Effect of a single methyl substituent on the electronic structure of cobaltocene studied by computationally assisted MATI spectroscopy.

Metallocenes represent archetypical organometallic compounds playing key roles in various fields of fundamental and applied chemistry. Many of their unique properties arise from low ionization energies (IE) which can be tuned by introducing substituents into the rings. Here we report the first mass-analyzed threshold ionization (MATI) spectrum of a methylmetallocene, (Cp')(Cp)Co (Cp' = η5-C5H4Me, Cp = η5-C5H5). The presence of a single Me group allows us to study the "pure" effect of methylation without the mutual influence of substituents. The MATI technique provides an extremely high accuracy in determining the adiabatic IE of (Cp')(Cp)Co which equals 5.2097(6) eV. The effect of a Me group on the IE of cobaltocene appears to be 36% stronger than that in bis(η6-benzene)chromium. The MATI spectrum of (Cp')(Cp)Co shows a rich vibronic structure from which vibrational frequencies of the free ion are determined. This information provides a solid basis for testing the quality of quantum chemical calculations. Various levels of the DFT and coupled cluster computations are used to describe the structural and electronic transformations accompanying the detachment of an elctron from (Cp')(Cp)Co. New aspects of the methyl substituent influence on the potential energy surfaces, as well as on the inhomogeneous changes in charge density and electrostatic potential caused by ionization, are discussed.

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.

Laser spectroscopic and computational insights into unexpected structural behaviours of sandwich complexes upon ionization.

Transition-metal sandwich complexes play key roles in various fields such as fundamental and applied chemistry; many of their unique properties arise from their ability to form stable or reactive ions. The first mass-analyzed threshold ionization (MATI) spectra of mixed sandwich compounds, (Ch)(Cp)Cr and (Cot)(Cp)Ti (Ch = η7-C7H7, Cp = η5-C5H5, Cot = η8-C8H8), presented in this work provide an extremely accurate description of the electron detachment. The ionization energies of the neutrals and stabilization energies of the metal-ligand interactions upon ionization are derived from the MATI data with an accuracy of 0.0006 eV. In combination with DFT calculations, laser threshold ionization spectroscopy reveals surprisingly different structural variations accompanying the detachment of the non-bonding dz2 electron from the sandwich molecules. The geometry of (Ch)(Cp)Cr remains practically unchanged while the ionization of (Cot)(Cp)Ti causes a noticeable shortening of the inter-ring distance, similar to that resulting from the ionization of a typical antibonding orbital. Electron density analysis throws light on the nature of these amazing effects.

Two-Color Resonant Two-Photon Mass-Analyzed Threshold Ionization of 2,4-Difluoroanisole and the Additivity Relation of Ionization Energy.

We used the two-color resonant two-photon ionization and mass-analyzed threshold ionization spectroscopic techniques to record the vibronic, photoionization efficiency, and cation spectra of 2,4-difluoroanisole. The cation spectra were obtained through ionization via seven intermediate vibronic states, which involved out-of-plane ring-F, in-plane ring-F, and ring-OCH3 bending vibrations as well as in-plane ring deformation vibrations. The band origin of the S1 ← S0 electronic transition of 2,4-difluoroanisole appeared at 35 556 ± 2 cm-1, and the adiabatic ionization energy was determined to be 67 568 ± 5 cm-1. The experimental data provided information on the active vibrations of aforementioned compound in the electronically excited state S1 and ground cationic state D0. A comparison of the experimental data obtained for 2,4-difluoroanisole in this study with the data obtained for other fluorine-substituted benzenes in previous studies indicated that the nature, location, and number of substituents influence electronic transition energy, ionization energy, and molecular vibration. In addition, a simple additivity relation might exist for predicting the ionization energy of multiply substituted benzene derivatives.

Cation Vibrations of 1-Methylnaphthalene and 2-Methylnaphthalene through Mass-Analyzed Threshold Ionization Spectroscopy.

Vibrationally resolved cation spectra of 1-methylnaphthalene (1MN) and 2-methylnaphthalene (2MN) were obtained using the two-color resonant two-photon mass-analyzed threshold ionization (MATI) spectroscopy. MATI spectra were obtained through ionization via several intermediate vibronic levels. Due to hindrance effect, no spectral features related to methyl torsion were observed in the MATI spectra of 1MN. By contrast, most of the in-plane ring deformation vibrations of the 2MN cation were found to couple with methyl torsion because of its small internal rotational barrier. These experimental findings were well supported by our theoretical calculations.

Rydberg state mediated multiphoton ionization of (η7-C7H7)(η5-C5H5)Cr: DFT-supported experimental insights into the molecular and electronic structures of excited sandwich complexes.

