Structure and energetics of the anisole-Ar(n) (n = 1, 2, 3) complexes: high-resolution resonant two-photon and threshold ionization experiments, and quantum chemical calculations.

We present a concerted experimental and theoretical study of the anisole···Arn complexes with n = 1-3. Experimentally, anisole was seeded into a pulsed supersonic argon jet producing a molecular beam. Resonant two-photon, two-colour ionisation (R2PI) spectra of anisole···Arn complexes with n = 1-3 were obtained. Also, the photodissociation of the (1 : 1) cluster was probed synchronously by - Zero Electron Kinetic Energy Photoelectron Spectroscopy (ZEKE) - and - Mass Resolved Threshold Ionization (MATI) - measuring electrons and ions obtained from pulsed field ionization of high-n Rydberg states upon two-colour laser excitation. The experimental results are compared to quantum chemical calculations at the DFT-D3 (B-LYP/def2-QZVP level with Grimme's D3 dispersion correction) level. Structure and energetics due to microsolvation effects by the direct interaction of the argon atoms with the π-system were evaluated. The experimental binding energy of the 1 : 1 cluster is finally compared to computational results; in the S0 ground state the theoretical value based on the "gold standard" CCSD(T)/CBS calculations lies within the error bars of the observed value. In the excited state the agreement between theory and experiment is not so spectacular but relative values of observed dissociation energies (D0) in the ground and excited states and of calculated ones agree well.

Mass analyzed threshold ionization detected infrared spectroscopy: isomerization activity of the phenol-Ar cluster near the ionization threshold.

The structure of the phenol-argon cluster (PhOH-Ar) in high-n Rydberg states is investigated by the newly developed technique of mass analyzed threshold ionization detected infrared (MATI-IR) spectroscopy. This method selectively measures IR spectra of molecular clusters in very high-n Rydberg states (n > 100) utilized in zero kinetic energy (ZEKE) photoelectron and MATI spectroscopy, whose ionic cores are essentially the same as the corresponding bare cation. The MATI-IR spectrum exhibits only the free OH stretching vibration (ν) when the π-bound cluster of the neutral ground electronic state (S0) is resonantly excited via the S1 origin to Rydberg states converging to its adiabatic ionization energy level, IE0(π). When Rydberg states converging to vibrationally excited levels of the local π-bound minimum are prepared, in addition to ν also the hydrogen-bonded OH stretching vibration (ν) of the H-bonded global minimum is observed in the MATI-IR spectra, even for vibrational excitation of only 14 cm(-1) above IE0(π). These results show that the π→ H site switching reaction of the Ar ligand from the aromatic ring to the OH group proceeds only from vibrationally excited states in the π-bound cation core with a small barrier of less than 14 cm(-1) from IE0(π). On the other hand, directly photoionized PhOH(+)-Ar shows both ν and ν in the IR spectra, even when it is just ionized to IE0(π). This result implies that the ionization-induced π→ H site switching occurs without excess energy in the H-bound or π-bound cations, in contrast to very high-n Rydberg states converging to levels of the π-bound cation. The different efficiencies of the site switching for the Rydberg ion core and the bare ion and the mechanism for the π→ H site switching are interpreted by direct ionization from the π-bound to the H-bound structures in addition to the conventional vertical ionization and transitions to high-n Rydberg states.

Ionization-induced π → H site-switching in phenol-CH4 complexes studied using IR dip spectroscopy.

IR spectra of phenol-CH4 complexes generated in a supersonic expansion were measured before and after photoionization. The IR spectrum before ionization shows the free OH stretching vibration (ν(OH)) and the structure of neutral phenol-CH4 in the electronic ground state (S0) is assigned to a π-bound geometry, in which the CH4 ligand is located above the phenol ring. The IR spectrum after ionization to the cationic ground state (D0) exhibits a red shifted ν(OH) band assigned to a hydrogen-bonded cationic structure, in which the CH4 ligand binds to the phenolic OH group. In contrast to phenol-Ar/Kr, the observed ionization-induced π → H migration has unity yield for CH4. This difference is attributed to intracluster vibrational energy redistribution processes.