中央研究院 原子與分子科學研究所

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 4p_z 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.

High-resolution mass-analyzed threshold ionization (MATI) spectra of (h6-Ph2)2Cr and (h6-Ph2)(h6-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 (h6-arene)2Cr+/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.

We applied two-color resonant two-photon ionization and mass-analyzed threshold ionization techniques to record the vibronic, photoionization efficiency, and cation spectra of the selected rotamers of 3-fluorostyrene. The adiabatic ionization energies of cis- and trans-3-fluorostyrene were determined to be 69 960 ± 5 and 69 856 ± 5 cm^-1, respectively. Cation vibrations 10a, 15, 6b, and 12 of both rotamers have been found to have frequencies of 218, 404, 452, and 971 cm^-1, respectively. This finding shows that the relative orientation of the vinyl group with respect to the F atom does not affect these vibrations of the 3-fluorostyrene cation. Our one-dimensional potential energy surface calculations support that the cis–trans isomerization of 3-fluorostyrene does not occur under the present experimental conditions.

Mass-analyzed threshold ionization spectra of jet-cooled [(h⁶-PhMe)(h⁶-PhH)Cr] and [(h⁶-Ph₂)(h⁶-PhH)Cr] reveal with unprecedented accuracy the effects of methyl and phenyl groups on the electronic structure of bis(h⁶- benzene)chromium. These “pure” substituent effects allow quantitative experimental determination of the ionization energy changes caused by the mutual substituent influence in bisarene systems. Two types of such influence have been revealed for the first time in bis(h⁶-toluene)- chromium.

We applied resonant two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) techniques to record the vibronic and cationic spectra of 2,5-difluorophenol. The distinct bands at 36448 and 36743 cm^-1 were confirmed as the origins of the S₁ ← S₀ electronic transition of the cis and trans rotamers, respectively. The corresponding adiabatic ionization energies were found to be 71164 and 71476 cm^-1 for these two rotameric species. The observed spectral features mainly result from the in-plane ring deformation and substituent-sensitive bending vibrations. Spectral analysis suggests that the molecular geometry and vibrational coordinates of the cation in the D₀ state resemble those of the neutral species in the S₁ state for both cis and
trans rotamers.

We applied the two-color resonant two-photon mass-analyzed threshold ionization (MATI) technique to record the cation spectra of trans-2-fluorostyrene by ionizing via six intermediate vibronic levels. The adiabatic ionization energy was determined to be 69304 ± 5 cm^-
1. The distinct MATI bands at 67, 124, 242, 355, 737, 806, 833, and 993 cm
^-1 were assigned to the active cation vibrations related to out-of-plane substituent-sensitive bending vibrations and in-plane ring deformation and bending motions. Many combination vibrations were also observed. Our experimental results suggest that the molecular geometry and vibrational coordinates of the trans-2-fluorostyrene cation in the D₀ state resemble those of the neutral species in the S₁ state.