The resonance-enhanced multiphoton ionization (REMPI) of a mixed sandwich complex has been achieved for the first time when exciting (η7-C7H7)(η5-C5H5)Cr via the Rydberg 4pz state. The REMPI spectrum is indicative of unexpectedly small changes of the sandwich geometry on excitation. Time-dependent DFT calculations reveal fine effects of the ligand nature on the molecular and electronic structure variations accompanying electronic excitation. Different trends are predicted for the sandwich geometry transformations in the mixed sandwich complex and its symmetric isomer, (η6-C6H6)2Cr, both on Rydberg excitation and ionization.

TD DFT insights into unusual properties of excited sandwich complexes: structural transformations and vibronic interactions in Rydberg-state bis(η6-benzene)chromium.

Sandwich compounds represent the only class of organometallics revealing vibronic structures of Rydberg transitions in their gas-phase absorption and ionization spectra. This provides rare possibilities of verifying computational results for Rydberg-state metal complexes by comparison with experimental spectroscopic data. In this work, the lowest Ryberg p state of bis(η6-benzene)chromium (1) corresponding to the 3dz2→ R4px,y transition has been modeled for the first time by TD DFT. The calculations were found to be able not only to estimate the energy of the Rydberg excitation in the 1 molecule but also to simulate its vibronic structure on the basis of the Rydberg-state optimized geometries and vibrational frequencies. The structural transformations caused by the Jahn-Teller effect in the excited 1 molecule appear to differ strongly from those in the degenerate-state benzene ion, cobaltocene or other metal-benzene complexes. The in-plane CH bending mode provides the main contribution to the JT distortion of the 1 excited-state D6h structure resulting in splitting of the R4px,y state into the R4px and R4py components belonging to the D2h point group. The calculations predict, however, a fluxional 1 behavior described by the D6h symmetry. Nevertheless, the JT effect leads to additional allowed vibronic components of the 3dz2→ R4px,y transition which is clearly revealed by the TD DFT simulation. The computational results correlate surprisingly well with the known experimental spectroscopic data and provide new insights into vibronic interactions in the Rydberg-state sandwich molecules.

DFT-Supported Threshold Ionization Study of Chromium Biphenyl Complexes: Unveiling the Mechanisms of Substituent Influence on Redox Properties of Sandwich Compounds.

High-resolution mass-analyzed threshold ionization (MATI) spectra of (η6 -Ph2 )2 Cr and (η6 -Ph2 )(η6 -PhMe)Cr demonstrate that the Ph groups work as electron donors, decreasing the ionization energy of the gas-phase bisarene complexes. In contrast to electrochemical data, a close similarity of the Ph and Me group effects on the oxidation of free sandwich molecules has been revealed. However, DFT calculations testify for the opposite shifts of the electron density caused by the Me and Ph substituents in the neutral complexes, the latter behaving as an electron-accepting fragment. On the contrary, in the bisarene cations, the Ph group becomes a stronger donor than methyl. This change provides the similar substituent effects observed with the MATI experiment. On the other hand, the well-documented opposite influence of the Me and Ph fragments on the redox potential of the (η6 -arene)2 Cr+/0 couple in solution appears to be a result of solvation effects but not intramolecular interactions as shown for the first time in this work.

Identification of four rotamers of m-methoxystyrene by resonant two-photon ionization and mass analyzed threshold ionization spectroscopy.

We report the vibronic and cation spectra of four rotamers of m-methoxystyrene, recorded by using the two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques. The excitation energies of the S1← S0 electronic transition are found to be 32 767, 32 907, 33 222, and 33 281 cm(-1), and the corresponding adiabatic ionization energies are 65 391, 64 977, 65 114, and 64 525 cm(-1) for these isomeric species. Most of the observed active vibrations in the electronically excited S1 and cationic ground D0 states involve in-plane ring deformation and substituent-sensitive bending motions. It is found that the relative orientation of the methoxyl with respect to the vinyl group does not influence the vibrational frequencies of the ring-substituent bending modes. The two dimensional potential energy surface calculations support our experimental finding that the isomerization is restricted in the S1 and D0 states.

Rotamers of 3,4-difluoroanisole studied by two-color resonant two-photon mass-analyzed threshold ionization spectroscopy.

We reported the vibronic and cation spectra of 3,4-difluoroanisole. The band origins of the S(1) ←S(0) electronic transition of the cis and trans rotamers appear at 35,505 ± 2 and 35,711 ± 2 cm(-1) and the adiabatic ionization energies are determined to be 67,780 ± 5 and 68,125 ± 5cm(-1), respectively. Most of the observed active vibrations in the electronically excited S(1) and cationic ground D(0) states mainly involve in-plane ring deformation and substituent-sensitive bending vibrations. Analysis of the obtained vibronic and cation spectra suggests that the molecular geometry, symmetry, and vibrational coordinates of the cation in the D(0) state are like those of the neutral species in the S(1) state for these two isomeric species.