties of metallocenes in metallopolymers

This review article combines the field of metallopolymers and stress-responsiveness on a molecular level, namely, metallocenes, as emerging stress-responsive building blocks for materials.

Substituent effects on the electronic structures of sandwich compounds: new understandings provided by DFT-assisted laser ionization spectroscopy of bisarene complexes

Recent advances on substituent effects in transition metal bisarene complexes studied with high-resolution threshold ionization spectroscopy are reviewed to demonstrate new aspects of the ligand influence on electronic structures of sandwich molecules. Unprecedented accuracy in the determination of ionization energies provided by the laser techniques makes it possible to reveal and describe quantitatively such fine phenomena as isotope effects, the mutual substituent influence or variations of substituent effects on replacing the central metal atom with its Group analogues. In combination with DFT calculations, laser ionization spectroscopy unveils mechanisms of the ligand influence on unique redox properties of sandwich complexes which are of key importance for their practical use.

Conformational structures of jet-cooled acetaminophen-water clusters: a gas phase spectroscopic and computational study

Jet-cooled acetaminophen (AAP)-water clusters, AAP-(H₂O)₁, were investigated by mass-selected resonant two-photon ionization (R2PI), ultraviolet-ultraviolet hole-burning (UV-UV HB), infrared-dip (IR-dip), and infrared-ultraviolet hole-burning (IR-UV HB) spectroscopy. Each syn- and anti-AAP rotamer has three distinctive binding sites (-OH, >CO, and >NH) for a water molecule, thus 6 different AAP-(H₂O)₁ conformers are expected to exist in the molecular beam. The origin bands of the AAP(OH)-(H₂O)₁ and AAP(CO)-(H₂O)₁ conformers (including their syn- and anti-conformers) in the R2PI spectrum are shifted to red and blue compared to those of the AAP monomer, respectively. These frequency shifts upon complexation between a water molecule and a specific binding site of AAP are also predicted by theoretical calculations. The spectral assignments of the origin bands in the R2PI spectra and the IR vibrational bands in the IR-dip spectra of the four lowest-energy conformers of AAP-(H₂O)₁, [syn- and anti-AAP(OH)-(H₂O)₁ and syn- and anti-AAP(CO)-(H₂O)₁], are aided by ab initio and time-dependent density functional theory (TDDFT) calculations. Further investigation of the IR-dip spectra has revealed a hydrogen-bonded NH stretching mode, supporting the presence of the syn-AAP(NH)-(H₂O)₁ conformer. Moreover, by employing IR-UV HB spectroscopy, we have reconfirmed the existence of the syn-AAP(NH)-(H₂O)₁ conformer, which happened to be buried underneath the broad background contributed by the AAP(OH)-(H₂O)₁ conformers. These observations have led us to conclude that all of the possible conformers of AAP-(H₂O)₁ have been found in this study.

Choosing the right precursor for thermal decomposition solution-phase synthesis of iron nanoparticles: tunable dissociation energies of ferrocene derivatives

Pathways to low-temperature thermal dissociation of ferrocene derivatives as iron nanoparticle precursors.

Theoretical Study of the Dissociation Energy of First-Row Metallocenium Ions

The bond dissociation energy of a series of metallocenium ions, i.e., the energy difference of the reaction MCp2(+) → MCp(+) + Cp· (with M = Ti, V, Cr, Mn, Fe, Co, and Ni), was studied by means of multiconfigurational perturbation theory (CASPT2, RASPT2, NEVPT2) and restricted coupled cluster theory (CCSD(T)). From a comparison between the results obtained from these different methods, and a detailed analysis of their treatment of electron correlation effects, a set of MCp(+)-Cp binding energies are proposed with an accuracy of 5 kcal/mol. The computed results are in good agreement with the experimental data measured by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy but disagree with the more recent threshold collision-induced dissociation (TCID) experiments.

Modeling unimolecular reactions in photoelectron photoion coincidence experiments

A computer program has been developed to model and analyze the data from photoelectron photoion coincidence (PEPICO) spectroscopy experiments. This code has been used during the past 12 years to extract thermochemical and kinetics information for almost a hundred systems, and the results have been published in over forty papers. It models the dissociative photoionization process in the threshold PEPICO experiment by calculating the thermal energy distribution of the neutral molecule, the energy distribution of the molecular ion as a function of the photon energy, and the resolution of the experiment. Parallel or consecutive dissociation paths of the molecular ion and also of the resulting fragment ions are modeled to reproduce the experimental breakdown curves and time-of-flight distributions. The latter are used to extract the experimental dissociation rates. For slow dissociations, either the quasi-exponential fragment peak shapes or, when the mass resolution is insufficient to model the peak shapes explicitly, the center of mass of the peaks can be used to obtain the rate constants. The internal energy distribution of the fragment ions is calculated from the densities of states using the microcanonical formalism to describe consecutive dissociations. Dissociation rates can be calculated by the RRKM, SSACM or VTST rate theories, and can include tunneling effects, as well. Isomerization of the dissociating ions can also be considered using analytical formulae for the dissociation rates either from the original or the isomer ions. The program can optimize the various input parameters to find a good fit to the experimental data, using the downhill simplex algorithm.